L. C. R. Wijewardhana

Does String Theory Solve the Puzzles of Black Hole Evaporation

We point out that the massive modes of closed superstring theories may play a crucial role in the last stages of black hole evaporation. If the Bekenstein-Hawking entropy describes the true degeneracy of a black hole — implying loss of quantum coherence and the unitary evolution of quantum states-it becomes entropically favorable for an evaporating black hole to make a transition to a state of massive string modes. This in turn may decay into massless modes of the string (radiation) avoiding the naked singularity exposed by black hole evaporation in the semiclassical picture. Alternatively, quantum coherence may be maintained if the entropy of an evaporating black hole is much larger than that given by the Bekenstein-Hawking formula. In that case, however, the transition to massive string modes is unlikely. String theories might thus resolve the difficulty of the naked singularity, but it appears likely that they will still involve loss of quantum coherence.

Superstring gravity and the early universe

Ten-dimensional superstring theories have been proposed as candidates for a unified description of all the forces of nature. These theories reduce to Einstein gravity coupled to Yang-Mills interactions at energy scales small compared to the string tension. The phenomenologically promising superstring theory, the heterotic string, is investigated at the high temperatures and short distances relevant in the early universe. The massive string modes alone constitute an unstable thermodynamic system with negative specific heat. The conditions for equilibrium between the massive string modes and the massless modes (radiation) are derived. The large energy fluctuations of the system require the use of the microcanonical ensemble. There is a maximum temperature which exceeds the temperature at which the canonical partition function becomes divergent. Above a critical volume there is a phase transition during which the massive string modes must evaporate. The possibilities of spontaneous compactification, large entropy production, and a solution of the horizon and flatness problems are discussed.

PHASE STRUCTURE OF NON-COMPACT QED3 AND THE ABELIAN HIGGS MODEL

We review the phase structure of a three-dimensional, non-compact Abelian gauge theory (QED3) as a function of the number

Chiral hierarchies and flavor-changing neutral currents in hypercolor.

N

Spontaneous chiral symmetry breaking in three-dimensional QED

of 4-component massless fermions. There is a critical

Superstrings at high temperature.

N_{c}

Chiral hierarchies from slowly running couplings in technicolor theories.

up to which there is dynamical fermion mass generation and an associated global symmetry breaking. We discuss various approaches to the determination of

Chiral hierarchies and chiral perturbations in technicolor theories

N_c

Induced Chern-Simons terms at high temperatures and finite densities.

, which lead to estimates ranging from

Fractional spin via canonical quantization of the O(3) nonlinear sigma model

N_c =1