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 $w$. 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 $s_c$ 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. $w\neq 1/3$. 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