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

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

Effect of Our Galaxy's Motion on Weak Lensing Measurements of Shear and Convergence

In this work we investigate the effect on weak-lensing shear and convergence measurements due to distortions from the Lorentz boost induced by our Galaxy's motion. While no ellipticity is induced in an image from the Lorentz boost to first order in beta = v/c, the image is magnified. This affects the inferred convergence at a 10 per cent level, and is most notable for low multipoles in the convergence power spectrum C {\kappa}{\kappa} and for surveys with large sky coverage like LSST and DES. Experiments which image only small fractions of the sky and convergence power spectrum determinations at l > 5 can safely neglect the boost effect to first order in beta.Comment: 4 pages, replaced to reflect changes made for publication to MNRA

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

Modifying gravity with the Aether: an alternative to Dark Matter

There is evidence that Newton and Einstein's theories of gravity cannot explain the dynamics of a universe made up solely of baryons and radiation. To be able to understand the properties of galaxies, clusters of galaxies and the universe on the whole it has become commonplace to invoke the presence of dark matter. An alternative approach is to modify the gravitational field equations to accommodate observations. We propose a new class of gravitational theories in which we add a new degree of freedom, the Aether, in the form of a vector field that is coupled covariantly, but non-minimally, with the space-time metric. We explore the Newtonian and non-Newtonian limits, discuss the conditions for these theories to be consistent and explore their effect on cosmology.Comment: Updated version: Notation improved - TG

Vector field models of modified gravity and the dark sector

We present a comprehensive investigation of cosmological constraints on the class of vector field formulations of modified gravity called Generalized Einstein-Aether models. Using linear perturbation theory we generate cosmic microwave background and large-scale structure spectra for general parameters of the theory, and then constrain them in various ways. We investigate two parameter regimes: a dark-matter candidate where the vector field sources structure formation, and a dark-energy candidate where it causes late-time acceleration. We find that the dark matter candidate does not fit the data, and identify five physical problems that can restrict this and other theories of dark matter. The dark energy candidate does fit the data, and we constrain its fundamental parameters; most notably we find that the theory's kinetic index parameter $n_{\mathrm{ae}}$ can differ significantly from its $\Lambda$CDM value.Comment: 16 pages, 11 figure