Research

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The research in the group ranges over the following areas:

  • Cosmology, Galaxy formation, evolution and mergers,
  • General relativity,
  • Properties of materials and the vacuum in ultrastrong magnetic fields,
  • Physics of neutron stars,
  • Magnetic and relativistic stellar structure,
  • Dynamics, formation and evolution of globular clusters

Presently our research is mostly focussed on the last four bullets. The theme running though our work is how how is our understanding of astrophysical phenomena connected with our understanding of fundamental physics. Although our work so far has emphasized compact objects (white dwarfs, neutron stars and black holes), the physics of the early universe and cosmology in general also provides a window onto fundamental physics.

Magnetars

Taken to the extreme, the magnetic fields surrounding a neutron star can range from 108 to 1015 Gauss (the magnetic field at the surface of the Earth averages 0.6 Gauss). At the high end of this range lie the magnetars whose fields exceed the quantum-electrodynamic critical value of 4.4 x 1013 G. In such strong fields, the typical energy of an electron spiral around a magnetic field line exceeds its restmass energy.

The study of isolated cooling neutron stars continues to be a mainstay of the research program.

Millisecond Pulsars and LMXBs

Weakly magnetized neutron stars usually reveal themselves as millisecond pulsars. These quickly rotating stars are thought to form when an ordinary neutron star accretes material from a low mass companion in a low-mass x-ray binary (LMXB). The neutron star is spun up by the accreting material. It is still unclear how the neutron stars lose their magnetic fields as they are spun up. Soon after the launch of RXTE, highly periodic oscillations were discovered in Type-I X-ray bursts, in which the fuel accumulated on the surface of a neutron star in an LMXB suddenly ignites and the flux of the star increases by several orders of magnitude for several seconds. The burning is thought to originate a particular point on the surface and quickly spread over the surface. Initially the oscillations were thought to be a signature of the hotspot as the star rotates. The frequency of the oscillation changes slightly during the burst which was thought to be due to the conservation of the angular momentum in the shearing atmosphere

General Theme

A general theme threads our research: how is our understanding of astrophysical phenomena connected with our understanding of fundamental physics. The gross properties of neutron stars such as their mass and radius which we might learn definitively from observations of the thermal radiation from their surfaces, probes nuclear physics in an otherwise inaccessible realm. The details of their kinematics may probe the quark-gluon phase transition, and the understanding the dynamics of material and light near their surfaces can verify QED and general relativity. The dynamics of black holes, the matter and fields surrounding them most directly probes general relativity, but perhaps also QED in the context of the central engine of gamma-ray bursts. I intend to continue my research in the exciting area of the physics of compact objects. Starting in the distant past and proceeding to the present and from large scales to small, some of the questions that we would like to address in my future research:

  • What is the nature of the initial singularity? Is it necessary?
  • Can generic principles help constrain the properties of dark energy in the distant past and today?
  • How does varying dark energy affect structure formation?

Now jumping down twenty orders of magnitude...

  • Does QED play a role in producing or processing the radiation from soft-gamma repeaters? In classical gamma-ray bursts?
  • What is the composition of neutron-star atmospheres (ionized, atomic or molecular)? What can we learn from their spectra?
  • How can geophysical analogues help us understand neutron-star phenomena?
  • What is the nature of nuclear matter in the core of a neutron star?
  • Is the quark-hadron phase transition important in neutron-star cores?
  • What can observations of neutron stars tell us about quantum chromodynamics?
  • What is the nature of neutron-star collapse? Does it have any important observable consequences?
  • How can black-hole electrodynamics help explain the observations and energetics of accreting black holes?