Dark Matter and the Fate of the Universe
If dark matter acts like cosmic glue, astronomers must be able to explain its existence in terms of the prevailing theory of universe formation. The big bang theory states that the early universe underwent an enormous expansion and is still expanding today. For gravity to clump galaxies together into walls or filaments, there must be large amounts of mass left over from the big bang, particularly unseen mass in the form of dark matter. In fact, supercomputer simulations of the formation of the universe show that galaxies, galactic clusters and larger structures can eventually form from aggregations of dark matter in the early universe.
Besides giving the universe structure, dark matter may play a role in its fate. The universe is expanding, but will it expand forever? Gravity will ultimately determine the fate of the expansion, and gravity is dependent upon the mass of the universe; specifically, there is a critical density of mass in the universe of 10-29 g/cm3 (equivalent to a few hydrogen atoms in a phone booth) that determines what might happen.
- Closed universe – If actual mass density is greater than critical mass density, the universe will expand, slow, stop and collapse back on itself into a "big crunch."
- Critical or flat universe – If actual mass density equals critical mass density, the universe will continue to expand forever, but the rate of expansion will slow more and more as time progresses. Everything in the universe will eventually become cold.
- Coasting or open universe – If actual mass density is less than critical mass density, the universe will continue to expand with no change in its rate of expansion.
Measurements of mass density must include both light and dark matter. So, it is important to know how much dark matter exists in the universe.
Recent observations of the motions of distant supernovae suggest that the universe's rate of expansion is actually accelerating. This opens up a fourth possibility, an accelerating universe, in which the all galaxies will move away from each other relatively rapidly and the universe will become cold and dark (faster than in the open universe, but still on the order of tens of billions of years). What causes this acceleration is unknown, but it has been called dark energy. Dark energy is even more mysterious than dark matter – and just another example of astronomy's darkness on the edge of town. Perhaps the universe, as Springsteen suggests, will carry its secrets for a long, long time:
Everybody's got a secret, Sonny,
Something that they just can't face,
Some folks spend their whole lives trying to keep it,
They carry it with them every step they take.