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AbstactWe quantize the tachyon field in a static two dimensional dilaton gravity black hole background,and we calculate the Hawking radiation rate.We find that the thermal radiation flux,due to the tachyon field, is larger than the conformal matter one.We also find that massive scalar fields which do not couple to the dilaton,do not give any contribution to the thermal radiation, up to terms quadratic in the scalar curvature.

We examine background solutions of black hole type arising from the string effective action in five dimensions. We derive the unique metric -dilaton vacuum which is a Schwarzschild type black hole. It is found that it can be extended to incorporate electric charge without changing the topology of the three space. Kalb-Ramond charge may also be introduced if the three space is closed. The basic features of the cosmology induced on a three brane evolving in this background are also discussed. 1

In an ungauged N = 1 supergravity theory defined on an arbitrary Kahlerian manifold we compute the divergent one-loop corrections to the bosonic part of the effective action. Although the theory is not renormalizable such a calculation may be of relevance in view of the fact that N = 1 supergravities emerge as effective nonrenormalizable theories in the low-energy limit of some superstring models. In our calculations we have committed ourselves neither to a particular four-dimensional geometry nor to a particular Kahlerian manifold. We pay special attention to the one-loop scalar potential of the theory. We show that, by a proper redefinition of the metric, geometric objects such as scalar curvature can be made not to interact with the scalars and the definition of the potential of the theory becomes in this way unambiguous.PACS number(s): 04.65. +e 534

In this work we find static black hole solutions in the context of two-dimensional dilaton gravity, which is modified by the addition of an R 2 term. This term arises from the one-loop effective action of a massive scalar field in its large mass approximation. The basic feature of this term is that it does not contribute to the Hawking radiation of the classical black hole backgrounds of the model. From this point of view, a class of the solutions derived are interpreted as describing back reaction effects. In particular it is argued that the evolution of a black hole via nonthermal signals is possible. Nevertheless, this evolution seems to be ''soft,'' in the sense that it does not lead to the evaporation of a black hole, leaving the Hawking radiation as the dominant mechanism for this process. ͓S0556-2821͑96͒05522-1͔

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We examine the cosmology induced on a brane moving in the background of a five-dimensional black hole, solution of the string effective action. The evolution, determined by the Israel junction conditions is found to be compatible with an accelerating universe with the present day acceleration coming after a decelerating phase. The possible species of the energy-momentum tensor, localized on the brane, for these solutions to be valid are discussed.

We study backreaction effects in two-dimensional dilaton gravity. The backreaction comes from an R 2 term which is a part of the one-loop effective action arising from massive scalar field quantization in a certain approximation. The peculiarity of this term is that it does not contribute to the Hawking radiation of the classical black hole solution of the field equations. In the static case we examine the horizon and the physical singularity of the new black hole solutions. Studying the possibility of time dependence we see the generation of a new singularity. The particular solution found still has the structure of a black hole, indicating that non-thermal effects cannot lead, at least in this approximation, to black hole evaporation.