1203.2891 (Martin Feix)

Revisiting metric perturbations in tensor-vector-scalar theory    [PDF]

Martin Feix

The cosmological behavior of modified gravity theories with additional degrees of freedom (DOFs) is typically complex and can give rise to non-intuitive results. A possible way of exploring such theories is to consider appropriate parameterizations of these new DOFs. Here I suggest using the algebraic structure of trivial identities, which typically occur at the level of the perturbed field equations, for defining such parameterizations. Choosing the example of Bekenstein's tensor-vector-scalar theory (TeVeS) and considering perturbations in the conformal Newtonian gauge, this parameterization is then used to study several aspects of the cosmological evolution in an Einstein-de Sitter universe. As a main result, I conclude that perturbations of the scalar field take a key role in generating enhanced growth if this enhancement is primarily associated with a gravitational slip. From this point of view, the previously found modified growth in TeVeS is truly a result of the complex interplay between both the scalar and the vector field. Since the occurrence of trivial identities of the above kind appears as a generic feature of modified gravity theories with extra DOFs, these parameterizations should generally prove useful to investigate the cosmological properties of other proposed modifications. As such parameterizations capture the full nature of modifications by construction, they also provide a suitable framework for developing semi-analytic models of cosmologically interesting quantities like, for instance, the growth factor, leading to various applications. Supplementary to numerical analysis, parameterizations based on trivial identities are thus an interesting tool to approach modified gravity theories with extra DOFs.

View original: http://arxiv.org/abs/1203.2891