Astronomy

Why Did The Universe Expand?

Why Did The Universe Expand?


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If all of the mass and energy in the universe was in a point smaller than the size of an electron at one time, why did it expand? Did it necessarily expand immediately on formation, or could it have remained in an initial state for some length of time until some phase threshold was reached?


It's an important question, because it tells us something important about the Big Bang model. Some think that the Big Bang explains the expansion, but actually, it takes expansion as an initial condition (and it is typical of dynamical models that they require an initial condition that is outside the model) and tracks how it proceeds. We generally don't explain why our models work, we just test our models. We would need some new model to test, one that currently does not exist or is highly speculative, to be able to say why the Big Bang model works, and why the universe is expanding.


Universe is Not Expanding After All, Controversial Study Suggests

This image shows a star forming region in a nearby galaxy known as the Large Magellanic Cloud. Image credit: ESA / Hubble.

In their study, the scientists tested one of the striking predictions of the Big Bang theory – that ordinary geometry does not work at great distances.

In the space around us, on Earth, in the Solar System and our Milky Way Galaxy, as similar objects get farther away, they look fainter and smaller. Their surface brightness, that is the brightness per unit area, remains constant.

In contrast, the Big Bang theory tells us that in an expanding Universe objects actually should appear fainter but bigger. Thus in this theory, the surface brightness decreases with the distance. In addition, the light is stretched as the Universe expanded, further dimming the light.

So in an expanding Universe the most distant galaxies should have hundreds of times dimmer surface brightness than similar nearby galaxies, making them actually undetectable with present-day telescopes.

But that is not what observations show, as demonstrated by this new study published in the International Journal of Modern Physics D.

The scientists carefully compared the size and brightness of about a thousand nearby and extremely distant galaxies. They chose the most luminous spiral galaxies for comparisons, matching the average luminosity of the near and far samples.

Contrary to the prediction of the Big Bang theory, they found that the surface brightnesses of the near and far galaxies are identical.

These results are consistent with what would be expected from ordinary geometry if the Universe was not expanding, and are in contradiction with the drastic dimming of surface brightness predicted by the expanding Universe hypothesis.

“Of course, you can hypothesize that galaxies were much smaller, and thus had hundreds of times greater intrinsic surface brightness in the past, and that, just by coincidence, the Big Bang dimming exactly cancels that greater brightness at all distances to produce the illusion of a constant brightness, but that would be a very big coincidence,” Mr Lerner said.

That was not the only startling result of their research. In order to apply the surface brightness test, first proposed in 1930 by physicist Richard C. Tolman, the team had to determine the actual luminosity of the galaxies, so as to match near and far galaxies.

To do that, the astrophysicists had to link the distance to the galaxies with their redshift. They hypothesized that the distance is proportional to the redshift at all distances, as is well verified to be the case in the nearby Universe.

They checked this relation between redshift and distance with the data on supernova brightness that has been used to measure the hypothesized accelerated expansion of the Universe.

“It is amazing that the predictions of this simple formula are as good as the predictions of the expanding Universe theory, which include complex corrections for hypothetical dark matter and dark energy,” said study co-author Dr Renato Falomo of the Osservatorio Astronomico di Padova, Italy.

Dr Riccardo Scarpa from the Instituto de Astrofısica de Canarias, Spain, who is a co-author of the study, added: “again you could take this to be merely coincidental, but it would be a second big coincidence.”

Therefore if the Universe is not expanding, the redshift of light with increasing distance must be caused by some other phenomena – something that happens to the light itself as it travels through space.

“We are not speculating now as to what could cause the redshift of light,” Mr Lerner said.

”However, such a redshift, which is not associated with expansion, could be observed with suitable spacecraft within our own Solar System in the future.”

Eric J. Lerner et al. UV surface brightness of galaxies from the local Universe to z

5. Int. J. Mod. Phys. D, published online May 02, 2014 doi: 10.1142/S0218271814500588


How the Big Bang Theory Works

Because of the limitations of the laws of science, we can't make any guesses about the instant the universe came into being. Instead, we can look at the period immediately following the creation of the universe. Right now, the earliest moment scientists talk about occurs at t = 1 x 10 -43 seconds (the "t" stands for the time after the creation of the universe). In other words, take the number 1.0 and move the decimal place to the left 43 times.

Cambridge University refers to the study of these earliest moments as quantum cosmology [source: Cambridge University]. At the earliest moments of the big bang, the universe was so small that classical physics didn't apply to it. Instead, quantum physics were in play. Quantum physics deal with physics on a subatomic scale. Much of the behavior of particles on the quantum scale seems strange to us, because the particles appear to defy our understanding of classical physics. Scientists hope to discover the link between quantum and classical physics, which will give us a lot more information about how the universe works.

At t = 1 x 10 -43 seconds, the universe was incredibly small, dense and hot. This homogenous area of the universe spanned a region of only 1 x 10 -33 centimeters (3.9 x 10 -34 inches). Today, that same stretch of space spans billions of light years. During this phase, big bang theorists believe, matter and energy were inseparable. The four primary forces of the universe were also a united force. The temperature of this universe was 1 x 10 32 degrees Kelvin (1 x 10 32 degrees Celsius , 1.8 x 10 32 degrees Fahrenheit). As tiny fractions of a second passed, the universe expanded rapidly. Cosmologists refer to the universe's expansion as inflation. The universe doubled in size several times in less than a second [source: UCLA].

As the universe expanded, it cooled. At around t = 1 x 10 -35 seconds, matter and energy decoupled. Cosmologists call this baryogenesis -- baryonic matter is the kind of matter we can observe. In contrast, we can't observe dark matter, but we know it exists by the way it affects energy and other matter. During baryogenesis, the universe filled with a nearly equal amount of matter and anti-matter. There was more matter than anti-matter, so while most particles and anti-particles annihilated each other, some particles survived. These particles would later combine to form all the matter in the universe.

A period of particle cosmology followed the quantum age. This period starts at t = 1 x 10 -11 seconds. This is a phase that scientists can recreate in lab conditions with particle accelerators. That means that we have some observational data on what the universe must have been like at this time. The unified force broke down into components. The forces of electromagnetism and weak nuclear force split off. Photons outnumbered matter particles, but the universe was too dense for light to shine within it.

Next came the period of standard cosmology, which begins .01 second after the beginning of the big bang. From this moment on, scientists feel they have a pretty good handle on how the universe evolved. The universe continued to expand and cool, and the subatomic particles formed during baryogenesis began to bond together. They formed neutrons and protons. By the time a full second had passed, these particles could form the nuclei of light elements like hydrogen (in the form of its isotope, deuterium), helium and lithium. This process is known as nucleosynthesis. But the universe was still too dense and hot for electrons to join these nuclei and form stable atoms.

That's a busy first second. Next we'll find out what happened over the next 13 billion years.

Saying that the universe is homogeneous and isotropic is another way of saying that every location in the universe is the same as every other one, and that there's no special or central spot for the universe. It's often called the Copernican or cosmological principle.


Contents

Around 1930, Edwin Hubble discovered that light from remote galaxies was redshifted the more remote, the more shifted. This was quickly interpreted as meaning galaxies were receding from Earth. If Earth is not in some special, privileged, central position in the universe, then it would mean all galaxies are moving apart, and the further away, the faster they are moving away. It is now understood that the universe is expanding, carrying the galaxies with it, and causing this observation. Many other observations agree, and also lead to the same conclusion. However, for many years it was not clear why or how the universe might be expanding, or what it might signify.

Based on a huge amount of experimental observation and theoretical work, it is now believed that the reason for the observation is that space itself is expanding, and that it expanded very rapidly within the first fraction of a second after the Big Bang. This kind of expansion is known as a "metric" expansion. In the terminology of mathematics and physics, a "metric" is a measure of distance that satisfies a specific list of properties, and the term implies that the sense of distance within the universe is itself changing. Today, metric variation is far too small an effect to see on less than an intergalactic scale.

The modern explanation for the metric expansion of space was proposed by physicist Alan Guth in 1979, while investigating the problem of why no magnetic monopoles are seen today. He found that if the universe contained a field in a positive-energy false vacuum state, then according to general relativity it would generate an exponential expansion of space. It was very quickly realized that such an expansion would resolve many other long-standing problems. These problems arise from the observation that to look like it does today, the Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang. Inflation theory largely resolves these problems as well, thus making a universe like ours much more likely in the context of Big Bang theory.

No physical field has yet been discovered that is responsible for this inflation. However such a field would be scalar and the first relativistic scalar field proven to exist, the Higgs field, was only discovered in 2012–2013 and is still being researched. So it is not seen as problematic that a field responsible for cosmic inflation and the metric expansion of space has not yet been discovered. The proposed field and its quanta (the subatomic particles related to it) have been named the inflaton. If this field did not exist, scientists would have to propose a different explanation for all the observations that strongly suggest a metric expansion of space has occurred, and is still occurring (much more slowly) today.

An expanding universe generally has a cosmological horizon, which, by analogy with the more familiar horizon caused by the curvature of Earth's surface, marks the boundary of the part of the Universe that an observer can see. Light (or other radiation) emitted by objects beyond the cosmological horizon in an accelerating universe never reaches the observer, because the space in between the observer and the object is expanding too rapidly.

