Probing Dark Energy Using Clusters of Galaxies

May 5, 2015

Of the four established ways to study dark matter astronomically, looking at the evolving properties of galaxy clusters is the most reliant on non-optical observations of our Universe. A KIPAC faculty member has proposed satellite observations for a new era of cluster constraints on dark energy.

One of the major questions in contemporary physics and astronomy is the nature of dark energy, the mysterious entity held responsible for the acceleration of the universe. Is it a universal and eternal cosmological constant as first proposed by Albert Einstein in 1917, or does it have more interesting properties? Cosmologist have come up with four ways to study it.  Two of these use supernova explosions and the distribution of galaxies to measure how fast the universe expanded.  Two more, involving measuring the shapes of galaxies and probing the properties of rich clusters of galaxies, test whether its gravitational properties are in accord with Einstein's general theory of relativity.
Astro-H is a major X-ray astronomy space mission led by the Japanese Space Agency, JAXA, with participation by NASA. It is scheduled for launch in 2014. It held its most recent collaboration meeting at SLAC on 25-27 August, organized by KIPAC. In a public talk at this meeting, KIPAC member Steve Allen outlined a proposal for using Astro-H to observe a carefully selected sample of rich clusters of galaxies.  
The intergalactic gas in galaxy clusters, which is heated to thousands of degrees, emits light in x-rays.  One of the strengths of Astro-H is that it will allow precise measurements of the temperature of the intergalctic gas, a characteristic that correlates tightly with cluster mass, which is an essential quantity to determine.  By providing this information for just a small fraction of the hundreds of thousands of galaxy clusters expected to be found in future surveys, the constraining power of the survey data could be boosted by a factor of order three.
Allen argues that if his proposal is adopted, then we will open up a new and complementary avenue for studying dark energy, and that this is important because it is difficult to remove the intrinsic biases inherent in all four approaches listed above. However, carrying out this program will require co-opting a sizable fraction of the available telescope time and so his proposal will be subject to very careful scrutiny.
Science Contact:
Steve Allen

Tidbit Author: Steve Allen and Jack Singal