Will we ever reach a point where all the light that was set free by recombination finally reaches us from our point of view and the CMB stops "shining" for observers on Earth?

up vote 15 down vote accepted

Light from recombination is not "constantly shining" and that's why you see it. At a given time in the universe's history (actually a slightly extended period but I'll keep things simple), and only at that time, photons decoupled from the ambient plasma and started travelling freely from all points in the universe. The photon background you see at any time in the universe's history corresponds to the decoupling photons that are just now arriving to you. In other words, at one time, all points in the universe emit a burst of light, and what you see at any given time is the light from just the right distance such that it is just now getting to you. What this means is that there is a constantly receding shell that corresponds to the CMB surface you're looking at. How far away from you that shell is depends on the age of the universe. There's no reason for it to ever stop completely (even if the universe is finite because it will likely have a periodic structure), but it will get fainter over time as the photons get redshifted.

In other words, CMB light will always be present at any time in the universe's history. The horizon of the visible universe can't "pass" the CMB since it's not some individual object. It's the light from an event that occurred everywhere at some early time, and as such you could pick any point in the universe at any time and find a shell around it such that the CMB light from that shell is just now getting to that point. The question "what point emitted light at decoupling that is just now getting to me" always has an answer. The only limitations on observation therefore come from the eternally decreasing energy of the individual photons due to redshifting.

  • 1
    @John You're really not getting this. Reread what I wrote. It doesn't mean anything to say "the edge of the observable universe eventually overtake the CMB". The CMB doesn't come from a single place. It is emitted from everywhere, and the part that you see is the part that just got to you, therefore necessarily within the observable universe. – ticster Aug 5 '14 at 19:59
  • 1
    @John Not to be a jerk, but I think you should really stop thinking you're right, because you're not, and it's stopping you from understanding a very simple point : every single point in the universe once emitted CMB light. Any point that enters your horizon will have at one point emitted that light, and so will shortly form (very soon after entering your horizon) the latest shell of CMB light that you observe. – ticster Aug 5 '14 at 20:24
  • 2
    @ticster, I think you should replace some of your uses of "point" by "cosmological time". It's confusing right now when you say that recombination happened only at a single point, and at all points. – benrg Aug 5 '14 at 20:45
  • 1
    @benrg Good point :P I'll edit it my answer to be less confusing on that... point. – ticster Aug 5 '14 at 20:46
  • 1
    @MSalters It has a finite photon density, but the total number is not necessarily finite. – ticster Aug 6 '14 at 8:44

If the Universe were "just expanding", there would be no limit to amount of CMB photons received. This is the gist of the Ant on a rubber rope puzzle: No matter how fast you expand a rubber rope / a Universe, it is always possible to travel an arbitrarily large distance in a finite time. And since CMB photons were emitted from everywhere in the Universe almost at once, they'd keep coming.

However, due to the accelerated expansion, there is a limit. If the acceleration is indeed how we think it is (due to a non-changing cosmological constant), the observable Universe asymptotically increases to a finite size in comoving coordinates. That is, the physical size will increases without limit, but the part from which we may receive information contains a finite number of galaxies, atoms, and photons.

This part — the maximum (comoving) size of the observable Universe — is today 63 billion lightyears (Glyr) in radius, whereas the Observable Universe today is only 46 Glyr in radius. Hence, we have already received $\mathbf{\sim(46/63)^3 = 40\%}$ of the CMB photons that we will ever be able see.

Thus, the CMB photons will become more and more redshifted, and arrive and an increasingly slower rate, until one sad and gray autumn day, our ancestors will see the very last CMB photon (which will have redshifted so far into the radio regime that it can hardly be called a photon anymore).

Classically, the CMB radiation would never completely disappear. It would just become more and more faint and redshifted.

Quantum mechanically, if ΛCDM cosmology is correct, only finitely many CMB photons will reach us, so there will be a last photon.

(edit: In general big bang cosmologies, there may be no last photon. I formerly said that the photon density is finite and that means there are finitely many photons per astronomical body, but ticster pointed out in a comment that there's no guaranteed last photon for any particular detector. In fact, I suppose in principle every astronomical body could receive an infinite number of CMB photons, infinities being what they are.

Also, it might be worth pointing out that CMB light mixes with light from other sources and photons are indistinguishable particles, so this question is not strictly meaningful unless the CMB is the only source of light in the universe.)

  • Thinking about computing the duration until that last photon due to logarithmic decrease (or whatever) makes my head ache. – Alfe Aug 5 '14 at 22:21
  • 1
    A finite photon density does not mean a finite number of photons can ever reach us, in particular if the universe is in infinite. Collisions would still play a role, but in a more complicated way than what you seem to think. You might end up with a non zero probability distribution for time of last photon (depending on whether certain quantities diverge or converge), but never a guaranteed last photon. The time scales involved would likely be unimaginibly gigantic. – ticster Aug 6 '14 at 8:49


"The spherical volume of space which will become the observable universe is 42 million light-years in radius at this time."

So essentially it is CMB light from every point from this "then size" of the Universe is reaching us and we happen to see at present or at any time CMB light from all such points within this universe that are at just the distance from you. But to my mind this is limited to the perimeter of the then Universe which was all of 42 million light-years in radius.

  • No, this is not correct. The spherical volume which today is our observable Universe, was 42 million lyr in radius when the CMB was emitted. But there's nothing special about this volume — for all we know there's just more outside. In 11 billion years from now, when the Universe has doubled in size, Wikipedia will say "The spherical volume of space which will become the observable universe is 110 million light-years in radius at this time" (the calculation of this number is slightly non-trivial). – pela May 7 at 13:23
  • Sorry, not 110 Mlyr, more like 50 Mlyr. But the point is that it's larger than the 42 Mlyr. – pela May 7 at 13:30

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.