# Gravity and Expansion

*Most recent answer: 12/02/2007*

2. What agent was at tug of war with, and immediately defeated, the gravity?

3. Is this the same agent which had generated elements out of spacetime?

4. Matter bends spacetime. Is it more precise to say matter is densely curved spacetime? Conversely, we may say, space is diluted matter. So, there must be another equivalence equation: S equals MC2 ?

- mehran

1. Although there is no center in any standard picture, you can think of gravity as pulling the parts together. Just like in the classical problem where pieces of matter fly away from an actual center, if the relative velocities are big enough the matter will escape rather than recollapse. So it’s a quantitative issue depending on the relative velocities vs the mass density.

2.There appears to have been a very large density of mass-energy ’stuck’ in space itself at an early stage. Oddly, given the ordinary equations for gravity a fixed density causes accelerating expansion, just the opposite of gravity’s effect when the mass itself is fixed.

3. The generation of ordinary matter occurred at a later, less mysterious stage.

4. In our current working picture, the properties of matter cannot be accounted for by spacetime curvature alone. It is possible, however, that if string theory succeeds all the features of matter will be understood as geometrical properties, but in a higher dimensional space.

Mike W.

For more information than you might want, see:

http://en.wikipedia.org/wiki/Big_Bang

For information on the intriguing idea of an oscillating unirverse, see:

http://en.wikipedia.org/wiki/Oscillatory_universe

LeeH

*(published on 12/02/2007)*

## Follow-Up #1: big bang or black hole?

- Larry Nestor (age 67)

Long Beach CA USA

It seems as if we've gotten this same good question numerous times, but since I can't find the answers, we'll have another round at it. Both the Big Bang picture and the black hole picture are solutions of exactly the same equations: General Relativity. A black hole is not just a region in which the mass within some volume exceeds a partcular threshold. That gives a black hole solution *under the condition that the mass density outside that region is low*. That condition was not even approximately met in the Big Bang, in which the density everywhere was high but nearly uniform. Different starting conditions give different solutions.

You may wonder how the mass density could have been high and nearly uniform. There are basically two ways. Either the universe is finite, wrapping around on itself, or it's infinite, extending without end. Either way is consistent with the equations. Current data do not tell us which is correct.

Mike W.

*(published on 09/13/2013)*