Matthew Walker-Dept of Physics - Carnegie Mellon University

Matthew Walker

Assistant Professor, Physics

Office: Wean Hall 8408
Phone: 412-268-2766
Fax: 412-681-0648


Ph.D., University of Michigan


There are two ways to measure the mass of a typical galaxy: 1) count the number of stars and multiply by the mass of a typical star, and 2) measure the orbital positions and speeds of those same stars, plug the numbers into the law of gravity (which relates mass to position and speed), and solve for mass. The problem is that the two answers don't agree. For nearly every galaxy that has been observed, the 'luminous mass' obtained by counting visible objects is smaller than the 'dynamical mass' inferred by applying (Newtonian) gravity to the motions of visible objects. The discrepancy between luminous and dynamical mass is called 'dark matter'---mass that is detected only via gravitation---and seems to represent ~85% of the mass in the Universe.

I study the astrophysical properties of dark matter, thus far via optical imaging, spectroscopy and dynamical modelling of the 'dwarf' galaxies that surround the Milky Way and neighboring Andromeda. The dwarf galaxies include the oldest, smallest and 'darkest' (i.e., composed almost entirely of dark matter) galaxies known, and currently represent the smallest physical scales (sizes of ~ 100 light years, speeds of a few kilometers per second, masses of ~100,000 Suns) that are associated empirically with dark matter. If dark matter is made from some kind of new fundamental particle, then the manner in which dark matter can form 'clumps' at such small scales can help to decide among various ideas about the properties of that particle. By measuring the spatial distribution of dark matter in dwarf galaxies, I aim to help figure out what the dark matter actually is. Thus my research has wandered into the intersection of dynamics, cosmology and particle physics.

For this work I use some of the world's largest optical telescopes, including the 6.5-meter Magellan telescopes at Las Campanas Observatory in Chile, the 6.5-meter MMT at Mt. Hopkins, Arizona, and the 8.2-meter Very Large Telescope at Cerro Paranal in Chile. I am also a member of the Sloan Digital Sky Survey IV collaboration.

Selected Publications