The observable universe is one causal patch of a much larger unobservable universe other parts of the Universe cannot communicate with Earth yet. These parts of the Universe are outside our current cosmological horizon. In the standard hot big bang model, without inflation, the cosmological horizon moves out, bringing new regions into view. [14] Yet as a local observer sees such a region for the first time, it looks no different from any other region of space the local observer has already seen: its background radiation is at nearly the same temperature as the background radiation of other regions, and its space-time curvature is evolving lock-step with the others. This presents a mystery: how did these new regions know what temperature and curvature they were supposed to have? They couldn't have learned it by getting signals, because they were not previously in communication with our past light cone. [15] [16]

Inflation answers this question by postulating that all the regions come from an earlier era with a big vacuum energy, or cosmological constant. A space with a cosmological constant is qualitatively different: instead of moving outward, the cosmological horizon stays put. For any one observer, the distance to the cosmological horizon is constant. With exponentially expanding space, two nearby observers are separated very quickly so much so, that the distance between them quickly exceeds the limits of communications. The spatial slices are expanding very fast to cover huge volumes. Things are constantly moving beyond the cosmological horizon, which is a fixed distance away, and everything becomes homogeneous.

As the inflationary field slowly relaxes to the vacuum, the cosmological constant goes to zero and space begins to expand normally. The new regions that come into view during the normal expansion phase are exactly the same regions that were pushed out of the horizon during inflation, and so they are at nearly the same temperature and curvature, because they come from the same originally small patch of space.

The theory of inflation thus explains why the temperatures and curvatures of different regions are so nearly equal. It also predicts that the total curvature of a space-slice at constant global time is zero. This prediction implies that the total ordinary matter, dark matter and residual vacuum energy in the Universe have to add up to the critical density, and the evidence supports this. More strikingly, inflation allows physicists to calculate the minute differences in temperature of different regions from quantum fluctuations during the inflationary era, and many of these quantitative predictions have been confirmed. [17] [18]

Space expands

In a space that expands exponentially (or nearly exponentially) with time, any pair of free-floating objects that are initially at rest will move apart from each other at an accelerating rate, at least as long as they are not bound together by any force. From the point of view of one such object, the spacetime is something like an inside-out Schwarzschild black hole—each object is surrounded by a spherical event horizon. Once the other object has fallen through this horizon it can never return, and even light signals it sends will never reach the first object (at least so long as the space continues to expand exponentially).

In the approximation that the expansion is exactly exponential, the horizon is static and remains a fixed physical distance away. This patch of an inflating universe can be described by the following metric: [19] [20]

This exponentially expanding spacetime is called a de Sitter space, and to sustain it there must be a cosmological constant, a vacuum energy density that is constant in space and time and proportional to Λ in the above metric. For the case of exactly exponential expansion, the vacuum energy has a negative pressure p equal in magnitude to its energy density ρ the equation of state is p=−ρ .

Inflation is typically not an exactly exponential expansion, but rather quasi- or near-exponential. In such a universe the horizon will slowly grow with time as the vacuum energy density gradually decreases.

Few inhomogeneities remain

Because the accelerating expansion of space stretches out any initial variations in density or temperature to very large length scales, an essential feature of inflation is that it smooths out inhomogeneities and anisotropies, and reduces the curvature of space. This pushes the Universe into a very simple state in which it is completely dominated by the inflaton field and the only significant inhomogeneities are tiny quantum fluctuations. Inflation also dilutes exotic heavy particles, such as the magnetic monopoles predicted by many extensions to the Standard Model of particle physics. If the Universe was only hot enough to form such particles before a period of inflation, they would not be observed in nature, as they would be so rare that it is quite likely that there are none in the observable universe. Together, these effects are called the inflationary "no-hair theorem" [21] by analogy with the no hair theorem for black holes.

The "no-hair" theorem works essentially because the cosmological horizon is no different from a black-hole horizon, except for philosophical disagreements about what is on the other side. The interpretation of the no-hair theorem is that the Universe (observable and unobservable) expands by an enormous factor during inflation. In an expanding universe, energy densities generally fall, or get diluted, as the volume of the Universe increases. For example, the density of ordinary "cold" matter (dust) goes down as the inverse of the volume: when linear dimensions double, the energy density goes down by a factor of eight the radiation energy density goes down even more rapidly as the Universe expands since the wavelength of each photon is stretched (redshifted), in addition to the photons being dispersed by the expansion. When linear dimensions are doubled, the energy density in radiation falls by a factor of sixteen (see the solution of the energy density continuity equation for an ultra-relativistic fluid). During inflation, the energy density in the inflaton field is roughly constant. However, the energy density in everything else, including inhomogeneities, curvature, anisotropies, exotic particles, and standard-model particles is falling, and through sufficient inflation these all become negligible. This leaves the Universe flat and symmetric, and (apart from the homogeneous inflaton field) mostly empty, at the moment inflation ends and reheating begins. [22]

Duration

A key requirement is that inflation must continue long enough to produce the present observable universe from a single, small inflationary Hubble volume. This is necessary to ensure that the Universe appears flat, homogeneous and isotropic at the largest observable scales. This requirement is generally thought to be satisfied if the Universe expanded by a factor of at least 10 26 during inflation. [23]

Reheating

Inflation is a period of supercooled expansion, when the temperature drops by a factor of 100,000 or so. (The exact drop is model-dependent, but in the first models it was typically from 10 27 K down to 10 22 K. [24] ) This relatively low temperature is maintained during the inflationary phase. When inflation ends the temperature returns to the pre-inflationary temperature this is called reheating or thermalization because the large potential energy of the inflaton field decays into particles and fills the Universe with Standard Model particles, including electromagnetic radiation, starting the radiation dominated phase of the Universe. Because the nature of the inflation is not known, this process is still poorly understood, although it is believed to take place through a parametric resonance. [25] [26]

Inflation resolves several problems in Big Bang cosmology that were discovered in the 1970s. [27] Inflation was first proposed by Alan Guth in 1979 while investigating the problem of why no magnetic monopoles are seen today he found that a positive-energy false vacuum would, according to general relativity, generate an exponential expansion of space. It was very quickly realised that such an expansion would resolve many other long-standing problems. These problems arise from the observation that to look like it does today, the Universe would have to have started from very finely tuned, or "special" initial conditions at the Big Bang. Inflation attempts to resolve these problems by providing a dynamical mechanism that drives the Universe to this special state, thus making a universe like ours much more likely in the context of the Big Bang theory.

Horizon problem

The horizon problem is the problem of determining why the Universe appears statistically homogeneous and isotropic in accordance with the cosmological principle. [28] [29] [30] For example, molecules in a canister of gas are distributed homogeneously and isotropically because they are in thermal equilibrium: gas throughout the canister has had enough time to interact to dissipate inhomogeneities and anisotropies. The situation is quite different in the big bang model without inflation, because gravitational expansion does not give the early universe enough time to equilibrate. In a big bang with only the matter and radiation known in the Standard Model, two widely separated regions of the observable universe cannot have equilibrated because they move apart from each other faster than the speed of light and thus have never come into causal contact. In the early Universe, it was not possible to send a light signal between the two regions. Because they have had no interaction, it is difficult to explain why they have the same temperature (are thermally equilibrated). Historically, proposed solutions included the Phoenix universe of Georges Lemaître, [31] the related oscillatory universe of Richard Chase Tolman, [32] and the Mixmaster universe of Charles Misner. Lemaître and Tolman proposed that a universe undergoing a number of cycles of contraction and expansion could come into thermal equilibrium. Their models failed, however, because of the buildup of entropy over several cycles. Misner made the (ultimately incorrect) conjecture that the Mixmaster mechanism, which made the Universe more chaotic, could lead to statistical homogeneity and isotropy. [29] [33]

Flatness problem

The flatness problem is sometimes called one of the Dicke coincidences (along with the cosmological constant problem). [34] [35] It became known in the 1960s that the density of matter in the Universe was comparable to the critical density necessary for a flat universe (that is, a universe whose large scale geometry is the usual Euclidean geometry, rather than a non-Euclidean hyperbolic or spherical geometry). [36] : 61

Therefore, regardless of the shape of the universe the contribution of spatial curvature to the expansion of the Universe could not be much greater than the contribution of matter. But as the Universe expands, the curvature redshifts away more slowly than matter and radiation. Extrapolated into the past, this presents a fine-tuning problem because the contribution of curvature to the Universe must be exponentially small (sixteen orders of magnitude less than the density of radiation at Big Bang nucleosynthesis, for example). This problem is exacerbated by recent observations of the cosmic microwave background that have demonstrated that the Universe is flat to within a few percent. [37]

Magnetic-monopole problem

The magnetic monopole problem, sometimes called the exotic-relics problem, says that if the early universe were very hot, a large number of very heavy [ why? ] , stable magnetic monopoles would have been produced. This is a problem with Grand Unified Theories, which propose that at high temperatures (such as in the early universe) the electromagnetic force, strong, and weak nuclear forces are not actually fundamental forces but arise due to spontaneous symmetry breaking from a single gauge theory. [38] These theories predict a number of heavy, stable particles that have not been observed in nature. The most notorious is the magnetic monopole, a kind of stable, heavy "charge" of magnetic field. [39] [40] Monopoles are predicted to be copiously produced following Grand Unified Theories at high temperature, [41] [42] and they should have persisted to the present day, to such an extent that they would become the primary constituent of the Universe. [43] [44] Not only is that not the case, but all searches for them have failed, placing stringent limits on the density of relic magnetic monopoles in the Universe. [45] A period of inflation that occurs below the temperature where magnetic monopoles can be produced would offer a possible resolution of this problem: monopoles would be separated from each other as the Universe around them expands, potentially lowering their observed density by many orders of magnitude. Though, as cosmologist Martin Rees has written, "Skeptics about exotic physics might not be hugely impressed by a theoretical argument to explain the absence of particles that are themselves only hypothetical. Preventive medicine can readily seem 100 percent effective against a disease that doesn't exist!" [46]

Precursors

In the early days of General Relativity, Albert Einstein introduced the cosmological constant to allow a static solution, which was a three-dimensional sphere with a uniform density of matter. Later, Willem de Sitter found a highly symmetric inflating universe, which described a universe with a cosmological constant that is otherwise empty. [47] It was discovered that Einstein's universe is unstable, and that small fluctuations cause it to collapse or turn into a de Sitter universe.

