Dark Matter and the Fate of the Universe
When astronomers Margaret Geller and Emilio E. Falco plotted the positions of galaxies and galactic clusters in the universe, it became clear that these objects were not randomly distributed. Instead, they were clumped together in long filaments (walls) interspersed with empty spaces (voids), thereby giving the universe a cobweblike structure. How did such a structure form? What holds it together?
 NASA/WMAP Science Team According to this timeline, the expansion of the universe is accelerating. |
The Big Bang theory of the formation of the universe states that the early universe underwent an enormous expansion and that it is still expanding today. The only explanation for this type of structure is that gravity is causing some of these galaxies to clump together into walls or filaments. For gravity to clump these galaxies together, there must be large amounts of mass left over from the Big Bang, particularly unseen mass (i.e. dark matter). In fact, supercomputer simulations of the formation of the universe show that galaxies, galactic clusters and larger structures can eventually form over time from aggregations of dark matter in the early universe. So, dark matter may be an important "glue" that holds this universal structure together. A question for future research is whether dark matter fills the entire universe, all the way to the galactic walls.
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 10-29g/cm3 (equivalent to a few hydrogen atoms in a phone booth) that determines what might happen.
- Closed universe -- If actual mass density is greater then 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; however, there must be lots of it to account for the acceleration of the universe.
Current research in cosmology centers on resolving these questions:
- What is the nature of dark matter?
- How much dark matter actually exists?
- What is the exact distribution of dark matter in the universe?
- What is dark energy?
Answers to these questions will improve our understanding of the origins, structure and fate of the universe.
