Overlay of the profile of the Bullet Cluster measured
using three different techniques. The light orange, round
galaxies that make up the cluster are seen clearly
in the image taken from optical telescopes. Overlaid is the
distribution of gas measured from X-ray observations in red
and the distribution of dark matter in blue. Composite Credit:
X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map:
NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Optical: NASA/STScI;Magellan/U.Arizona/D.Clowe et al.;
Dark matter continues to be a hot topic at my dinner table. I am continually amazed by how my non-physicist friends are interested in the research on this elusive material that makes up 25 percent of the universe. Just last night I was bombarded with questions like "How do we know it’s out there?" and "Can we see it?"
In my last post, I tried to explain what makes observations of dark matter so difficult. We see ordinary matter through the very strong and efficient electromagnetic force. The problem is that this force does not appear to affect dark matter.
That leaves gravity as the most obvious force for tracking dark matter, but the effects of gravity are really difficult to observe without enormous masses. This difficulty is the main reason that the existence of dark matter wasn’t generally accepted until the 1970’s.
To study dark matter in detail, we have to learn how to pit the forces of gravity against the biggest objects in the universe. One such trick was used in a project at Stanford University, at the Kavli Institute for Particle Astrophysics and Cosmology. In a press release from August of last year, Marusa Bradac describes some pretty convincing observations of dark matter in a massive cluster of galaxies known as the Bullet Cluster.