Testing General Relativity in Cosmology (1806.10122v2)

Abstract: We review recent developments and results in testing general relativity (GR) at cosmological scales. The subject has witnessed rapid growth during the last two decades with the aim of addressing the question of cosmic acceleration and the dark energy associated with it. However, with the advent of precision cosmology, it has also become a well-motivated endeavor by itself to test gravitational physics at cosmic scales. We overview cosmological probes of gravity, formalisms and parameterizations for testing deviations from GR at cosmological scales, selected modified gravity (MG) theories, gravitational screening mechanisms, and computer codes developed for these tests. We then provide summaries of recent cosmological constraints on MG parameters and selected MG models. We supplement these cosmological constraints with a summary of implications from the recent binary neutron star merger event. Next, we summarize some results on MG parameter forecasts with and without astrophysical systematics that will dominate the uncertainties. The review aims at providing an overall picture of the subject and an entry point to students and researchers interested in joining the field. It can also serve as a quick reference to recent results and constraints on testing gravity at cosmological scales.

• The paper provides a comprehensive review of methodologies and observational tests of General Relativity on cosmological scales.
• It examines cosmological probes, parameterizations, and modified gravity theories to address cosmic acceleration and dark energy.
• The study outlines current constraints and future prospects, emphasizing the need for precise data in refining gravitational models.

Testing General Relativity in Cosmology

Abstract

The paper authored by Mustapha Ishak, titled "Testing general relativity in cosmology," provides an extensive review of the methodologies, current constraints, and ongoing research aimed at testing the predictions of General Relativity (GR) on cosmological scales. Over the past two decades, significant advancements have been made in leveraging cosmological observations to validate or challenge GR, especially in light of cosmic acceleration and the postulation of dark energy and dark matter. This paper examines various cosmological probes, parameterizations, modified gravity theories, and the computational tools employed in these efforts, thereby offering a comprehensive picture to researchers and students interested in the domain of cosmological gravity tests.

Overview

The paper details the evolution of cosmological tests of GR in response to the discovery of cosmic acceleration, primarily traced to observations of distant supernovae, and the theoretical consequences of dark energy. The goal is to understand whether GR, which has been incredibly successful from solar system scales to galaxy clusters, also holds true at the largest cosmic scales. Precision cosmology has advanced testing frameworks and developed modifications or alternatives to GR, known as Modified Gravity (MG) theories, that could account for cosmological observations without evoking dark energy.

Key Elements and Approaches

1. Cosmological Probes of Gravity:
   • This encompasses the paper of cosmological structures through observations such as the cosmic microwave background (CMB), galaxy clustering, lensing, and supernovae. These observations are employed to test deviations from GR using frameworks designed to measure cosmic expansion, geometry, and structure growth.
2. Formalism and Parameterization:
   • The paper reviews formalisms and parameterizations used to test GR, such as the Parameterized Post-Friedmann (PPF) formalism and modifications of the Einstein-Hilbert action through terms that account for potential deviations. These frameworks are vital for interpreting observational data in terms of possible modifications to gravitational theory.
3. Modified Gravity Theories:
   • MG models are explored, including scalar-tensor models like $f(R)$ gravity and Horndeski theories, which allow for extensions of GR to facilitate self-accelerating solutions to the cosmic acceleration problem. These theories often incorporate screening mechanisms to satisfy stringent local GR tests while allowing deviations on cosmological scales.
4. Gravitational Screening Mechanisms:
   • Techniques like the chameleon, symmetron, and Vainshtein mechanisms are discussed. These are essential to reduce the strength of scalar fields in high-density regions like the solar system, a necessary condition to pass local gravitational tests while admitting modifications at larger scales.
5. Current Results and Constraints:
   • The paper details how recent observations increasingly constrain MG parameters, with analyses showing that current data from CMB, large-scale structure (LSS), and gravitational lensing remains largely consistent with GR, though some tensions encourage the continuous exploration of MG theories.
6. Implications and Future Prospects:
   • Advancements in observational technologies and methodologies promise to further tighten constraints on MG models. Projects like the Dark Energy Survey (DES), Large Synoptic Survey Telescope (LSST), and forthcoming CMB missions will provide data to critically test GR on cosmological scales.

Implications and Speculation

The overall implications of these efforts are profound as they address fundamental questions about the nature of gravity and the universe's acceleration. The paper underscores the potential of cosmological observations to provide evidence for new physics, advancing our understanding beyond the standard model of cosmology, or confirming the robustness of GR in regimes not yet tested. Future paths involving increasingly precise cosmological data, coupled with advancements in theoretical models and computational methods, may soon illuminate whether the framework of GR or some MG variant better describes our universe.

Conclusion

Mustapha Ishak's paper serves as both a review and a resource for current efforts in cosmology intended to test the limits of GR. The methodical overview and synthesis of theoretical and observational work underscore the critical role of cosmology in testing fundamental physics. It encapsulates the potential for future discoveries that could redefine our understanding of gravity and alter the landscape of cosmological physics.