The Evidence for Dark Energy

If the creation and evolution of the Universe could be compared to baking a cake, then, in the context of ΛCDM plus inflation, the most important ingredient would be yeast – the substance causing an exponential expansion – a de Sitter phase. There are two such episodes of the dominance of yeast: the first is at the beginning, less than 10-32 seconds after the Big Bang. The modern creation story tells us that a fast-acting yeast causes the rapid expansion that inflates the Universe by a factor of possibly 1029, wiping out any significant initial curvature (pushing the density toward the critical density), extending the causally connected region to well beyond the currently observable Universe, and creating the small fluctuations that become the observed structure. In the reheating that occurs after this inflation, the Universe is literally recreated; the initial conditions that applied before this episode, whatever they were, are irrelevant. Everything that the Universe is now observed to be, all of the existent ingredients in their present abundances, appeared out of the vacuum at the end of this initial yeast-dominated period. The microphysics of this event – the nature of the yeast – is unknown, but in a naturalistic world (one in which physical effects follow physical causes) there is substantial reason to believe that such an early de Sitter phase has actually occurred. We should keep in mind, however, that in the absence of primary evidence (such as gravitational-wave-like fluctuations) and without a specific physical mechanism (the breaking of supersymmetry, for example), this is essentially an act of faith.

The second age of yeast-dominance is now. This is the current era (beginning about five billion years ago) when dark energy (the yeast) took over from cold matter (the dough) and again is driving an accelerated exponential expansion of the world, but much more slowly than in the early inflationary epoch. These periods of vacuum-energy-dominated exponential expansion, as well as those of radiation andmatter dominance, are shown schematically in the time-line of Figure 6.1. Here we see that, while inflation was a short episode (≈ 10-32 seconds), the Universe expanded by a large factor, possibly 1029, during this period. The current de Sitter phase has lasted five billion years so far, but the Universe has only expanded by a factor of two.