Many Worlds in One: The Search for Other Universes by Vilenkin Alex

Author:Vilenkin, Alex [Vilenkin, Alex]
Language: eng
Format: epub, mobi
Publisher: Farrar, Straus and Giroux
Published: 2007-07-10T00:00:00+00:00

THE COINCIDENCE PROBLEM

“Any coincidence,” said Miss Marple to herself, “is worth noticing. You can throw it away later if it is only a coincidence.”

—AGATHA CHRISTIE, Nemesis

It came as a total surprise when in the late 1990s two teams of astronomers announced that they had evidence for a nonvanishing cosmological constant. As we discussed in Chapter 9, this discovery was great news for the theory of inflation. The mass (energy) density of the vacuum provided precisely the amount that was missing to make the universe flat. But it was dreadful news for the particle theory.

The goal of solving the cosmological constant problem with a beautiful symmetry appeared now even more elusive. A symmetry would do a perfect job; it would not leave even a trace of vacuum energy uncompensated. But that was not all. The actual value of the cosmological constant that was obtained from the data looked extremely suspicious—so much so that most particle physicists and cosmologists refused to believe it and hoped that it would somehow go away.

The observed mass density of the vacuum is slightly more than twice the average density of matter. The puzzle is that the two densities are comparable, in the sense that one is not very much greater or smaller than the other. This is surprising, because the matter density and the vacuum density behave very differently with the expansion of the universe. The vacuum density does not change at all (as long as we stay in the same vacuum), while the matter density decreases as the volume grows. If the two densities are more or less the same today, then at the time of last scattering the matter density was a billion times greater than the vacuum density, and at 1 second A.B. it was 1045 times greater. In the distant future the pattern will be reversed and the density of matter will become much smaller than that of the vacuum. For example, a trillion years from now it will be 1050 times smaller.

Thus, throughout most of the history of the universe the density of matter is strikingly different from that of the vacuum. Why, then, do we happen to live at the very special epoch when the two densities are close to each other? Considering the huge range of variation of the matter density, the coincidence is so extraordinary that it’s very hard to dismiss it as “only a coincidence.”

It looked as if nature were trying to tell us something. But, in her usual manner, she refused to make it easy for us to understand. Why would a fundamental constant of nature, like the cosmological constant, be related to the matter density at the particular epoch when we humans happen to be around? The idea of some connection between these two quantities appeared totally ridiculous. The particle physics community was in disarray.

And then there was a remarkable fact that made the situation even more peculiar. A nonzero cosmological constant, not far off the observed value, had been theoretically predicted years before the observations were made.

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