# How can there be anything “beyond” the CMB?

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Two things we take, for this purpose, to be axiomatic:

1. The CMB is the oldest thing we can observe directly
2. The cosmological red shift tells us how "old" something we are observing is

Yet we also have a third axiom:

1. There are some parts of the universe we can never observe because they are receding away from us at a superluminary speed.

But this surely also implies that we can see beyond the CMB if we see anything which has a red shift indicating an expansion speed very close to $c$.

This suggests axiom 2 is incorrect - so what should it be instead?

Yet we also have a third axiom:

1. There are some parts of the universe we can never observe because they are receding away from us at a superluminary speed.

This is simply false, so of course it gets you in trouble if you insist on taking it as axiomatic. The average recession velocity goes luminal at redshift $zapprox 1.4$, while there are observed galaxies and objects at $zsim 8$, e.g., Z8 GND 5296 dwarf galaxy and the GRB 090423 gamma ray burst. Additionally, there are at least some candidates for objects much farther than even that, possibly $zsim 12$.

That means that other than the CMB itself, the most distant and most ancient object we might have observed hails from when the universe as around $370,mathrm{Myr}$ old, which is around a thousand times older than the recombination epoch when the universe first became transparent and hence when the cosmic background radiation was emitted. In terms of redshift, the CMB has $zgtrsim 1100$ or so.

But this surely also implies that we can see beyond the CMB if we see anything which has a red shift indicating an expansion speed very close to $c$.

Something at redshift $zgtrsim 1100$ has a recession velocity of $vgtrsim 3.2c$.

This suggests axiom 2 is incorrect - so what should it be instead?

Since (3) says incorrect things, it's best to throw it out instead.

## If the Universe is an outdoor pool, is the CMB the pool walls or simply the edge of a sphere within the pool corresponding to our 13.8 bn years eye sight? Could there be many such spheres in the pool, none of them seeing the actual pool walls?

The heart of my question is the cosmic microwave background (CMB). I know what it is (thank you Internet), but what does it represent? Is it the end of what we see, or an actual border (but then: a border of what?).

If it's just the end of lightspeed vision, could there be a number of universes happily dangling around in a greater area (the "pool")?

Then comes the (I think) unanswered question: what is all this composed of? The Universe sphere we know well. What about the space between the spheres? What about the pool walls?Beyond the pool walls?

I apologize for the semantics, these are hard concepts for me to grasp. Thank you!

This is actually a very good question with not a simple answer.

Here is a graphic of the history of the Universe. Between the beginning and the region marked "Afterglow Light Pattern", the universe was opaque to light. Any photons emitted by matter were almost instantly reabsorbed by other matter, so no photons could travel freely. At the point marked "Afterglow Light Pattern", the universe cooled to the point where the matter no longer interacted strongly with the photons, so the light was free to escape. Sometimes we call this "the surface of last scattering".

The Cosmic Microwave Background is the light that escaped at this moment. The CMB, in your pool analogy, isn't the walls of the pool - it's the water itself. The CMB permeates all of space as the remnant of the light that escaped from the surface of last scattering.

So, where does the surface of last scattering fit in with your pool analogy? The surface of last scattering is the most distant part of the pool you can see. Looking at great distances also means looking back in time. As well look farther and farther into the Universe, we see older and older galaxies. If we could look all the way back, we see the surface of last scattering, because we can't see anything from before then (when the Universe was opaque).

## How is the CMB be the most distant thing we can see when there is a cosmic horizon?

I'm watching the PBS Spacetime series on the Cosmic Microwave Backround and something's nagging at me that I was hoping you could resolve. I get the gist of the following facts:

Because of the speed of light, when we look out into space we are actually looking into the past. So when we're looking at something that's a billion light years away, we're really seeing it as it was a billion years ago.

The universe is expanding, and the further into the past we look, the faster the objects we see are moving away from us (Hubble's law right?). This creates a cosmic horizon there are objects that are so far away, that they are moving away from us so quickly that their light will never have a chance to reach us, the universe is expanding too quickly for us to ever see them.

The most distant thing we can see is the Cosmic Microwave Background, a frozen radio (err.. microwave?) emissions from when the universe was in it's infancy some 13.2 billion years ago (or whatever).

