26 research outputs found
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
Truncation of Einstein equations through Gravitational Foliation
In previous works, we suggested considering a (3+1)D quantum gravitational
field as an evolution of a (2+1)D renormalized quantum gravitational field
along the direction of the gravitational force. The starting point of the
suggestion is a derivation of a unique hypersurface which looks effectively
like (2+1)D from the point of view of Einstein equations in (3+1)D. In this
paper, we derive such unique hypersurfaces for different kinds of stationary
spherical metrics. We find that these hypersurfaces exist whenever all the
components of the gravitational force field vanish on the hypersurface. We
discuss the implication of this result and the necessary further work.Comment: 26 pages, no figure
Proposed evolution in Marolf-Maxfield toy model obtained through correspondence to spontaneous collapse theory
The Marolf-Maxfield topological toy model for 2D gravity gives the full
spectrum of boundary theories, but can not describe any evolution. In order to
obtain the expected evolution from Hartle-Hawking state to one of the
superselection sectors, we suggest to consider a correspondence between models
of evaporating black holes and models of collapsing wave functions. This note
explores this correspondence by equating the Marolf-Maxfield topological toy
model and to the Bonifacio model of spontaneous collapse theory. The expected
evolution of a matrix element due to a generator of a parameter in
Marolf-Maxfield model is obtained
Evaluating the Wald Entropy from two-derivative terms in quadratic actions
We evaluate the Wald Noether charge entropy for a black hole in generalized
theories of gravity. Expanding the Lagrangian to second order in gravitational
perturbations, we show that contributions to the entropy density originate only
from the coefficients of two-derivative terms. The same considerations are
extended to include matter fields and to show that arbitrary powers of matter
fields and their symmetrized covariant derivatives cannot contribute to the
entropy density. We also explain how to use the linearized gravitational field
equation rather than quadratic actions to obtain the same results. Several
explicit examples are presented that allow us to clarify subtle points in the
derivation and application of our method
Islands in the Fluid: Islands are Common in Cosmology
We discuss the possibility of entanglement islands in cosmological spacetimes
with a general perfect fluid with an equation of state . We find that flat
universes with time-symmetric slices where the Hubble parameter vanishes always
have islands on that slice. We then move away from such slices, considering
still universes with a general perfect fluid. Under the local thermal
equilibrium assumption, the comoving entropy density is constant. As a
result, the conditions for an island become an inequality between the energy
density (or Hubble parameter) and the temperature at some time of
normalization. The consequences are that islands can exist for practically all
fluids that are not radiation, i.e. . We also discuss the
ramifications of our results for universes with spatial curvature. Finally, we
show that islands occur in the Simple Harmonic Universe model which has no
classical singularity at the background level, in contrast to all previous
examples where islands occurred only in space-times with singularities.Comment: 26 pages, 5 figure