In the early 1970s Zeldovich noticed the flatness and horizon problems of Big Bang cosmology before his work, cosmology was presumed to be symmetrical on purely philosophical grounds. [ citation needed ] In the Soviet Union, this and other considerations led Belinski and Khalatnikov to analyze the chaotic BKL singularity in General Relativity. Misner's Mixmaster universe attempted to use this chaotic behavior to solve the cosmological problems, with limited success.

False vacuum

In the late 1970s, Sidney Coleman applied the instanton techniques developed by Alexander Polyakov and collaborators to study the fate of the false vacuum in quantum field theory. Like a metastable phase in statistical mechanics—water below the freezing temperature or above the boiling point—a quantum field would need to nucleate a large enough bubble of the new vacuum, the new phase, in order to make a transition. Coleman found the most likely decay pathway for vacuum decay and calculated the inverse lifetime per unit volume. He eventually noted that gravitational effects would be significant, but he did not calculate these effects and did not apply the results to cosmology.

The universe could have been spontaneously created from nothing (no space, time, nor matter) by quantum fluctuations of metastable false vacuum causing an expanding bubble of true vacuum. [48]

Starobinsky inflation

In the Soviet Union, Alexei Starobinsky noted that quantum corrections to general relativity should be important for the early universe. These generically lead to curvature-squared corrections to the Einstein–Hilbert action and a form of f(R) modified gravity. The solution to Einstein's equations in the presence of curvature squared terms, when the curvatures are large, leads to an effective cosmological constant. Therefore, he proposed that the early universe went through an inflationary de Sitter era. [49] This resolved the cosmology problems and led to specific predictions for the corrections to the microwave background radiation, corrections that were then calculated in detail. Starobinsky used the action

which corresponds to the potential

in the Einstein frame. This results in the observables: n s = 1 − 2 N , r = 12 N 2 . =1->,quad quad r=>>.> [50]

Monopole problem

In 1978, Zeldovich noted the monopole problem, which was an unambiguous quantitative version of the horizon problem, this time in a subfield of particle physics, which led to several speculative attempts to resolve it. In 1980 Alan Guth realized that false vacuum decay in the early universe would solve the problem, leading him to propose a scalar-driven inflation. Starobinsky's and Guth's scenarios both predicted an initial de Sitter phase, differing only in mechanistic details.

Early inflationary models

Guth proposed inflation in January 1981 to explain the nonexistence of magnetic monopoles [51] [52] it was Guth who coined the term "inflation". [53] At the same time, Starobinsky argued that quantum corrections to gravity would replace the initial singularity of the Universe with an exponentially expanding de Sitter phase. [54] In October 1980, Demosthenes Kazanas suggested that exponential expansion could eliminate the particle horizon and perhaps solve the horizon problem, [55] [56] while Sato suggested that an exponential expansion could eliminate domain walls (another kind of exotic relic). [57] In 1981 Einhorn and Sato [58] published a model similar to Guth's and showed that it would resolve the puzzle of the magnetic monopole abundance in Grand Unified Theories. Like Guth, they concluded that such a model not only required fine tuning of the cosmological constant, but also would likely lead to a much too granular universe, i.e., to large density variations resulting from bubble wall collisions.

Guth proposed that as the early universe cooled, it was trapped in a false vacuum with a high energy density, which is much like a cosmological constant. As the very early universe cooled it was trapped in a metastable state (it was supercooled), which it could only decay out of through the process of bubble nucleation via quantum tunneling. Bubbles of true vacuum spontaneously form in the sea of false vacuum and rapidly begin expanding at the speed of light. Guth recognized that this model was problematic because the model did not reheat properly: when the bubbles nucleated, they did not generate any radiation. Radiation could only be generated in collisions between bubble walls. But if inflation lasted long enough to solve the initial conditions problems, collisions between bubbles became exceedingly rare. In any one causal patch it is likely that only one bubble would nucleate.

. Kazanas (1980) called this phase of the early Universe "de Sitter's phase." The name "inflation" was given by Guth (1981). . Guth himself did not refer to work of Kazanas until he published a book on the subject under the title "The inflationary universe: the quest for a new theory of cosmic origin" (1997), where he apologizes for not having referenced the work of Kazanas and of others, related to inflation. [59]

Slow-roll inflation

The bubble collision problem was solved by Linde [60] and independently by Andreas Albrecht and Paul Steinhardt [61] in a model named new inflation or slow-roll inflation (Guth's model then became known as old inflation). In this model, instead of tunneling out of a false vacuum state, inflation occurred by a scalar field rolling down a potential energy hill. When the field rolls very slowly compared to the expansion of the Universe, inflation occurs. However, when the hill becomes steeper, inflation ends and reheating can occur.

Effects of asymmetries

Eventually, it was shown that new inflation does not produce a perfectly symmetric universe, but that quantum fluctuations in the inflaton are created. These fluctuations form the primordial seeds for all structure created in the later universe. [62] These fluctuations were first calculated by Viatcheslav Mukhanov and G. V. Chibisov in analyzing Starobinsky's similar model. [63] [64] [65] In the context of inflation, they were worked out independently of the work of Mukhanov and Chibisov at the three-week 1982 Nuffield Workshop on the Very Early Universe at Cambridge University. [66] The fluctuations were calculated by four groups working separately over the course of the workshop: Stephen Hawking [67] Starobinsky [68] Guth and So-Young Pi [69] and Bardeen, Steinhardt and Turner. [70]

Inflation is a mechanism for realizing the cosmological principle, which is the basis of the standard model of physical cosmology: it accounts for the homogeneity and isotropy of the observable universe. In addition, it accounts for the observed flatness and absence of magnetic monopoles. Since Guth's early work, each of these observations has received further confirmation, most impressively by the detailed observations of the cosmic microwave background made by the Planck spacecraft. [71] This analysis shows that the Universe is flat to within 0.5 percent, and that it is homogeneous and isotropic to one part in 100,000.

Inflation predicts that the structures visible in the Universe today formed through the gravitational collapse of perturbations that were formed as quantum mechanical fluctuations in the inflationary epoch. The detailed form of the spectrum of perturbations, called a nearly-scale-invariant Gaussian random field is very specific and has only two free parameters. One is the amplitude of the spectrum and the spectral index, which measures the slight deviation from scale invariance predicted by inflation (perfect scale invariance corresponds to the idealized de Sitter universe). [72] The other free parameter is the tensor to scalar ratio. The simplest inflation models, those without fine-tuning, predict a tensor to scalar ratio near 0.1. [73]

Inflation predicts that the observed perturbations should be in thermal equilibrium with each other (these are called adiabatic or isentropic perturbations). This structure for the perturbations has been confirmed by the Planck spacecraft, WMAP spacecraft and other cosmic microwave background (CMB) experiments, and galaxy surveys, especially the ongoing Sloan Digital Sky Survey. [74] These experiments have shown that the one part in 100,000 inhomogeneities observed have exactly the form predicted by theory. There is evidence for a slight deviation from scale invariance. The spectral index, ns is one for a scale-invariant Harrison–Zel'dovich spectrum. The simplest inflation models predict that ns is between 0.92 and 0.98. [75] [73] [76] [77] This is the range that is possible without fine-tuning of the parameters related to energy. [76] From Planck data it can be inferred that ns=0.968 ± 0.006, [71] [78] and a tensor to scalar ratio that is less than 0.11. These are considered an important confirmation of the theory of inflation. [17]

Various inflation theories have been proposed that make radically different predictions, but they generally have much more fine tuning than should be necessary. [75] [73] As a physical model, however, inflation is most valuable in that it robustly predicts the initial conditions of the Universe based on only two adjustable parameters: the spectral index (that can only change in a small range) and the amplitude of the perturbations. Except in contrived models, this is true regardless of how inflation is realized in particle physics.

Occasionally, effects are observed that appear to contradict the simplest models of inflation. The first-year WMAP data suggested that the spectrum might not be nearly scale-invariant, but might instead have a slight curvature. [79] However, the third-year data revealed that the effect was a statistical anomaly. [17] Another effect remarked upon since the first cosmic microwave background satellite, the Cosmic Background Explorer is that the amplitude of the quadrupole moment of the CMB is unexpectedly low and the other low multipoles appear to be preferentially aligned with the ecliptic plane. Some have claimed that this is a signature of non-Gaussianity and thus contradicts the simplest models of inflation. Others have suggested that the effect may be due to other new physics, foreground contamination, or even publication bias. [80]

An experimental program is underway to further test inflation with more precise CMB measurements. In particular, high precision measurements of the so-called "B-modes" of the polarization of the background radiation could provide evidence of the gravitational radiation produced by inflation, and could also show whether the energy scale of inflation predicted by the simplest models (10 15 –10 16 GeV) is correct. [73] [76] In March 2014, the BICEP2 team announced B-mode CMB polarization confirming inflation had been demonstrated. The team announced the tensor-to-scalar power ratio r was between 0.15 and 0.27 (rejecting the null hypothesis r is expected to be 0 in the absence of inflation). [81] However, on 19 June 2014, lowered confidence in confirming the findings was reported [82] [83] [84] on 19 September 2014, a further reduction in confidence was reported [85] [86] and, on 30 January 2015, even less confidence yet was reported. [87] [88] By 2018, additional data suggested, with 95% confidence, that r is 0.06 or lower: consistent with the null hypothesis, but still also consistent with many remaining models of inflation. [81]

Other potentially corroborating measurements are expected from the Planck spacecraft, although it is unclear if the signal will be visible, or if contamination from foreground sources will interfere. [89] Other forthcoming measurements, such as those of 21 centimeter radiation (radiation emitted and absorbed from neutral hydrogen before the first stars formed), may measure the power spectrum with even greater resolution than the CMB and galaxy surveys, although it is not known if these measurements will be possible or if interference with radio sources on Earth and in the galaxy will be too great. [90]

Is the theory of cosmological inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflaton field giving rise to inflation?

