3,223 research outputs found
Nonsingular Dilaton Cosmology in the String Frame
We consider the theory obtained by adding to the usual string frame dilaton
gravity action specially constructed higher derivative terms motivated by the
limited curvature construction of [MukhanovET1992a] and determine the spatially
homogeneous and isotropic solutions to the resulting equations of motion. All
solutions of the resulting theory of gravity with these symmetries are
nonsingular and all curvature invariants are bounded. For initial conditions
inspired by the pre-big-bang scenario solutions exist which correspond to a
spatially flat Universe starting in a dilaton-dominated superinflationary phase
with and having a smooth transition to an expanding Friedmann
Universe with . Hence, the graceful exit problem of pre-big-bang
cosmology is solved in a natural way.Comment: 8 pages, 3 figures, LaTex; minor changes; published in JHEP 9909:003,
199
Nonsingular Lagrangians for Two Dimensional Black Holes
We introduce a large class of modifications of the standard lagrangian for
two dimensional dilaton gravity, whose general solutions are nonsingular black
holes. A subclass of these lagrangians have extremal solutions which are
nonsingular analogues of the extremal Reissner-Nordstrom spacetime. It is
possible that quantum deformations of these extremal solutions are the endpoint
of Hawking evaporation when the models are coupled to matter, and that the
resulting evolution may be studied entirely within the framework of the
semiclassical approximation. Numerical work to verify this conjecture is in
progress. We point out however that the solutions with non-negative mass always
contain Cauchy horizons, and may be sensitive to small perturbations.Comment: 27 pages, three figures, RU-92-61. (Replaced version contains some
corrections to incorrect equations. The zero temperature extremal geometry
(the conjectured end-point of the Hawking evaporation) is not as stated in
the previous version, but rather is a nonsingular analogue of the zero
temperature Reissner-Nordstrom space-time.
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