Astronomy

When the universe expands does it create new space, matter, or something else?

When the universe expands does it create new space, matter, or something else?


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I am wondering what exactly is meant when it is said the universe expands. Does it simply create new space for matter to fill, does it also create new matter/dark matter to fill that space, or am I way off? Thank you for any help!


Yes, space is constantly being created. The new space does not hold any matter (like atoms) or dark matter. This means that the density of normal and dark matter decreases at the same rate as the volume increases. However, dark energy, which is something completely different and thought to be a property of vacuum itself, is being created with the new space, so the density of dark energy stays constant.

This in turns mean that while the early Universe (i.e. from it was 70,000 years old and until it was almost 10 billion years) was dominated by matter, the Universe is now dominated by dark energy.

And it will only get worse.


The expansion of spacetime does not involve the creation of anything. What is happening in the process of expansion is the enlargement of the metric itself of the spacetime.

Imagine this as any arbitrary coordinate system with geometric points a constant distance apart. As spacetime expands, the distance between these geometric points expand for the same distance for every point. Basically every region of spacetime is expanding away from every other region at the same rate and distance no matter where you happen to be situated.

This is why thinking of new matter being "created" as the method of expansion is not just incorrect, but not helpful at all at it introduces the idea that spacetime has a concrete edge where new stuff is always coming into existence that could conceivably be thought of as a certain distance from us. It isn't so. Spacetime expansion is happening everywhere as the distances between every point on our coordinate system expand.

At our scale (and indeed on any scale smaller than galaxy clusters) we don't see this isotropic expansion as gravitational forces between objects overcome following this coordinate expansion. Remember that Andromeda is currently heading towards us as a reminder. Galaxy clusters are following the expansion and are distancing themselves from each other presently. Care should be taken here though. It isn't the distances between the galaxies in the cluster that are expanding, it is the clusters themselves distancing from each other.

During the 'big bang', the distances between coordinates was zero. This is where the idea of the initial singularity comes from, as the density of spacetime instantaneously becomes infinite with the claim of no geometric space existing at all (look up the definition of singularity for further clarification). Most will currently tell you that the notion of the singularity existing is doubtful and that General Relativity's descriptions of spacetime don't adhere to what is happening on the quantum level, but that is outside the scope of this question.


This question also occurred in Lecture 4 of Leonard Susskind's Modern Physics in relation to a model that he showed to explain expansion.

"Little bits of space, little bits of newly formed space, that's one way to think about it. The other way to think about it is just trough the equations of general relativity… "

I believe that it would be wrong to think of space being 'created'. Or at least, that would be more like a metaphysical concept. It raises the question what is space and what is creation? I do not think that it has much place in general relativity (at least not without considering deeper models that try to unify general relativity with quantum field theory). Is space created or is it just stretched (compare with a rubber band, do we create rubber band when we stretch a rubber band)? Those concepts may be ways to think of it but they do not precisely describe what they "are", or at least they go beyond the physics and mathematical description. The current equations (at least to the equations of general relativity) neither tell whether space is stretched or whether space is created. Those concepts are irrelevant.

There isn't some physical mechanism by which we can say 'this or that is creating space'. An interesting contrasting view that puts the expansion and "creation" of space in a different light is the viewpoint that the expanding universe and the relativistic equations can already be derived using only Newtonian dynamics (first described by McCrea and Milne mid 30s). That framework, albeit leading to the same equations, is a Newtonian notion and has a different interpretation. Good to mention is that in Newtonian mechanics space is absolute (so when a volume expands then one could say there must be something like creation of space), but in general relativity this is not the case. What looks big to you can look small to a different observer.


In General Relativity the expansion of the universe can be described by the Einstein Field equations like any other motion. You could compare it to the Newtonian cosmological model and see the expansion of the universe as described by equations of motion (ie. there is no magical entity that is causing expansion by "creating" space, it is just ordinary motion as described by the Einstein field equations).

These equations can be very briefly stated as:

  • Note: the following equations can be written in many different ways. I have used the notation from Jerzy Plebanski Andrzej Krasinski, in An Introduction to General Relativity and Cosmology

    The expanding universe can be described by the Einstein field equations.

    $ R^{mu u} - frac{1}{2}g^{mu u}R+Lambda g^{mu u} = G^{mu u} + Lambda g^{mu u} = kappa T^{mu u} $

    where $kappa = frac{8pi G}{c^4}$ is Einstein's constant and $Lambda$ the (unknown) cosmological constant.

    One of many specific solutions are the Friedman equations (for a homogeneous isotropic universe) that use the Robertson-Walker geometry

    $egin{array}{rcl} d s^2 &=& g_{mu u} dx^{mu} dx^{ u} &=& d t^2 - R(t) left( frac{d r^2}{1-kr} + r^2(d heta^2 + sin^2 heta d psi^2) ight) end{array}$

    with $R(z)$ a time dependent scale factor and $k$ the curvature index.

