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By considering linear-order departures from general relativity, we compute a novel expression for the weak lensing convergence power spectrum under alternative theories of gravity. This comprises an integral over a kernel of general relativistic quantities multiplied by a theory-dependent "source" term. The clear separation between theory-independent and -dependent terms allows for an explicit understanding of each physical effect introduced by altering the theory of gravity. We take advantage of this to explore the degeneracies between gravitational parameters in weak lensing observations. [ABSTRACT FROM AUTHOR]

In the quasistatic regime, generic modifications to gravity can give rise to novel scale dependence of the gravitational field equations. Crucially, the detectability of the new scale-dependent terms hinges upon the existence of an effective mass scale or length scale at which corrections to general relativity become relevant. Starting from only a few basic principles, we derive the general form of this scale dependence. Our method recovers results previously known in the specific case of Homdeski gravity, but also shows that they are valid more generally, beyond the regime of scalar field theories. We forecast the constraints that upcoming experiments will place on the existence of a new fundamental mass scale or length scale in cosmology. [ABSTRACT FROM AUTHOR]

The growth rate of the large-scale structure of the Universe has been advocated as the observable par excellence for testing gravity on cosmological scales. By considering linear-order deviations from general relativity, we show that corrections to the growth rate, f, can be expressed as an integral over a "source" term, weighted by a theory-independent "response kernel." This leads to an efficient and accurate "plug-and-play" expression for generating growth rates in alternative gravity theories, bypassing lengthy theory-specific computations. We use this approach to explicitly show that f is sensitive to a degenerate combination of modified expansion and modified clustering effects. Hence the growth rate, when used in isolation, is not a straightforward diagnostic of modified gravity. [ABSTRACT FROM AUTHOR]

A unified framework for theories of modified gravity will be an essential tool for interpreting the forthcoming deluge of cosmological data. We present such a formalism, the parameterized post- Friedmann framework (PPF), which parameterizes the cosmological perturbation theory of a wide variety of modified gravity models. PPF is able to handle spin-U degrees of freedom from new scalar, vector, and tensor fields, meaning that it is not restricted to simple models based solely on cosmological scalar fields. A direct correspondence is maintained between the parameterization and the underlying space of theories, which allows us to build up a "dictionary" of modified gravity theories and their PPF correspondences. In this paper we describe the construction of the parameterization and demonstrate its use through a number of worked examples relevant to the current literature. We indicate how the formalism will be implemented numerically, so that the dictionary of modified gravity can be pitted against forthcoming observations. [ABSTRACT FROM AUTHOR]

Parametrized frameworks for modified gravity are potentially useful tools for model-independent tests of general relativity on cosmological scales. The toy model of an Einstein-de Sitter universe provides a safe test bed in which to improve our understanding of their behavior. We implement a mathematically consistent parametri/ation at the level of the field equations, and use this to calcúlale the evolution of perturbations in an Einstein-de Sitter scenario. Our parametrization explicitly allows for new scalar degrees of freedom, and we compare this to theories in which the only degrees of freedom come from the metric and ordinary matter. The impact on the integrated Sachs-Wolfe effect and canonically conserved superhorizon perturbations is considered. [ABSTRACT FROM AUTHOR]

There is a distinct possibility that current and future cosmological data can be used to constrain Einstein's theory of gravity on the very largest scales. To be able to do this in a model-independent way, it makes sense to work with a general parameterization of modified gravity. Such an approach would be analogous to the Parameterized Post-Newtonian (PPN) approach which is used on the scale of the Solar System. A few such parameterizations have been proposed and preliminary constraints have been obtained. We show that the majority of such parameterizations are only exactly applicable in the quasistatic regime. On larger scales they fail to encapsulate the full behavior of typical models currently under consideration. We suggest that it may be possible to capture the additions to the 'Parameterized Post-Friedmann' (PPF) formalism by treating them akin to fluid perturbations. [ABSTRACT FROM AUTHOR]

We propose a consistency test of gravity based on the weak lensing signal of cosmic voids. For a given void profile, as traced by galaxies, the lensing signal can vary in different gravity theories. Thus the comparison of the lensing shear profile of such voids with the general relativistic prediction can test for deviations from general relativity (GR). For concreteness, we calculate the expected lensing signal in two gravity theories involving scalar fields with derivative couplings. We find that the scalar field has the potential to boost the tangential shear both within and outside the void radius. Reversing the method, one can infer the void central density parameter from the lensing signal, and compare to the value estimated independently using the galaxy tracer profiles of voids. Hence, one can check for consistency between the behavior of light and matter under the assumption of GR. We use voids traced by luminous red galaxies in SDSS to demonstrate our methodology, finding that the void central density parameter can shift from its GR value by up to 20% in some Galileon gravity models. Although Galileon gravity is now disfavored as a source of cosmic acceleration by other data sets, the methods we demonstrate here can be used to test for more general fifth force effects with upcoming void lensing data. [ABSTRACT FROM AUTHOR]

We investigate the potential of high-energy astrophysical events, from which both massless and massive signals are detected, to probe fundamental physics. In particular, we consider how strong gravitational lensing can induce time delays in multimessenger signals from the same source. Obvious messenger examples are massless photons and gravitational waves, and massive neutrinos, although more exotic applications can also be imagined, such as to massive gravitons or axions. The different propagation times of the massive and massless particles can, in principle, place bounds on the total neutrino mass and probe cosmological parameters. Whilst measuring such an effect may pose a significant experimental challenge, we believe that the "massive time delay" represents an unexplored fundamental physics phenomenon. [ABSTRACT FROM AUTHOR]