When I think about it, points two and three seem to be in conflict with each other. It seems like the CMB will always be the most distant thing we can see since the light it released was the first light ever produced in our universe. So a billion years ago the CMB was the most distant thing able to be detected, and a billion years from now the CMB will still be most distant thing view-able, since the the universe began releasing light more or less everywhere at once, and so we're seeing today the CMB at a distance of 13.2 B light years, a billion years ago one would be able to see it at a distance of 12.2 B, and a billion years from now one will be able to view it at a distance of 14.2 B light years.

But doesn't this contradict the idea of a cosmic horizon? If, in the future, we will actually be able to see less of the universe because it is expanding faster, wouldn't the furthest away objects we see actually be closer to us today than they will be (lets say) a billion years from now? Wouldn't the CMB "we" see in a billion have had to have been released from a point closer to us than the CMB we are seeing today since the universe is expanding and today's most distant objects will have fallen behind the cosmic horizon, and we would therefore have seen it already, and not a billion years from now? My apologies if I'm being a bit inarticulate, even speaking about this in a way that makes sense to me is difficult, so let me know if I can clarify any part of my question.

Please ELI've taken an intro to physics and a few calc classes, along with having watched some PBS Spacetime episodes.

You are quite correct that we can't see what happened before the CMB (this time is known as recombination) but this is not unusual in Physics. For example we can't see what happens at collisions in the Large Hadron Collider. All we can see is the debris that comes flying out of the collisions. But we understand the physics involved so by measuring the properties of the debris we can calculate what happened in the collision. That's how the Higgs boson was discovered. It wasn't directly observed but its existence was shown by precise measurements of the particles that we can detect.

And the same applies to the universe. The CMB is the debris that came flying out of the Big Bang, so by measuring the properties of the CMB we can calculate what happened at times before recombination.

The obvious question is how we know our calculations are correct. The way we approach this is to try calculating the same thing in different ways. For example Higgs bosons can be detected in several different ways, and if those different measurements gave different masses for the Higgs boson we'd know at least some of our calculations must be wrong. This is harder for the universe since we only have the one universe and the creation of the universe isn't an experiment we can repeat. But we can still cross check various different calculations and at least make sure they are consistent, which is exactly what is done.

Recombination happened about 370000 years after the Big Bang, and in fact the physical properties of the universe at this time are easy to understand. The density and temperature are in the range that we can recreate in the lab so we can directly probe the properties of plasma under these conditions. Indeed even as far back as nucleosynthesis, which happened only a few minutes after the Big Bang we still understand the physics well from experiment.

For example you mention a time $10^<-12>$ seconds after the Big Bang, and this time is normally taken to be the end of the electroweak epoch. From this time on the interactions between particles in the universe occur at energies that can be probed in colliders so we can experimentally determine what would be happening from this time onwards. Incidentally the temperature at this time was more like $10^<15>$ K than $10^<12>$ K.

But it is certainly true that as we go back towards the Big Bang there comes a point where the density and temperature exceed anything we can study experimentally, and we can be less sure what happened then. This is still an active area of research.

It isn't the oldest thing we can see.

Most of the hydrogen, helium and deuterium nuclei that are around in the universe now, were created in the time period between a few seconds and about 15 minutes after the big bang.

The abundances of these nuclei in the universe is a direct probe of the physical conditions, and time evolution of those conditions, at the epoch of primordial nucleosynthesis.

The only important free parameter in the standard big bang model, as far as these abundances are concerned, is the ratio of baryons to photons, which in turn can be found from fluctuations in the cosmic microwave background formed hundreds of thousands of years later.

There is complete concordance in the estimated primordial abundances of He and D. Spectacularly precisely in the case of D, where the primordial abundance is quite sensitive to the nucleosynthetic conditions and the primordial abundance can be estimated precisely.

However, aside from this, there is the fact that we don't need to see something to know what has occurred. The cosmic microwave background and the fluctuations within it are the consequence of events that happened at earlier times. Unless one wants to abandon physical reasoning, then there is no difficulty in accepting that the cosmic microwave background, and it's evolving temperature with time (which has been measured) is very strong evidence that the universe was much denser and hotter in the past, with all the physical consequences that would imply.

Of course you can push that too far. There are details of the physics itself that are poorly understood before about $10^<-12>$ s, although things like the baryon to photon ratio, measured in the CMB, encode the mysterious matter/anti-matter asymmetry and allow it to be probed even though it cannot be "seen".

But post $10^<-12>$ s the physics is reasonably understood, so if we have a good idea of what the conditions are in the period of a few seconds after the big bang (from primordial nucleosynthesis) and a few hundred thousand years after the big bang (from the CMB), then we can reasonably extrapolate back to $10^<-12>$ s.