    This will lead to a solution for the scale factor $R$

    $egin{array}{rcrcl} G_{00} &=& frac{3k}{R^2} + 3 frac{dot R^2}{R^2} &=& kappa epsilon - Lambda -G_{11} = -G_{22} = -G_{33} &=& frac{k}{R^2} + frac{dot R^2}{R^2} + 2 frac{ddot R}{R} &=& -kappa p + Lambda end{array} $

    where $p$ is the pressure and $epsilon$ is the energy density.


The expansion can be seen as a kinematic effect (like with the expansion in Newtonian Cosmology) and is when described by the equations of motion in the framework of general relativity equivalent to a change, in time, of the 'metric'. This change of the metric happens to be the way that we can describe motion, gravitation and electromagnetism in a relativistic way.

If you wish you could call that effect something like 'creation of space', but then you would, imo, need to be consistent and call any other relativistic length contraction/stretch as 'space being created/eliminated' (e.g. space is removed from Michelson's and Morley's interferometer when it is moving, or when gravitational waves pass by then we experience a repeated removal and addition of space).

So the next time you see an apple fall from a tree you should imagine that, from the point of view of that apple, space is being contracted, thus space is being removed.


Since the second part of your question is a duplicate I'll address just the first part. however I suspect you're going to be disappointed because my answer is that your question doesn't have an answer.

The problem is that spacetime isn't an object and isn't being stretched. We're all used to seeing spacetime modelled as a rubber sheet, but while this can be a useful analogy for beginners it's misleading if you stretch it too far. In general relativity spacetime is a mathematical structure not a physical object. It's a combination of a manifold and a metric. At the risk of oversimplifying, the manifold determines the dimensionality and the topology, and the metric allows us to calculate distances.

We normally approximate our universe with the FLRW metric, and one of the features of this metric is that it's time dependant. Specifically, if we use the metric to calculate the distance between two comoving points we find that the distance we calculate increases with time. This is why we say the universe is expanding. However nothing is being stretched or created in anything like the usual meaning of those words.

Short answer: Unlike stretchy materials, spacetime lacks a measure of stretchedness.

Long answer: Let's see where the rubber sheet analogy holds and where it fails. When you stretch a rubber sheet, two points on it change distance. You can also formulate a differential version of this property by saying it about infinitesimally close points. Similar is happening for the spacetime. In fact, description how infinitesimally close points change their (infinitesimal) distance is a complete geometric description of a spacetime in general relativity (GR). Even more, it is complete description of a spacetime in GR (up to details about the matter content in it).

This is described by a quantity called metric tensor $g_$, or simply metric for short. It is $n$ by $n$ matrix, where $n$ is the number of spacetime dimensions (so it is usually 4 unless you are dealing with theories with different number of dimensions). Square of the infinitesimal distance is given by: egin ds^2=sum_mu sum_ u g_dx^mu dx^ u end

In Einstein summation convention, summation over repeated indices is assumed, so this is usually written simply as $ds^2=g_dx^mu dx^ u$.

For flat spacetime $g_$ is a diagonal matrix with -1 followed by 1s or 1 followed by -1s, depending on the convention (it gives the same physics in the end). For a flat Euclidean space, metric tensor is simply a unit matrix, so for 3-dimensional space this gives: egin ds^2=dx^2+dy^2+dz^2 end which is just a differential expression for Pythagorean theorem.

Where does the analogy fail?

Well, as I said before, metric is a complete description of the spacetime in GR. This means that the spacetime doesn't have any additional property. Rubber, on the other hand, has a measure of how much it has been stretched. This is closely related to rubber having a measure amount of rubber per unit space. But there is no such thing as "amount of space per unit space", so once you stretch the spacetime, it won't go back. Or you could say that stretching and creating spacetime is the same thing, because you end up with more volume "for free" (meaning that, unlike rubber, it has no way to "remember" it used to have less volume).

Expansion of space means simply that distances between the points become larger, without any movement implied. It is simpler that with stretchy materials, but less intuitive, because our intuition forces us to imply additional properties which are not there.


Famine

Because the universe expanded the energy density grew less and less. And those particles that had a large mass (which after all is 'solidified' energy) became tougher to produce, while the killing continued at much the same rate. This period in our universe's history has been dubbed the era of the quark slaughter, although the larger leptons befell the same fate and died out. The killing went on until only the smallest particles remained: all the gluons, the photon, the Up and Down quarks, the electron and its e-neutrino.

And for some reason which is still unknown, the balance of matter and antimatter shifted slightly in favor of regular matter. This shift resulted in a minute excess of regular matter and while all other matter and antimatter cancelled each other out, the excess of regular matter survived and went on to live happily ever after. All of the matter that makes up our universe today, including planet earth and her dwellers was present at the birth of time and survived the fiercest era of destruction our realm has ever known.

Think about it. If particles were sentient beings they could all remember being part of this incredible turmoil, a world in which fellow particles arose and were killed off before they could come to make a difference. And suddenly something happened that no one could have anticipated. The dying ceased. Vast planes of space stretched out ahead in distances no one could have known were there. Suddenly there was room to travel as the survivors were a truly minute part of all the particles that ever existed. Like solely surviving a great disease that wiped out an entire city, being left with nothing but space. Alive in a realm of unprecedented grandness alive without the prospect of ever having to die.

