On the growth of structure in theories with a dynamical preferred frame

We study the cosmological stability of a class of theories with a dynamical preferred frame. For a range of actions, we find cosmological solutions which are compatible with observations of the recent history of the Universe: a matter dominated era followed by accelerated expansion. We then study the evolution of linear perturbations on these backgrounds and find conditions on the parameters of the theory which allow for the growth of structure sourced by the new degrees of freedom

Discrete and continuous symmetries in multi-Higgs-doublet models

We consider the Higgs sector of multi-Higgs-doublet models in the presence of simple symmetries relating the various fields. We construct basis invariant observables which may in principle be used to detect these symmetries for any number of doublets. A categorization of the symmetries into classes is required, which we perform in detail for the case of two and three Higgs doublets.Comment: 13 pages, RevTex, references adde

Tensor Microwave Anisotropies from a Stochastic Magnetic Field

We derive an expression for the angular power spectrum of cosmic microwave background anisotropies due to gravity waves generated by a stochastic magnetic field and compare the result with current observations; we take into account the non-linear nature of the stress energy tensor of the magnetic field. For almost scale invariant spectra, the amplitude of the magnetic field at galactic scales is constrained to be of order 10^{-9} Gauss. If we assume that the magnetic field is damped below the Alfven damping scale, we find that its amplitude at 0.1 h^{-1}Mpc, B_\lambda, is constrained to be B_\lambda<7.9 x10^{-6} e^{3n} Gauss, for n-3/2, where n is the spectral index of the magnetic field and H_0=100h km s^{-1}Mpc^{-1} is the Hubble constant today.Comment: 6 pages, 1 figure, accepted for publication in Phys. Rev.

The Vector-Tensor nature of Bekenstein's relativistic theory of Modified Gravity

Bekenstein's theory of relativistic gravity is conventionally written as a bi-metric theory. The two metrics are related by a disformal transformation defined by a dynamical vector field and a scalar field. In this comment we show that the theory can be re-written as Vector-Tensor theory akin to Einstein-Aether theories with non-canonical kinetic terms. We discuss some of the implications of this equivalence.Comment: Updated version: Notation cleaned up and some typos corrected-TG

Einstein's Theory of Gravity and the Problem of Missing Mass

The observed matter in the universe accounts for just 5 percent of the observed gravity. A possible explanation is that Newton's and Einstein's theories of gravity fail where gravity is either weak or enhanced. The modified theory of Newtonian dynamics (MOND) reproduces, without dark matter, spiral-galaxy orbital motions and the relation between luminosity and rotation in galaxies, although not in clusters. Recent extensions of Einstein's theory are theoretically more complete. They inevitably include dark fields that seed structure growth, and they may explain recent weak lensing data. However, the presence of dark fields reduces calculability and comes at the expense of the original MOND premise -- that the matter we see is the sole source of gravity. Observational tests of the relic radiation, weak lensing, and the growth of structure may distinguish modified gravity from dark matter.Comment: 11 pages, 3 figures. As published (with corrected typos in caption of Figure 1 and address of one author). Figures much better in published versio