In the same way that if you follow the final part of the path of a launched projectile, it is perfectly reasonable to measure that trajectory and follow it back to identify where the launch site was.

Interesting question! Let me see if I can shed some light with an analogy. Btw, I shall be referring to John's answer at points.

Studying or researching in astronomy is very much like criminal investigation. You have the crime, you search for clues which are used to reconstruct the events of the crime. Here, we have a crime, the construction of the universe, with clues spread here and there. One of the very well known clue is the CMB, and the oldest clue we can find (or have found, more on this later). As John tells us, we can recreate this clue and see what follows. A natural deduction. But can we work backwards from this clue, leading to the crime?

Working backwards, we need a method that creates the CMB. We have a theory, that at one point matter was coupled to light, the universe was opaque and exactly 380,000 years later, they decoupled, and light was finally free to travel the universe, and this is what we see as the CMB. Is this correct? Prossibly, and Sherlock shall tell you, that by the balance of probability, it's more probable than possible because the black body spectrum and anisotropies of the CMB predicted by this theory very precisely match that is observed. We have taken a very important step!

Now, how come they decoupled? Because at this time "recombination" occurred, electrons and protons formed hydrogen, which was transparent to light. Voila!

But before this? Nucleosynthesis, we know this since once again, balance of probability, and experiments. Before this? Formation of protons and neutrons and other composite subatomic particles. And finally before this, we expect the CvB (Cosmic neutrino Background), when the neutrinos decoupled from matter. We are trying to observe CvB to see if we are on the right track.

Here we have, as John again mentions, the end of the electroweak epoch. Since he mentions this, let us go and see if we can't trace it further back. Here we run into trouble. There are several ways to reach this stage. Which is correct?

Here's an analogy that tells us what we can do right now. Watson notes about Sherlock says "I nearly fell into the error of supposing that you were typewriting. Of course, it is obvious that it is music. You observe the spatulate finger-end, Watson, which is common to both professions? There is a spirituality about the face however. which the typewriter does not generate. This lady is a musician."

Our clue is that we have to reach the end of the electroweak epoch, this is our "spatulate finger-end". And hence we create theories, our "typewriters" and "musicians", all of which predict this. But we are missing our "spirituality about the face", and this is what we are trying to find, and what John mentions as "an active area of research".

There's one thing that pops to mind. There's a theory that states that the universe began as the collision of two "branes", as opposed to a sudden inflation out of a pinpoint of energy (or uncertainty, which is again debatable). Researchers say they can settle this by observing the gravitational waves formed during that event. If they are mild enough, then the former theory gets credibility, if not, then the latter.

## How can there be anything &ldquobeyond&rdquo the CMB? - Astronomy

I am preparing a presentation to 16 year old students about "How big is the Universe", mainly about how it is measured. I wondered if you know what is the current record holder for the most distant object from Earth. If you could point me at a URL with pictures I would be grateful.

You might say that the most distant object visible from Earth is the Cosmic Microwave Background (CMB), the remaining heat from the early Universe which is visible all around us. The CMB radiation that is reaching us today traveled to us from a part of the Universe that is now over 45 billion light years away. It appears as a smooth, even distribution of thermal radiation with a temperature of about 3 degrees Kelvin, that we see coming from all directions. NASA and ESA have used 3 satellites to observed the CMB, COBE, WMAP and Planck, click on the names for more info on how the CMB is observed.

If you're picky about what you call an "object," the CMB may not qualify. The most distant objects observed are generally galaxies that are undergoing periods of rapid star formation, and are gravitationally lensed by a foreground object, making them even brighter and thus easier to detect. Although the most distance known object changes quite frequently, at the moment the most distant known object is a galaxy called EGSY8p7 (great name, I know). It has a redshift of 8.68, which corresponds to a distance of about 30 billion light years.

#### Mike Jones

Mike is a fourth year astronomy graduate student at Cornell, where he works with Professors Martha Haynes and Riccardo Giovanelli on the ALFALFA survey, a blind survey of gas-rich galaxies in the local Universe carried out with the 305m Arecibo telescope in Puerto Rico.

