# Proof of the expansion of the universe-cosmological redshift?

1. Aug 23, 2011

### tunafish

Hi everybody!! I'm going to make this quick, so i won't waste your time!
SO:
What are the proofs of the expansion of the universe??
And what were the firsts?
Is the cosmological redshift a proof of that?

A little more complicated one:

From where, in the einstein equations, should i see that tere is the gravitational attraction which pushes thing togheter?

Thanks a lot friends!

2. Aug 23, 2011

### Clever-Name

3. Aug 23, 2011

### bapowell

You're essentially asking how to recover Newton's law of gravity from the Einstein field equations of general relativity. You can make contact with Newtonian gravity in the so-called "weak field limit", in which the gravitational metric is taken to be static and to be "close" to the Minkowski metric of flat space. In this approximation, the Einstein equations reduce to a Poisson's equation involving the gravitational potential -- the potential being treated as a perturbation. Newton's law of gravitational attraction follows from Poisson's equation. I can certainly add more mathematical detail if you're interested...

4. Aug 23, 2011

### Chronos

I'm confused, did you mean Poisson or Poincaire?

5. Aug 23, 2011

### marcus

Brian Powell already answered this but I will throw in a URL just in case you like the approach. John Baez has an intuitive explanation of the Einstein Field Equation. It is described in terms of what happens to a cloud of coffee grounds, as I recall.
The title is something like "The meaning of Einstein's equation".
It's pretty elementary. He's a good explainer. You can usually trust him to find a simple way to explain something if there is a way.

You can get it if you google "Baez meaning einstein"
so you don't really need the link but here it is:
http://math.ucr.edu/home/baez/einstein/

It might be wise to get Brian's explanation though. Might have a neat one.

Last edited: Aug 23, 2011
6. Aug 23, 2011

### bapowell

No -- Poisson. For gravitational potential, $\phi$, and energy density, $\rho$, one has Poisson's equation:
$$\nabla^2\phi = 4\pi G \rho$$

7. Aug 23, 2011

### Chronos

Sometimes I feel like the village idiot.

8. Aug 23, 2011

### bapowell

If you don't ever feel like the village idiot, then you're doing something wrong.

9. Aug 23, 2011

### Chronos

I feel safe on that count.