Richard Watkins

Calculations of relic densities in the early universe

The cosmological relic densities of stable particles are calculated to high precision. The results depend on assumptions made about the quark-hadron phase transition. We discuss in detail the calculation of the thermally averaged annihilation cross section. We consider the specific cases of photinos, higgsinos, and Dirac and Majorana neutrinos.

Solutions to the strong CP problem in a world with gravity

We examine various solutions of the strong-CP problem to determine their sensitivity to possible violations of global symmetries by Plauck scale physics. While some solutions remain viable even in the face of such effects. Violations of the Peccei-Quinn (PQ) symmetry by non-renormalizable operators of dimension less than 10 will generally shift the value of {bar {theta}} to values inconsistent with the experimental bound {bar {theta}} {approx_lt} 10{sup {minus}}9. We show that it is possible to construct axion models where gauge symmetries protect PQ symmetry to the requisite level.

Gravitational Stability of Scalar Matter

The question of the stability of matter against gravitational collapse is of general interest to astrophysics. In this work we investigate the stability against small radial oscillations of equilibrium configurations of cold, gravitationally bound states of complex scalar fields, known as boson stars. These equilibrium configurations exhibit a mass profile against central density which is very similar to that of ordinary neutron stars, with a pronounced maximum mass at Mc = 0.633MPl2/m, where MPl is the Planck mass, for a certain value of the central density σc(0). We give analytical and numerical proof that configurations with central densities greater than σc(0) are unstable against radial perturbations by studying the behavior of the eigenfrequencies of the perturbations for different values of σ(0).

Phase transitions with sub-critical bubbles

We study the dynamics of cosmological phase transitions initiated from a state of thermal equilibrium. If the effective potential satisfies certain general conditions, a homogeneous phase of false vacuum will form as the Universe expands, and the transition will proceed by well-known bubble nucleation processes. If such conditions do not hold, the Universe may instead be filled with a two-phase emulsion. The evolution of the transition will be determined by the free energy difference between the two phases and by the expansion rate of the Universe. Thermal fluctuations between the phases will determine the final distribution of regions of the Universe in each phase as they freeze out. We develop a method to study the dynamics of such fluctuations, which we call sub-critical bubbles, and apply it to several situations of interest, including the symmetric and asymmetric double-well, and the Coleman-Weinberg scalar potentials. We show that in certain cases it is possible to avoid supercooling, with the transition being completed by sub-critical fluctuations. Possible applications to the electroweak phase transition are briefly discussed.

Coherent Couplings of Neutralinos to Nuclei From Squark Mixing

Abstract We show that left-right scalar quark mixing results in a coherent coupling of the lightest neutralino to nuclei which is usually comparable to (and often larger than) that due to gaugino/higgsino mixing. This has implications for dark matter searches.

Metastable cosmic strings in realistic models

The stability of the electroweak Z-string is investigated at high temperatures. The results show that, while finite temperature corrections can improve the stability of the Z-string, their effect is not strong enough to stabilize the Z-string in the standard electroweak model. Consequently, the Z-string will be unstable even under the conditions present during the electroweak phase transition. Phenomenologically viable models based on the gauge group SU(2)[sub L] [times] SU(2) [sub R] [times] U(1)[sub B-L] are then considered, and it is shown that metastable strings exist and are stable to small perturbations for a large region of the parameter space for these models. It is also shown that these strings are superconducting with bosonic charge carriers. The string superconductivity may be able to stabilize segments and loops against dynamical contraction. Possible implications of these strings for cosmology are discussed.

Aspects of reheating in first order inflation

Studied here is reheating in theories where inflation is completed by a first-order phase transition. In the scenarios, the Universe decays from its false vacuum state by bubble nucleation. In the first stage of reheating, vacuum energy is converted into kinetic energy for the bubble walls. To help understand this phase, researchers derive a simple expression for the equation of state of a universe filled with expanding bubbles. Eventually, the bubble walls collide. Researchers present numerical simulations of two-bubble collisions clarifying and extending previous work by Hawking, Moss, and Stewart. The researchers results indicate that wall energy is efficiently converted into coherent scalar waves. Also discussed is particle production due to quantum effects. These effects lead to the decay of the coherent scalar waves. They also lead to direct particle production during bubble-wall collisions. Researchers calculate particle production for colliding walls in both sine-Gordon and theta (4) theories and show that it is far more efficient in the theta (4) case. The relevance of this work for recently proposed models of first order inflation is discussed.