Attack of MOG

Here at Colorado, instead of simply requiring graduate students to attend colloquia as they do at BYU, we are required to take "seminar" classes for each of our first four semesters. These are designed to run like a journal club on steroids. A group of professors selects a topic and a paper for each class, then the students take turns presenting that paper to the class in a presentation/discussion format. It's a lot of work, but I have found it to be very rewarding.

This semester, I am in a seminar on the largest, gravitationally bound objects in the universe, galaxy clusters. Galaxy clusters are interesting for a number of reason, but probably the hottest reason to study them right now is cosmology. This has resulted in about half of the classes being devoted to cosmology. A few weeks ago we studied the now famous paper by Clowe et al. on the Bullet Cluster and the existence of dark matter. This paper produced the stunning image on the right that I imagine you have all seen. This was seen as very strong evidence for the existence of dark matter, as opposed to modified theories of gravity (e.g. modified Newtonian dynamics (MOND) and modified gravity (MOG)) that attempt to explain the evidence for dark matter by saying that the problem is with our theories of gravity rather than missing mass. The Bullet Cluster observation was seen as a nail in the coffin for those theories.

Some people, like Dr. John Moffat at the University of Waterloo (Canada), refuse to give up on MOG despite the evidence. There is a story about him on space.com (found here). He has published a paper attempting to explain the bullet cluster observation using a modified version of general relativity (MOG) and he claims that MOG can explain it. For those of you who'd like to read it, you can find it here.

I respect his efforts and I think people should question the current cosmological models, but at some point you have to give up on disproven theories. MOG already suffers from the problem that you have to fine tune the parameters one way to get galaxy rotation curves to work and another way to get galaxy cluster observations to match. Besides that, MOG doesn't mesh with other cosmological evidence, like the flat universe from WMAP, without further changing the parameters. At some point, enough in enough and we all just need to declare MOG dead and get on with cosmology. I think that point was the picture above by Clowe et al.