# Critical mass

1. Jul 30, 2009

### chi22

According to widespread modern knowledge, the universe possesses about 0.4 of the required mass to collapse back into it's original state, in a sort of 'big crush',
As the current predictable future of the universe is to expand in an exhilarating speed while 'tearing up' the matter into the most elementary values.
Since the universe is consently expanding in an exhilarating speed, why didn't it collapse back to the singularity at the first moments of the big bang, when it's extention speed was significantly lower?

- chi, IL.

2. Jul 30, 2009

### nicksauce

The universe isn't at 0.4 of the critical density, it is at about (1% margin of error) the critical density (~26% matter ~74% dark energy). This means that the universe is flat (will expand forever), although we can't rule out an open universe or a closed universe.

If the universe is at the critical density, then it will always be at the critical density. To see this, look at the Friedmann equation governing the expansion of the universe:
$$(\frac{\dot{a}}{a})^2 = \frac{8\pi G\rho}{3} - \frac{k}{a^2}$$

K is a constant related to the curvature of the universe. If the universe is at the critical density, then K = 0. Since we know today we are at the critical density, then K = 0, and it always was since it's a constant. Therefore there is no reason to believe the universe would re-collapse moments after the big bang.