Does the fact that we are able to see CMBR implies that universe expanded faster than light?

Question

Supposedly, the universe underwent rapid expansion immediately after the big bang, surpassing the speed of light. If we can detect remnants from that era, does this suggest they moved faster than light, allowing us to observe the cosmic microwave background radiation originating from that time?

Answer 1

We do not detect any CMBR from the inflationary epoch. The CMBR originated about 370000 years later, at recombination. Before that time the universe was opaque, like a star or substantially hotter. The universe had to cool down sufficiently to allow light to travel unimpeded. This was not until substantially after inflation.

Answer 2

No, the fact that we see the CMB simply means that the radius of the observervable universe is at least 13 billion light years.

If you imagine the early universe as a 3D infinite sea of bright orange plasma, somewhat similar to the sun, the point of last scattering or recombination epoch is the instant when all the plasma everywhere suddenly becomes transparent at the same instant, i.e. space becomes "black" as we know it today. Due to the finite speed of light, an observer would witness this as a transparency or blackness that appears in his immediate neighborhood and then steadily recedes into the distance. Or, perhaps more accurately, the background color of space in all directions would be an orange plasma that continually recedes, and reveals planets, stars, your home galaxy, and eventually other galaxies as it recedes.

Now, if the universe were expanding arbitrarily slowly, the background we would see today would still be the orange plasma glow, behind all galaxies in all directions. It would be the Cosmic Visible Light Background. But the expansion of space causes a redshift of light passing through it, which is more severe the more distance it traveled through expanding space. So now an immortal observer who was around since recombination would have still seen the orange plasma recede in all directions, but as time went on, as it receded it would have seemed to cool to dark orange, red, infrared, and then microwave range. It was at this epoch in time that human beings came awake and began looking at the sky.

Back to your question: if the universe were finite, and at some point less than 13 billion ly radius just "ended," or else looped back on itself, then in the 13 billion years since recombination, the light from recombination from everywhere would have dissipated everywhere, and we would see fully formed galaxies in every direction, endlessly (or else a truly "black" backdrop where the edge of all matter lay).

The fact that we still see the recombination afterglow (albeit redshifted) implies that the observervable universe is at least 13 billion ly in radius. It would be precisely this radius if the expansion rate were extremely slow. As it is, the real radius is more like 46 billion ly.

(Note, in my hypothetical slow-moving universe, I'm assuming we know for independent reasons that recombination was 13 billion yrs ago, glossing over the fact that inferred redshift from the CMB is the main reason we know that recombination was that long ago.)

Answer 3

Although another answer accurate points out that we don't see radiation from arbitrarily early times, we could in principle without violating relativity. For example, the possibility has been considered that the stochastic gravitational wave background reported last year could have originated before the Universe's first second.

But the thing to realize is that this radiation only comes from arbitrarily early times as measured by the target. If your friend flies away from you at some speed arbitrarily close to $c$, they could get arbitrarily far from you before even 1 second passes on their clock, due to time dilation. That's essentially what happens in cosmology (although the coordinates are a bit more complicated).