Robertson-Walker metric and time expansion

[Imax]

Does an FRW metric allow possibilities like a twin paradox? What's a hypersurface of time?

 

[WannabeNewton]

Does an FRW metric allow possibilities like a twin paradox? What's a hypersurface of time?

Not sure on the first question sorry. A hypersurface of time is, in this case, essentially the 3D surface you get when you keep time constant. So its like if you look at the universe at some instant of time.

 

[Chalnoth]

Does an FRW metric allow possibilities like a twin paradox?

Well, yeah. The twin paradox (though not actually a paradox) is intrinsic to the nature of relativity, and since the FRW metric is based upon General Relativity, certainly it can apply.

What's a hypersurface of time?

Well, a set of points in space that all have the same time coordinate. Note that such a hypersurface depends upon what sort of time coordinate you use.

 

[Imax]

So, an FRW metric can allow for time dilation, time contraction (i.e. time expansion).

Is there any evidence that time can expand at the same rate as space?

 

[Chalnoth]

So, an FRW metric can allow for time dilation, time contraction (i.e. time expansion).

Is there any evidence that time can expand at the same rate as space?

Not in any absolute sense. With normal FRW coordinates, there is no expansion of time, just space. But you don't need to describe the system with the same coordinates. The expansion will always be there no matter what coordinates you pick, but precisely how it manifests itself depends upon those coordinates.