14 Gyrs it went down by a factor of

1100, it'll only go down by a factor of

2.5 after another 14 Gyrs (to approx 1K).
Here's a snapshot from Jorrie's calculator giving times for order of magnitude decreases:

#### Attachments

For fun, I decided to do a back-of-the-envelope calculation for what this would be. Assuming I'm doing things correctly (the source I used glibly ignored units in its calculations, and decided not to say which dimensionless convention they were using), the eventual temperature of the cosmological horizon will be approximately ##10^<-28>K##, which will be achieved when the scale factor is approximately ##10^<28>## times greater than it is today. If we just take the expansion rate to be a constant determined by the cosmological constant (since for most of the period in question it will be), then the expansion will follow the formula (using the convention ##a(0) = 1##):

Using the above formula, the CMB temperature and the horizon temparture will be the same order of magnitude in about a trillion years. According to this Wikipedia article, star formation is currently projected to cease at around 100 trillion years, indicating that there may be some civilizations which rise long after the CMB is undetectable.

14 Gyrs it went down by a factor of

1100, it'll only go down by a factor of

2.5 after another 14 Gyrs (to approx 1K).
Here's a snapshot from Jorrie's calculator giving times for order of magnitude decreases:
View attachment 222464
Considering the numbers above, and the timescale of the technological advancement on Earth, the hypothetical future civilisations will have plenty of time to develop sufficiently precise measurements.
Other than that, the question is going to be difficult to answer, as it involves assuming something about such qualifiers as 'too faint', 'difficult', 'undetectable', etc., all while talking about civilisations whose history, biology, environment and capabilities can be pretty much anything.

The CMB temperature drops the same way the frequency of all photons moving freely in the universe drops because of cosmological redshift. The CMB photons are redshifted by a factor of about 1000, so the CMB temperature has dropped by the same factor. That factor is also the ratio of the scale factor of the universe now to the scale factor of the universe at the time of CMB emission. The same evolution will continue into the future.

Bear in mind that the accelerated expansion is a description of how objects in our universe are moving away from one another at an accelerating rate.

The rate of expansion is decreasing (slowly), and seems to be approaching a constant value. A constant rate of expansion results in objects accelerating away from one another because the recession velocity is the rate of expansion multiplied by distance. If the rate of expansion is constant, increasing distance means increasing recession velocity.

Right now the rate of expansion is not quite constant, but it's decreasing more slowly than distances increase, resulting in an increase in recession velocity.

Thanks. Well I asked because Bandersnatch said that the temperature drop happened the most in the early epochs of the universe and that as the age grows the amount of redshift became proportionally smaller (or so I understood from his answer). So I understand that the temperature drop is not linear with the expansion rate, and if so that's why I asked for a more graphical way of grasping how the temperature drop evolves with the universe age, I guess it would not draw a straight sloped line but some kind of curve?.

"That factor is also the ratio of the scale factor of the universe now to the scale factor of the universe at the time of CMB emission. The same evolution will continue into the future". Yes but since the expansion is accelerating, for a layman like me who can not have a visual intuition for the scale factor evolution, it's difficult to get some understanding about how the temperature actually evolves, and will evolve, in time.

BTW, if I understand well the accelerating expansion of the universe means that spacetime coordinates which are now close to the edge of our observable universe, inhabited by some very early stars and proto-galaxies, may eventually disappear from our horizon as they cross the point where their expansion rate exceeds the speed of light from our frame of reference. If so, would not the very CMB emitting surface, the surface of last scattering, also become eventually far enough that light from it would not be able to reach us anymore because that surface would be receding from us at an FTL rate?

In the future, please mark threads on topics you're a beginner in as 'B'. 'I' indicates undergraduate-level prior knowledge. Thanks. :)

The calculator is a great tool for learning cosmology visually, as it can graph many more relationships. If you're interested in using it, follow the tutorial.

This is actually more complicated than that. The cosmic event horizon means that there exist a distance from beyond which light will never reach us, no matter how long we wait (i.e. even after infinite time). This is a feature of accelerated expansion, but it doesn't have much to do with recession velocities exceeding the numerical value equal to the speed of light. E.g. the regions from which CMBR was emitted has never been receding slower than 3c, and yet we can see them.
Furthermore, everything that once becomes observable, stays observable forever, at least in principle, and there's always more light from further away to be seen (not counting such things as being swamped by horizon radiation, or the wavelength of light becoming so stretched, that it is too large for any reasonably-sized detector). So there'll always be CMBR.

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
(best enjoyed with some maths knowledge)

#### Attachments

The CMB emitting surface (technical term: surface of last scattering) isn't an object. A galaxy, proto-galaxy, or other object is located at a particular place (though it will move over time). The surface of last scattering occurred everywhere. This surface is marked by the plasma in the early universe cooling to become a transparent gas, which occurred at a temperature of approximately 3000K, when the universe was roughly 300,000 years old. Every location in our universe cooled to this temperature. As our universe ages, the CMB which is observed stems from plasma which was cooling to a gas further away.

