If the CMB is all around us 13.8B years away, why isn’t the universe considered 13.8B years old and 27.6B years wide?
I understand why it would most likely be impossible to physically observe the other 13.8B years, but theoretically the geometric properties of a radius should apply to physics.
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Space itself expands. If it didn’t then you’d get measurements like you posted, but it does so the universe is quite a bit more spread out than simply twice the age*speed of light.
13.8 B is just how far out in every direction we can see into the observable universe. The "big bang" doesn’t have a center or direction.
The universe expands. The CMB light we receive today was emitted by matter 42 million light years away from us at that time, the same matter is now 46 billion light years away. That is the radius of the observable universe, the overall universe is larger and potentially infinite.
Don Lincoln explains the expanding observable universe is 92 billion years in diameter. Clear explanation.
Space expands by 73 kilometers per second per 3.26 million lightyears. (1 megaparsec)
So the observable universe has a radius of 46 billion lightyears, thats the furthest we can see in any direction, and that distance corresponds to just a few hundred thousand years after the Big Bang, when the universe first became transparent. Objects that far away are redshifted almost to being nonvisible, and they get gradually less so as you look closer and closer.
Because of the expansion though, we could never reach these objects even if we travelled at lightspeed, because we are only seeing the light they emitted 13.8 billion years ago, the actual objects themselves are much further away than that, and the expansion of the universe between us would have exceeded lightspeed by now.
While there are other problems like ignoring cosmic inflation that others have already pointed out, they already corrected that. I’ll go with your numbers for ease of understanding.
You’re Right in your understanding that a radius and geometric properties apply. The Big Bang, and thus the CMB happened everywhere, including where we are, so if we look in one direction we can see 13.8 billion years back and if we turn around and look in the other direction, we can see those “other” 13.8 billion years you mention, for a total of 27.6 billion years. The radius and diameter of the universe (even if the actual numbers are different due to inflation).
But it gets a bit weird. If we change our reference point to another planet – to Earth2 at the edge of the observable universe 13.8 billion years away – by our understanding, they would still see 13.8 billion years in all directions and we would be at the edge of their observable universe.
The universe is probably, possibly infinite, this is just part we can observe because the light has had time to reach us and that time indicates the age of the universe.
Because the universe is functionally infinite (i. e. infinite or sufficiently vast that we can’t tell the difference)
So how far we can see – the observable universe, out to the Hubble limit, is determined by the age of the universe (has light had time to reach us) and by expansion effects (has expansion changed the size of the Hubble limit faster than the expansion of the universe.
So we can see 13.8 billion lightyears, but where that was is now much further away because while the light has been travelling from there to here, that place has been moving away from us at nearly the speed of light due to the expansion of the space in between.