In Guth's early proposal, it was thought that the inflaton was the Higgs field, the field that explains the mass of the elementary particles. [52] It is now believed by some that the inflaton cannot be the Higgs field [91] although the recent discovery of the Higgs boson has increased the number of works considering the Higgs field as inflaton. [92] One problem of this identification is the current tension with experimental data at the electroweak scale, [93] which is currently under study at the Large Hadron Collider (LHC). Other models of inflation relied on the properties of Grand Unified Theories. [61] Since the simplest models of grand unification have failed, it is now thought by many physicists that inflation will be included in a supersymmetric theory such as string theory or a supersymmetric grand unified theory. At present, while inflation is understood principally by its detailed predictions of the initial conditions for the hot early universe, the particle physics is largely ad hoc modelling. As such, although predictions of inflation have been consistent with the results of observational tests, many open questions remain.

Fine-tuning problem

One of the most severe challenges for inflation arises from the need for fine tuning. In new inflation, the slow-roll conditions must be satisfied for inflation to occur. The slow-roll conditions say that the inflaton potential must be flat (compared to the large vacuum energy) and that the inflaton particles must have a small mass. [ clarification needed ] [94] New inflation requires the Universe to have a scalar field with an especially flat potential and special initial conditions. However, explanations for these fine-tunings have been proposed. For example, classically scale invariant field theories, where scale invariance is broken by quantum effects, provide an explanation of the flatness of inflationary potentials, as long as the theory can be studied through perturbation theory. [95]

Linde proposed a theory known as chaotic inflation in which he suggested that the conditions for inflation were actually satisfied quite generically. Inflation will occur in virtually any universe that begins in a chaotic, high energy state that has a scalar field with unbounded potential energy. [96] However, in his model the inflaton field necessarily takes values larger than one Planck unit: for this reason, these are often called large field models and the competing new inflation models are called small field models. In this situation, the predictions of effective field theory are thought to be invalid, as renormalization should cause large corrections that could prevent inflation. [97] This problem has not yet been resolved and some cosmologists argue that the small field models, in which inflation can occur at a much lower energy scale, are better models. [98] While inflation depends on quantum field theory (and the semiclassical approximation to quantum gravity) in an important way, it has not been completely reconciled with these theories.

Brandenberger commented on fine-tuning in another situation. [99] The amplitude of the primordial inhomogeneities produced in inflation is directly tied to the energy scale of inflation. This scale is suggested to be around 10 16 GeV or 10 −3 times the Planck energy. The natural scale is naïvely the Planck scale so this small value could be seen as another form of fine-tuning (called a hierarchy problem): the energy density given by the scalar potential is down by 10 −12 compared to the Planck density. This is not usually considered to be a critical problem, however, because the scale of inflation corresponds naturally to the scale of gauge unification.

Eternal inflation

In many models, the inflationary phase of the Universe's expansion lasts forever in at least some regions of the Universe. This occurs because inflating regions expand very rapidly, reproducing themselves. Unless the rate of decay to the non-inflating phase is sufficiently fast, new inflating regions are produced more rapidly than non-inflating regions. In such models, most of the volume of the Universe is continuously inflating at any given time.

All models of eternal inflation produce an infinite, hypothetical multiverse, typically a fractal. The multiverse theory has created significant dissension in the scientific community about the viability of the inflationary model.

Paul Steinhardt, one of the original architects of the inflationary model, introduced the first example of eternal inflation in 1983. [100] He showed that the inflation could proceed forever by producing bubbles of non-inflating space filled with hot matter and radiation surrounded by empty space that continues to inflate. The bubbles could not grow fast enough to keep up with the inflation. Later that same year, Alexander Vilenkin showed that eternal inflation is generic. [101]

Although new inflation is classically rolling down the potential, quantum fluctuations can sometimes lift it to previous levels. These regions in which the inflaton fluctuates upwards expand much faster than regions in which the inflaton has a lower potential energy, and tend to dominate in terms of physical volume. It has been shown that any inflationary theory with an unbounded potential is eternal. There are well-known theorems that this steady state cannot continue forever into the past. Inflationary spacetime, which is similar to de Sitter space, is incomplete without a contracting region. However, unlike de Sitter space, fluctuations in a contracting inflationary space collapse to form a gravitational singularity, a point where densities become infinite. Therefore, it is necessary to have a theory for the Universe's initial conditions.

In eternal inflation, regions with inflation have an exponentially growing volume, while regions that are not inflating don't. This suggests that the volume of the inflating part of the Universe in the global picture is always unimaginably larger than the part that has stopped inflating, even though inflation eventually ends as seen by any single pre-inflationary observer. Scientists disagree about how to assign a probability distribution to this hypothetical anthropic landscape. If the probability of different regions is counted by volume, one should expect that inflation will never end or applying boundary conditions that a local observer exists to observe it, that inflation will end as late as possible.

Some physicists believe this paradox can be resolved by weighting observers by their pre-inflationary volume. Others believe that there is no resolution to the paradox and that the multiverse is a critical flaw in the inflationary paradigm. Paul Steinhardt, who first introduced the eternal inflationary model, [100] later became one of its most vocal critics for this reason. [102] [103] [104]

Initial conditions

Some physicists have tried to avoid the initial conditions problem by proposing models for an eternally inflating universe with no origin. [105] [106] [107] These models propose that while the Universe, on the largest scales, expands exponentially it was, is and always will be, spatially infinite and has existed, and will exist, forever.

Other proposals attempt to describe the ex nihilo creation of the Universe based on quantum cosmology and the following inflation. Vilenkin put forth one such scenario. [101] Hartle and Hawking offered the no-boundary proposal for the initial creation of the Universe in which inflation comes about naturally. [108] [109] [110]

Guth described the inflationary universe as the "ultimate free lunch": [111] [112] new universes, similar to our own, are continually produced in a vast inflating background. Gravitational interactions, in this case, circumvent (but do not violate) the first law of thermodynamics (energy conservation) and the second law of thermodynamics (entropy and the arrow of time problem). However, while there is consensus that this solves the initial conditions problem, some have disputed this, as it is much more likely that the Universe came about by a quantum fluctuation. Don Page was an outspoken critic of inflation because of this anomaly. [113] He stressed that the thermodynamic arrow of time necessitates low entropy initial conditions, which would be highly unlikely. According to them, rather than solving this problem, the inflation theory aggravates it – the reheating at the end of the inflation era increases entropy, making it necessary for the initial state of the Universe to be even more orderly than in other Big Bang theories with no inflation phase.

Hawking and Page later found ambiguous results when they attempted to compute the probability of inflation in the Hartle-Hawking initial state. [114] Other authors have argued that, since inflation is eternal, the probability doesn't matter as long as it is not precisely zero: once it starts, inflation perpetuates itself and quickly dominates the Universe. [5] [115] : 223–225 However, Albrecht and Lorenzo Sorbo argued that the probability of an inflationary cosmos, consistent with today's observations, emerging by a random fluctuation from some pre-existent state is much higher than that of a non-inflationary cosmos. This is because the "seed" amount of non-gravitational energy required for the inflationary cosmos is so much less than that for a non-inflationary alternative, which outweighs any entropic considerations. [116]

Another problem that has occasionally been mentioned is the trans-Planckian problem or trans-Planckian effects. [117] Since the energy scale of inflation and the Planck scale are relatively close, some of the quantum fluctuations that have made up the structure in our universe were smaller than the Planck length before inflation. Therefore, there ought to be corrections from Planck-scale physics, in particular the unknown quantum theory of gravity. Some disagreement remains about the magnitude of this effect: about whether it is just on the threshold of detectability or completely undetectable. [118]

Hybrid inflation

Another kind of inflation, called hybrid inflation, is an extension of new inflation. It introduces additional scalar fields, so that while one of the scalar fields is responsible for normal slow roll inflation, another triggers the end of inflation: when inflation has continued for sufficiently long, it becomes favorable to the second field to decay into a much lower energy state. [119]

In hybrid inflation, one scalar field is responsible for most of the energy density (thus determining the rate of expansion), while another is responsible for the slow roll (thus determining the period of inflation and its termination). Thus fluctuations in the former inflaton would not affect inflation termination, while fluctuations in the latter would not affect the rate of expansion. Therefore, hybrid inflation is not eternal. [120] [121] When the second (slow-rolling) inflaton reaches the bottom of its potential, it changes the location of the minimum of the first inflaton's potential, which leads to a fast roll of the inflaton down its potential, leading to termination of inflation.

Relation to dark energy

Dark energy is broadly similar to inflation and is thought to be causing the expansion of the present-day universe to accelerate. However, the energy scale of dark energy is much lower, 10 −12 GeV, roughly 27 orders of magnitude less than the scale of inflation.

