NSF funds Mo's search for 'missing' mass in universe

Astronomy professor Houjun Mo recently received notice of a three-year, $428,000 National Science Foundation grant to allow him to study the distribution of approximately 80 percent of the “missing” mass in the universe, or “reconstructing the cosmic density field to understand the local intergalactic medium and its relation to galaxies.”

Answers will provide more information about how galaxies are formed. Despite impressive progress by astronomers and physicists working to fully characterize the Universe in the past 15 years, major questions remain, Mo said.

Only about 20 percent of the universe’s mass is accounted for in stars, planets, diffuse gas and other mass we think of as “normal,” Mo said. Among astronomers, this material is collectively known as baryons. An analogy with icebergs can be helpful here: At present, we can only “see” the tip, about 10 percent of total baryonic makeup. Clues from the observed behavior of stars and planets suggest the rest must be there, but “the problem is how to detect material that has not yet formed stars,” Mo says. “It contains massive gas clouds, but even these do not account for all the mass up to a critical value we know must be present.”

To narrow the search for what he and fellow theoretical astronomers propose must be the abundant and dominant material, known as cold dark matter (CDM), Mo will use observations by the Hubble Space Telescope, integrated with information on the shape and size of known structure obtained from the Sloan Digital Sky Survey, and facts derived from earlier work showing the universe has a flat shape. “My interest here is to develop statistical methods to characterize the clustering properties of galaxies and to understand how the results can help us to find the ‘missing’ mass and to constrain galaxy formation and evolution,” said Mo.

Pushed and pulled by gravitational and other forces, CDM density distribution—the unseen bottom of the iceberg—change and grow with time to form clumps called dark matter halos. These are assumed to be the “gravitational potential wells” or a kind of hive where stars and galaxies form.

“A first step in finding the ‘missing’ mass and in understanding galaxy distribution in the universe is to understand how CDM halos are distributed in space,” Mo said.

He’ll use both analytical models and numerical simulations to understand how dark matter halo distribution is related to the ‘missing’ mass and to overall dark matter distribution.

June 5, 2009.