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First of all, what's the difference between gravity & gravitation? Anyway that's not the actual question. If every matter attracts each other toward itself, then wasn't the universe supposed to be compact at a point rather than expanding?

nabla
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Farhan Fuad
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3 Answers3

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EDIT As John Rennie points out, my answer here is incorrect. To quote John

I have to say I think this answer is entirely wrong. You give the impression that the universe is expanding outwards from a point, which it isn't, and the expansion rate is an initial condition. In fact gravity was slowing the expansion until a few billion years ago when dark energy took over and started accelerating the expansion again.

As I can't delete the answer below and, rather than edit it, I would prefer to leave it as is in order to correct a misconception about the big bang that might be widely shared.

Please read my answer in the light of John's comment and his post Did the big bang happen at a point?. END EDIT

You mean, just immediately after the big bang, is the universe thought to have been compact? Yes.

If you assume, as we do, that the expansion of space of the universe following the big bang was a (much) larger "outward" force than the attractive (but very weak) force of gravity, that might answer your question

For example, I, as just a not very strong human, can easily pull a small rock away from the ground, despite the whole earth trying to pull it back. That's how weak gravity is. (Or the gravitational force, as in your first line).

Also, at the time immediately after the big bang, matter as we now know it did not exist, it was energy/radiation rather than anything like the stars and galaxies we see today.

There are lots of similar questions to yours on this site, have a search, or look at Expanding Universe

Community
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  • I have to say I think this answer is entirely wrong. You give the impression that the universe is expanding outwards from a point, which it isn't, and the expansion rate is an initial condition. In fact gravity was slowing the expansion until a few billion years ago when dark energy took over and started accelerating the expansion again. – John Rennie Nov 20 '15 at 17:51
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Your question assumes that the universe started out as a point at the Big Bang and then expanded outwards, however this is not the case. Have a look at my answer to Did the Big Bang happen at a point? for more on this.

However your main point remains, that is shouldn't gravity be slowing the expansion, and indeed it did until a few billion years ago.

The expanding universe is described by a solution to Einstein's equations called the FLRW metric. When we solve the equations we find out that the universe had to start out expanding. We describe the expansion by a scale factor $a$, and the expansion rate can be described by the rate of change of the scale factor $\dot{a}$. The equations tells us that the expansion rate $\dot{a}$ has indeed slowed down with time as the mutual gravity of all the matter in the universe opposes the expansion.

Until recently we expected that this slowing would continue and almost but not quite bring the expansion to a complete stop. However we have since discovered dark energy, which has the opposite effect to matter and causes the expansion to speed up not slow down. Dark energy only dominates when the matter density falls below a certain value and this happened a few billion years ago. So for the last few billion years the expansion has been speeding up and we expect it to carry on speeding up fo the forseeable future.

If you're interested to know more I describe how the scale factor is calculated in How does the Hubble parameter change with the age of the universe?.

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John Rennie
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  • I agree with all the above. Just a little remark: the force between two bodies being attractive does not imply that the bodies move towards each other (they will move in conic sections). Therefore, even if the Big Bang had actually happened locally, at just one point, with (simplest case) only two masses, the two bodies will still rotate in conic sections rather then collapse onto each other (unless they accidentally happened to have velocities in the same directions). – gented Nov 20 '15 at 22:54
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You are correct that the gravity of everythig in the Unuverse should have contracted the universe from everything we know, but the fact is we don't have all the answers. For example, what caused the big bang? What cause the sudden inflation? what is dark energy?

String theory has some potential answers for the initial inflation but string theory has run into problems when it tries to unify quantum mechanics with General Relativity. Dark energy is still a mystery but it is the only way the behavior of the expansion of the universe can be explained. In order for a theory to be valid the thoery has to be able to predict things that can be verified independantly of the problem the theory is trying to solve. it's not enough to say thet there is dark energy causing the expansion. Dark energy has to be explained and predictions from the explanation have to be verified.

It's a very exciting time for physics where hopefully some new ideas will emerge.

Peter R
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