The estimates computed by WMAP and Planck state that the Big Bang occured about 13.82 billion years ago and recombination happened 379,000 years after that, which is when the universe first became transparent and the first detectable light was born.

Considering the following few things then....

  • Big Bang was the start of the universe
  • CMB is the first known remnant of the Big Bang (seen in all possible directions)
  • We can see far back into the past, upto the time where the light first became visible / detectable
  • What we see the farthest is also (one of) the first thing(s) to have ever become visible after the birth of the universe. Not only is it the first visible thing to have ever been formed, but also a virtual marker to the time when space and time themselves were born a "little while ago".

Why is it that the Cosmic Microwave Background not formally considered as the edge of the universe as a whole?

(Just to add a note, please, think I get the difference between "universe" and the "observable universe". However, in this case, the dilemma is still about the universe and not its subset — observable universe)

In other words, I guess the question could probably also include some sub-pointers such as:

  1. Is there an assumption that there could be some matter beyond the CMB — except the hot, dense and opaque soup of plasma?
  2. Do we know with reasonable certainty that there are galaxies (or matter) that have already receded away from us forever — without any detection yet and without any chance of being detected in future (which means, it's already drifting away faster than the speed of light, owing to the accelerated expansion of the universe)?
  3. If answer to the above is yes, and if those galaxies (or matter), that were born ONLY AFTER the Recombination Era (CMB), have already drifted away beyond the sphere of the observable universe, why is the CMB still visible? Is it not like saying that we see the "old", but we don't and won't see the "new"? How did some matter (formed later) overtake the CMB (created before it) in the race of ever receding space?

I've read through the following questions and the links that they further point to. However, I'm not absolutely sure if the question here completely overlaps them:

Can we observe an edge of the universe?
Will the CMB eventually recede outside our observable universe?


  • $\begingroup$ Have you understood the answer to the questions you quote, that every space point was at the mathematical (0,0,0) point of the start of the universe? You were at the center of the universe ? $\endgroup$
    – anna v
    Commented Jan 10, 2017 at 4:45
  • $\begingroup$ We may be able to "see" other radiation (gravitational, neutrinos...) from before the CMB. $\endgroup$
    – hdhondt
    Commented Jan 10, 2017 at 4:47
  • $\begingroup$ @annav Thanks for the comment. To be honest, I didn't precisely consider the idea that any chosen point was the center of the universe per se. Was thinking that, since everything was squished together at one infinitesimally small place, associating the directions with it wasn't really going to help. Hope that was somewhere close to how it was actually supposed to be understood? However, this changes dramatically with the initial expansion of the universe after the big bang. I've tried to add the new understanding about the issue in the comments section of this very helpful answer below. $\endgroup$ Commented Jan 10, 2017 at 22:04
  • $\begingroup$ @hdhondt Thanks... Didn't know that before, but it's amazing to learn about the technique and the way it tries to circumvent the problem! $\endgroup$ Commented Jan 10, 2017 at 22:06

1 Answer 1


The CMB origin at about 380,000 years after the Big Bang is indeed the furthest we can see, IN THE ELECTROMAGNETIC spectral domain. And you are right that this is not about the full universe vs the observable universe, you are talking about a portion of the observable universe which is simply occluded from us not in principle, but because photons could not propagate from freely out until then.

So, theoretically the universe is about 13.8 billion years old, and we can 'see' into the past only to 380,000 years after the Big Bang.

The reason we don't stop there, in either theory or in understanding what's behind that apparent 'wall', is that 1) we know a lot about what happened before the 380,000 year 'wall' from what needed to be there in order for us to see what we see after, AND maybe more important 2) for those who don't believe what they can't see, we will be able to see behind the 'wall' with gravitational waves.

Gravitational waves (GWs) are affected little by that 'wall' and all we need to do is build a large enough interferometer pair, to see them. LIGO which detected GWs from black holes merging, cannot detect those cosmologically originated GWs because their wavelengths are much larger. We need space based interferometers with legs a million Kms or larger -- that's in the planning for the next decade, with 2 or 3 satellites forming the 1 or 3 legs (funding dependent). And later bigger ones. We spect to see behind the wall using that gravitational astronomy.

As for your 3 questions:

  1. Matter behinds the wall. We know there had to be matter, but it was mostly uncondensed and very energetic charged particles, mostly electrons and protons. At 380,000 years they recombined into hydrogen atoms and a few other things, and the photons we see now as the CMB could escape. We know actually a lot more, eg, about the very small inhomogeneities and anisotropies in the CMB which came from the same on the density of matter, and which served as seeds of galaxies and stars. Before electrons and protons it was even hotter, and it was quarks, gluons and electrons and a few other particles, and before that particles we have not seen in the lab. We know the basic physics for those things but still expect there will be more energetic particles, perhaps remnants of the Big Bang that became dark matter, and other exotic particles. As it gets hotter it's quantum gravity like string theory claims, and for which we still don't know what the right theory is.

