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Why is the Universe flat and not spherical?

My question involves the Universe is flat theory. If there was a big bang why would the Universe be flat? I would think that the Big Bang would result in a sphere shape mass of energy, light, matter, heat, radiation, etc. and whatever else expanding outward while maintaining the shape of the sphere.

The meaning of the Big Bang has been very often misunderstood. It is thought that something exploded somewhere and then the exploded part expanded to where we are currently. This is not correct. Before the Big Bang, there was no space or time. So, there is nothing "outside" the Big Bang. The Universe simply expanded from a very small volume into a huge volume, and this expansion is occuring even today. So, the place where we are right now corresponds to some place in a very small volume in the very early Universe. Hence, the Big Bang occured EVERYWHERE in the Universe. It occured at all places including the place where we are right now.

Why does the Universe look flat? This was one of the perplexing questions in cosmology for a long time. Today, most astronomers believe in the theory of inflation (and there are pieces of evidence supporting this). According to this theory, the Universe underwent exponential expansion about 10-30 seconds after the Big Bang. The result was that something of the size of an atom expanded to the size of the solar system by the end of the inflationary epoch.

If this were the case, irrespective of the original geometry of the Universe, it would appear flat to us. The analogy will be to take a balloon; we can easily see it to be rounded; now blow the balloon to a very large volume and then put a small ant on its surface. The ant will think that it is on a sheet; it cannot detect the curvature. To put this in another way, the distances that we probe are way too small to detect any possible curvature in the Universe.

If as you say "the distances that we probe are way too small to detect any possible curvature in the Universe" ... how can we accept recent "proofs" of a flat universe? Are all attempts to prove the flatness or otherwise of the universe limited to data collected from the observable universe? If so, and we suppose our view to be equivalent that of a short sighted ant on earth, surely it must be an impossibility to find such a proof, unless of course information can travel faster than light.

First, you have to distinguish between "universe" and "observable universe". Technically, "universe" constitutes everything that exists, while "observable universe" constitutes everything that exists within our horizon (that is, the volume of the universe within which light has had time to reach us). Every observation we can ever make is confined to the observable universe, and we have no way of knowing for sure what's happening beyond the horizon. But many people use "universe" as shorthand for "observable universe", which can create some confusion. So when we say "WMAP provides strong evidence that the universe is flat", we really mean "WMAP provides strong evidence that the observable universe is flat".

However, according to inflationary theory, even if the universe has some curvature, the observable universe should be flat at the level at which we're capable of measuring it. But we don't *know* that inflationary theory is correct. So yes, it is important to do experiments like the one performed by WMAP. If we were to detect deviations from flatness in the observable universe, then it would provide evidence against inflation.

August 2002, Jagadheep D. Pandian (more by Jagadheep D. Pandian) (Like this Answer), Christopher Springob (more by Christopher Springob) (Like this Answer)

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