How to find that the universe is expanding from general theory of relativity

[manvirsingh]

1 how to find that the universe is expanding from general theory of relativity

2 how to apply general theory of relativity to find that the universe is expanding

 

[HallsofIvy]

How are (1) and (2) different?

 

[PatrickPowers]

1 how to find that the universe is expanding from general theory of relativity

2 how to apply general theory of relativity to find that the universe is expanding

As far as I know general relativity doesn't predict that the Universe is expanding. The expansion was discovered by astronomers. Galaxies in all directions are for the most part moving away from the Earth, and the farther away the faster they move. So the Universe is expanding.

It is NOT however expanding into empty space. Every part of the Universe has more or less the same number of galaxies as every other part. They are getting farther apart: space is stretching so that there is more of it while the amount of matter stays the same.

 

[Naty1]

The original equations of general relativity implied either an expanding or contracting universe...Einstein tried to make his formulation match what he thought was observed at the time, a static universe, by introducing the cosmological constant.

More here:
http://en.wikipedia.org/wiki/Cosmological_constant#History

 

[manvirsingh]

how to apply "The original equations of general relativity"

 

[PeterDonis]

how to apply "The original equations of general relativity"

I think what Naty1 meant was that if you take the Einstein Field Equation without a cosmological constant, which was how it was originally formulated, there are no solutions that admit a static universe; put another way, there are no static solutions with a non-zero stress-energy tensor, so if there is matter in the universe (which we believe there is ), then according to the original EFE the universe cannot be static; it must be either expanding or contracting. There are other reasons to believe that the universe is not contracting, so the original EFE basically predicts that the universe is expanding. At the time, the universe was believed to be static, so Einstein added the cosmological constant term to the EFE to allow static solutions with non-zero stress-energy.

 

[tom.stoer]

Even with non-zero cc a static universe is not possible b/c this solution corresponds is unstable (like a saddle-point); any small perturbation of such a solution would either create expansion or contraction.

 

[Naty1]

...it must be either expanding or contracting.

That is exactly what I meant...and post #7 adds the final description...so

how to find that the universe is expanding from general theory of relativity

you can't...or that's not the way it happened...the expanding (rather than contracting) universe was shown by the observations of Hubble and Slipher.