  2. We do think we know that there are galaxies that we can not see now. Even many of the ones we see now, emitted their light long ago, and will not see their light emittEd now ever. They are traveling away from us now too fast, and light emitted from them will never reach us. But we are seeing the light from many such galaxies now, that they emitted billions of years ago. Yes, the cosmological horizon is, we think, real

  3. Nothing overtook the CMB. Galaxies and stars were formed maybe a few million years after tHe CMB broke free. Remember the universe was expanding, so if they are younger than the CMB they were created closer to us, and it's why we can see them. General Relativistic geometry can be tricky, but for cosmology it's good to think in terms of time from the Big Bang or back from us. Keep in mind the CMB was released everywhere in space, and what we see now are photons that reached us now. They traveled for 13.8 billion minus 380,000 years. We have seen galaxies going back to a couple hundred million years from the Big Bang (but sorry, I may not have the number exactly right, or most updated).

For an intro to the chronology of the universe see the wiki article at https://en.m.wikipedia.org/wiki/Chronology_of_the_universe

It's got the different cosmological periods or epochs, including the recombination time (the 'wall') and other important cosmological times. We still have a lot to learn, but the most mysterious epochs from our knowledge of elementary particle physics are those that are the earliest: the Planck epoch (we just don't know what makes thing up then, maybe string theory or other quantum gravity theory will get to it sometime), the strong unification era (we know a little bit after how and when the stron and electroweak force unify, but still plenty uncertainty), and the inflationary epoch (we have inflation theories, some version seems right but we're not sure which, or the field that caused it). We tend to know a lot about the rest, from theory and observation, but still we think we'll find surprises.

Your final two questions:

A. The current observable universe is about 46 billion light years in radius. We see pretty far out, but have not seen the edge, or what is called the horizon (we would not fall off). Unfortunately, if anybody is around in a quite few billions of years we will see even the closest galaxies get too far from us to be able to see them (or their successors) because the expansion will have taken them past our then horizon

B. There will always be CMB around as they were created everywhere in space. However, they will be way way redshifted- right now they've been redshifted by a factor of 1100, and we see it as high microwaves, 100 Ghz range. Another factor of a million say they'll be 100 KHz but much weaker, and eventually they'll get too weak and low frequency for us to detect.

  • $\begingroup$ Thanks very much for such a long, detailed answer! It's mind-boggling and fascinating to know what we know, what we don't know and especially how we plan to "peek" beyond the CMB wall! Whilst I think I understand a good part of what the answer says, the most difficult bit was to grasp Nothing overtook the CMB. Galaxies and stars were formed maybe a few million years after the CMB broke free. Remember the universe was expanding ..... and it's why we can see them ANDWe do think we know that there are galaxies that we can not see now in conjunction. With thanks to the link you shared.. $\endgroup$ Commented Jan 10, 2017 at 21:49
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    $\begingroup$ ... think I landed upon something while trying to connect the dots further and find something that perhaps explains this visually. If we think those green bubbles are CMB, then based on the "Horizon" lines, will it be correct to assume that, There are regions of CMB too (on the darker sides of the horizon lines) that have receded away from us forever, just as some of the galaxies have ? $\endgroup$ Commented Jan 10, 2017 at 21:54
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    $\begingroup$ Absolutely right. Good picture. Just like there is CMB everywhere in our observable universe, there is also for that which has receded form us and is now behind the horizon. As you say, on the darker sides of the horizon. The stuff behind the horizon should not be different than what we see - it just receded from us too far. $\endgroup$
    – Bob Bee
    Commented Jan 11, 2017 at 0:53
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    $\begingroup$ @DhruvSaxena Look up LISA to get an idea of the space based grav wave interferometer project BobBee speaks of. LISA may or may not get off the ground (politics and funding make it uncertain), but a great deal of thought has gone into the project and a phased array of grav wave detectors would be able to image somewhat like a radio telescope. My guess is that now gravitation waves have been experimentally proven to exist, projects like this will go ahead. $\endgroup$ Commented Jan 11, 2017 at 1:10
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    $\begingroup$ @RodVance. I agree, and see it as inevitable, particularly if they start detecting a few each year at LIGO and the other terrestrial interferometers coming online. There's a nice review in one of the Relativity Living Reviews about LISA etc, and a graphic of what size and kinds of objects it may see at different frequencies, as well as what other interferometer sizes should see. Interesting that on the larger cosmological scales they would be sensitive to grav waves for instance from macroscopic large domain walls and other exotic structures. Can also see inside neutron stars with higher SNRs $\endgroup$
    – Bob Bee
    Commented Jan 12, 2017 at 4:34

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