Inflation and string cosmology

The discovery of flux compactifications opened the way for reconciling inflation and string theory. [122] Brane inflation suggests that inflation arises from the motion of D-branes [123] in the compactified geometry, usually towards a stack of anti-D-branes. This theory, governed by the Dirac-Born-Infeld action, is different from ordinary inflation. The dynamics are not completely understood. It appears that special conditions are necessary since inflation occurs in tunneling between two vacua in the string landscape. The process of tunneling between two vacua is a form of old inflation, but new inflation must then occur by some other mechanism.

Inflation and loop quantum gravity

When investigating the effects the theory of loop quantum gravity would have on cosmology, a loop quantum cosmology model has evolved that provides a possible mechanism for cosmological inflation. Loop quantum gravity assumes a quantized spacetime. If the energy density is larger than can be held by the quantized spacetime, it is thought to bounce back. [124]

Other models have been advanced that are claimed to explain some or all of the observations addressed by inflation.

Big bounce

The big bounce hypothesis attempts to replace the cosmic singularity with a cosmic contraction and bounce, thereby explaining the initial conditions that led to the big bang. [125] The flatness and horizon problems are naturally solved in the Einstein-Cartan-Sciama-Kibble theory of gravity, without needing an exotic form of matter or free parameters. [126] [127] This theory extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the torsion tensor, as a dynamical variable. The minimal coupling between torsion and Dirac spinors generates a spin-spin interaction that is significant in fermionic matter at extremely high densities. Such an interaction averts the unphysical Big Bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting. The rapid expansion immediately after the Big Bounce explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic. As the density of the Universe decreases, the effects of torsion weaken and the Universe smoothly enters the radiation-dominated era.

Ekpyrotic and cyclic models

The ekpyrotic and cyclic models are also considered adjuncts to inflation. These models solve the horizon problem through an expanding epoch well before the Big Bang, and then generate the required spectrum of primordial density perturbations during a contracting phase leading to a Big Crunch. The Universe passes through the Big Crunch and emerges in a hot Big Bang phase. In this sense they are reminiscent of Richard Chace Tolman's oscillatory universe in Tolman's model, however, the total age of the Universe is necessarily finite, while in these models this is not necessarily so. Whether the correct spectrum of density fluctuations can be produced, and whether the Universe can successfully navigate the Big Bang/Big Crunch transition, remains a topic of controversy and current research. Ekpyrotic models avoid the magnetic monopole problem as long as the temperature at the Big Crunch/Big Bang transition remains below the Grand Unified Scale, as this is the temperature required to produce magnetic monopoles in the first place. As things stand, there is no evidence of any 'slowing down' of the expansion, but this is not surprising as each cycle is expected to last on the order of a trillion years.

String gas cosmology

String theory requires that, in addition to the three observable spatial dimensions, additional dimensions exist that are curled up or compactified (see also Kaluza–Klein theory). Extra dimensions appear as a frequent component of supergravity models and other approaches to quantum gravity. This raised the contingent question of why four space-time dimensions became large and the rest became unobservably small. An attempt to address this question, called string gas cosmology, was proposed by Robert Brandenberger and Cumrun Vafa. [128] This model focuses on the dynamics of the early universe considered as a hot gas of strings. Brandenberger and Vafa show that a dimension of spacetime can only expand if the strings that wind around it can efficiently annihilate each other. Each string is a one-dimensional object, and the largest number of dimensions in which two strings will generically intersect (and, presumably, annihilate) is three. Therefore, the most likely number of non-compact (large) spatial dimensions is three. Current work on this model centers on whether it can succeed in stabilizing the size of the compactified dimensions and produce the correct spectrum of primordial density perturbations. [129] The original model did not "solve the entropy and flatness problems of standard cosmology", [130] although Brandenburger and coauthors later argued that these problems can be eliminated by implementing string gas cosmology in the context of a bouncing-universe scenario. [131] [132]

Varying c

Cosmological models employing a variable speed of light have been proposed to resolve the horizon problem of and provide an alternative to cosmic inflation. In the VSL models, the fundamental constant c, denoting the speed of light in vacuum, is greater in the early universe than its present value, effectively increasing the particle horizon at the time of decoupling sufficiently to account for the observed isotropy of the CMB.

Since its introduction by Alan Guth in 1980, the inflationary paradigm has become widely accepted. Nevertheless, many physicists, mathematicians, and philosophers of science have voiced criticisms, claiming untestable predictions and a lack of serious empirical support. [5] In 1999, John Earman and Jesús Mosterín published a thorough critical review of inflationary cosmology, concluding, "we do not think that there are, as yet, good grounds for admitting any of the models of inflation into the standard core of cosmology." [6]

In order to work, and as pointed out by Roger Penrose from 1986 on, inflation requires extremely specific initial conditions of its own, so that the problem (or pseudo-problem) of initial conditions is not solved: "There is something fundamentally misconceived about trying to explain the uniformity of the early universe as resulting from a thermalization process. [. ] For, if the thermalization is actually doing anything [. ] then it represents a definite increasing of the entropy. Thus, the universe would have been even more special before the thermalization than after." [133] The problem of specific or "fine-tuned" initial conditions would not have been solved it would have gotten worse. At a conference in 2015, Penrose said that "inflation isn't falsifiable, it's falsified. [. ] BICEP did a wonderful service by bringing all the Inflation-ists out of their shell, and giving them a black eye." [7]

A recurrent criticism of inflation is that the invoked inflaton field does not correspond to any known physical field, and that its potential energy curve seems to be an ad hoc contrivance to accommodate almost any data obtainable. Paul Steinhardt, one of the founding fathers of inflationary cosmology, has recently become one of its sharpest critics. He calls 'bad inflation' a period of accelerated expansion whose outcome conflicts with observations, and 'good inflation' one compatible with them: "Not only is bad inflation more likely than good inflation, but no inflation is more likely than either [. ] Roger Penrose considered all the possible configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation [. ] Other configurations lead to a uniform, flat universe directly – without inflation. Obtaining a flat universe is unlikely overall. Penrose's shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation – by a factor of 10 to the googol (10 to the 100) power!" [5] [115] Together with Anna Ijjas and Abraham Loeb, he wrote articles claiming that the inflationary paradigm is in trouble in view of the data from the Planck satellite. [134] [135] Counter-arguments were presented by Alan Guth, David Kaiser, and Yasunori Nomura [136] and by Andrei Linde, [137] saying that "cosmic inflation is on a stronger footing than ever before". [136]


Why Is The Expansion Of The Universe Always Drawn Like A Cylinder?

I have gotten a lot of questions about diagrams of the Universe's expansion. I must say the number of questions on this topic is of great credit to how widely circulated one particular diagram from the WMAP team has been. After my recent article about tracing the Big Bang back to its original location, there was another burst of questions about the setup of this particular diagram:

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the . [+] earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light it also forms a backlight for later developments of the universe. Image Credit: NASA / WMAP Science Team

It’s true that while we had a long discussion about how the Big Bang was an even expansion of space itself in every possible direction, the diagrams usually give us a more directional vision of the evolution of the universe. It’s not just this one diagram, either, though the WMAP image is probably the most familiar - if you’ve seen any of these diagrams, it’s probably that one.

The fundamental issue is that the Universe is an evolving four dimensional entity, and an artist has two dimensions to work with, and compressing by two dimension is really hard to do. Artists are pretty good at compressing three dimensions into two dimensions - we can imply a lot of depth with clever use of perspective. And in fact the artist who’s constructed the WMAP image is doing just that by giving you a cylinder of space, which we have all successfully parsed as “has some volume”.

WMAP observes the first light of the universe- the afterglow of the Big Bang. This light emerged . [+] 375,000 years after the Big Bang. Patterns imprinted on this light encode the events that happened only a tiny fraction of a second after the Big Bang. In turn, the patterns are the seeds of the development of the structures of galaxies we now see billions of years after the Big Bang. Image Credit: NASA / WMAP Science Team

Here’s the issue: how do you draw and illustrate a changing three dimensional object? You can draw it at different stages, like a biologist’s illustrations of a jellyfish in different stages of life. You could make a video out of it, of course, but if your aim is to make an illustration, you’re stuck with a single image. The other option is to try and take a slice of the whole object, and show how that section evolves over time. It’s definitely incomplete, but it might give you a better sense of the changes going on, particularly if you can make the assumption that every other section you might have chosen is doing pretty much the same thing.

That’s what’s happened with the cylinder view. We’ve taken, effectively, a narrow cylinder of current-day space, and shown you how that evolves backwards in time. In this case, the circular sliver of space that we’re looking at slowly shrinks, and the galaxies that lived in that space in earlier times become smaller and brighter, and less separated, and if we trace that region of space even further backwards, we hit the Cosmic Microwave Background - the oldest light in the Universe. If we were to keep going, we’d expect this sliver of space to shrink rapidly as we go backwards in time through inflation, and would eventually become infinitely small, as it joins with all other pieces of space we could have selected at the start, in the singularity. It’s because we’re showing time along the long direction of the cylinder that it looks like there’s directionality here, but in actual fact the expansion is evenly distributed within that cylinder - the expansion of the Universe isn’t “off to the right.”

This diagram, and the others like it are giving you a small slice of the universe to look at, rather than attempting to show the evolution of the entire universe, if such a thing were possible. This is a simplification of how the entire Universe has changed and evolved over time, but you could make a similar slice of any other piece of space that exists today - in tracing it back, you’d see the same sorts of changes.


Era of Atoms

The Era of Atoms (380,000 years – 1 billion years or so) began as the universe finally cooled and expanded enough for the nuclei to capture free electrons, forming fully-fledged, neutral atoms. Previously trapped photons were finally free to move through space, and the universe became transparent for the first time. These photons have been passing through space ever since, forming the cosmic microwave background. The expansion since the origin of the universe has redshifted the initially energetic photons to microwave wavelengths. The CMB also marks the furthest point back in time we can observe — the time before is sometimes referred to as the dark ages.

