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Why were random variations introduced in the spherically symmetric universe after Big Bang which made it non-symmetrical. Since the outcome of a coin toss depend on factors such as torque applied, air resistance etc. By the same logic a spherically symmetric universe should have been symmetrical and the outcomes of quantum processes should have been also same on both the opposite ends.

Qmechanic
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jaskaran
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    First of all, the universe is assumed to be homogeneous and isotropic on large enough scales, although some observations cast doubt on this. Second, why would you assume symmetry? A random quantum fluctuation is that-- random. Why would a quantum fluctuation at location $x$ imply a similar one at location $-x$? – Sean Nov 16 '14 at 14:36
  • To Sean: we do not know where is the limit between quantum behavior and classical behavior. Can we put the local asymmetry of the universe on the quantum randomness? To be specific, the fact that looking in one direction we see the constellation Centaurus, and in another direction the constellation Lyra, can be put on quantum randomness? I am not sure. – Sofia Nov 16 '14 at 15:04
  • Could it be that the point from which the Big Bang theory says that the universe was born, wasn't exactly a dimensionless point, but had some non spherically-symmetrical structure? – Sofia Nov 16 '14 at 15:08
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    Can you really say that the universe was spherical after the Big Bang if it was infinite in size? (rhetorical question) – HDE 226868 Nov 16 '14 at 15:14
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    @Sofia The Big Bang did not happen at a point. – HDE 226868 Nov 16 '14 at 16:00
  • @Sean What are these doubt-casting observations? – zibadawa timmy Nov 17 '14 at 00:53
  • @zibadawatimmy: see this Wikipedia page in the Low multipoles and other anomalies section. There may be anomalies in the CMB on such a large scale that they threaten the concept of large scale homogeneity. However I must stress that this is still very much an open question. – John Rennie Nov 17 '14 at 05:31

2 Answers2

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First off, spherical symmetry isn't really the best description. Cosmological models usually assume that the universe is (approximately) homogeneous and isotropic. That's a higher degree of symmetry than spherical symmetry. Spherical symmetry would normally be used to describe something that has a lower degree of symmetry, so that there is a center. The universe doesn't have a center.

So the question should really be why the universe isn't perfectly homogeneous and isotropic. We don't have any naturalistic principle that prescribes initial conditions for the universe. The perfectly symmetric universe you describe would be a possible set of initial conditions.

Many physicists actually have the opposite feeling compared to yours. They are concerned about the horizon problem, which is that different regions of the universe have the same temperature and other properties, even though they were not in causal contact in the early universe and therefore couldn't come to thermal equilibrium with each other. Inflation gives one possible solution to the horizon problem.

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Fluctuations of density in the universe naturally become greater with time because matter is attracted to regions that are denser than average, and as a result they get denser still and the other regions less dense. So if there were even tiny density fluctuations in the early universe they would have grown into the density variations we see today - variations like planets, stars, galaxies, clusters, superclusters, etc.

So the question is what caused the original tiny density fluctuations in the early universe? This is one of the (many) issues addressed by the theory of inflation. The density differences were produced by quantum fluctuations in the inflaton field. See the Wikipedia article on primordial fluctuations for more details.

So if the theory of inflation is correct the current inhomogeneities were caused by the uncertainty principle. This is a temptingly plausible explanation, though the theory of cosmic inflation remains only a hypothesis.

John Rennie
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  • Minor clarification: the density fluctuations we see today weren't caused by the inflaton collapsing (I'm assuming you are referring to the end of inflation and reheating). Rather, it was quantum fluctuation that occurred during inflation. These fluctuations then get stretched to superhorizon scales at which point they "freeze" due to Hubble friction. Then after inflation, the horizon starts growing again, and the fluctuations re-enter as density fluctuations. – asperanz Nov 16 '14 at 22:23