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Phys. Rev. D 82, 063506 (2010) [5 pages]
Dark matter constraints from a cosmic index of refraction

Susan Gardner1,2,* and David C. Latimer2
1Center for Particle Astrophysics and Theoretical Physics Department, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
2Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506-0055, USA
Received 29 January 2010; published 3 September 2010

The dark matter candidates of particle physics invariably possess electromagnetic interactions, if only via quantum fluctuations. Taken en masse, dark matter can thus engender an index of refraction which deviates from its vacuum value. Its presence is signaled through frequency-dependent effects in the propagation and attenuation of light. We discuss theoretical constraints on the expansion of the index of refraction with frequency, the physical interpretation of the terms, and the particular observations needed to isolate its coefficients. This, with the advent of new opportunities to view gamma-ray bursts at cosmological distance scales, gives us a new probe of dark matter and a new possibility for its direct detection. As a first application we use the time delay determined from radio afterglow observations of distant gamma-ray bursts to realize a direct limit on the electric charge-to-mass ratio of dark matter of |ε|/M<1×10-5  eV-1 at 95% C.L.
© 2010 The American Physical Society