158 research outputs found
String Cosmology: An Update
A string cosmology scenario ("pre-big-bang") postulates that the evolution of
the Universe starts from a state of very small curvature and coupling,
undergoes a long phase of dilaton-driven kinetic inflation and at some later
time joins smoothly standard radiation dominated cosmological evolution, thus
giving rise to a singularity free inflationary cosmology. I report on recent
progress in understanding some outstanding issues such as initial conditions,
graceful exit transition and generation of inhomogeneity perturbations.Comment: 11 pages, 1 eps figure, Latex, based on talk at COSMO9
Causal Boundary Entropy From Horizon Conformal Field Theory
The quantum theory of near horizon regions of spacetimes with classical
spatially flat, homogeneous and isotropic Friedman-Robertson-Walker geometry
can be approximately described by a two dimensional conformal field theory. The
central charge of this theory and expectation value of its Hamiltonian are both
proportional to the horizon area in units of Newton's constant. The statistical
entropy of horizon states, which can be calculated using two dimensional state
counting methods, is proportional to the horizon area and depends on a
numerical constant of order unity which is determined by Planck scale physics.
This constant can be fixed such that the entropy is equal to a quarter of the
horizon area in units of Newton's constant, in agreement with thermodynamic
considerations.Comment: 11 pages, no figure
Dark Matter in Models of String Cosmology
The origin of dark matter in the universe may be weakly interacting scalar
particles produced by amplification of quantum fluctuations during a period of
dilaton-driven inflation. We present two interesting cases, the case of small
fluctuations, and the resulting nonthermal spectrum, and the case of large
fluctuations of a field with a periodic potential, the QCD axion.Comment: 13 pages LaTeX, uses aipproc.sty, talk presented by R. Brustein at
COSMO 98, Asilomar, California, November 199
Classical Corrections in String Cosmology
An important element in a model of non-singular string cosmology is a phase
in which classical corrections saturate the growth of curvature in a
deSitter-like phase with a linearly growing dilaton (an `algebraic fixed
point'). As the form of the classical corrections is not well known, here we
look for evidence, based on a suggested symmetry of the action, scale factor
duality and on conformal field theory considerations, that they can produce
this saturation. It has previously been observed that imposing scale factor
duality on the corrections is not compatible with fixed point
behavior. Here we present arguments that these problems persist to all orders
in . We also present evidence for the form of a solution to the
equations of motion using conformal perturbation theory, examine its
implications for the form of the effective action and find novel fixed point
structure.Comment: 30 pages, Latex, epsfig, 6 figure
Black hole entropy divergence and the uncertainty principle
Black hole entropy has been shown by 't Hooft to diverge at the horizon. The
region near the horizon is in a thermal state, so entropy is linear to energy
which consequently also diverges. We find a similar divergence for the energy
of the reduced density matrix of relativistic and non-relativistic field
theories, extending previous results in quantum mechanics. This divergence is
due to an infinitely sharp division between the observable and unobservable
regions of space, and it stems from the position/momentum uncertainty relation
in the same way that the momentum fluctuations of a precisely localized quantum
particle diverge. We show that when the boundary between the observable and
unobservable regions is smoothed the divergence is tamed. We argue that the
divergence of black hole entropy can also be interpreted as a consequence of
position/momentum uncertainty, and that 't Hooft's brick wall tames the
divergence in the same way, by smoothing the boundary.Comment: Added clarifications and explanation
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