But that wasn't all. There was something else.


King of the early universe

Tired of the Big Bang Theory and want your own version of cosmology? That's fine, but you'll have to explain things like the expansion of the universe and the splotches in the baby picture of cosmos. In other words, you have to do a better job at explaining the universe than inflation does.

This seems easy, but it isn't. The pressure, density and temperature differences in the universe's early years has bedeviled many alternative cosmologies, including one of the most popular let's-go-bigger-than-the-big-bang ideas, known as (are you ready for this), Ekpyrotic universe. The word ekpyrotic comes from the Greek for word for "conflagration," which refers to an ancient philosophical idea of a constantly repeating universe.

In the Ekpyrotic scenario, the universe … constantly repeats. Under that perspective, we are currently in a "bang" phase, which will eventually (somehow) slow down, stop, reverse, and crunch back down to incredibly high temperatures and pressures. Then, the universe will (somehow) bounce back and re-ignite in a new big bang phase.

The trouble is, it's hard to replicate the blotches and splotches in the baby picture of the universe in an Ekpyrotic universe. When we attempt to put together some vague physics to explain the crunch-bounce-bang cycle (and I do emphasize "vague" here, because these processes involve energies and scales that we aren't even coming close to understanding with known physics), everything just comes out too … smooth. No bumps. No wiggles. No splotches. No differences in temperature, pressure or density.

And that doesn't just mean the theories don't match observations of the early universe. It means that these cosmologies don't lead to a universe filled with galaxies, stars or even people.

So that's kind of a bummer.


The universe is expanding too fast, and that could rewrite cosmology

AT FIRST, it was a whisper. Now it has become a shout: there is something seriously wrong with our understanding of the cosmos. When we measure the rate at which the universe is expanding, we get different results depending on whether we extrapolate from the early universe or look at exploding stars in nearby galaxies. The discrepancy means that everything is speeding apart more quickly than we expect.

The problem originally surfaced a few years ago, and the hope was that it would fade away with more precise observations. In fact, the latest measurements have made it impossible to ignore. “It is starting to get really serious,” says Edvard Mörtsell, a cosmologist at Stockholm University in Sweden. “People must have really screwed up for this not to be real in some sense.”

Cosmologists have been scrabbling for answers. They have played around with the properties of dark energy and dark matter, those two well-known, yet still mysterious, components of our standard model of cosmology. They have imagined all manner of new exotic ingredients – all to no avail.

The conclusion could hardly be starker. Our best model of the cosmos, a seemingly serenely sailing ship, might be holed beneath the water line. That has led some researchers to suggest taking the ultimate step: abandoning that ship and building a new standard model from the ground up, based on a revised understanding of gravity. It is hardly the first such attempt. Now, however, it comes with a twist – almost literally. By putting a quantum spin on Einstein’s theories of space and time, we might finally make sense of &hellip

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How will the universe end, and could anything survive?

Don't panic, but our planet is doomed. It's just going to take a while. Roughly 6 billion years from now, the Earth will probably be vaporized when the dying Sun expands into a red giant and engulfs our planet.

But the Earth is just one planet in the solar system, the Sun is just one of hundreds of billions of stars in the galaxy, and there are hundreds of billions of galaxies in the observable universe. What's in store for all of that? How does the universe end?

The science is much less settled on how that will happen. We're not even sure if the universe will come to a firm, defined end, or just slowly tail off. Our best understanding of physics suggests there are several options for the universal apocalypse. It also offers some hints on how we might, just maybe, survive it.

Our first clue to the end of the universe comes from thermodynamics, the study of heat. Thermodynamics is the wild-eyed street preacher of physics, bearing a cardboard placard with a simple warning: "THE HEAT DEATH IS COMING".

The heat death is far worse than being burnt to a crisp

Despite the name, the heat death of the universe isn't a fiery inferno. Instead, it's the death of all differences in heat.

This may not sound scary, but the heat death is far worse than being burnt to a crisp. That's because nearly everything in everyday life requires some kind of temperature difference, either directly or indirectly.

For instance, your car runs because it's hotter inside its engine than outside. Your computer runs on electricity from the local power plant, which probably works by heating water and using that to power a turbine. And you run on food, which exists thanks to the enormous temperature difference between the Sun and the rest of the universe.

However, once the universe reaches heat death, everything everywhere will be the same temperature. That means nothing interesting will ever happen again.

Heat death looked like the only possible way the universe could end

Every star will die, nearly all matter will decay, and eventually all that will be left is a sparse soup of particles and radiation. Even the energy of that soup will be sapped away over time by the expansion of the universe, leaving everything just a fraction of a degree above absolute zero.

In this "Big Freeze", the universe ends up uniformly cold, dead and empty.

After the development of thermodynamics in the early 1800s, heat death looked like the only possible way the universe could end. But 100 years ago, Albert Einstein's theory of general relativity suggested that the universe had a far more dramatic fate.

