Knocking on Heaven's Door - Lisa Randall [174]
Inflation also partially answers why there is something rather than nothing. Some of the enormous energy density stored during inflation was converted (consistently with E = mc2) to matter, and that is the matter that evolves to what we see today. As I discuss at the close of this chapter, we physicists still would like to know why there is more matter than antimatter in the universe. But whatever the answer to that question, the matter we know began evolving according to Big Bang theory predictions as soon as cosmological inflation ended.
Inflation was derived as a bottom-up theory. It solved important problems for the conventional Big Bang explosion, but only a few really believed any of the actual models for how it came about. No compelling high-energy theory seemed to obviously imply inflation. Since it was so challenging to make a credible model, many physicists (including those at Harvard when I was a graduate student) doubted the idea could be right. On the other hand, Andrei Linde, a Russian-born physicist now at Stanford, and one of the first to work on inflation, thought it had to be correct simply because no one had found any other solution to the puzzles about the size, shape, and uniformity of the universe that inflation addressed.
Inflation was an interesting example of the truth-beauty connection—or lack thereof. Whereas the exponential expansion of the universe beautifully and succinctly explained many phenomena about how the universe started, the search for a theory that naturally yields the exponential expansion led to many not-so-pretty models.
Recently, however, most physicists—even though not yet satisfied with most models—have become convinced that inflation, or something very similar to inflation, did occur. Observations of the last several years have confirmed the cosmological picture of Big Bang cosmology preceded by inflation. Many physicists now trust that Big Bang evolution and inflation have occurred because predictions based on these theories have been confirmed with impressive precision. The true model underlying inflation is still an open question. But the exponential expansion has a lot of evidence supporting it at this point.
One type of evidence for cosmological inflation has to do with the deviations from perfect uniformity in the cosmic microwave background radiation that the previous chapter introduced. The background radiation tells us much more than just that the Big Bang occurred. The beauty of it is that because it is essentially a snapshot of the universe very early on—before stars had time to form—it lets us look back directly into the beginnings of structure at the time when the universe was still very smooth. Cosmic microwave background (CMB) measurements also revealed tiny departures from perfect homogeneity. Inflation predicts this because quantum mechanical fluctuations caused inflation to end at slightly different times in different regions of the universe, giving rise to tiny deviations from absolute uniformity. The satellite-based Wilkinson Microwave Anisotropy Probe (WMAP), named for the Prince ton physicist David Wilkinson who pioneered the project, made detailed measurements that distinguished inflationary predictions from other possibilities. Despite the fact that inflation happened long ago at incredibly high temperatures, theory based on inflationary cosmology nonetheless predicts the exact statistical properties of the pattern of temperature variations that should be imprinted on the radiation in the sky today. WMAP measured the small inhomogeneities in temperature and energy density with more accuracy and on smaller angular scales than had been done before, and the pattern conformed to inflationary expectations.
The chief confirmation of inflation that WMAP gave us was the measurement of the universe’s extreme flatness. Einstein taught us that space can be curved. (See Figure 73 for