Why can we still 'see' the CMB

Question

I read that the CMB were all the photons left free when the universe was cold enough for electrons got bound with protons (recombination) and the photons free in that moment continued traveling in the directions they were.

So I imagine:

• the CMB as a sphere (with incredibly thick walls) increasing its radius continuously away from the 'center' of the universe. When the Milky Way formed, hadn't this sphere of photons already passed the space in which the Milky Way formed? How can we still see them if they are moving away from us.

• the rate at which the space grew was bigger than the speed of these photons, so the Milky Way and our solar system had time to be formed before being reached by this photons lagged behind the speed in which the space increased.

On the other hand, how can we continuously measure the CMB if is not being generated continuously as a stream of photons like looking at a distant star. We should be able to see them only once when they passed by earth.

Answer 1

Firstly, there is no centre to the universe and the universe seems to continue indefinitely in all directions. It is best to imagine the universe as infinite in size.

Now we need to explain why the CMB appears to be a sphere surrounding us.

Let's imagine, for a moment that I have filled the entire infinite universe with (magic) mud. This mud glows, and because its magic mud, it doesn't change the orbits of the Moon, Earth or anything else in the universe. But if I hold up my my hand in front of my face I can't see it, because of the mud.

Now then, at one moment all the mud vanishes, everywhere, throughout the whole universe at the same time. One second later, if I hold my hand up I can see it. But if I look in the sky I can't see the moon. The moon is still hidden in mud. In fact if I look at the sky, all I see is mud at 1 light-second (300,000km) distant. And because it is glowing mud, the sky appears to glow. It looks like I am in a middle of sphere with radius 300000, surrounded by glowing mud. But this is an effect of the fact that light travels at 300000km/s

After 1.2 seconds, the moon becomes visible, after about 8 minutes, the sun becomes visible too. But it takes 4½ years for the first stars to be visible. At this point it would look as if I'm in the middle of sphere with radius 4.5 light years surrounded by mud.

Now what really happened is that 13.5 billion years ago the whole infinite universe was filled with hot gas that blocked light. As this cooled this became transparent. The whole universe became transparent at the same time, and while it wasn't instantaneous, it happened pretty quickly. So what we see now is that we seem to be in the middle of a sphere of hot gas 13.5 billion light-years across.

Except that in those 13.5 billion years, space has stretched. This has stretched the sphere from 13.5 billion light-years to 45 billion light years, and it has stretched the light from that hot gas, so it appears to be very cold.

The photons that we see in the CMB now did not come from the hot gas that once filled this part of the universe, but from hot gas that was in part of the universe that is now 45 billion light years away. That being so, The photons that we saw last year came from gas that was in part of the universe that is now 44,999,999,999 light years away. Since we can't detect structures as small as few light years at that distance, the CMB appears as a constant even pattern in the sky, changing very little if at all.

Answer 2

TL;DR:
The light we now see from the Sun was emitted eight minutes prior, from the closest star, about 4.22 years prior. Observational astronomy inherently looks back in time, and the further away something is, the further we're looking back in time. We can still see the cosmic microwave background radiation because we can (at least conceptually) still see all the way back in time to the big bang itself. The time of last scattering is close to but not quite that far back in time.

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So I imagine the CMB as a sphere (with incredibly thick walls) increasing its radius continuously away from the 'center' of the universe.

This represents a very common misunderstanding regarding big bang theory. The big bang was not an explosion in space. A better way of looking at it is that the big bang was instead an explosion of space that took place everywhere. While the observable universe is finite and was very small moments after the big bang, the universe as a whole may well be infinite, and if that is the case, it was infinite moments after the big bang. (Science doesn't know what happened at the very instance the big bang started.) What this means is that every point in the universe appears to be the center of the universe, or better put, there is no center of the universe.

The early universe was very hot and very dense. The expansion of the universe made the universe cool from this state. Neutrinos decoupled from matter a second after the big bang. Future scientists might be able to detect the resulting cosmic neutrino background radiation, just as they can now detect the cosmic microwave background radiation that formed 380,000 years later.

Despite the universe having cleared optically about 380,000 years after the big bang, the universe of 370,000 years after the big bang versus 390,000 years after the Big Bang wouldn't have looked all that much different. The universe of 370,000 years after the big bang was a rather thin and almost uniform warmish glowing gas that was slightly ionized. The universe of 20,000 years later (390,000 years after the big bang) was locally clear, but the only thing that was visible was a warmish glowing gas a bit over 20,000 light years away, in every direction.

That warmish glowing gas was still visible a few hundred million years later, but now in the deep infrared thanks to the continued expansion of the universe. The intervening space contained the first stars and perhaps the first galaxies. That warmish glowing gas remains visible to this day, but now in the microwave thanks to the continued expansion of the universe. The intervening space contains lots and lots of stars and galaxies.

Whether the surface of last scattering will always remain observable depends on the nature of the future expansion of the universe and dark energy. One possible far future outcome is that the expansion continues such that the distance to furthest into the past that is possible to "see" by any means (the particle horizon) is shorter than the distance to the surface of last scattering. The cosmic microwave background radiation (by then, the cosmic radio background radiation) will wink out of observability if that outcome is the case.