velomont I am no expert but the observable universe is constrained by the time that light has had to travel vs, the expansion of the universe (including inflation). Also the early universe, before stars, was opaque.
“In practice, we can see light only from as far back as the time of photon decoupling in the recombination epoch. That is when particles were first able to emit photons that were not quickly re-absorbed by other particles.”
I prefer the term Horrendous Space Kablooie, or HSK.
The universe is expanding. This makes distant objects appear to be moving away from us, although they are not “moving” in the normal sense, merely being carried along by the expansion of space. The rate that distant objects “move” away from us is proportional to their distance, which means that some objects are so far away that they are “moving” from us as greater than the speed of light and will never be visible to us. (This does not violate relativity because, as I mentioned above, they are not actually moving.)
The visible universe is the sphere centered on Earth that contains objects that are receding at less than the speed of light.
Practically, however, what we can see is more limited than that. When we look far away we are also looking backwards in time. If we look far enough away we see back before the formation of stars, so there’s nothing giving off light to observe. If we look even further away, we see back to the time shortly after the Big Bang when the entire universe was filled with hot plasma that was opaque to light. The Cosmic Microwave Background is our very-red-shifted view of the wall of hot plasma we see in every direction out past the earliest stars.
Over time this wall of hot plasma gets farther and farther away. Eventually it will be so far away that it will be receding from us faster than the speed of light and it will be outside the observable universe. Intelligent beings in that future universe will not be able to see the CMB and will not have access to an important piece of information about the Big Bang. Lucky us!
The universe has always been infinite in size. When it came into existence it was very, very dense. Now it is much less dense.
This is a common source of confusion (see here). The Hubble radius (~14 Glyr) is the distance at which objects recede from us at c, the radius of the observable Universe (aka the particle horizon) is actually quite a bit larger (~45 Glyr). Also new objects are constantly entering the observable Universe and the distance beyond which we will never see objects is actually the co-moving radius which the observable Universe asymptotically approaches (~60 Glyr) or this radius plus the radius of the cosmological event horizon if we are allowed to get into a spaceship to increase the distance we can see (~77 Glyr).
This discussion has been extremely interesting and, coincidentally, I’ve seen a few interesting documentaries on TV in the last few weeks about this.
And, Asympotically fat, thanks for the reference. I’ve taken an initial look at it and, to be honest, it’s somewhat (ok, a lot) beyond me but I’ll certainly learn something from this.
