Looking at Inner Space to Understand Outer Space,"
Looking at Inner Space to Understand Outer Space
As much as 95 percent of the matter in the universe is unseen by astronomers—"dark matter" of a new and unknown type. This surprising result arises from several different ways of comparing the observed, visible mass of galaxies with the mass inferred from their gravity. Professor Benjamin Wandelt and graduate student Richard Cyburt, in collaboration with Professor Brian Fields and Vasiliki Pavlidou, are taking a new look at the nature of this mysterious stuff (R.H. Cyburt, B.D. Fields, V. Pavlidou, and B. Wandelt, Phys. Rev. D. 65, 123503 [2002]). Their calculations reveal some possible characteristics of dark matter and may lead to new understanding of how structures formed in the early universe.
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In the most popular dark-matter model, weakly interacting massive particles (WIMPs) interact only through gravity. But there is growing evidence that suggests that the distribution of matter in galaxies is not in line with what one expects if galaxies are mostly made of these feeble particles.
The difficulties of the WIMP theory has sparked interest in alternative theories, in which dark matter particles can interact with one another through forces other than gravity. Numerical simulations have shown that this self-interacting dark matter does indeed predict galaxy matter distributions in better agreement with observations. But, if dark matter particles can interact strongly with each other, can they interact with ordinary matter ("baryons") as well?
The Illinois team investigated the consequences of such interactions between ordinary, visible matter and the unseen dark matter in the early universe and in our Galaxy today. During the first three minutes after the big bang, the universe gave rise to fusion reactions in which the lightest elements were formed from protons and neutrons. Dark matter interactions with the normal matter could spoil this process, but the Illinois researchers showed that during these epochs, the dark matter is remarkably benign.
In the present-day Milky Way Galaxy, however, strong interactions between dark matter and normal matter would not go undetected. Our Galaxy acts as a natural particle accelerator, and is filled with high-energy cosmic rays, most of which are protons. If the cosmic rays could interact with the dark matter, the collisions would be a prodigious source of high-energy radiation—gamma rays. These gamma rays would be 100 times brighter than actual observations by existing gamma-ray observatories. This discrepancy has important implications for dark matter. One possibility is that it doesn't react strongly with normal matter after all. An alternative possibility is that the strength of the interaction varies with the collision energies. If this is the case, then the gamma-ray measurements allow the Illinois group to begin to map the microscopic properties of the dark matter. Modern particle physics and cosmological theory together allow us to connect microphysics with astronomical observations of such different phenomena as the matter distribution in galaxies, the abundances of the lightest elements, and the gamma ray sky—the true meaning of using outer space to probe inner space.