The galaxy weighs in

Our little neighborhood

In an exciting move, the European Space Agency will be launching Gaia in 2011 for the most comprehensive mapping survey of the galaxy ever. They will be measuring speeds and positions for about 1 billion nearby stars. From the link:

By establishing where things are in our galaxy, the spacecraft will help scientists measure the weight and distribution of mass in the Milky Way in much greater detail than ever before. These measurements are vital for models that attempt to describe how the pull of dark matter has shaped our galaxy.

It will be interesting to find what these results have to tell us about the distribution of dark matter, which in turn could yield clues as to how and when it formed. Most matter we know about interacts with the electromagnetic field making it possible for us to detect it. Thus, we can find out about things like stars and dust. However, the observed mass was not enough to account for things like rotational speeds of galaxies, orbital velocities of galaxies in clusters, and so forth. So a new form of matter, appropriately called dark matter was proposed. Another suggestion was modified gravity at large scales. However, dark matter has more explanatory power such as with anisotropy of the cosmic microwave background.

A hopeful test for is that if dark matter (hypothesized to be WIMPs, or weakly interacting massive particles) have a certain mass ranges, we could expect to create them in colliders such as the LHC. It is suggested that the mass of a WIMP is on the TeV scale. That is a Tera Electron Volt, which is 10**12 (10 to the 12th power). By comparison, the proton is about 938 MeV (Mega Electron Volts or order of 10**6). If you know that electron volts are measurements of energy and you’re not used to these types of units, it may puzzle you that I’m using mass and energy interchangeably. But than you’ll remember Einstein’s famous equation which relates mass to energy and you’ll know that they can indeed be interchanged. That is an enlightening discussion in and of itself, and perhaps the topic of a future post, but for now I’ll refer you to The Wiki if you want to dig into it a bit more.

So, going back to Gaia, I don’t know about you, but once we have a better map of the galaxy, I won’t feel so lost anymore.