Moduli stabilization and supersymmetry breaking in effective theories of strings.

In the "central" region of moduli space of string theories, which is most likely to describe the "real world," the string coupling is about unity and the volume of compact dimensions is about the string volume. We map this region, assuming that the string coupling and compact volume moduli are chiral superfields of N = 1 supergravity in four dimensions, and requiring that the supersymmetry (SUSY) breaking scale is about the weak scale and that the cosmological constant be acceptably small. We find that the SUSY breaking field has a weak scale mass and a flat potential, that the potential is steep in other directions, and that there are additional nearby minima with a large negative cosmological constant.

Causal boundary entropy from horizon conformal field theory.

The effective quantum theory of near horizon regions of classical four-dimensional spatially flat, Friedman-Robertson-Walker spacetimes is shown to be approximately a two-dimensional conformal field theory. The central charge and expectation value of the Hamiltonian of this theory, and the statistical entropy of horizon states which can be calculated using Cardy's formula, are all proportional to the horizon area in units of Newton's constant. The proportionality constant which is determined by Planck scale physics 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.

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.

Generalized second law in cosmology from causal boundary entropy

A classical and quantum mechanical generalized second law of thermodynamics in cosmology implies constraints on the effective equation of state of the universe in the form of energy conditions, obeyed by many known cosmological solutions, forbids certain cosmological singularities, and is compatible with entropy bounds. This second law is based on the conjecture that causal boundaries and not only event horizons have geometric entropies proportional to their area. In string cosmology the second law provides new information about nonsingular solutions.

Causal entropy bound for a spacelike region.

The identification of a causal-connection scale motivates us to propose a new covariant bound on entropy within a generic spacelike region. This "causal entropy bound," scaling as sqrt[EV], and thus lying around the geometric mean of Bekenstein's S/ER and holographic S/A bounds, is checked in various "critical" situations. In the case of limited gravity, Bekenstein's bound is the strongest while naive holography is the weakest. In the case of strong gravity, our bound and Bousso's holographic bound are stronger than Bekenstein's, while naive holography is too tight, and hence typically wrong.