Ram Brustein

Challenges for superstring cosmology

Abstract We consider whether current notions about superstring theory below the Planck scale are compatible with cosmology. We find that the anticipated form for the dilaton interaction creates a serious roadblock for inflation and makes it unlikely that the universe ever reaches a state with zero cosmological constant and time-independent gravitational constant.

Limitations in Using Luminosity Distance to Determine the Equation of State of the Universe

Supernova searches have been been suggested as a method for determining precisely the current value and time variation of the equation of state, w, of the dark energy component responsible for the accelerated expansion of the Universe. We show that the method is fundamentally limited by the fact that luminosity distance depends on w through a multiple integral relation that smears out information about w and its time variation. The effect degrades the resolution of w that can be obtained from current data.

Measuring the equation of state of the universe: Pitfalls and prospects

We explore various pitfalls and challenges in determining the equation of state

Relic Gravitational Waves from String Cosmology

(w)

Moduli potentials in string compactifications with fluxes: Mapping the discretuum

of the dark energy component that dominates the universe and causes the current accelerated expansion. We demonstrated in an earlier paper the existence of a degeneracy that makes it impossible to resolve well the value of w or its time derivative with supernovae data. Here we consider standard practices, such as assuming that w is constant or greater than

The Graceful exit problem in string cosmology

\ensuremath{-}1,

Wald's entropy is equal to a quarter of the horizon area in units of the effective gravitational coupling

and show that they also can lead to gross errors in estimating the true equation of state. We further consider combining measurements of the cosmic microwave background anisotropy and the Alcock-Paczynski test with supernovae data and find that the improvement in resolving the time derivative of w is marginal, although the combination can constrain its present value perhaps to 20% uncertainty.

A model of graceful exit in string cosmology

A large class of string-cosmology backgrounds leads to a spectrum of relic stochastic gravitational waves, strongly tilted towards high frequencies, and characterized by two basic parameters of the cosmological model. We estimate the required sensitivity for detection of the predicted gravitational radiation and show that a region of our parameter space is within reach for some of the planned gravitational-wave detectors.

Generalized second law in cosmology from causal boundary entropy

We find de Sitter and flat space solutions with all moduli stabilized in four dimensional supergravity theories derived from the heterotic and type II string theories, and explain how all the previously known obstacles to finding such solutions can be removed. Further, we argue that if the compact manifold allows a large enough space of discrete topological choices then it is possible to tune the parameters of the four dimensional supergravity such that a hierarchy is created and the solutions lie in the outer region of moduli space in which the compact volume is large in string units, the string coupling is weak, and string perturbation theory is valid. We show that at least two light chiral superfields are required for this scenario to work, however, one field is sufficient to obtain a minimum with an acceptably small and negative cosmological constant. We discuss cosmological issues of the scenario and the possible role of anthropic considerations in choosing the vacuum of the theory. We conclude that the most likely stable vacuua are in or near the central region of moduli space where string perturbation theory is not strictly valid, and that anthropic considerations cannot help much in choosing a vacuum.

Ratio of shear viscosity to entropy density in generalized theories of gravity

Abstract Isotropic string cosmology can be classified into duality related branches:one can be connected smoothyto an expanding Friedman-Robertson-Walker Universe, the describes accelerated inflation or contraction. We showthat, if the dilation potential has certain generic properties, the two branches can evolve smoothy into each other. We find,however, that just the effects of a potential do not allow for a graceful exit from accelerated inflation in the weak curvatureregime. We explain how conformal field theory techniques could be used to give a decisive answer on whether graceful exitfrom accelerated inflation is possible at all.