Time according to whom, after the big bang?

[komodekork]

When physicist talk about time after the big bang, what do they mean? Time is relative, so which frame of reference are they talking about?

Could anyone please explain?

 

[phinds]

When physicist talk about time after the big bang, what do they mean? Time is relative, so which frame of reference are they talking about?

Could anyone please explain?

Current cosmological models indicate that the U was once tiny and has since expanded. Various experiments have shown that there definitely is light reaching the Earth that has been traveling for something like 13.7 billion years. and that that radiation started about 480 thousand years after the universe started not being infinitesimal. Thus the belief that the "big bang", whatever that was, happened 13.7 billion years ago (maybe plus or minus the 408 thousand, I forget).

The relative nature of time that you are talking about is a relativistic effect of motion of two objects in relation to each other. It doesn't apply to the concept that the big bang happened when it did.

 

[cosmik debris]

When physicist talk about time after the big bang, what do they mean? Time is relative, so which frame of reference are they talking about?

Could anyone please explain?

They are talking about time in their own frame. In a way it can't be anything else, it is the time of the observers frame. However cosmological time in some theoretical spacetimes is like an absolute cosmological time. I know this contradicts relativity but in some of these models (like the RLFW) the universe has a symmetry (killing vectors) with a constant time surface.

 

[phinds]

... cosmological time in some theoretical spacetimes is like an absolute cosmological time. I know this contradicts relativity ...QUOTE]

Uh ... how is that? Relatively has to do with the relative motion of two bodies. How does time since the big bang have anything to do with that?

Thanks,

Paul

 

[IsometricPion]

Time is relative, so which frame of reference are they talking about?

In general, there are no frames of reference in GR that cover all of space-time (like they do in SR). However, one can usually come up with coordinate systems that cover all of space-time. Cosmologists often use a coordinate system wherein the CMB is at rest. So, the amount of time you mentioned is the amount of proper time elapsed since the big bang in the http://en.wikipedia.org/wiki/Synchronous_coordinates" in which the CMB, ideally, is a purely spacelike hypersurface.

 

[bapowell]

Uh ... how is that? Relatively has to do with the relative motion of two bodies. How does time since the big bang have anything to do with that?

No it doesn't. It has to do with reference frames. In general relativity, all reference frames are equally appropriate -- the Lorentz symmetry enjoyed by inertial observers is no longer a preferred symmetry. What cosmik is saying, however, is that some spacetimes possesses sufficient symmetry to define a global time variable (just like one does for inertial frames in special relativity). In these spacetimes, this time variable is a 'natural' choice to run a clock by. In our FRW universe, observers comoving with the expansion (at reast wrt the CMB) are in one such frame. We are (very closely) comoving observers, and so this is the clock used by modern cosmologists. But this time is still no better than any other -- one can choose whichever frame one wishes; some are simply more convenient than others. (To see what I mean by convenience, imagine using galaxy-centric coordinates to plan satellite trajectories around the Earth -- obviously one should use geocentric coordinates for such a feat.)

 

[cosmik debris]

No it doesn't. It has to do with reference frames. In general relativity, all reference frames are equally appropriate -- the Lorentz symmetry enjoyed by inertial observers is no longer a preferred symmetry. What cosmik is saying, however, is that some spacetimes possesses sufficient symmetry to define a global time variable (just like one does for inertial frames in special relativity). In these spacetimes, this time variable is a 'natural' choice to run a clock by. In our FRW universe, observers comoving with the expansion (at reast wrt the CMB) are in one such frame. We are (very closely) comoving observers, and so this is the clock used by modern cosmologists. But this time is still no better than any other -- one can choose whichever frame one wishes; some are simply more convenient than others. (To see what I mean by convenience, imagine using galaxy-centric coordinates to plan satellite trajectories around the Earth -- obviously one should use geocentric coordinates for such a feat.)

Yes a much better explanation.