Heywood Tam

Instabilities in the aether

We investigate the stability of theories in which Lorentz invariance is spontaneously broken by fixed-norm vector “aether” fields. Models with generic kinetic terms are plagued either by ghosts or by tachyons, and are therefore physically unacceptable. There are precisely three kinetic terms that are not manifestly unstable: a sigma model (∂_µA_ν)^2, the Maxwell Lagrangian F_µνF^µν, and a scalar Lagrangian (∂_µA^µ)^2. The timelike sigma-model case is well defined and stable when the vector norm is fixed by a constraint; however, when it is determined by minimizing a potential there is necessarily a tachyonic ghost, and therefore an instability. In the Maxwell and scalar cases, the Hamiltonian is unbounded below, but at the level of perturbation theory there are fewer degrees of freedom and the models are stable. However, in these two theories there are obstacles to smooth evolution for certain choices of initial data.

Axion dark matter and cosmological parameters.

We observe that photon cooling after big bang nucleosynthesis but before recombination can remove the conflict between the observed and theoretically predicted value of the primordial abundance of ^{7}Li. Such cooling is ordinarily difficult to achieve. However, the recent realization that dark matter axions form a Bose-Einstein condensate provides a possible mechanism because the much colder axions may reach thermal contact with the photons. This proposal predicts a high effective number of neutrinos as measured by the cosmic microwave anisotropy spectrum.

{gamma} rays from muon capture in I, Au, and Bi

A significant improvement has been made in the identification of {gamma} rays from muon capture in I, Au, and Bi, all monisotopic elements. The ({mu}{sup -},{nu}n) reaction was clearly observed in all nuclei, but the levels excited do not correlate well with the spectroscopic factors from the (d,{sup 3}He) reaction. Some ({mu}{sup -},{nu}2n), ({mu}{sup -},{nu}3n), ({mu}{sup -},{nu}4n), ({mu}{sup -},{nu}5n) and other reactions have been observed at a lower yield. The muonic x-ray cascades have also been studied in detail.

Lorentz violation in Goldstone gravity

We consider a theory of gravity in which a symmetric two-index tensor in Minkowski spacetime acquires a vacuum expectation value (vev) via a potential, thereby breaking Lorentz invariance spontaneously. When the vev breaks all the generators of the Lorentz group, six Goldstone modes emerge, two linear combinations of which have properties that are identical to those of the graviton in general relativity. Integrating out massive modes yields an infinite number of Lorentz-violating radiative-correction terms in the low-energy effective Lagrangian. We examine a representative subset of these terms and show that they modify the dispersion relation of the two propagating graviton modes such that their phase velocity is direction dependent. If the phase velocity of the Goldstone gravitons is subluminal, cosmic rays can emit gravi-Cherenkov radiation, and the detection of high-energy cosmic rays can be used to constrain these radiative-correction terms. Test particles in the vicinity of the Goldstone gravitons undergo longitudinal oscillations in addition to the usual transverse oscillations as predicted by general relativity. Finally, we discuss the possibility of having vevs that do not break all six generators and examine in detail one such theory.

Vortices and the entrainment transition in the two-dimensional Kuramoto model

We study synchronization in the two-dimensional lattice of coupled phase oscillators with random intrinsic frequencies. When the coupling K is larger than a threshold K{E} , there is a macroscopic cluster of frequency-synchronized oscillators. We explain why the macroscopic cluster disappears at K{E} . We view the system in terms of vortices, since cluster boundaries are delineated by the motion of these topological defects. In the entrained phase (K>K{E}) , vortices move in fixed paths around clusters, while in the unentrained phase (K<K{E}) , vortices sometimes wander off. These deviant vortices are responsible for the disappearance of the macroscopic cluster. The regularity of vortex motion is determined by whether clusters behave as single effective oscillators. The unentrained phase is also characterized by time-dependent cluster structure and the presence of chaos. Thus, the entrainment transition is actually an order-chaos transition. We present an analytical argument for the scaling K{E}∼K{L} for small lattices, where K{L} is the threshold for phase locking. By also deriving the scaling K{L}∼log N , we thus show that K{E}∼log N for small N , in agreement with numerics. In addition, we show how to use the linearized model to predict where vortices are generated.

{gamma} rays from muon capture in {sup 27}Al and natural Si

Muon capture

The Color of High Energy Density Gold

\ensuremath{\gamma}

Cosmic axion thermalization

rays have been observed for the first time in

Optical properties in nonequilibrium phase transitions

^{27}\mathrm{Al}

Unitary Evolution and Cosmological Fine-Tuning

, and a significant improvement has been made in the identification of \ensuremath{\gamma}-rays from muon capture in natural Si. The (