The differences in density seen in the CMB provided the seeds for galaxy formation. The first galaxies formed when the universe was roughly 1 billion years old and heralded the current Era of Galaxies.


Readers reply: the universe is expanding – but what is it expanding into?

What’s beyond the universe? Simples! All those objects you’ve mislaid somewhere and never set eyes on since. Pens, glasses, wallets, keys, phones, penknives, combs, diaries, umbrellas, handkerchiefs … you name it!

Don’t people realise that all space around is riddled with tiny wormholes, into which these objects sink, never to be seen by mortal eyes again? FirmlyDirac

The universe is a clowns balloon that’s still in the blowing up stage and is about to be made into the shape of a sausage dog. Jamessss

As a teacher, this is a question that often comes up in my physics lessons. Part of the problem is one of perspective. Human brains work best when thinking about things in as few dimensions as possible. We reduce the curved and lumpy surface of the Earth to two-dimensional maps, or two-dimensional streets to one-dimensional systems such as street numbers or mile markers.

Our minds are not able to intuitively conceive of what the universe is really like. We see a balloon expand, and we see that it expands into the air around it, and we assume that the universe does the same. This is incorrect. The universe does not expand into anything because, as far as any evidence we have goes, it is everything that exists. In other words, there is nothing outside of the universe.

We often think of the big bang as happening in space in the centre of the universe, but this is only partly true. What really happened is that the big bang is the universe. It was not an explosion in space space is the explosion. The space between objects has been expanding ever since. The fact that this is unfathomable should make it all the more amazing. Andrew Busch

Instead of thinking of the universe as inflating like a balloon, I think of it as a giant ball of dough being stretched in all directions by several chefs. So there’s always dough still at the original starting point. And with that, I’m hungry. Eva_Brick

The question starts with an incorrect assumption – that the universe has as “edge”, a boundary between “universe” and “non universe”.

All we know, and this is what we mean by the expansion of the universe, is that, on average, every galaxy is getting increasingly far away from every other galaxy, at an increasing rate, with no central point. This does not mean that, at any local level, space-time is “stretching” like a rubber sheet (a common misunderstanding), nor does it mean that at local scales stars and galaxies can’t still fly into each other.

The problem with your question is you’re imagining something from your experience – say an inflating balloon – and asking the seeming reasonable: “What’s outside the ballon?” from the vantage point of being outside the balloon and being able to see it’s a balloon.

Instead, imagine this: you’re a cat, inside a flat. You’ve never been outside. So, as far as you know, nothing exists outside the flat. Now imagine you’re shrinking. From your feline perspective, you’re staying the same size, but all the walls and furniture seem to be moving further away from you – your universe is expanding. But you wouldn’t ask: “Where is it expanding into?” because as far as you know – and can ever know – the flat is all there is, it’s just spreading out (presumably, that’s just what flats do, because the only one you can ever observe does it). That’s the position we’re in and why the question doesn’t make sense. HaveYouFedTheFish

The universe is everything (as far as we know), so it doesn’t make sense to say “what is it expanding into”. It just is everything. The big bang happened “everywhere” at the same time. Is it infinite or finite? We don’t know. Even if it is finite, it might not have an “edge”. There are many unanswered or even unanswerable questions at the moment. We may never know, because all our measurements are constrained to the observable universe. csjjl1

Just because we don’t understand something doesn’t mean that it must therefore be the work of some kind of supernatural entity. That’s absurd. There are limits to what the human brain can comprehend – it is constrained by its evolutionary context. It’s simply not acceptable to explain the stuff we can’t get our heads around by invoking a cloud-dwelling divinity. Well, it might be acceptable to some people, but not to me. Oh, and the universe is expanding into itself. FirebirdV

My theory is that the universe has to expand to contain Brian Cox’s sense of self satisfaction. DonerCard

According to Men in Black, we’re in a big marble being thrown around by aliens. Doesn’t explain how the marble is expanding though, granted. AleYarse

Has anyone noticed this thread is expanding? What I want to know is: what is it expanding into? Plovdiv12

We call the origin point of our universe the big bang event. This is the point, from our frame of reference, where new time and space began unfolding the universe. It is located about 90bn light years* in every direction within our 13.8bn-year-old universe. This counterintuitive observation is due to the fact that new time and space is continuously being created within the singularity** everywhere at once. The origin point of the universe has receded beyond our observable horizon note that every other point in the universe shares a similar observed reference frame***.

Keep in mind that although there is no such thing as an “outside” to our singularity universe, there is no reason why an infinite number of singularities couldn’t exist, each with its own universe of continuously unfolding spacetime within.

Mainstream cosmology is of the opinion that new time and space will continue to unfold within the singularity for ever the universe will gradually grow cold and dark as static mass becomes more and more diffuse and energy undergoes continuous entropy (the “big rip” theory).

* The edge of the “observable” universe is around 46.6bn light years in every direction the distance to the big bang event, which is a much more speculative distance, is probably around 90bn light years in every direction.

** Most cosmologists believe the universe did not begin as a “mathematical singularity”, but instead as something better described as very small and dense. Stephen Hawking demonstrated that we cannot learn anything about the origin of the universe until it has aged at least 10⁻³² seconds, because no information has been created yet.

*** Every point within the universe shares the same frame of reference of observing itself to be the oldest, most central and most distant point from the big bang event, as compared with any other point within the entire singularity. Even though new time and space is continuously unfolding our universe, it still maintains characteristics of the singularity of our origin. Chris Ducey

I think Prof Harvey Keitel said it best in Mean Streets, when considering the meaning of an eternal spiritual life, the universe and everything: “Ya don’t fuck around with the infinite”. dylan37

To say that “the universe is expanding” because what we can see and observe from our part of it that it looks to be moving away from a certain point sounds to me like hubris. haemodroid

It is not “moving away from a certain point” – rather, all points are expanding away from all other points. So it’s not hubris at all our place in the universe is as non-special and unremarkable as all other places. Readout_Noise

I think it helps to realise the inseparable relationship between things and the physical dimensions of space and time. Any measurement of space involves looking at the distances between things and any measurement of time involves looking at the movement of things relative to each other. If you had a completely empty space, you would not be able to measure space or time. So, in that sense, a region of the universe that has nothing in it doesn’t even really exist before something moves into it. Shortordercook

Einstein once said people used to think if you took everything out of the universe then you’d be left with space and time. Relativity says space and time would come out with the things. SidneyLotterby

42. PunkyRooster (and Parcival)

Hey, you beat me to it! That’s not fair! Nelliev

In a joke that a quantum physicist might be able to explain, I said 42 before either of you. And after and at the same time, but in a different place. Sort of … Florton66


What Makes the Universe Expand?

So, the Universe started with a bang. Everything was hot, dense, and expanding.

It's 13.7 billion years later now, and our Universe is cold and sparse. The temperature of the leftover glow from the big bang -- which used to be over 10^30 degrees -- is now down to 2.7 Kelvin, just barely above absolute zero. The Universe used to be denser than the center of the Sun. Now, on average, the density of the Universe is only about one proton per cubic meter, with mass clumped into stars and galaxies separated by trillions of miles.

But, for all of it, the Universe is still expanding. What's amazing is, despite all of the things that could cause this expansion, there's only one thing that the expansion rate depends on. Know what it is?

The energy density of the Universe. Or, in other words, the total amount of matter, radiation, neutrinos, and all other forms of energy in the entire Universe divided by the Universe's volume. I'll say it again: energy density is the only thing that determines the rate of expansion of the Universe.

This is remarkable, and here's why. First off, the Universe could have existed in one of three ways. It could have been curved like a sphere (what we call positive), curved like a saddle (what we call negative), or completely flat (what we call zero curvature).

If the Universe were either positively or negatively curved, the three angles of triangles in space would not add up to 180 degrees! And the expansion rate would be dramatically changed by the presence of curvature. The fact that we have none is still a mystery for us, although perhaps inflation explains why.

The fact that the expansion rate depends on energy density means something else: at different times in the Universe, different things were important. Think about this: if you have a Universe where you double the volume, the matter density halves. Right? Same mass, twice the volume, therefore the density is half. I mean, look, the coins on the bigger ballon are clearly less dense, right? Same number of coins, but they're spread out over a larger volume:

But radiation -- things like light -- not only dilute like matter does, it gets redshifted, which means its energy density drops faster than matter does. So when the Universe was very young and very hot and very dense, radiation was more important than matter was!

As the Universe cooled, matter became more important, and dominated the expansion rate for billions of years. But there's something else in the Universe, something that doesn't dilute as space expands. It appears that there's an energy inherent in space itself, and this is what we call dark energy. Some people call it a cosmological constant. Why constant? Because its energy density never drops. Just a few billion years ago, dark energy became the dominant form of energy in the Universe. Matter gets less dense, but dark energy never does. If we graphed them all together -- radiation, matter, and dark energy -- you'd see which one dominated the expansion rate of the Universe.

And that's it. That's all that causes the Universe's expansion: the total amount of energy that's in it. From here to infinity, as far as we can tell, dark energy will continue to dominate the Universe.

Some of you are inclined towards math, and will want an equation that relates the Hubble expansion rate (H) to the energy density (ρ). Well, I hate getting too technical, so here you go:

Want the more detailed equations? Go here, if you can't wait. I'll do a special, simple math post tomorrow for those of you who can.


