Revisiting metric perturbations in tensor-vector-scalar theory

I revisit cosmological perturbations in Bekenstein's tensor-vector-scalar theory (TeVeS). Considering only scalar modes in the conformal Newtonian gauge, the extra degrees of freedom are expressed in a way suitable for studying modifications at the level of the metric potentials. Assuming a universe in the matter-dominated phase, I discuss the mechanism responsible for boosting structure growth, and confirm the vector field as its key ingredient. Using a semi-analytic approach, I further characterize the evolution of density perturbations and the potentials on sub- and superhorizon scales.Comment: 12 pages, 6 figures; updated Figs. 3, 5, and 6, improved discussion of potential and density growth, accepted versio

Growth rate of cosmological perturbations at z ~ 0.1 from a new observational test

Spatial variations in the distribution of galaxy luminosities, estimated from redshifts as distance proxies, are correlated with the peculiar velocity field. Comparing these variations with the peculiar velocities inferred from galaxy redshift surveys is a powerful test of gravity and dark energy theories on cosmological scales. Using ~ 2 $\times$ 10$^{5}$ galaxies from the SDSS Data Release 7, we perform this test in the framework of gravitational instability to estimate the normalized growth rate of density perturbations f$\sigma_{8}$ = 0.37 +/- 0.13 at z ~ 0.1, which is in agreement with the $\Lambda$CDM scenario. This unique measurement is complementary to those obtained with more traditional methods, including clustering analysis. The estimated accuracy at z ~ 0.1 is competitive with other methods when applied to similar datasets.Comment: 4 pages, 2 figures, matches version accepted for publication in PR

Extragalactic and cosmological tests of gravity theories with additional scalar or vector fields

Despite the many successes of the current standard model of cosmology on the largest physical scales, it relies on two phenomenologically motivated constituents, cold dark matter and dark energy, which account for approximately 95% of the energy-matter content of the universe. From a more fundamental point of view, however, the introduction of a dark energy (DE) component is theoretically challenging and extremely fine-tuned, despite the many proposals for its dynamics. On the other hand, the concept of cold dark matter (CDM) also suffers from several issues such as the lack of direct experimental detection, the question of its cosmological abundance and problems related to the formation of structure on small scales. A perhaps more natural solution might be that the gravitational interaction genuinely differs from that of general relativity, which expresses itself as either one or even both of the above dark components. Here we consider different possibilities on how to constrain hypothetical modifications to the gravitational sector, focusing on the subset of tensor-vector-scalar (TeVeS) theory as an alternative to CDM on galactic scales and a particular class of chameleon models which aim at explaining the coincidences of DE. Developing an analytic model for nonspherical lenses, we begin our analysis with testing TeVeS against observations of multiple-image systems. We then approach the role of low-density objects such as cosmic filaments in this framework and discuss potentially observable signatures. Along these lines, we also consider the possibility of massive neutrinos in TeVeS theory and outline a general approach for constraining this hypothesis with the help of cluster lenses. This approach is then demonstrated using the cluster lens A2390 with its remarkable straight arc. Presenting a general framework to explore the nonlinear clustering of density perturbations in coupled scalar field models, we then consider a particular chameleon model and highlight the possibility of measurable effects on intermediate scales, i.e. those relevant for galaxy clusters. Finally, we discuss the prospects of applying similar methods in the context of TeVeS and present an ansatz which allows to cast the linear perturbation equations into a more convenient form

Tracing the cosmic velocity field at z ~ 0.1 from galaxy luminosities in the SDSS DR7

Spatial modulations in the distribution of observed luminosities (computed using redshifts) of ~ 5 $\times$ 10$^5$ galaxies from the SDSS Data Release 7, probe the cosmic peculiar velocity field out to z ~ 0.1. Allowing for luminosity evolution, the r-band luminosity function, determined via a spline-based estimator, is well represented by a Schechter form with M$^{\star}$(z) - 5log$_{10}$h = -20.52 - 1.6(z - 0.1) $\pm$ 0.05 and $\alpha^{\star}$ = -1.1 $\pm$ 0.03. Bulk flows and higher velocity moments in two redshift bins, 0.02 < z < 0.07 and 0.07 < z < 0.22, agree with the predictions of the $\Lambda$CDM model, as obtained from mock galaxy catalogs designed to match the observations. Assuming a $\Lambda$CDM model, we estimate $\sigma_{8}$ $\approx$ 1.1 $\pm$ 0.4 for the amplitude of the linear matter power spectrum, where the low accuracy is due to the limited number of galaxies. While the low-z bin is robust against coherent photometric uncertainties, the bias of results from the second bin is consistent with the ~ 1% magnitude tilt reported by the SDSS collaboration. The systematics are expected to have a significantly lower impact in future datasets with larger sky coverage and better photometric calibration.Comment: 21 pages, 11 figures, accepted versio

Lopsidedness of self-consistent galaxies by the external field effect of clusters

Adopting Schwarzschild's orbit-superposition technique, we construct a series of self-consistent galaxy models, embedded in the external field of galaxy clusters in the framework of Milgrom's MOdified Newtonian Dynamics. These models represent relatively massive ellipticals with a Hernquist radial profile at various distances from the cluster centre. Using $N$-body simulations, we perform a first analysis of these models and their evolution. We find that self-gravitating axisymmetric density models, even under a weak external field, lose their symmetry by instability and generally evolve to triaxial configurations. A kinematic analysis suggests that the instability originates from both box and non-classified orbits with low angular momentum. We also consider a self-consistent isolated system which is then placed in a strong external field and allowed to evolve freely. This model, just as the corresponding equilibrium model in the same external field, eventually settles to a triaxial equilibrium as well, but has a higher velocity radial anisotropy and is rounder. The presence of an external field in MOND universe generically predicts some lopsidedness of galaxy shapes.Comment: 24 pages, 20 figures. Accepted for publication in Ap

An analytic model for non-spherical lenses in covariant MOdified Newtonian Dynamics

Strong gravitational lensing by galaxies in MOdified Newtonian Dynamics (MOND) has until now been restricted to spherically symmetric models. These models were able to account for the size of the Einstein ring of observed lenses, but were unable to account for double-imaged systems with collinear images, as well as four-image lenses. Non-spherical models are generally cumbersome to compute numerically in MOND, but we present here a class of analytic non-spherical models that can be applied to fit double-imaged and quadruple-imaged systems. We use them to obtain a reasonable MOND fit to ten double-imaged systems, as well as to the quadruple-imaged system Q2237+030 which is an isolated bulge-disc lens producing an Einstein cross. However, we also find five double-imaged systems and three quadruple-imaged systems for which no reasonable MOND fit can be obtained with our models. We argue that this is mostly due to the intrinsic limitation of the analytic models, even though the presence of small amounts of additional dark mass on galaxy scales in MOND is also plausible.Comment: 10 pages, 6 figures, references update