Sunday, February 27, 2011

Modified Gravity works well for Gas Rich Galaxies

The motion of stars and gas in galaxies surprisingly is not so easily explained by the gravity they produce. They tend to follow flat rotation curves - after a certain distance the velocity tends to be the same whether a star is nearer or farther from the center of the galaxy. It had been expected that more distant stars and gas would be moving slower. The most popular explanation for this is dark matter , that 80% of the mass of galaxies like our Milky Way is some new unseen form that doesn't interact much, except by gravity. Another possible explanation is MOND, a theory that modifies gravity when it is very weak. A recent preprint shows that MOND works very well for gas-rich galaxies while dark matter is rather less natural and convincing: A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies.

Saturday, February 26, 2011

An Unusual Earlier Ice Age

There was an earlier ice age in the southern supercontinent Gondwana at the boundary between the Ordovician and Silurian periods around 445 million years ago. This ice age was unusual in that it occurred while CO2 levels were high and also coincided with a major extinction event. See The Magnitude and Duration of Late Ordovician–Early Silurian Glaciation in Science.

Thursday, February 03, 2011

The Downsizing Problem for Galaxies

It was commonly thought that galaxies form and grow larger by mergers of smaller masses. However, it has been observed that galaxies with high redshifts (which indicates that they are being observed at an early time) are on average larger than galaxies at low redshifts. This is the Downsizing Problem. The preprints Feedback in Galaxy Formation and The Many Manifestations of Downsizing: Hierarchical Galaxy Formation Models confront Observations discuss this and other problems in galaxy formation theory.