The puzzle of isotropy
It remains a puzzle why the universe is so isotropic and, at least in its early stages, so uniform in density. When we look at the cosmic microwave background in opposite directions on the sky, the regions we are looking at seem to have had no causal contact with each other – each lies far beyond the other’s horizon – and yet they appear identical. A second problem is that the universe today seems to be quite close to being spatially flat. This requires the universe to have been incredibly close to flatness in its early stages. The horizon and flatness problems are partially solved by the idea that the universe went through a period of exponential expansion, “inflation”, at or close to the Big Bang itself, driven by a phase with a very strong cosmological constant (dark energy). This period of inflation could have driven the universe closer to isotropy and uniformity.
Some cosmologists speculate that prior to the Big Bang the universe existed as a shadowy, chaotic quantum world. Some small region underwent a fluctuation and found itself with a very high vacuum energy, which triggered the exponential expansion of that region and the emergence of the universe we see today. In this picture there would be other expanding universes beyond our horizon, the multiverse. This allows scope for the anthropic principle, which argues that the properties of the universe have to be compatible with our presence, otherwise we would not be around to see it. In the multiverse model, most of the parallel universes would then be devoid of life.
Since the physics that we really know about extends back to the era of quark confinement, but no further, these ideas are essentially pure speculation.
● The Big Bang J Silk (1980, W H Freeman)
● Cosmology M Rowan-Robinson (2005, 4th edition, OUP)
● The First Three Minutes S Weinberg (1993, Basic Books)
● Nine Numbers of the Cosmos M Rowan-Robinson (2001, OUP)
● Life in the Universe RAS leaflet
● Cambridge Cosmology
● Ned Wright’s Cosmology Tutorial
● WMAP’s Universe
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