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Dr. Siopsis has been investigating the scattering of various modes (scalar, photons, etc) by black holes. Whether the black hole comes from an underlying M-theory or not, scattering cross sections exhibit a universal behavior [2] which is even common with non-relativistic scattering by a Newtonian potential. This behavior does not even require supersymmetry. Dr. Siopsis extended these results to the study of absorption of scalars by a distribution of D3-branes [3]. He included a larger set of supergravity backgrounds than had previously been considered and arbitrary partial waves. He solved the wave equation in the respective backgrounds for various D-brane distributions in the extremal limit. He also extended the analysis to the case of non-extremal D3-branes. In general, the waves become singular at the positions of the D-branes. Dr. Siopsis found that the absorption coefficients exhibit a universal behavior similar to the form of the gray-body factors in the case of black-hole scattering. His calculations included the troublesome spherically symmetric D3-brane distribution where resonances arise for an infinite number of frequencies of the incident wave. He showed that if reflection is forbidden, the absorption coefficients exhibit the same behavior as in the cases where no resonances arise. It appears that the Schwarzschild-like coordinates one works with in supergravity are more physically relevant than the AdS space coordinates one uses to establish the AdS/CFT correspondence. Dr. Siopsis and his student Suphot Musiri extended these results to non-extremal D-branes [4]. They solved the wave equation for scalars in the supergravity background of certain distributions of branes and computed the absorption coefficients. The form of these coefficients is similar to the gray-body factors associated with black-hole scattering. They are given in terms of two different temperature parameters, indicating that fields (open string modes) do not remain in thermal equilibrium as we move off extremality. They discussed the microscopic implications of this result in terms of the longitudinal and transverse to the brane string modes. This should shed some light on the origin of the disagreement between the supergravity and conformal field theory results on the free energy of a system of non-coincident D-branes. |
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