Ambiguous Tests of General Relativity on Cosmological Scales

Abstract

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.

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• The Parameterized Post-Friedmann Framework for Theories of Modified Gravity: Concepts, Formalism and Examples
• Linear perturbations in Galileon gravity models
• Extending cosmological tests of General Relativity with the Square Kilometre Array
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• Parametrized modified gravity constraints after Planck
• Parameterizing dark sector perturbations via equations of state
• Assessing the impact of bulk and shear viscosities on large scale structure formation
• $\text{Hi-COLA}$: Fast, approximate simulations of structure formation in Horndeski gravity
• Computing model independent perturbations in dark energy and modified gravity
• A cosmological exclusion plot: Towards model-independent constraints on modified gravity from current and future growth rate data
• Signatures of Modified Gravity on the 21-cm Power Spectrum at Reionisation
• A general theory of linear cosmological perturbations: bimetric theories
• Future CMB ISW-Lensing bispectrum constraints on modified gravity in the Parameterized Post-Friedmann formalism
• Constraining dark sector perturbations I: cosmic shear and CMB lensing
• Material models of dark energy
• Simple numerical implementation of general dark energy models
• Modified gravity versus shear viscosity: imprints on the scalar matter perturbations
• Revisiting metric perturbations in tensor-vector-scalar theory
• K-mouflage at high $k$: extending the reach of $\text{Hi-COLA}$
• Minimal parameterizations for modified gravity