General relativity says that matter and energy warp space and time. This relationship between space-time and matter-energy (stuff) &mdash between the stage and the actors on it &mdash extends to the entire universe. The stuff in the universe, according to Einstein, determines the ultimate fate of the universe itself.

The universe began as something incredibly small, and then expanded incredibly quickly

The theory predicted that the universe as a whole must either be expanding or contracting. It could not stay the same size. Einstein realized this in 1917, and was so reluctant to believe it that he fudged his own theory.

Then in 1929, the American astronomer Edwin Hubble found hard evidence that the universe was expanding. Einstein changed his mind, calling his previous insistence on a static universe the "greatest blunder" of his career.

If the universe is expanding, it must once have been much smaller than it is now. This realization led to the Big Bang theory: the idea that the universe began as something incredibly small, and then expanded incredibly quickly. We can see the "afterglow" of the Big Bang even today, in the cosmic microwave background radiation &ndash a constant stream of radio waves, coming from all directions in the sky.

The fate of the universe, then, hinges on a very simple question: will the universe continue to expand, and how quickly?

If there's too much stuff, the expansion of the universe will slow down and stop

For a universe containing normal "stuff", such as matter and light, the answer to this question depends on how much stuff there is. More stuff means more gravity, which pulls everything back together and slows the expansion.

As long as the amount of stuff doesn't go over a critical threshold, the universe will continue to expand forever, and eventually suffer heat death, freezing out.

But if there's too much stuff, the expansion of the universe will slow down and stop. Then the universe will begin to contract. A contracting universe will shrink smaller and smaller, getting hotter and denser, eventually ending in a fabulously compact inferno, a sort of reverse Big Bang known as the Big Crunch.

For most of the 20th century, astrophysicists weren't sure which of these scenarios would play out. Would it be the Big Freeze or the Big Crunch? Ice or fire?

Dark energy pulls the universe apart

They tried to perform a cosmic census, adding up how much stuff there is in our universe. It turned out that we're strangely close to the critical threshold, leaving our fate uncertain.

That all changed at the end of the 20th century. In 1998, two competing teams of astrophysicists made an astonishing announcement: the expansion of the universe is speeding up.

Normal matter and energy can't make the universe behave this way. This was the first evidence of a fundamentally new kind of energy, dubbed "dark energy", which didn't behave like anything else in the cosmos.

Dark energy pulls the universe apart. We still don't understand what it is, but roughly 70% of the energy in the universe is dark energy, and that number is growing every day.

The existence of dark energy means that the amount of stuff in the universe doesn't get to determine its ultimate fate.

Instead, dark energy controls the cosmos, accelerating the expansion of the universe for all time. This makes the Big Crunch much less likely.

But that doesn't mean that the Big Freeze is inevitable. There are other possibilities.

One of them originated, not in the study of the cosmos, but in the world of subatomic particles. This is perhaps the strangest fate for the universe. It sounds like something out of science fiction, and in a way, it is.

In Kurt Vonnegut's classic sci-fi novel Cat's Cradle, ice-nine is a new form of water ice with a remarkable property: it freezes at 46 °C, not at 0 °C. When a crystal of ice-nine is dropped into a glass of water, all the water around it immediately patterns itself after the crystal, since it has lower energy than liquid water.

There's nowhere for the ice to start forming

The new crystals of ice-nine do the same thing to the water around them, and in the blink of an eye, the chain reaction turns all the water in the glass &mdash or (spoiler alert!) all of Earth's oceans &mdash into solid ice-nine.

The same thing can happen in real life with normal ice and normal water. If you put very pure water into a very clean glass, and cool it just below 0°C, the water will become supercooled: it stays liquid below its natural freezing point. There are no impurities in the water and no rough patches on the glass, so there's nowhere for the ice to start forming. But if you drop a crystal of ice into the glass, the water will freeze rapidly, just like ice-nine.

Ice-nine and supercooled water may not seem relevant to the fate of the universe. But something similar could happen to space itself.

Quantum physics dictates that even in a totally empty vacuum, there is a small amount of energy. But there might also be some other kind of vacuum, which holds less energy.

The new vacuum will "convert" the old vacuum around it

If that's true, then the entire universe is like a glass of supercooled water. It will only last until a "bubble" of lower-energy vacuum shows up.

Fortunately, there are no such bubbles that we're aware of. Unfortunately, quantum physics also dictates that if a lower-energy vacuum is possible, then a bubble of that vacuum will inevitably dart into existence somewhere in the universe.

When that happens, just like ice-nine, the new vacuum will "convert" the old vacuum around it. The bubble would expand at nearly the speed of light, so we'd never see it coming.

Inside the bubble, things would be radically different, and not terribly hospitable.

Humans, planets and even the stars themselves would be destroyed

The properties of fundamental particles like electrons and quarks could be entirely different, radically rewriting the rules of chemistry and perhaps preventing atoms from forming.

Humans, planets and even the stars themselves would be destroyed in this Big Change. In a 1980 paper, Physicists Sidney Coleman and Frank de Luccia called it "the ultimate ecological catastrophe".

