What Reference Frame Determines the Age of the Universe?

[Eaglespike]

The experts say the age of the universe is known
with considerable accuracy. What frame of reference are
they using and why doesn't it violate the idea
of relativity to claim to have a preferred frame?

 

[Chalnoth]

The experts say the age of the universe is known
with considerable accuracy. What frame of reference are
they using and why doesn't it violate the idea
of relativity to claim to have a preferred frame?

Well, there is no preferred frame, but it is perfectly reasonable to pick a frame of reference. The frame of reference we pick is that of the matter around us. That matter has experienced about 13.7 billion years worth of history since the big bang.

 

[phinds]

And eaglespike, just to add to what Chalnoth said, that does NOT mean that any macro objects such as galaxies and stars and planets are 13.7 billion years old. Macro objects didn't even START forming until about 400,000 years after the singularity (which is the "starting point" for the 13.7 billion year age)

 

[Eaglespike]

Thank you Chalnoth and phinds. However, the age would presumably depend on
the frame so have you any idea what the difference would be for
an observer at the limits of our visible universe and an observer nearby?
It seems to me that when they give the 13.2 billion figure they are assuming
that they can step outside the universe and watch it evolve.
Any comments would be appreciated!

 

[phinds]

Thank you Chalnoth and phinds. However, the age would presumably depend on
the frame so have you any idea what the difference would be for
an observer at the limits of our visible universe and an observer nearby?
It seems to me that when they give the 13.2 billion figure they are assuming
that they can step outside the universe and watch it evolve.
Any comments would be appreciated!

They would see the same age. There IS no "outside".

 

[Chalnoth]

Thank you Chalnoth and phinds. However, the age would presumably depend on
the frame so have you any idea what the difference would be for
an observer at the limits of our visible universe and an observer nearby?
It seems to me that when they give the 13.2 billion figure they are assuming
that they can step outside the universe and watch it evolve.
Any comments would be appreciated!

Basically, we can set an overall reference frame by using the temperature of the CMB. An observer anywhere in our universe, no matter their reference frame, can look at the CMB and measure its temperature (provided they have the right instrument).

We can define a global reference frame by the following two criteria:
1. An observer in the global reference frame sees a CMB which is has the same average temperature in every direction (that is, it has no dipole). This means that the observer is at rest with respect to the CMB.
2. At a given time t, the every observer sees the same temperature of the CMB. That is to say, we can define a global "now" by saying that all observers "now" see the same temperature of the CMB that we see (2.725K). These observers will see the total time passed since the big bang as being the same, so we can use the same time coordinate.

This is perhaps a bit technical. But the upshot is that the fact that our universe is, on average, the same in every location and in every direction means that there is a convenient choice of reference frame. With this convenient choice, we can talk about things like the age of the universe in a sensible way, in a way that observers on far-away galaxies will agree with.