1999

Ph.D

University of Madras

Some aspects of D-brane physics and quantum gravity are studied in this thesis. D-branes on Calabi-Yau manifolds from the point of view of gauged linear sigma models are studied. An appropriate set of boundary conditions, on the fields of the theory from the variation of the action under ordinary and supersymmetric variation is obtained. These boundary conditions that define the D-brane are studied in the gauged linear sigma model as well as its infra red limit. It is found that, a consistent set of boundary condition could be obtained, describing D0-branes and also D-branes wrapping middle cycles of Calabi-Yau, the description of D2, D4 and D6 branes on Calabi-Yau manifolds appear to be more difficult. An application of D-branes to a certain class of String theoretic black holes is studied. The emission of scalar particles from a class of near extremal five dimensional black holes and the corresponding D-brane configuration at high energies are studied; And it is shown that the distribution functions and the black hole grey body factors are modified in the high energy regime of the Hawking spectrum in such a way that the emission rates exactly match in both descriptions. The results are extended to charged scalar emission in five dimensions and to neutral and charged scalar emission in four dimensions. The application of holography, a generic principle of quantum gravity and hence of string theory, to inflationary cosmology are also studied in this thesis. It is found that the models of extended inflation type, and also of new inflation type, a naive application of the holographic principle leads to its violation, whereas the correct procedure that restores the holographic principle, leads to a lower bound on the density fluctuations.