Adding insult to injury, dark energy would probably behave differently after the Big Change. Rather than driving the universe to expand faster, dark energy might instead pull the universe in on itself, collapsing into a Big Crunch.

There is a fourth possibility, and once again dark energy is at centre stage. This idea is very speculative and unlikely, but it can't yet be ruled out. Dark energy might be even more powerful than we thought, and might be enough to end the universe on its own, without any intervening Big Change, Freeze, or Crunch.

Dark energy has a peculiar property. As the universe expands, its density remains constant. That means more of it pops into existence over time, to keep pace with the increasing volume of the universe. This is unusual, but doesn't break any laws of physics.

However, it could get weirder. What if the density of dark energy increases as the universe expands? In other words, what if the amount of dark energy in the universe increases more quickly than the expansion of the universe itself?

This idea was put forward by Robert Caldwell of Dartmouth College in Hanover, New Hampshire. He calls it "phantom dark energy". It leads to a remarkably strange fate for the universe.

If phantom dark energy exists, then the dark side is our ultimate downfall, just like Star Wars warned us it would be.

Atoms themselves would shatter, a fraction of a second before the universe itself ripped apart

Right now, the density of dark energy is very low, far less than the density of matter here on Earth, or even the density of the Milky Way galaxy, which is much less dense than Earth. But as time goes on, the density of phantom dark energy would build up, and tear the universe apart.

In a 2003 paper, Caldwell and his colleagues outlined a scenario they called "cosmic doomsday". Once the phantom dark energy becomes more dense than a particular object, that object gets torn to shreds.

First, phantom dark energy would pull the Milky Way apart, sending its constituent stars flying. Then the solar system would be unbound, because the pull of dark energy would be stronger than the pull of the Sun on the Earth.

Finally, in a few frantic minutes the Earth would explode. Then atoms themselves would shatter, a fraction of a second before the universe itself ripped apart. Caldwell calls this the Big Rip.

The Big Rip is, by Caldwell's own admission, "very outlandish" &ndash and not just because it sounds like something out of an over-the-top superhero comic.

This is a remarkably grim portrait of the future

Phantom dark energy flies in the face of some fairly basic ideas about the universe, like the assumption that matter and energy can't go faster than the speed of light. There are good reasons not to believe in it.

Based on our observations of the expansion of the universe, and particle physics experiments, it seems much more likely that the ultimate fate of our universe is a Big Freeze, possibly followed by a Big Change and a final Big Crunch.

But this is a remarkably grim portrait of the future &mdash aeons of cold emptiness, finally terminated by a vacuum decay and a final implosion into nothingness. Is there any escape? Or are we doomed to book a table at the Restaurant at the End of the Universe?

There's certainly no reason for us, individually, to worry about the end of the universe. All of these events are trillions of years into the future, with the possible exception of the Big Change, so they're not exactly an imminent problem.

Also, there's no reason to worry about humanity. If nothing else, genetic drift will have rendered our descendants unrecognizable long before then. But could intelligent feeling creatures of any kind, human or not, survive?

If the universe is accelerating, that's really bad news

Physicist Freeman Dyson of the Institute for Advanced Studies in Princeton, New Jersey considered this question in a classic paper published in 1979. At the time, he concluded that life could modify itself to survive the Big Freeze, which he thought was less challenging than the inferno of the Big Crunch.

But these days, he's much less optimistic, thanks to the discovery of dark energy.

"If the universe is accelerating, that's really bad news," says Dyson. Accelerating expansion means we'll eventually lose contact with all but a handful of galaxies, dramatically limiting the amount of energy available to us. "It's a rather dismal situation in the long run."

The situation could still change. "We really don't know whether the expansion is going to continue since we don't understand why it's accelerating," says Dyson. "The optimistic view is that the acceleration will slow down as the universe gets bigger." If that happens, "the future is much more promising."

But what if the expansion doesn't slow down, or if it becomes clear that the Big Change is coming? Some physicists have proposed a solution that is solidly in mad-scientist territory. To escape the end of the universe, we should build our own universe in a laboratory, and jump in.

One physicist who has worked on this idea is Alan Guth of MIT in Cambridge, Massachusetts, who is known for his work on the very early universe.

You would jump-start the creation of an entirely new universe

"I can't say that the laws of physics absolutely imply that it's possible," says Guth. "If it is possible, it would require technology vastly beyond anything that we can foresee. It would require huge amounts of energy that one would need to be able to obtain and control."

The first step, according to Guth, would be creating an incredibly dense form of matter &mdash so dense that it was on the verge of collapsing into a black hole. By doing that in the right way, and then quickly clearing the matter out of the area, you might be able to force that region of space to start expanding rapidly.

In effect, you would jump-start the creation of an entirely new universe. As the space in the region expanded, the boundary would shrink, creating a bubble of warped space where the inside was bigger than the outside.

That may sound familiar to Doctor Who fans, and according to Guth, the TARDIS is "probably a very accurate analogy" for the kind of warping of space he's talking about.