Perhaps a better way to think of the CMB itself is to not worry about how it was emitted, but to think of it as a photon gas.

Back before the emission of the CMB, the normal matter in the universe consisted of a hot plasma of mostly hydrogen. As a plasma is defined by its atoms being ionized, and an ionized Hydrogen atom is basically just a proton, the early universe was mostly a gas of protons, electrons, and the photons they interact with (with a good number of Helium atoms and tiny amount of other light elements thrown in). All of these were bouncing off one another all the time, exchanging energy, keeping them all together in thermal equilibrium. As this plasma cooled, the protons and electrons combined to form neutral atoms, which don't interact as readily with light. This caused the mostly-hydrogen gas and the photons to evolve independently (roughly 90% of the photons which we currently observe from the CMB never bounced off anything else in the intervening 14 billion years until they were absorbed by our detectors). So instead of a plasma, you now have a photon gas which cools as the universe expands, and a mostly-hydrogen gas which both cools and collapses to form things like galaxies.

## How is the CMB be the most distant thing we can see when there is a cosmic horizon?

I'm watching the PBS Spacetime series on the Cosmic Microwave Backround and something's nagging at me that I was hoping you could resolve. I get the gist of the following facts:

Because of the speed of light, when we look out into space we are actually looking into the past. So when we're looking at something that's a billion light years away, we're really seeing it as it was a billion years ago.

The universe is expanding, and the further into the past we look, the faster the objects we see are moving away from us (Hubble's law right?). This creates a cosmic horizon there are objects that are so far away, that they are moving away from us so quickly that their light will never have a chance to reach us, the universe is expanding too quickly for us to ever see them.

The most distant thing we can see is the Cosmic Microwave Background, a frozen radio (err.. microwave?) emissions from when the universe was in it's infancy some 13.2 billion years ago (or whatever).

When I think about it, points two and three seem to be in conflict with each other. It seems like the CMB will always be the most distant thing we can see since the light it released was the first light ever produced in our universe. So a billion years ago the CMB was the most distant thing able to be detected, and a billion years from now the CMB will still be most distant thing view-able, since the the universe began releasing light more or less everywhere at once, and so we're seeing today the CMB at a distance of 13.2 B light years, a billion years ago one would be able to see it at a distance of 12.2 B, and a billion years from now one will be able to view it at a distance of 14.2 B light years.

But doesn't this contradict the idea of a cosmic horizon? If, in the future, we will actually be able to see less of the universe because it is expanding faster, wouldn't the furthest away objects we see actually be closer to us today than they will be (lets say) a billion years from now? Wouldn't the CMB "we" see in a billion have had to have been released from a point closer to us than the CMB we are seeing today since the universe is expanding and today's most distant objects will have fallen behind the cosmic horizon, and we would therefore have seen it already, and not a billion years from now? My apologies if I'm being a bit inarticulate, even speaking about this in a way that makes sense to me is difficult, so let me know if I can clarify any part of my question.

Please ELI've taken an intro to physics and a few calc classes, along with having watched some PBS Spacetime episodes.

Today, stars don't emit the CMB – they emit much more energetic (hotter, higher-frequency, shorter-wavelength) radiation in general. CMB was emitted by "everything" that could emit radiation at the relevant time, around 400,000 years after the Big Bang. At that time, the radiation was in thermal equilibrium with everything else, so its distribution to frequencies was fully described by the Planck black body curve.

The universe was expanding and the wavelengths were just being expanded linearly with the size of the universe. The temperature was dropping inversely proportionally to the size of the Universe. At some moment, the radiation got decoupled so it started to live its own life, with the temperature no longer linked to the temperature of the matter around. The temperature continued to drop inversely proportionally to the size of the universe while the temperature of the stars etc. stayed higher and independent because the two subsystems no longer interacted.

None of these things has anything to do with the speed of divergence of mutually distant galaxies. The temperature is a local quantity and it's determined by the local physics while the mutual speed of distant galaxies is not a local effect. It is a global effect and an illusion of a sort, too. The increasing distance between the galaxies is due to the expansion of the space in between them. However, all the calculations of the temperature and its evolution may be applied locally at each place of the universe, independently of distant places of the universe, and this calculation of the temperature and its evolution is moreover the same at every place because the universe is uniform at cosmological scales.

