My research interests cover a wide range within the fields of theoretical cosmolosy and gravitational physics. In particular, I am currently investigating topics in cosmic radiation backgrounds, large scale structure formation, gravitational lensing, and gamma ray bursts. In addition physics education is of interest to me: I have just written a textbook, First Principles of Cosmology (Addison-Wesley 1997), on the advanced undergraduate to graduate level, am beginning to think about an introductory approach to teaching scientific literacy and physics based on conservation principles, and am becoming involved with developing a summer school on large scale structure.
The propagation behavior of the electromagnetic radiation by which we observe the universe involves gravitational interactions on all scales, providing valuable information on the distribution of matter inhomogeneities. I have focused on the gravitational effects of a realistically clumpy universe, helping develop a clear, relativistically rigorous parametrization scheme detailing deviations from the global Friedmann-Roberson-Walker model due to density fluctuations, without employing any kind of averaging procedure or a priori assumptions about the magnitude of the density inhomogeneities. The resulting Green function solution directly relates the density field to an effective gravitational potential, which can then be applied to problems of light propagation and astrophysical observations.
Some applications include gravitational lensing of the cosmic microwave background radiation, the distance-redshift relation, image distortions, and time delays in a clumpy universe, and postlinear calculation of the Sachs-Wolfe effect for generating microwave background anisotropies from density fluctuations.
Gamma ray bursts present an astrophysical puzzle as to their mechanism, source and even approximate distance. If they are of cosmological origin they would be a new class of source, isotropically distributed, located at great distance, and with luminosities exceeding quasars, thus having great potential to add to our understanding of cosmology and large scale structure. In the absence of a sufficiently robust clue to their nature from individual burst time and spectral structure, or observation of a non gamma ray wavelength counterpart, I investigate their statistical distributions in count and angular space. This is made more challenging by the transient nature of the bursts and their poor localization on the sky (a few degrees). The cosmological tools of log N-log S relations, angular correlation functions, and multipole moments can impose severe constraints on source models and evolutionary scenarios, however.
linder@kestrel .phast.umass.edu
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Last Update:6/4/97