Lawrence Widrow

My academic career took me from my home town in Connecticut to Stanford University where I completed a Bachelor of Science degree in 1984. From there I went to the University of Chicago where I was a PhD student under the supervision of Michael Turner. My PhD thesis was on Superconducting Cosmic Strings. While at Chicago, I also had the oppurtunity to work on inflation and the origin of primordial magnetic fields. After Chicago, I spent two years as a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics and then two and a half years as a senior research associate at the University of Toronto's Canadian Institute for Theoretical Astrophysics.

Spiral galaxies such as the Milky Way are thought to possess extended massive halos of dark matter. The evidence comes primarily from rotation curves: Without an unseen component, disk galaxies would fly apart within a few 10's of millions of years. Dark matter is also necessary to hold groups and rich clusters of galaxies together.

Despite compelling evidence of its existence, the nature of dark matter remains a mystery. An appealing possibility is that dark matter is made of ordinary baryonic material (i.e., protons, neutrons, etc.) that simply doesn't radiate. Examples of baryonic dark matter candidates include white dwarfs, neutron stars, and cold dense gas clouds. Results from studies of light element production in the early Universe indicate that some, but not all of the dark matter is baryonic.

More exciting is the possibility that dark matter is composed of exotic elementary particles that interact only weakly, if at all, with ordinary matter. The leading candidates for non-baryonic dark matter are WIMPs (weakly interacting massive particles) and axions. Both of these candidates fit naturally within the framework of the Cold Dark Matter model, widely regarded as the leading scenario for the formation of structure in the Universe.

I am interested in the structure of dark matter halos and implications for dark matter detection experiments. At present, over twenty groups around the world have deployed or are in the process of building terrestrial dark matter detectors. David Stiff, a PhD student at Queen's under my supervision, Josh Frieman (Fermilab), and I have studied the structure of dark halos and high resolution simulations performed at the Queen's High Performance Computing Center together with novel numerical techniques. Our work suggests that future dark matter detection experiments should be able to see features in energy and angular spectra that are associated with the structure of the Milky Way's dark halo.

I am also a member of the international collaboration MEGA. This experiment is searching for gravitational microlensing events toward the Galaxy Andromeda. Gravitational microlensing is a brightening of a distant star when a massive object (e.g., white dwarf, black hole) passes close to the line of sight. Our experiment should be able to determine whether the Andromeda Galaxy is surrounded by a halo of massive compact objects or MACHOs. We are currently analysing three years worth of observations taken at several telescopes around the world.

Magnetic fields are ubiquitous components of astrophysical objects. They are found in stars, molecular clouds, spiral and elliptical galaxies, and galaxy clusters. The magnetic fields found in galaxies and clusters are relatively weak (field strengths are typically of order a microgauss) but are ordered over scales of tens of kiloparsecs. The origin of these large scale magnetic fields remains a mystery. In fact, there are two questions: where did the very first fields come from and how were these fields amplified and maintained.

The origin of magnetic fields has been an interest of mine since my early work with my thesis advisor Michael Turner. We proposed that primordial magnetic fields were generated in the very early Universe during inflation. More recently, George Davies (a recent PhD student) and I studied the production of magnetic fields during the formation of a galactic disk.

At present, Steve Toews, an MSc student at Queen's, and I are developing a new numerical method to study magnetic field. The method is based on the thin flux tube approximation: We follow magnetic flux tubes as they move under the influence of fluid flows, a gravitational field, and their own magnetic tension.

• Lawrence M. Widrow 2002, Origin of Galactic and Extragalactic Magnetic Fields, Rev. Mod. Phys. (in press) Online Version
• David Stiff, Lawrence M. Widrow and Joshua Frieman, 2001, Signatures of Hierarchical Clustering in Dark Matter Detection Experiments, Phys. Rev. D, 64, 083515
• Crotts, A., Uglesich, R., Gould, A., Gyuk, G., Sackett, P., Kuijken, K., Sutherland, W., Widrow, L., 2001, Microlensing in M31 - The MEGA Survey's Prospects and Initial Results, in Microlensing 2000: A New Era of Microlensing Astrophysics, ASP Conference Proceedings, Vol. 239. Edited by J. W. Menzies and Penny D. Sackett. San Francisco: Astronomical Society of the Pacific ADS Link
• Lawrence M. Widrow, 2000 Distribution Functions for Cuspy Dark Matter Density Profiles, Astrophysical Journal Supplements, 131, 39-46
• George Davies and Lawrence M. Widrow, 2000, A Possible Mechanism for Generating Galactic Magnetic Fields, Astrophysical Journal, 540, 755-764
• Judith A. Irwin, Lawrence M. Widrow and Jayanne English, 2000, An Observational Test of Dark Matter as Cold Fractal Clouds, Astrophysical Journal, 529, 77-87

Graduate Students

• David Stiff, PhD in progress
• Steve Toews, MSc in progress
• Joseph MacMillan, MSc 2001, PhD in progress
• Michael Seymour, MSc 2000
• Gil Holder, MSc 1996
• L. Jonathan Dursi, MSc 1996
• George Davies, MSc 1995, PhD 2002