Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

I have a few related questions:

  • Where is the CMB coming (emitted/reflected/remitted) from?
  • When CMB hits the earth, is that the first thing those photons hit since they were emitted 400 thousand years after the big bang?
  • Why isn't the CMB at the edges of the universe? Why is it flying around in the middle? Has the trajectory of the photons been bent by masses in the universe until it bends back inward? Or is the theory that the universe wraps around on itself?
share|improve this question
Do you mean the Cosmic Microwave Background, or you are interested in cosmic rays? –  alemi Aug 11 '14 at 2:39
Possibly related: physics.stackexchange.com/q/25070 and physics.stackexchange.com/q/16757 –  alemi Aug 11 '14 at 2:41
@alemi Yes, I did mean cosmic microwave background. I edited the question to be clearer, thanks. –  B T Aug 11 '14 at 3:42
Did the linked questions help? It seems as if your question has a lot of overlap with those, and may be closed as a duplicate. –  alemi Aug 11 '14 at 3:46
Those questions have interesting answers, but don't answer my questions exactly. Or maybe I just don't understand the parts of the answers that answer my questions ; ) –  B T Aug 11 '14 at 3:46

2 Answers 2

up vote 6 down vote accepted

At a basic level:

The universe, in the beginning was very hot. So hot in fact that there were no atoms, only electrons and protons and neutrons and photons flying around. The photons were scatting off of the electrons and protons, as they interacted strongly because the electrons and protons are charged. The universe was much like the plasma you find in plasma balls, but turned up to 11. It was opaque. You could not see through it.

As the universe expanded, it cooled and at around 380,000 years after the big bang, it was cold enough that stable atoms could form. At this point, all of the photons that were flying around suddenly stopped reacting with all of the free electrons and protons, since they started to form atoms that had no net charge, behaving much like a very dilute gas, like the air. The universe became transparent. Just as we can see through air, at this point the photons could travel unimpeded. This is referred to as the "surface of last scattering", but you shouldn't think of it as a surface, you should think of it as a moment in time where the universe went from being opaque to light to being mostly transparent to light.

Having suddenly nothing to interact with, those photons just starting travelling in straight lines. Some of those photons were just the right distance from us and were pointed in just the right direction that they are hitting us just now. In fact, they are hitting us continuously since the entire universe was filled with this photons just before the universe went "transparent".

So, the CMB isn't at the edges, its everywhere, its all of the photons that are still to this day flying off in every which direction. Occasionally those photons hit something, but since the universe is mostly empty space, the fraction that hit something is completely negligible. It is safe to assume they have not interacted with anything since the "surface of last scattering" nearly 14 billion years ago.

Nowadays, those photons are long in wavelength, nearly 1 mm, because as the universe has continued to expand, they continue to cool and stretch in wavelength.

share|improve this answer
So when you say the photons were "just the right distance from us", do you mean that they traveled in a basically straight line to us from the moment they were created? Would this mean that the matter that made up the earth was 13.4 billion light years away from those particular CMBR photons during recombination? –  B T Aug 11 '14 at 4:21
Essentially, yes. –  alemi Aug 11 '14 at 4:25
@BT The matter that made the Earth was closer to the CMB photons at that time. But as time passed, it got further away due to the expansion of the universe. –  mpv Aug 11 '14 at 8:28
@mpv Interesting, that would make some sense I suppose. Do you have any idea how close they were to "us" during recombination? –  B T Aug 11 '14 at 19:42
@BT It was about 42 million light years. See here: en.wikipedia.org/wiki/Observable_universe and search the text "at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away." –  mpv Aug 11 '14 at 20:50

I will reply to

Why isn't the CMB at the edges of the universe? Why is it flying around in the middle?

The occurrence of space time and matter after the Big Bang happened to all points in our universe. The expansion of space happened at the same rate outwards for all points of the universe. All points of the universe 380.000 years ago had electromagnetic radiation decoupled from matter, when neutral matter was formed, at all points of the universe.

Light that at 380.000 years after BB started from the point where we are now, is traveling on the expanding space with the velocity of light away from us. What reaches us is light from all the other points of space that have taken the appropriate light years to reach us.

The Cosmic Microwave Background for this reason, the expansion of all points in space, is mostly isotropic. The small variations in intensity are interpreted as accumulation of light coming from distant regions of the Universe,

The glow is very nearly uniform in all directions, but the tiny residual variations show a very specific pattern, the same as that expected of a fairly uniformly distributed hot gas that has expanded to the current size of the universe. In particular, the spectral radiance at different angles of observation in the sky contains small anisotropies, or irregularities, which vary with the size of the region examined.

CMB is the main observational tool that confirms the Big Bang model.

Has the trajectory of the photons been bent by masses in the universe

A bit of bending happens if the photon has passed strong gravitational fields,

until it bends back inward?

No. It is that the expansion starts and continues on all points of the universe. All points of the universe are the center of the universe, in that sense.

Or is the theory that the universe wraps around on itself?

No. See above.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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