We don't really know if it's possible or not

Eventually, the outside would shrink to nothingness, and the new baby universe would pinch off from our own, spared from whatever fate our universe may meet.

It's far from certain that this scheme would actually work. "I would have to say that it's unclear," says Guth. "We don't really know if it's possible or not."

However, Guth also points out that there is another source of hope beyond the end of the universe &ndash well, hope of a sort.

Guth was the first to propose that the very early universe expanded astonishingly fast for a tiny fraction of a second, an idea known as "inflation". Many cosmologists now believe inflation is the most promising approach for explaining the early universe, and Guth's plan for creating a new universe relies on recreating this rapid expansion.

The multiverse as a whole is genuinely eternal

Inflation has an intriguing consequence for the ultimate fate of the universe. The theory dictates that the universe we inhabit is just one small part of a multiverse, with an eternally inflating background continually spawning "pocket universes" like our own.

"If that's the case, even if we're convinced that an individual pocket universe will ultimately die through refrigeration, the multiverse as a whole will go on living forever, with new life being created in each pocket universe as it's created," says Guth. "In this picture, the multiverse as a whole is genuinely eternal, at least eternal into the future, even as individual pocket universes live and die."

In other words, Franz Kafka may have been right on the money when he said that there is "plenty of hope, an infinite amount of hope&mdashbut not for us."

This is a bit of a bleak thought. If it upsets you, here is a picture of a cute kitten.


Q: How does the expansion of space affect the things that inhabit that space? Are atoms, people, stars, and everything else getting bigger too?

Physicist: Way back in the day Edwin Hubble (of telescope fame) noticed that the farther away a galaxy is, the faster it’s moving away from us. From this he figured out that the universe is expanding, but in a very specific, weird way. Rather than things just flying apart (like debris from an explosion), the space between things is actually increasing on its own.

There’s some detail on what the difference is over here.

You’d think that, what with space itself expanding, everything else would expand with it. After all, the expansion of space is roughly analogous to a stretching rubber sheet. If you stretch the sheet anything drawn on it will stretch just as much.

The expansion of space doesn’t cause the things in space to expand, just move apart.

But in fact, while the space between and inside everything increases, the things themselves don’t. Or at least, they snap back faster than they can be stretched. The size of atoms, their chemical bonds, and by extension everything that’s composed of them, is determined by physical laws and constants. For example, the size of electron orbitals is scaled by the Bohr radius, a0,which is just fit to pop with physical constants. , where , , , , and are all constants, etched indelibly into the fabric of the universe, and none of them are terribly concerned with the amount of space around.

So everything around is the size it’s “supposed to be”. At least, everything solid. Fluffier things, like stars, gas clouds, and whatnot tend to have a particular stable size. As space expands a star in that space will expand as well. However, with a drop in density comes a drop in the fusion rate, the core cools a little, and the star is free to collapse back into its preferred equilibrium size. The same idea applies to chemical bonds: atoms in any given molecule like to be a set distance from each other, and while the expansion of space may move them slightly farther apart then they’d like, they have no trouble at all returning to their original distance.

It’s worth noting that this isn’t the sort of thing that anyone would need to worry about / include in any calculations / talk about publicly. Right now the universe is expanding at the rate of approximately 72 (km/s)/Mpc (“kilometers per second per megaparsec”). This rate is called the “Hubble Constant“, which is a weird name, considering that over the history of the universe it hasn’t been constant. Unlike other physical constants, which are constant. This expansion rate means that distances increase in size by about 0.0000000074% every year. On the scale of the universe (45 billion of light years, give or take) that expansion is important. On the scale of our galaxy (100,000 light years), and especially on the scale of people ( light years), that expansion doesn’t mean anything. Your hair grows about 1 billion times faster than the universe “expands you”, and your atoms don’t naturally compensate for hair growth.

That all being said, the Hubble constant doesn’t seem to be constant. In fact it’s increasing. So, in the future the expansion may be noticeable on a smaller scale. At some point, in the inconceivably distant future, the expansion of space may be fast enough to overcome the forces that return matter to equilibrium. Once the gravitational force of a star is overcome it’ll fly apart. Once the electrical forces that maintain chemical bonds is overcome, there goes everything else. This unfortunate occasion, is known as the “Big Rip” to juxtapose it with the “Big Bang”. The jury’s still out on when and if the Big Rip will happen, but it’s a very long way off if it does happen.


6 Comments

THANK YOU! This clarified some of my random questions. !

The best book to read is The Atom and The Universe: Theories and Facts Unfold published by xlibris.com.

This book is excellent to understand Higgs bosons and weak energy.

