Astronomers measuring the masses of galaxies and clusters of galaxies are faced with a similar situation -- there is not enough visible matter to account for the gravitational motions of galaxies.

Having determined that nearly 95 percent of the mass in the universe consists of an invisible, mysterious form of dark matter, they naturally would like to know what the stuff is, where it came from, and how it behaves.

Astronomy professors Brian Fields and Benjamin Wandelt and graduate students Richard Cyburt and Vasiliki Pavlidou, all at the University of Illinois at Urbana-Champaign, have taken a new look at the nature of this odd material.

Their calculations, to be reported in the June 15 issue of Physical Review D, shed some light on the characteristics of dark matter, and may lead to new theories about how structure formed in the early universe.

"We endowed dark matter particles with certain microscopic properties, and then we saw what the consequences would be for galaxies," said Wandelt, who also is a professor of physics at Illinois. "So, in a sense, we looked at what inner space has to say about outer space."

In the most popular dark matter model, Weakly Interacting Massive Particles interact only through gravity.

"But there is growing evidence that suggests the structure of galaxies -- the distribution of visible matter embedded in large halos of dark matter -- is not what WIMP theories predict," Wandelt said. "There doesn't appear to be as much mass piled up in galactic centers as we would expect if dark matter was composed of WIMPs."

To alleviate this problem, some astronomers have proposed a different picture -- one 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 halo cores in better agreement with the observations," Wandelt said. "But, if dark matter particles can interact strongly with each other, then similar interactions might be expected between them and ordinary baryonic matter as well."

The Illinois team investigated the potential impacts such interactions could have on Big Bang nucleosynthesis and on the production of high-energy gamma rays.

Big Bang nucleosynthesis is the process by which the primordial elements, consisting mainly of deuterium (an isotope of hydrogen), helium and lithium, were produced. In the standard scenario, dark matter plays no role in the creation of these light elements.

"If dark matter can interact strongly with baryons, however, it is possible that those interactions could destroy the newly forming deuterium, and thereby delay the onset of nucleosynthesis," Fields said. "This would reduce the abundances of the light elements, of course, which is contrary to what is observed."

The researchers' calculations show that the effect of dark matter-baryon interactions is much smaller than the normal destruction of deuterium by photons. In Big Bang nucleosynthesis, deuterium is broken apart by photons until the universe has cooled to a certain temperature. Even if strongly interacting dark matter was present, its effect would be swamped by that of photons.

"We see, therefore, that Big Bang nucleosynthesis cannot place strong constraints on dark matter," Fields said. "In other words, strongly interacting dark matter is completely compatible with the production of elements in the early universe."

Although the production of light elements would be unaffected by strongly interacting dark matter, the production of gamma rays would skyrocket.

The collisions of cosmic rays with baryonic matter -- wisps of interstellar gas and dust -- produces gamma rays. If dark matter-baryon interactions were allowed, similar processes would occur between cosmic rays and bits of dark matter, contributing to the gamma ray background.

When the researchers compared their calculations of this gamma ray background with satellite observations of high-energy gamma rays, they found a problem.

"With strongly interacting dark matter, the gamma ray production would be more than 100 times greater than what we actually observe," Fields said. "This tells us that dark matter either doesn't react strongly with baryons after all, or that the strength of the interaction is energy-dependent."

If the strength of dark matter-baryon interactions does drop with energy, so that baryon-interacting dark matter remains allowed, "it could lead to a new theory of how structure formed in the universe," Wandelt said. "These interactions would heat the dark matter. Our calculations suggest that this could affect the clustering properties of the early universe, and could impact what we see today in large-scale structure. We and others are in the midst of looking at that." - By James E. Kloeppel


[Contact: James E. Kloeppel]

05-Jun-2002