# Changeover from deceleration to acceleration

1. Mar 9, 2010

### Ranku

Did the universe changeover from deceleration to acceleration exactly after the moment when the increasing repulsive force became equal to the decreasing gravitational force?

[Would like to confirm if that would be consistent with the fact that acceleration starts when the mass density of the vacuum becomes bigger than half of mass density of matter.]

2. Mar 9, 2010

### Ich

It's a constant repulsive "force", if you will, not an increasing one. Otherwise you're correct.

3. Mar 9, 2010

### Ranku

But doesn't the repulsive force of the cosmological constant increase in proportion to distance?

4. Mar 9, 2010

### Ich

So does gravitation. I was referring to DE being constant over time, while matter gets diluted.

5. Mar 9, 2010

### Ranku

I know ;). So then I assume I can confirm my original query about the universe accelerating just after repulsive force overtakes gravitational force.

6. Mar 9, 2010

### Ich

Yes. Expansion accelerates when $\Omega_{\Lambda}>\Omega_M/2$.

7. Mar 9, 2010

### Chronos

SNAP suggests expansion began accelerating a few billion years ago. You can search for SNAP on Arxiv.

8. Mar 9, 2010

### Wallace

SNAP was a proposed experiment that hasn't been built, let along put into space. It's currently not likely that it will ever fly (though something similar probably will at some point). It certainly hasn't taken any data yet though....

If you're after a good, reasonably current, review of observational evidence and probable future surveys try http://arxiv.org/abs/0803.0982" [Broken].

Last edited by a moderator: May 4, 2017