Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: gravity and expansion

Learn more physics!

Most recent answer: 09/13/2013
Q:
1. As soon as matter and energy were created, right at the big bang, gravity must have tried to pull them into a center, which, as a side effect, having stopped the generation of spacetime, should have turned into the ultimate black hole: the Singularity. Why the universe; why not a vibrating Singularity to Bang to Singularity to Bang?

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
A:
Hi Mehran- good to hear from you again. Sorry for reformatting your question; our program was having trouble and it took a while to figure out that the problem was that it couldn’t handle an apostrophe.

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?

Q:
If all the mass of the universe was in one small place, as the big bang believers say, then why isn't it a black hole, and not let any mass or energy get out. I think either the concept of a black hole is wrong, or the concept of a big bang is wrong. I know Hawking suggested a way for energy to escape from a black hole, but would this apply to something as heavy as a proton?
- Larry Nestor (age 67)
Long Beach CA USA
A:

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)

Follow-up on this answer.