Knocking on Heaven's Door - Lisa Randall [179]
[ FIGURE 77 ] The universe has expanded differently over time. During the inflationary phase it quickly expanded exponentially. The conventional Big Bang expansion took over when inflation ended. Dark energy now makes the expansion rate accelerate again.
FURTHER MYSTERIES
The necessity for dark energy and dark matter tells us that we can’t be as smug about our understanding of the evolution of the universe as the incredible agreement of cosmological theory with cosmological data might suggest. Most of the universe is stuff whose identity remains a mystery. Twenty years from now, people might smile at our ignorance.
And these are not the only puzzles evoked by the energy of the universe. The value of the dark energy, in particular, is actually the tail end of a much larger mystery: why is the energy that pervades the universe so small? Had the amount of dark energy been greater, it would have dominated matter and radiation much earlier in the evolution of the universe, and structure (and life) would not have had time to form. On top of that, no one knows what was responsible earlier on for the large energy density that triggered and fueled inflation. But the biggest problem with the energy of the universe is the cosmological constant problem.
Based on quantum mechanics, we would have expected a much larger value for dark energy—both during inflation and now. Quantum mechanics tells us that the vacuum—the state with no permanent particles present—is actually filled with ephemeral particles that pop in and out of existence. These short-lived particles can have any energy. They sometimes can have energy so large that gravitational effects can no longer be neglected. These highly energetic particles contribute an extremely large energy to the vacuum—much larger than the long evolution of our universe would allow. In order for the universe to look like the one we see, the value of the vacuum energy has to be an astonishing 120 orders of magnitude smaller than the energy that quantum mechanics would lead us to expect.
And there is yet a further challenge associated with this problem. Why do we happen to live in the time when the energy densities of matter, dark matter, and dark energy are comparable? Certainly dark energy dominates over matter, but it’s by less than a factor of three. Given that these energies in principle have entirely different origins and any one of them could have overwhelmed the others, the fact that their densities are so close seems very mysterious. The peculiarity of this coincidence is especially notable because it is only (roughly speaking) in our time that this coincidence is true. Earlier in the universe, dark energy was a much smaller fraction of the whole. And later on it will be a much greater fraction. Only today are the three components—ordinary matter, dark matter, and dark energy—comparable.
The questions of why the energy density is so extraordinarily tiny and why these different energy sources contribute similar amounts today are entirely unsolved. In fact, some physicists believe that there is no true explanation. They think we live in a universe with such an incredibly unlikely value for the vacuum energy because any larger value would have prevented the formation of galaxies and structure—and us—in the universe. We wouldn’t be here to ask about the value of the energy in any universe with a somewhat larger value of the cosmological constant. Those physicists believe that there are many universes, and each of these universes contains a different value of the