[ QUOTE ]
Thanks for the reply. I do not know the techniques cosmologists use, but from my limited understanding of relativity (I've never formally studied GR), it doesn't seem like this:
"The measured age is, to all intents and purposes, an 'absolute' value. It does not formally depend on the velocity or the acceleration of an observer."
can be true. Again, you may be completely correct, but I'm having trouble understanding how it could be true.
Is it true that, if we had a hypothetical observer that has been around since the big bang, they would measure the exact same age of the universe no matter where their location in spacetime post-expansion? A photon emitted at the big bang is the same age now that it was at the big bang, therefore it doesn't seem like this photon would measure the same time that, say, an observer attached to whatever matter that becomes Earth would.
Do cosmologists just have an agreed upon "standard" for measuring the age of the universe, and in this sense it is absolute? But wouldn't it be possible that a separate group of people could agree on a different "standard" and come up with an entirely different answer? Or is it really "absolute" in every sense of the word (this disagrees with my intuition regarding relativity); i.e. every observer measures the same time no matter where their location in spacetime?
[/ QUOTE ]
Well, theres a few caveats to all of this ;-) First, and most importantly, any measured age is the age of the 'observable' Universe. That is, the age of the region whose size is determined by the distance a photon can travel during the time since the big bang, together with any expansion the Universe has undergone. Second, any age we derive is not formally the age of 'existence', but rather the time since the Universe was in a very hot, very dense state - just after the Big Bang.
With that said:
Based on our current understanding of GR, then two observers in different galaxies but living at the same time as each other, will both derive the same age for the Universe as a whole. As they are in different galaxies though, each observer will see the OTHER observer as they weer some time in the past though. So if observer A measures the age, and then transmits that age to observer B, then observer B will receive an age from observer A that is equal to the age that B measures, minus the time it takes for light to get from A to B.
Now, the reason that both observers get the same age for the Universe is because theyre measuring that age using the Cosmic Microwave Background radiation. The CMB doesn't take 'time' to go from one place to another - it pervades the entire Universe. Its a relic from a period in the Universes history called 'decoupling', which I don't really have time to go into here, but wikipedia has a good writeup on the subject.
Now, the universe is STILL expanding - not as fast as it did at very early times, but the expansion is still happening (and probably accelerating). But that doesn't affect the age measured from the CMB. For practical purposes, this CMB *is* photons that were emitted 'at' the big bang (if you want to be pedantic then it would be about a few thousand years after the big bang, but thats not really a biggie ;-)). To fully understand this, you really need to read up on decoupling, which is a fascinating, if slightly subtle topic:
http://en.wikipedia.org/wiki/Cosmic_...ave_Background
There are many ways to measure the age of the universe, so in that sense theres no agreed, single 'standard' way. However, nearly all these methods, several of which are effectively indeendent of each other, give an age of around 13 billion years, so that age is pretty robust.