1. The reasons why the data that have been gathered for red shift and blue shift from the observation of galaxies through the use of telescope might not be served as a guide that the world would be expanding:
a) The accuracy of the telescope that has been used to determine whether the galaxies would be in red shift and blue shift in order to conclude that the galaxies would moving away or towards the earth could be in question. In short distance of viewing an object, the telescope could identify accurately the change of the size of planet from big to small or small to big so as to give signal whether it should be in blue shift or red shift. However, if the object is placed very far away from telescope, the object that is shown in the screen on the telescope would be very small. The telescope might turn up to show one signal as a result of its inability to identify the accuracy of change of size of the object as if that all the galaxies are moving far away from the earth. Or in other words, it might have given wrong signal that the world would be expanding due to the inaccuracy of the telescope since it might be accurate in short distance with a big object and yet it might not be accurate if if would be in very small and tiny object that would appear on the screen when it would be placed many miles far away from the earth. Thus, the accuracy of the telescope might be in question since it has not been tested whether it could be accurate when objects would appear to be very tiny and small on the screen..
b) The telescope might have been tested on earth to be accurate in short distance and yet it has not been tested from one galaxy to another so as to determine whether it is still accurate to measure the movement of object in the galaxies that is located in many miles far away from the earth.
c) If you would blow a balloon, all the substances in the balloon would be shaken and vibrated. Even if they would be creatures inside the balloon, all the creatures would feel the strong pressure, i.e. wind, pulling them towards the corner of this balloon. Why is it that we that are on earth would not feel the pressure that the earth would be expanding? As we know if we blow the balloon, all the things in the balloon would fly away and would turn up to be in messy order. Question has to be raised. Why is that the air would still remain on earth despite the great pressure that has caused galaxies to advance as a result of expanding? No matter the pressure would externally influence as a result of the world expanding, nothing has affected the earth and it seems to be that something is controlling the earth to make it a secure place. Religious people call it, God.
d) If you blow a balloon, all the substance would go travel towards the corner of this balloon. Let’s use blowing balloon to explain the galaxies. Let’s assume that you blow from the Sun, you would certainly see blue shift as well as red shift since some galaxies would move towards the earth from Sun. If you would blow from the Pluto, the same, you would still see some galaxies moving towards the earth since there are some galaxies from the sun would move towards the earth from the Pluto. However, if you would blow from the earth as a centre outwards, you would then see all galaxies would be moving far away from the earth. Now question has to be raised. The assumption that all galaxies would have been moving far away from the earth seems to presume that the earth would be stagnant and all galaxies would be advancing away from the earth. As the earth would turn up to be the centre of the universe, it turns up that a person would view from any side of the earth would turn up to be that all galaxies seem to moving away from earth. This seems to be weird and irrational.
The reliability of data gathered from scientist that the world would be expanding is in question.

Well, I think I know what she means by saying space (or spacetime) is flat, but why does she keep trying to explain it in terms of a donut? Donuts are not flat!
BTW: the transript has ‘Taurus’, where she said ‘torus’. Since she did say ‘donut’, there is no doubt: she really did mean, ‘torus’, the donut-shaped figure you get by moving one circle along a larger circle, keeping the former’s center on the latter, and keeping it perpendicular to the tangent on the latter at all times.

How can it be ascertained that a galaxy is located within fixed coordinates in space and that the space between the galaxy and a neighboring galaxy is “stretching”? Why can it not be that a galaxy is simply accelerating away from a neighbouring galaxy into a relatively distant region of space and thus located within continuously varying coordinates in a space that is not “stretching”?


The future of humanity: can we avert disaster?

Climate change and artificial intelligence pose substantial — and possibly existential — problems for humanity to solve. Can we?

  • Just by living our day-to-day lives, we are walking into a disaster.
  • Can humanity wake up to avert disaster?
  • Perhaps COVID was the wake-up call we all needed.

Does humanity have a chance for a better future, or are we just unable to stop ourselves from driving off a cliff? This was the question that came to me as I participated in a conference entitled The Future of Humanity hosted by Marcelo's Institute for Cross-Disciplinary Engagement. The conference hosted an array of remarkable speakers, some of whom were hopeful about our chances and some less so. But when it came to the dangers facing our project of civilization, two themes appeared in almost everyone's talks.

And here's the key aspect that unifies those dangers: we are doing it to ourselves.


Ask Ethan: If the Universe is expanding, why aren’t we?

“The Universe is expanding the way your mind is expanding. It’s not expanding into anything you’re just getting less dense.” -Katie Mack

One of the biggest scientific surprises of the 20th century was the discovery that the Universe itself is expanding. Distant galaxies recede from us and from one another more quickly than the nearby ones, as though the fabric of space itself is being stretched. On the largest scales, the matter and energy densities of the Universe has been dropping for billions of years, and continues to do so as time goes on. And if we look to large enough distances, we’ll find galaxies that are being pushed away so rapidly by the expansion of space that nothing we send out today will ever reach them, not even at the speed of light. But doesn’t that create a paradox back here? That’s what Kent Hudson wants to know:

If the universe is expanding at rates in excess of the speed of light, why does it not appear to affect our solar system and the planetary distances from the sun, etc.? And why would the relative distances of stars in our galaxy not appear to be increasing… or are they?

Kent’s hunch is right, and the solar system, planetary and stellar distances all aren’t increasing as the Universe expands. So what’s actually expanding in the expanding Universe? Let’s find out.

