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Space expansion is a cosmological phenomenon wherein the proper distance between two spatial points for a given inertial reference frame increases from one moment of time to another. That is, space itself expands; the added distance is not due to relative motion of points or objects.

3
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Spacetime isn't a thing. Or to be more precise in the description provided by General Relativity spacetime is not a thing. Spacetime is a purely mathematical object. It is a combination of a manifold …
answered Nov 5 '17 by John Rennie
2
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The furthest we could possibly see using light waves is the so called surface of last scattering (because before this time the universe was opaque). This is the distance that the cosmic microwave back …
answered Oct 24 '13 by John Rennie
1
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The large circles have exactly the same number of points as the smaller circles, which in turn is the same as the number of points on the real line. All contain $\aleph_1$ points. See the Wikipedia ar …
answered Nov 18 '15 by John Rennie
4
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In general relativity a free particle moves on a trajectory called a geodesic and to make it diverge from that geodesic you need to apply a force to it. To take an everyday example, an object momentar …
answered May 29 '16 by John Rennie
0
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This paper on the Arxiv claims to have measured an anisotropy at the $2\sigma$ level. I'm not familiar enough with the subject to pass judgement, though I get the impression that the general view is i …
answered Sep 9 '14 by John Rennie
1
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If you go back to the recombination era, when the cosmic microwave background was emitted, the universe was extremely smooth. Indeed the inhomogeneities in the CMB radiation are only one part in a hun …
answered Oct 28 '13 by John Rennie
11
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The balloon analogy is useful in some respects, but it is misleading in one important respect. In the balloon analogy the curavture of the balloon surface is extrinsic while in GR the curvature of the …
answered Feb 6 '14 by John Rennie
4
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As Antonio says in his comment, by setting $ds = 0$ all you are doing is writing the equation of a null geodesic, i.e. the trajectory of a ray of light. And you deduce that (again as Antonio has point …
answered Jun 16 '14 by John Rennie
3
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The FLRW metric for a flat universe is beguilingly simple: $$ ds^2 = -c^2dt^2 + a^2(t)(dx^2 + dy^2 + dz^2) $$ and using the fact that light follows null geodesics, i.e. $ds = 0$, allows us to immedi …
answered Mar 12 '17 by John Rennie
2
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If we look around the distribution of matter in the universe seems to be roughly isotropic and homogeneous (if we go to large enough scales). General relativity tells us that matter distributed in thi …
answered Mar 18 '16 by John Rennie
2
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There is no centre of mass for the universe because it is isotropic and homogeneous. That is every point in the universe is like every other point so there is no one point you can point to and say thi …
answered Jun 9 '16 by John Rennie
3
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Outside of its event horizon the gravity from a black hole is exactly the same as the gravity from any object that isn't a black hole. So black holes just make up part of the overall stuff in the univ …
answered Jul 27 '16 by John Rennie
1
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So if every object is polarized (positive and negative separation), then on a bigger level shouldn't galaxy's have this too? In general macroscopic objects aren't polarised. For example a single …
answered Sep 6 '14 by John Rennie
1
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We'll need a bit of background to explain this. We describe the expansion of the universe by a scale factor, that we write as $a(t)$. Note that this scale factor is a function of time, and by conventi …
answered Oct 29 '16 by John Rennie
3
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As Stefan has mentioned in his comment, there are already several questions on this issue. However I'd guess from the way you have phrased your question that the existing answers might be a bit techni …
answered Mar 19 '13 by John Rennie

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