# Problem with my universe!

1. Apr 5, 2006

### shaan_aragorn

In his book "The Brief History Of Time", Stephen Hawking says that if the universe is finite, as assumed by Newton et al, the astronomical bodies would fall on one point. Now I am having a really tough time imagining this. Wouldnt the gravitational force be too weak to pull these bodies on one point over such enormous distances?

Last edited: Apr 5, 2006
2. Apr 5, 2006

### Staff: Mentor

No. In order for that to be correct, the gravitational pull would have to be exactly zero. Otherwise, with any net pull, regardless of how small, the matter in the universe would eventually fall to the center of mass. The strength of the pull just determines how long it takes.

The key is that if the universe started static and had a center, every object would have some net force pulling it toward that center. It wouldn't stay static for long.

3. Apr 5, 2006

### pi-r8

That's not true. It could rotate, the way the planets in the solar system do. Those aren't going to fall in to the sun, and they're not expanding their orbit either.

4. Apr 5, 2006

### complexPHILOSOPHY

Are you postulating that the universe itself is rotating?

The reason our planets orbit the sun without being 'pulled' towards it, is because the sun's mass is relatively static which means it's gravitational pull on our solar system is also relatively static.

Last edited: Apr 5, 2006
5. Apr 5, 2006

### Staff: Mentor

How can a universe rotate and still be considered static? Can a universe, by definition, even be rotating?

Also, merely rotating wouldn't be enough: that implies a constant angular momentum, and with a constant angular momentum, you only get an orbit at one specific radius - every object further away spirals out and every object closer in spirals into the center. Or maybe some just end up in highly elipitcal orbits - my celestial mechanics isn't that great. Either way, it's tough to call that situation "static".

Last edited: Apr 5, 2006
6. Apr 6, 2006

### shaan_aragorn

Is this situation analogous to two magnets (or charges/poles) kept in vacuum with zero gravity (for cutting out external effects)? Will such two magnets also attract each other (assuming they are of opposite polarities) over an infinite distance?

7. Apr 6, 2006

### pi-r8

I was simply objecting to the notion that "if the universe is finite, it must fall on one point." I never said anything about it being static. I'm also not actually postulating that it is actually rotating (as in, ever object is rotating around a common center of mass), I simply wanted to point out that such a situation is possible.

8. Apr 6, 2006

### Staff: Mentor

Hmm - rereading the OP, I think we'll need to clarify what Hawking was talking about. I assumed he also meant static. The question seems to be asking why the universe couldn't be static.

Last edited: Apr 6, 2006
9. Apr 11, 2006

### Chronos

Hawking proposed the universe is finite, but unbounded, in his book. He never intended to infer the universe is finite in the sense of having definable boundaries [or a center].

10. Apr 11, 2006

### Reshma

The universe is isotropic and homogenous. The universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places. Stars exhibit red shift and blue shift when observed from the earth but a majority of them exhibit red shift, indicating that the stars are moving away. This observation will be true from any region of the universe.

Last edited: Apr 11, 2006