Inflation

The inflationary epoch was a possibly very brief but certainly spectacular period when the space inside a region of the universe which includes our own observable patch (i.e., the part of the universe that we can observe today) expanded with blistering speed — completely unbelievable speed. The expansion rate was so big that it sounds totally insane. And the only thing that keeps it from being insane is that the theory of inflation makes predictions which, so far, agree with our measurements of the cosmos . (Including those by BICEP2.) That doesn’t mean it’s right, but it does mean that

  • there are good reasons to think it might be right, and
  • no one can currently show that it’s wrong.

Let me say again: the space expanded. Things didn’t rush into the space: the space simply became much larger. It’s not one bit like an explosion. Click here to read more about the difference between an explosion of something into space and an expansion of space itself.

How insane is this rate of expansion? A patch of the universe no larger than your computer screen expanded to the size of the observable patch of the universe, or larger, in less than the time it takes for a quark to cross from one side of proton to the other. I won’t even bother to tell you the numbers, partly because we don’t actually know how long inflation lasted, but also the numbers are too big in size and too small in time for humans to think about them. Basically, a giant chunk of universe was created from a tiny one almost instantaneously.

What was the universe like during the period of this expansion? Empty. Extremely empty. Much, much, much emptier than space is now. Extremely cold. Extremely dark. Anything which might have been there before inflation started would have been pulled apart and dragged to great distances in an instant. [Small Caution: There is a moderately important and very subtle caveat to the empty/dark/cold statement, and I haven’t figured out how to write a comprehensible article on it yet. Rather than “extremely”, it would have been more accurate to say that the universe was “maximally'” empty, dark and cold — empty of everything except quantum fluctuations.]

What happened before inflation, and how inflation got started, we don’t know. There are a number of reasonable scientifically-grounded theoretical ideas, but they’re all speculation until someone thinks of a way to test them by making measurements. There may not even have been a “before inflation”, either because inflation is always going on somewhere in the universe, or because time doesn’t really make any sense if you go back too far, or for some other reason. But in many contexts it almost doesn’t matter, as I’ll now explain through a set of figures, answering some frequently asked questions along the way.

The cause was a large amount of what is often called

  • “dark energy” (but it’s not energy, it’s energy and negative pressure in the right combination) or
  • the “cosmological constant” (Einstein’s [non]-blunder: but fortunately it wasn’t constant, or the universe would have inflated forever) or
  • “dark smooth tension” (which is correct but it’s kind of clunky-sounding and not any clearer.)

Anyway, the universe has some of this stuff now, which is why the universe’s expansion rate has started to increase in the past few billion years. But (we suspect!) at some point, for some reason, it had a lot, lot more. And this caused the region containing our part of the universe to expand with a rate that accelerated enormously… i.e., caused it to “inflate”. See Figures 1,2,3, which contain a wild and surely wrong guess as to how inflation started, but by Figure 4, the details of the guess have become completely irrelevant.

Fig. 1: A completely wild and unjustified guess about what one region of the universe might have looked like before inflation began. In the grey region, for some unknown reason, there is a very substantial amount of dark energy. I’ve also drawn a few objects inside the grey region as green and red dots. I have no idea what’s outside the grey region, but as you’ll see, it doesn’t matter very much in the end.

Where did this huge amount of “dark energy” come from?

We don’t know. There are various suggestions … some of which have been ruled out by recent data. We hope to learn more about this question in the coming decade.

Fig. 2: Dark energy causes the grey region to begin expanding. The objects in the grey region (green and red dots) are carried apart as the space with the dark energy expands, becoming more spacious without actually moving into the exterior region outside the grey region.

Why doesn’t the rate of expansion slow down as the dark energy becomes diluted by the expansion?

Curiously and surprisingly, as the universe inflates and its volume grows, the amount of dark energy per unit volume stays the same. That means it will inflate and inflate and inflate, without slowing down, until something makes the dark energy go away.

Fig. 3: Since dark energy, unlike ordinary materials, does not become diluted as space expands, but remains constantly dense, the grey region continues to expand. By now all but one of the green and red dots has receded from view. Whatever the temperature of the expanding region was to start with, it is becoming extremely cold [more precisely, as cold as is possible under the circumstances].

Doesn’t that incredible expansion mean that things moved apart faster than the speed of light … the universal speed limit?

And doesn’t that violate Einstein’s theory of relativity?

No it doesn’t. Einstein’s theory says that if two objects pass each other at the same point, an observer moving with one of them will measure the other to be traveling below or at the universal speed limit, and never faster. But two objects at two different points can move apart faster than the speed of light if space itself expands… which is what happens in the expanding universe. Read more here about the expansion of space, and how and why it is completely different from an explosion.

Fig. 4: The heart of the inflationary epoch. By now, inflation has moved all the objects that were in the original grey region of Figure 1 (green and red dots) to extremely great distances from one another. The grey region has grown to a incomprehensibly vast size, profoundly empty and cold [though note the above caveat]. And the expansion may go on and on for many stages. The original guess illustrated in Figures 1 and 2 is now completely irrelevant to the properties of this region of the universe if we had started with a quite different guess in Figures 1 and 2, we would still have arrived at this same Figure 4.

That’s right. [Meh. Kinda right. As cold as it could possibly get but there’s those quantum fluctuations around that make this statement subtle.] The universe became hot after inflation see below for more on this. Whether it was also hot at some period before inflation is completely speculative there’s no evidence one way or another. But during inflation, the temperature dropped to a tiny fraction of a degree above absolute zero [but… but… this needs an article…].

Fig. 5: The expansion of the inflating region is slowing down. And what will, over time, become the observable patch of our universe has now become big enough to draw, outlined in red dashes.

Why did inflation stop?

We don’t know. But again, there are a number of scientifically grounded suggestions, ones with equations and predictions and ways to test them, at least in part. We may learn more soon from ongoing studies of the cosmos.

What happened when inflation stopped?

The best guess as to what happened (and our equations show this is possible, but don’t tell us the details) is that all that dark energy got turned into particles — including particles we’re made from, and lots of other types of particles we know about, and perhaps lots of particles we don’t know about. And when this happened, the universe became very hot, and very dense — and it continued expanding, though much more slowly.

Fig. 6: As inflation ends, the dark energy that fills the formerly inflating region is turned into the motion-energy and mass-energy of particles, which appear in enormous abundance, making the universe extremely hot. The larger the amount of dark energy per unit volume was during inflation, the hotter the universe can become after it heats up. A large region, extending far beyond what is shown, and including what will become our observable patch, is filled with a nearly uniform hot dense soup of particles. From here, the universe will continue expanding, but much more slowly than during inflation, and it will slowly cool. The Hot Big Bang is described in a separate article.

This was what is the origin of the Hot Big Bang. Some people (including me) simply say: “This moment is the start of the Big Bang”. Others say that the Big Bang includes the Hot Big Bang and inflation, though this is odd, since inflation is more of a Whoosh than a Bang. Some say that inflation is what put the “Bang” into “Big Bang”, by first making the universe large and expanding, and then making it hot. Still others say that it includes the Hot Big Bang, inflation, and everything that came before it… but this is risky, because before inflation there might have been something that does not in any sense deserve the term “Bang” (which implies a very energetic, intense and sudden event.)

Since this terminology hasn’t settled yet, what you decide to call “The Big Bang” is kind of up to you. It’s just important to know that you have different options, and that different scientists and websites may use different meanings for “Big Bang”.


A history of the history of the universe

The interdisciplinary nature of the field helps explain its comparatively late start. Our modern picture of the universe started to come together only in the 1920s, shortly after Albert Einstein developed the theory of general relativity, a mathematical framework that describes gravity as a consequence of the bending of space and time.

"Before you understand the nature of gravity, you can't really make a theory of why things are the way they are," Steinhardt said. Other forces have greater effects on particles, but gravity is the major player in the arena of planets, stars and galaxies. Isaac Newton's description of gravity often works in that realm too, but it treats space (and time) as a rigid and unchanging backdrop against which to measure events. Einstein's work showed that space itself could expand and contract, shifting the universe from stage to actor and bringing it into the fray as a dynamic object to study.

In the mid-1920s, astronomer Edwin Hubble made observations from the recently built 100-inch (254 centimeters) Hooker telescope at the Mount Wilson Observatory in California. He was attempting to settle a debate about the location of certain clouds in space that astronomers could see. Hubble proved that these "nebulae" weren't small, local clouds but instead were vast, distant star clusters similar to our own Milky Way &mdash "island universes" in the parlance of the time. Today, we call them galaxies and know that they number in the trillions.

The biggest upheavals in cosmic perspective were yet to come. Hubble's work in the late 1920s suggested that galaxies in every direction are speeding away from us, triggering decades of further debate. Eventual measurements of the cosmic microwave background (CMB) &mdash light left over from the universe's early years and since stretched into microwaves &mdash in the 1960s proved that reality matched one of the possibilities suggested by general relativity: Starting out small and hot, the universe has been getting bigger and colder ever since. The concept became known as the Big Bang theory, and it rattled cosmologists because it implied that even the universe could have a beginning and an end.

But at least those astronomers could see the galaxies' motion in their telescopes. One of cosmology's most seismic shifts, said Farrar, is the idea that the vast majority of the stuff out there is made of something else, something completely invisible. The material we can see amounts to little more than a cosmic rounding error &mdash only about 5% of everything in the universe.

The first denizen of the other 95% of the universe, what's come to be called the "dark sector," reared its head in the 1970s. Back then, astronomer Vera Rubin realized that galaxies were pinwheeling around so fast they ought to spin themselves apart. More than hard-to-see matter, Farrar said, the stuff keeping galaxies together had to be something totally unknown to physicists, something that &mdash except for its gravitational pull &mdash completely ignores ordinary matter and light. Later mapping revealed that the galaxies we see are simply nuclei in the center of colossal "dark matter" spheres. The filaments of visible matter that stretch across the universe hang on a dark frame that outweighs visible particles five to one.

