Why can we observe the Cosmic Microwave Background no matter the direction we look?

I often read that the CMB was released from everywhere in the Universe, in every direction.

If that statement is true, can someone elaborate what "everywhere in the universe, in every direction" means ? If not, why can we observe it in every direction ?

• – John Rennie Feb 5 '16 at 16:05
• It's harder to observe if you're looking down. ;-) – David Richerby Feb 5 '16 at 17:01

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and electrons couldn't form neutral hydrogen, since every time it "tried", an energetic photon would knock off the electron.

This gas was everywhere. And photons traveled and scattered in all directions:

Photons (purple) scatter on free electrons (green), and both are mixed with protons (red).

380,000 years after the Big Bang, the temperature had fallen sufficiently that neutral atoms could form (this is called recombination). The radiation, which until now had scattered continously on free electrons, could now stream freely between the atoms (this is called decoupling).

So they did. Still in all directions:

This free streaming is still taking place. Photons travel in all directions, and are everywhere. The photons that you are able to see, are the ones that started out at a particular distance from you, and in a particular direction, but other photons started out at smaller and larger distances, and in other directions. You just don't see them, because you happen to be right here. But a person in another place of the Universe would see the same as you.

The photons that we observe as the CMB come from a region we call the surface of last scattering, because it corresponds to the surface of a shell centered at us. But there is nothing special about this "surface", except that is consists of all points in the Universe that are so far from us, that it takes a photon roughly 13.8 billion years to travel. And because of the expansion, these points are now roughly 47 billion lightyears away from us.

In the figure below, the arrows show CMB photons. All have the same length; they start where they were emitted and end where they are today. What we observe as the CMB are all arrows that end at the Milky Way (in the center). Other arrows may be observed by other observers in other galaxies that have their own surface of last scattering around them.

• Pela's answer makes sense if we are at the precise center of the expanding shell. But the Copernican principle states that we are not privileged observers. Frankly I've puzzled this question often, and cannot find an answer that is consistent with the standard definition of the CMB, which is that it is the cooled Big Bang. – Alan Adler Oct 14 '19 at 0:40
• Also, it's actually not true. You would observe exactly the same (statistically speaking) if you were at any other place in the Universe. Any observer at any point in space and time has a "shell" around them consisting of the photons that were emitted 380,000 yr after BB. But there is nothing special about neither that shell, nor our position in the Universe. This is entirely consistent with the Copernican principle. – pela Oct 14 '19 at 9:03

Yes. That's why we called the CMB isotropic until a decade or two ago; isotropic meaning 'the same in every direction'. Since that time we've had the Wilkinson Microwave Anisotropy Probe, and its successors which show slight differences in it depending on where we point our antenna. We can easily measure our velocity with respect to this nearly uniform background radiation: Speeding through the universe. Subtracting that, we get Speed corrected WMAP data showing hot and cold spots. That's why we now call the CMB anisotropic.

Whichever direction we look in the universe, we are looking back in time. If we observe a galaxy that is $$10$$ million light years away then we are observing photons that were emitted $$10$$ million years ago. If we observe a galaxy that is $$1$$ billion light years away then we are observing photons that were emitted $$1$$ billion years ago.

And the further away in distance (and so further back in time) we go, the more the photons are red-shifted because of the expansion of the universe while the photons were in flight.

Eventually, in every direction, we reach the point where we are observing photons that were emitted $$380,000$$ years after the Big Bang. Any photons emitted earlier than this will have been scattered by the ionized gas that filled the universe before this time. So the oldest photons that we can observe in any direction are photons emitted $$380,000$$ years after the Big Bang. These photons have been red-shifted to microwave wavelengths so they form what we call the Cosmic Microwave Background Radiation or CMBR.