Tessa Baker

Testing general relativity with present and future astrophysical observations

One century after its formulation, Einsteins general relativity (GR) has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that GR should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of GR. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einsteins theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

Strong constraints on cosmological gravity from GW170817 and GRB 170817A

The detection of an electromagnetic counterpart (GRB 170817A) to the gravitational-wave signal (GW170817) from the merger of two neutron stars opens a completely new arena for testing theories of gravity. We show that this measurement allows us to place stringent constraints on general scalar-tensor and vector-tensor theories, while allowing us to place an independent bound on the graviton mass in bimetric theories of gravity. These constraints severely reduce the viable range of cosmological models that have been proposed as alternatives to general relativistic cosmology.

Beyond ΛCDM: Problems, solutions, and the road ahead

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

Ambiguous tests of general relativity on cosmological scales

There are a number of approaches to testing General Relativity (GR) on linear scales using parameterized frameworks for modifying cosmological perturbation theory. It is sometimes assumed that the details of any given parameterization are unimportant if one uses it as a diagnostic for deviations from GR. In this brief report we argue that this is not necessarily so. First we show that adopting alternative combinations of modifications to the field equations significantly changes the constraints that one obtains. In addition, we show that using a parameterization with insufficient freedom significantly tightens the apparent theoretical constraints. Fundamentally we argue that it is almost never appropriate to consider modifications to the perturbed Einstein equations as being constraints on the effective gravitational constant, for example, in the same sense that solar system constraints are. The only consistent modifications are either those that grant near-total freedom, as in decomposition methods, or ones which map directly to a particular part of theory space.

Linking Tests of Gravity On All Scales: from the Strong-Field Regime to Cosmology

The current eort to test General Relativity employs multiple disparate formalisms for dierent observables, obscuring the relations between laboratory, astrophysical and cosmological constraints. To remedy this situation, we develop a parameter space for comparing tests of gravity on all scales in the universe. In particular, we present new methods for linking cosmological large-scale structure, the Cosmic Microwave Background and gravitational waves with classic PPN tests of gravity. Diagrams of this gravitational parameter space reveal a noticeable untested regime. The untested window, which separates small-scale systems from the troubled cosmological regime, could potentially hide the onset of corrections to General Relativity. Subject headings: gravitation { dark energy

OBSERVATIONAL SIGNATURES OF MODIFIED GRAVITY ON ULTRA-LARGE SCALES

Extremely large surveys with future experiments like Euclid and the SKA will soon allow us to access perturbation modes close to the Hubble scale, with wavenumbers

New gravitational scales in cosmological surveys

k \sim \mathcal{H}

Phi Zeta Delta: Growth of Perturbations in Parameterized Gravity for an Einstein-de Sitter Universe

. If a modified gravity theory is responsible for cosmic acceleration, the Hubble scale is a natural regime for deviations from General Relativity (GR) to become manifest. The majority of studies to date have concentrated on the consequences of alternative gravity theories for the subhorizon, quasi-static regime, however. In this paper we investigate how modifications to the gravitational field equations affect perturbations around the Hubble scale. We choose functional forms to represent the generic scale-dependent behaviour of gravity theories that modify GR at long wavelengths, and study the resulting deviations of ultra large-scale relativistic observables from their GR behaviour. We find that these are small unless modifications to the field equations are drastic. The angular dependence and redshift evolution of the deviations is highly parameterisation- and survey-dependent, however, and so they are possibly a rich source of modified gravity phenomenology if they can be measured.

A general theory of linear cosmological perturbations: stability conditions, the quasistatic limit and dynamics

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 Horndeski 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.

The Parameterized Post-Friedmann Framework for Theories of Modified Gravity: Concepts, Formalism and Examples.

Parameterized 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 (EdS) universe provides a safe testbed in which to improve our understanding of their behaviour. We implement a mathematically consistent parameterization at the level of the field equations, and use this to calculate the evolution of perturbations in an EdS scenario. Our parameterization 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.