Individual photons certainly don't have a rest frame. However, there is a rest frame in which the CMB is almost perfectly isotropic (the deviations from a perfect blackbody spectrum are of the order of 1 part in 100,000), and for convenience we call that the rest frame of the CMB.

That frame is essentially the rest frame of the plasma which emitted the CMB, i.e. the surface of last scattering, adjusted for the Hubble flow.

Our motion causes anisotropy through simple Doppler shifting: the CMB photons coming from the direction we're currently heading towards get blueshifted, the photons in the opposite direction get redshifted.

The Earth's velocity with respect to that frame is a little complicated, because we're orbiting the Sun, which is orbiting within the galaxy, which has its own motion in the local group, etc. Of course all of those motions are operating at different time scales, and different speeds. The shortest period effect is of course due to our orbit around the Sun, but our orbit speed is pretty sedate compared to the other motions I mentioned. So there's noticeable annual variation in the exact amount and location of the anisotropy, but the long period high velocity motions are the major factors controlling the anisotropy.

shows the CMB after it's had the anisotropy corrected. The 1 in 100,000 parts variations I mentioned above are amplified enormously, otherwise the image would look totally uniform. This amplification can only be done after the anisotropy compensation, otherwise the anisotropy would totally dominate the image.

## Space is obviously dark, but how dark can it get?

"Space. The final frontier." The way Patrick Stewart says those words in the intro to Star Trek: The Next Generation is almost ominous, making the journey of the Enterprise seem like a venture into an impenetrable void. That voice will always give you chills.

Between the planets and stars that come out at night, space may look like a vast expanse of darkness, but it is still lit up by a background glow from stars and galaxies. This is the cosmic optical background (COB). As opposed to the invisible cosmic microwave background or CMB (leftovers from the Big Bang), the COB tells us about all the galaxies that have formed from the moment after the Big Bang to 450,000 later. These galaxies keep the universe from experiencing a total blackout.

#### More New Horizons

Faraway stars and galaxies could also give us an idea of how many galaxies possibly exist—something that may tell us how dark it really is out there.

Because new research using data New Horizons spacecraft has found far fewer of these galaxies than were thought to exist, space may be darker than we thought. Unlike Hubble, which for all its far-out vision, still orbits Earth, The New Horizons Spacecraft is far away enough to really gauge the darkness of space. If it could observe Arrokoth, it can see things Hubble (which imaged distant stars above) can’t even dream of. New Horizons data has suggested the darkest reaches of space are ten times darker than the darkest areas Hubble can see. Instead of trillions of unseen galaxies, think in the realm of hundreds of billions.

“The cosmic optical background (COB) is the average flux of visible light photons averaged over the volume of the observable Universe,” said astronomer Marc Postman, who coauthored a study recently published in The Astrophysical Journal. “It reflects, at least in part, an integral over the cosmological history of star formation occur- ring in recognizable galaxies, proto-galaxies, and star clusters, as well as mass accretion by black holes.”

Even black holes keep the lights on—or at least their accretion discs, which are made mostly of glowing star stuff, do. Photons, or particles of light, swirl around the accretion disc of a black hole until they finally make it past its event horizon. This is the point of no return. There is thought to be true blackness beyond that danger zone, because not even light can escape.

The COB can be difficult to observe. Hubble’s vision is often distorted by light pollution around Earth. Either sunlight or artificial light can reflect off particles of space dust (mostly the remnants of smashed comets and asteroids), causing a glare that gets in the way of its observations, and telescopes obviously can’t squint. For something orbiting our planet, zodiacal light, or the unearthly white light that comes from between planets and emerges about an hour before dawn or after sunset, is also a nuisance. New Horizons is not as distracted by that. Any telescope or spacecraft is going to have issues with measuring the COB.

If you really want to know how bright the COB is, you have to factor out all these distractions. Doing that requires going further out from the inner solar system. This is where New Horizons is particularly useful, because it is over 4 billion miles away, where the sky can get much darker. Getting an accurate read on the COB also meant leaving out any light from galaxies thought to exist but are too faint for their existence to be confirmed. Light pollution from stars in our own galaxy was the most annoying issue for the astronomers, who had to keep it from messing with the real brightness of the COB.

It turned out that once all the other noise was cancelled out, the COB was actually pretty faint. So what actually is that light? Maybe galactic refuse. Maybe rogue stars. Maybe neither. When NASA’s James Webb Space Telescope finally takes off, it could finally give more insight into the darkness of the final frontier.