Astronomers Searching for the "Neptune" of the Universe

The planet Neptune was once "dark matter," University of Michigan Astronomy professor Mario Mateo says. Before the term was even coined, astronomers predicted its existence based on an anomaly in the orbit of Neptune's neighbor Uranus. They knew just where to look for Neptune. For the past quarter century, astronomers have been looking for the Neptune of the universe, so to speak. Mateo believes this "Neptune" may exist in the form of dwarf galaxies.

Dark matter is a substance astronomers have not directly observed, but they deduce it exists because they detect its gravitational effects on visible matter. Based on these measurements, the prevailing theory in astronomy and cosmology is that the visible parts of the universe make up only a fraction of its total matter and energy.

Dark matter could take the form of dwarf stars and planets, elementary particles including neutrinos, or hypothetical and as-yet undetected particles that don't interact with visible light or other parts of the electromagnetic spectrum. Dark matter is believed to hold galaxies together.

The gravitational force of the visible matter is not considered strong enough to prevent stars from escaping. Other theories exist to explain these discrepancies, though. For example, modified Newtonian dynamics, Mateo said, proposes that gravitational forces become stronger when accelerations are very weak. While their results align with current dark matter models, Mateo  says they also bolster this less-popular explanation.

"Dwarf galaxies are not much to look at," Mateo continued, "but they may really alter our fundamental views on the nature of dark matter and, perhaps, even gravity."

Stars in dwarf spheroidal galaxies behave in a way that suggests the galaxies are utterly dominated by dark matter. Mateo and post-doctoral researcher Matthew Walker measured the velocity of 6,804 stars in seven of the 14 dwarf satellite galaxies orbiting the Milky Way: Carina, Draco, Fornax, Leo I, Leo II, Sculptor and Sextans. They found that, contrary to what Newton's law of gravity predicts, stars in these galaxies do not move slower the farther they are from their galaxy's core.

Posted by Casey Kazan.

Story Link:

http://www.ns.umich.edu/htdocs/releases/story.php?id=6125