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randybryan

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- Thread starter randybryan
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randybryan

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mfb

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Space does not have to be infinite, but it is probably without a boundary. This is not a contradiction, see the surface of earth for example: finite area, but no boundary.

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randybryan

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UltrafastPED

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This the slightest imbalance, anywhere, or at anytime will start the gravitational agglomeration of matter.

But such a slight imbalance at any time during the Big Bang, early or late, will result in inhomogeneity at some level - this is where detailed models must be compared to observational evidence in order to refine our understanding.

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randybryan

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UltrafastPED

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

But the argument is simple:

One object alone exerts gravity on nothing but itself.

Two objects exert gravity on each other, and are attracted. If they were originally still they will collide; if they had their own motions they will follow a conic section: parabolic (a bullet), hyperbolic (some comets), elliptical (planets)

What if you arrange 3 in a line? Then maybe you can cancel the net gravity on the middle one, but the other two are still going to move.

Keep adding one more, and every arrangement is unstable.

In the limit you can generate an infinite array - like a giant salt crystal, but with stars instead of atoms of Cl and Na - with an equal number of stationary attractors in each direction.

But once the perfect alignment is broken, you will have worlds in collision!

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Chronos

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randybryan

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Awesome. Thanks for the reply. Definitely going to read up on some cosmology

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Superposed_Cat

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Space does not have to be infinite, but it is probably without a boundary. This is not a contradiction, see the surface of earth for example: finite area, but no boundary.

sooo if you get to the edge of the universe you go to the beginning?

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There is no "edge of the universe".

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Superposed_Cat

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I thought space-time was finite

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UltrafastPED

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The surface area of a sphere is finite ... but there is no edge.

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George Jones

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I thought space-time was finite

In what sense?

There are models of the universe for which space finite, but I know of no seriously considered models of the universe for which spacetime is finite. In fact, all spacetimes that satisfy a certain finiteness condition have closed timelike curves.

- #14

bapowell

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It's the other way around: isotropy implies homogeneity (the easy example is a homogeneous electric field pointing in a given direction: it's uniform but not isotropic). While we happen to observe that isotropy holds on relatively large scales today, we can't easily extrapolate that back to the early universe, for reasons of instability mentioned above. To get around these, one must fine tune the initial curvature of the universe to an immense degree. The idea that the very early universe emerging from the big bang was flat and isotropic is therefore by no means a natural proposal. Inflation, a period of accelerated expansion in the very early universe, can be invoked to dynamically push the universe to flatness and isotropy.

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