When Newton first conceived of the Universe, he pictured space as a grid. It was an absolute, fixed entity filled with masses that gravitationally attracted one another. But when Einstein came along, he recognized that this imaginary grid wasn’t fixed, wasn’t absolute and wasn’t at all like Newton had imagined. Instead, this grid was like a fabric, and the fabric itself was curved, distorted and forced to evolve over time by the presence of matter and energy. Moreover, the matter and energy within it determined how this spacetime fabric was curved.

But if all you had within your spacetime was a bunch of masses, they would inevitably collapse to form a black hole, imploding the entire Universe. Einstein didn’t like that idea, so he added a “fix” in the form of a cosmological constant. If there were this extra term — this extra energy permeating empty space — it could repel all of these masses and hold the Universe static. It would prevent a gravitational collapse. By adding this extra feature, Einstein could make the Universe exist in a near-constant state for all eternity.

But not everyone was so wedded to the idea that the Universe needed to be static. One of the first solutions was by a physicist named Alexander Friedmann. He showed that if you didn’t add this extra cosmological constant, and you had a Universe that was filled with anything energetic — matter, radiation, dust, fluid, etc. — there were two classes of solutions: one for a contracting Universe and one for an expanding Universe.

The mathematics tells you about the possible solutions, but you need to look to the physical Universe to find which one of these describes us. That came in the 1920s, thanks to the work of Edwin Hubble. Hubble was the first to discover that individual stars could be measured in other galaxies, determining their distance. By combining those measurements with the work of Vesto Slipher, which showed that these objects had their atomic signatures shifted, an incredible result popped out.

Either all of relativity was wrong, we were at the center of the Universe and everything was moving symmetrically away from us, or relativity was right, Friedmann was right, and the farther away a galaxy was from us, on average, the faster it appeared to recede from our perspective. With one fell swoop, the expanding Universe went from being an idea to being the leading idea describing our Universe.

The way the expansion works is a little counterintuitive. It’s as though the fabric of space itself is getting stretched over time, and all the objects within that space are being dragged apart from one another. The farther away an object is from another, the more “stretching” occurs, and so the faster they appear to recede from each other. If all you had was a Universe filled uniformly and evenly with matter, that matter would simply get less dense and would see everything expand away from everything else as time went on.

But the Universe isn’t perfectly even and uniform. It has overdense regions, like planets, stars, galaxies and clusters of galaxies. It has underdense regions, like great cosmic voids where there are virtually no massive objects present at all. The reason for this is that there are other physical phenomena at play besides the Universe’s expansion. On small scales, like animal-sized and below, electromagnetism and nuclear forces dominate. On larger scales, like that of planets, solar systems and galaxies, gravitational forces dominate. The big competition on the largest scales of all — on the scale of the entire Universe — is between the Universe’s expansion and the gravitational attraction of all the matter and energy present within.

On the largest scales of all, the expansion wins. The most distant galaxies are expanding away so quickly that no signals we send out, even at the speed of light, will ever reach them. The superclusters of the Universe — these long, filamentary structures lined with galaxies and stretching for over a billion light years — are being stretched and pulled apart by the Universe’s expansion. In the relatively short term, they will cease to exist. And even the Milky Way’s nearest large galaxy cluster, the Virgo cluster, at just 50 million light years away, will never pull us into it. Despite a gravitational pull that’s more than a thousand times as powerful as our own, the expansion of the Universe will drive all of this apart.

But there are also smaller scales, where the expansion has been overcome, at least locally. The Virgo cluster itself will remain gravitationally bound. The Milky Way and all the local group galaxies will stay bound together, and eventually merge under their own gravity. Earth will remain orbiting the Sun at the same distance, Earth itself will remain the same size, and the atoms making up everything on it will not expand. Why? Because the expansion of the Universe only has any effect where another force — whether gravitational, electromagnetic or nuclear — hasn’t overcome it. If some force can successfully hold an object together, even the expanding Universe can’t affect a change.

The reason for this is subtle, and is related to the fact that the expansion itself isn’t a force, but rather a rate. Space is really still expanding on all scales, but the expansion only affects things cumulatively. There’s a certain speed that space will expand at between any two points, but if that speed is less than the escape velocity between those two objects — if there’s a force binding them — there’s no increase in the distance between them. And if there’s no increase in distance, that impetus to expand has no effect. At any instant, it’s more than counteracted, and so it never gets the additive effect that shows up between the unbound objects. As a result, stable, bound objects can survive unchanged for eternity in an expanding Universe.

As long as the Universe has the properties we measure it to have, this will remain the case forever. Dark energy may exist and cause the distant galaxies to accelerate away from us, but the effect of the expansion across a fixed distance will never increase. Only in the case of a cosmic “Big Rip” — which the evidence points away from, not towards — will this conclusion change.

The fabric of space itself may still be expanding everywhere, but it doesn’t have a measurable effect on every object. If some force binds you together strongly enough, the expanding Universe will have no effect on you. It’s only on the largest scales of all, where all the binding forces between objects are too weak to defeat the speedy Hubble rate, that expansion occurs at all. As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”