# Meaning of age of universe

1. Nov 22, 2004

### jzmckay

Meaning of "age of universe"

This question may have been addressed somewhat indirectly in other posts, but I'm interested in specifics.

Exactly what assumptions about time (and the universe) are implied by a statement like, "The age of the universe is X years old"? In particular, does this implicitly assume some very general notion of an "absolute" spacetime reference of a sort? What would that reference frame be?

A couple specific problems I have:

(1) Particles, stars, and even entire galaxies move around each other in complex manners at speeds that approach the speed of light relative to each other (and due to gravitational effects, in non-inertial frames with respect to each other). When I say the universe is X years old, am I just making that statement for my local region of the universe? In other words, wouldn't the clock somewhere else (e.g., another galaxy which has moved at high speeds in a complex relation to our galaxy) claim a different age for the universe at the "present"?

(2) I've asked this question of others, and one response I've received is that the "age estimate" somehow assumes the reference frame of the earth, i.e., that the earth is X years old. But the earth and its constituent particles do not constitute one particular path going from the Big Bang until now. For instance, suppose an extreme Twin Paradox example, where one person (or at least a ship) is sent out at speeds close to the speed of light. Suppose the speed is high enough that when it returns to earth, the ship (and any passengers) are significantly younger. In order for this to be relevant for current age of universe estimates, the "significantly younger" would have to mean, say, 100 million years or so younger. At high enough speeds, this is possible. So, let's say that the folks on earth reckoned the universe to be 13.7 billion years old when the ship left. When the ship returns, the folks on earth reckon the universe to be something like 13.8 billion years old. However, the ship folks reckon only a few years to have passed, so they still estimate the universe to be about 13.7 billion years old.

But, since the "age of the universe" when it is mentioned is based on astronomical observation, the ship returning would have to revise the estimate up to 13.8 billion years... at least according to modern methods we use to determine the "age of the universe." (I recognize that for the ship, time really has elapsed some 100 million years less, but how does this relate back to the way we come up with our "age" estimate?)

Obviously this is a very extreme case, but since every particle that makes up the earth has its own history in spacetime since the Big Bang, arguably parts of the earth are significantly "younger" in some sense than other parts (perhaps not by hundreds of millions of years, but maybe by millions or at least thousands of years). I suspect that there are some arguments about homogeneity of the universe that allow us to discount such extremes in estimating the "age," but what exactly are those assumptions?

In other words, if the reference frame is not earth or our solar system tracing a direct spacetime path back to the Big Bang, then what does it mean to say that the universe is X years old? In exactly WHAT REFERENCE FRAME would you have to be from the Big Bang until the present in order to make a claim of being the age X that we use in our current "age of the universe" estimates? I'm just looking for the explicit assumptions we make here about the meaning of "age", whether we're taking data from particular types of stars or supernovae, or whether we're looking at background radiation.

I hope this question isn't too convoluted, and I ask for any thoughts (or links to places these questions may have been addressed). Thanks so much.

Last edited: Nov 22, 2004
2. Nov 22, 2004

### Garth

The frame is that selected by the assumptions of homogeneity and isotropy, which is co-moving with the ‘cosmological representative gas’. It may be identified as that in which the Cosmological Microwave Background is globally isotropic.

You are correct to point out that galaxies are moving with respect to each other and would have different relative clock rates, however as the Milky Way is moving at 0.2%c relative to the surface of last scattering of the CMB, the difference to the cosmological age for an observer in our galaxy would only be 2 x 10^-6.

Garth

3. Nov 22, 2004

### jzmckay

Thanks so much. I think I get the idea. Also, thanks for pointing out the relative insignificance of the speeds of different galaxies; it makes much more sense now.