The Hubble Space Telescope then uncovered signs of an unexpected variety of energy &mdash which cosmologists now say accounts for the remaining 70% of the universe after accounting for dark matter (25%) and visible matter (5%) &mdash in the 1990s, when it clocked the expansion of the universe as speeding up like a runaway train. "Dark energy," possibly a type of energy inherent to space itself, is pushing the universe apart faster than gravity can draw the cosmos together. In a trillion years, any astronomers left in the Milky Way will find themselves in a true island universe, enveloped by darkness.

"We are at a transition point in the history of the universe, from where it's dominated by matter to where it's dominated by a new form of energy," Steinhardt said. "Dark matter determined our past. Dark energy will determine our future."


The origin of the universe

In 1927 Belgian physicist and cleric Georges Lemaître published a paper that put the theoretical and empirical squarely together under the title “Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques” (“A Homogeneous Universe of Constant Mass and Growing Radius, Accounting for the Radial Velocity of the Extragalactic Nebulae”). Lemaître began with a study of the dynamical solutions of Einstein’s model (with the cosmological constant included)—that is, those solutions with a cosmic radius that varies with time. He treated the Doppler shifts of the spiral nebulae as evidence of a cosmic expansion and used the redshifts and distances of 42 nebulae to deduce a value for the slope of the velocity-distance graph. At the time, Lemaître’s paper had little impact, partly because it had been published in the rather obscure Annales de la Societe Scientifique de Bruxelles (“Annals of the Scientific Society of Brussels”), and it was fully appreciated only a few years later, when cosmologists and astronomers had become more open to the idea of an expanding universe.

In 1929, building on Friedmann’s work, American mathematician and physicist Howard P. Robertson summarized the most general space-time metric that is possible under the assumption that the universe is homogeneous (of the same density everywhere) and isotropic (the same in all spatial directions). (A metric is a generalization of the Pythagorean theorem that describes the inherent geometry of space-time.) Similar results were obtained by English mathematician Arthur G. Walker, so this metric is called the Robertson-Walker metric. The Robertson-Walker metric and the expansion of the universe (as revealed by the galactic redshifts) were the twin foundations on which much of 20th-century cosmology was constructed.

American astronomer Edwin Hubble was the most influential observer of his generation. Using the 100-inch (254-cm) reflector at the Mount Wilson Observatory, in 1923 Hubble identified a Cepheid variable star in the Andromeda Nebula. From this he was able to determine a more-precise distance to the nebula, using the Cepheid variable as a much better standard candle. The Cepheids vary in brightness in a regular and easily identifiable way, with a quick increase in brightness followed by a slower decline. In 1908 American astronomer Henrietta Leavitt had found a relationship between the period and the brightness: the brighter the Cepheid, the longer its period. Ejnar Hertzsprung and American astronomer Harlow Shapley went on to calibrate the relationship in terms of absolute magnitudes. Hubble could easily measure the Cepheid’s period. He could then use the calibration curve to determine the star’s absolute magnitude, or intrinsic brightness, and the intrinsic brightness compared with the observed brightness gave the distance of the star. This measurement established beyond question that the Andromeda Nebula is outside the Milky Way and is a galaxy in its own right. Further work by Hubble with Cepheid variables in other spiral nebulae confirmed the island-universe theory.

When Hubble turned to the problem of the distance-redshift relationship, he soon superseded Slipher’s work. In 1929 Hubble published a paper showing a clear linear relationship between distance and redshift, which he interpreted as a velocity. He used Slipher’s velocities but added more that had been measured at Mount Wilson by American astronomer Milton Humason. Distances of the nearer nebulae were found by using Cepheids as standard candles. At greater distances Hubble used as a standard candle the brightest individual stars that could be resolved (assuming that these would be of the same brightness in all galaxies), and at greater distances yet, the luminosities of the nebulae themselves were the standard candle. Hubble’s paper led to a rapid acceptance of the distance-redshift (or distance-velocity) relation in the astronomical community, and this relationship is known as “Hubble’s law,” although, as discussed above, it had been several times anticipated.

Hubble himself was quite cautious about what the distance-velocity relationship implied about the history of the universe, but the natural conclusion to draw was that in the remote past all the galaxies had been close together. The distance-velocity relationship being linear, if galaxy B was 10 times farther away than galaxy A, it would be receding at 10 times the speed. By the same token, if the galactic clock was run backward to the beginning, both A and B would be at the same point (galaxy B retracing the greater distance at greater speed). Hubble’s value for the slope of the line in the velocity-versus-distance graph (today known as the Hubble constant) was 500 km (300 miles) per second per million parsecs (megaparsec). (A parsec is about 3.26 light-years and is the distance at which the radius of Earth’s orbit would subtend an angle of one second.) With this value for the Hubble constant, the universe appeared to be about two billion years old.

Subsequent studies indicated that this estimate was far too young. The study of radioactive isotopes in rocks suggested that Earth had to be 4.5 billion years old, which would make the universe younger than some of the objects in it. The value of the Hubble constant has been revised repeatedly. A major correction was made in 1952 when American astronomer Walter Baade discovered that Hubble had seriously underestimated galactic distances, because there are actually two different kinds of Cepheids. Baade’s recalibration resulted in a halving of the Hubble constant. A further major correction by American astronomer Allan Sandage in 1958 brought it down to about 100 km (60 miles) per second per megaparsec. Sandage, who was Hubble’s former observing assistant, showed that what Hubble had taken as the brightest individual stars in a galaxy were actually tight clusters of bright stars embedded in gaseous nebulae. For several decades the value of the constant was (according to different researchers) in the range 50–100 km (80–160 miles) per second per megaparsec. The currently accepted value for Hubble’s constant is around 71 km (44 miles) per second per megaparsec, with a margin of error of about 5 percent. The associated age of the universe, tightly constrained by many types of observations, is about 13.7 billion years.

Several astronomers proposed mechanisms to explain the redshifts without accepting the expansion of the universe. In 1929 the Swiss astrophysicist Fritz Zwicky proposed that photons gradually give up their energy to the intergalactic matter through which they travel, through a process analogous to Compton scattering, leading to a progressive reddening of the light. Others simply suggested various versions of the reddening of light with distance (collectively these were called the “tired light” hypothesis) without attempting to provide a physical explanation. These proposals never commanded a wide following, and during the 1930s astronomers and cosmologists increasingly embraced the expansion of the universe.

The general-relativistic cosmological models and the observed expansion of the universe suggest that the universe was once very small. In the 1930s astronomers began to explore evolutionary models of the universe, a good example being Georges Lemaître’s primeval atom. According to Lemaître, the universe began as a single atom having an atomic weight equal to the entire mass of the universe, which then decayed by a super-radiative process until atoms of ordinary atomic weight emerged.

A pioneering study of elemental abundances in the stars had been made by British-born American astronomer Cecilia Payne in her doctoral thesis of 1925. The amount of each element present in a star can be inferred from the strengths of the absorption lines in the star’s spectrum, if these are controlled for the temperature and pressure of the star. One fact that emerged early on was that stars did not have the same composition as Earth and were predominantly hydrogen and helium. In 1938 Norwegian mineralogist Victor Goldschmidt published a detailed summary of data on cosmic abundances of the elements, running over most of the periodic table.

Although it is possible to see Lemaître’s theory as a progenitor of the “big bang” theory, it was a paper of 1948 by American physicist Ralph Alpher and his dissertation supervisor, George Gamow, that changed the direction of research by putting nuclear physics into cosmology. As a joke, Gamow added the name of physicist Hans Bethe in order to preserve the Alpher-Bethe-Gamow sequence of (almost) Greek letters. In the aßγ paper, which was only one page long, Alpher and Gamow maintained that the formation of the elements (nucleosynthesis) began about 20 seconds after the start of the expansion of the universe. They supposed that the universe began with a hot dense gas of neutrons, which started to decay into protons and electrons. The building up of the elements was due to successive neutron capture (and readjustments of charge by ß-decay). Using recently published values for the neutron-capture cross-sections of the elements, they integrated their equations to produce a graph of the abundances of all the elements, which resulted in a smooth-curve approximation to the jagged abundance curve that had been published by Goldschmidt.

In another paper in 1948, Alpher and American physicist Robert Herman argued that electromagnetic radiation from the early universe should still exist, but with the expansion it should now correspond to a temperature of about 5 K (kelvins, or −268 °C [−451 °F]) and thus would be visible to radio telescopes. In a 1953 paper, Alpher, Herman, and American physicist James Follin provided a stage-by-stage history of the early universe, concluding that nucleosynthesis was essentially complete after 30 minutes of cosmic expansion. They deduced that if all the neutrons available at the end of nucleosynthesis went into making helium only, the present-day hydrogen-to-helium ratio would be between 7:1 and 10:1 in terms of numbers of atoms. This would correspond to a present-day universe that was between 29 and 36 percent helium by weight. (Because some neutrons would go into building other elements, the helium figures would be upper limits.) They pointed out that these figures were of the same order as the hydrogen-to-helium ratios measured in planetary nebulae and stellar atmospheres, though these showed quite a large range.

The Gamow-Alpher theory largely ceased development after 1953, and it failed to attract a following, in spite of the fact that they had published in highly prominent journals and had made detailed, testable predictions. Unfortunately, it was not until the 1960s that the hydrogen-to-helium ratio became known precisely enough to test the theory. More crucially, Alpher and Gamow failed to interest radio astronomers in looking for the 5-K background radiation, and their prediction was soon forgotten.