Einstein equations don't work at large scale?

[Ibix]

As Bill Clinton once testified, “That depends what your definition of ‘is’, is.” Other than “exists at my point in spacetime” the other definitions seem fungible.

The first line in deriving the FLRW cosmological model is to assume that everything is the same everywhere at any given time. This inevitably leads to the universe being the same age everywhere because we insisted that the universe be the same everywhere, and following through the Einstein Field Equations adds that this age is finite.

However, GR is a theory of spacetime, not space and time. There is nothing in the physics requiring you to use the definition of time used in the initial derivation, not in any spacetime, and other definitions of time don't necessarily share the same definition of "now".

That said, anybody talking about "the" age of the universe must be using a definition of time where there is a unique age. And in cosmology, AFAIK everybody uses either cosmological or conformal time, and both share the same definition of "now" that has the universe being the same age everywhere. Cosmological time corresponds directly to clock time.

 

[Sagittarius A-Star]

here Claudia de Rham talk about some problems:

Her related paper is described in the Wikipedia article under "Ghost-free massive gravity":
https://en.wikipedia.org/wiki/Massive_gravity#Ghost-free_massive_gravity

 

[gen x]

What theory are you referring to? Can you give a reference?

Bear in mind that there is no way with our current technology to even test predictions about gravitons.

Sagittarius A-Star post some material at post #84

 

[martinbn]

Sagittarius A-Star post some material at post #84

In the other thread yous said you were not interested in theories that will not be around in 100+ years. Well, this one has not been around, as an experimentaly tested theory, for 0 years. Chances are it will never be a part of accepted science. Why are you interested in it?

 

[Dale]

I am interested in Einstein Cartan theory, but for different reasons. To me the issue is that it needs to be experimentally tested and the only places where it differs from GR are inside matter. So, I anticipate that it will be some time before we can test it, probably not in my lifetime.

 

[PeterDonis]

The first line in deriving the FLRW cosmological model is to assume that everything is the same everywhere at any given time.

To rephrase this in a way that might make the point clearer: we assume that there exists a set of spacelike hypersurfaces that foliate the spacetime (i.e., every event is on one and only one such hypersurface) and which are homogeneous and isotropic. We then construct a coordinate chart on the spacetime that uses those hypersurfaces as its surfaces of constant time, and we scale the time coordinate so that it directly represents the proper time of observers at rest in the spatial coordinates, who always directly observe the homogeneity and isotropy. In other words, we assume a particular geometric property of the spacetime that ensures that there are coordinates in which "everything is the same everywhere at any given time".

 

[pervect]

@Dale
@Ibix

I would like that some day it turns out that time is absolute, it better suit human intuition.

If time is relative as we learn today, universe is old 13.8 billions years, looking from witch frame if absolute frame at rest dont exist?

So universe age is different from different places in space?

That's going to be very difficult (if it is possible at all) to reconcile with existing evidence. Specifically, the constancy of the speed of light is well established at this point, and there is no known way to reconcile that with absolute time. My understanding is it's already been proved impossible, actually, but I'll settle for the weaker statement since I don't have a quotable reference in front of me. The argument might need additional assumptions that I haven't mentioned, like isotropy, as well.