From the November 2015 issue

Black holes inhale vast quantities of dust and gases. Does dark matter also get consumed?

Don Gensler, Umpqua, Oregon
By | Published: November 23, 2015
Black hole flare
These images, taken with NASA’s Galaxy Evolution Explorer (GALEX) and the Pan-STARRS1 telescope in Hawaii, show a galaxy that brightened suddenly, caused by a flare from its nucleus. The flare is a signature of the galaxy’s central black hole shredding a star that wandered too close. The top left image, taken by GALEX in 2009, shows the galaxy before the flare, when it wasn’t visible in ultraviolet light. In the top right image, taken by GALEX on June 23, 2010, the galaxy had become 350 times brighter in ultraviolet light. The bottom left image, taken by Pan-STARRS1, shows the galaxy (the bright dot in the center) in 2009 before the flare’s appearance. The bottom right image, taken by Pan-STARRS1 from June to August 2010, shows the flare from the galaxy nucleus. Note how the light from the flare is much bluer (hotter) than the host galaxy light.
NASA/S. Gezari (JHU)/A. Rest (STScI)/R. Chornock (Harvard-Smithsonian CfA)
Black holes capture matter through their gravitational pull. Dark matter is the name astronomers use for matter that does not interact through the electromagnetic or nuclear forces in physics, but which scientists still see through its gravitational effects. Since it interacts gravitationally, dark matter behaves no differently near black holes than any other type of matter, so black holes certainly can consume it.

Although this process sounds exotic, black holes in general relativity do not care whether they feed on regular or dark matter. The only properties a black hole has are mass, angular momentum (spin), and electric charge. Any other information carried by the material that falls into a black hole, for example what type of particle it is, is lost forever. This is called the “no hair” theorem: Black holes are completely specified by three numbers and have no extra properties (“hair”). For this reason, black holes are the simplest macroscopic objects in the universe.

The idea of information being lost from material falling into black holes is uncomfortable from a physics standpoint and as a result has long been a subject of intense debate (the “information paradox”). We hope that its resolution could be an important step toward reconciling the classical physics of general relativity with quantum mechanics.

Jason Dexter

Max Planck Institute

Garching, Germany