Cosmological Tests of Gravity (1902.10503v3)

Abstract: Cosmological observations are beginning to reach a level of precision that allow us to test some of the most fundamental assumptions in our working model of the Universe. One such an assumption is that gravity is governed by the General Theory of Relativity. In this review we discuss how one might go about extending General Relativity and how such extensions can be described in a unified way on large scales. This allows us to describe the impact of modified gravity on the growth and morphology of the large scale structure of the Universe. On smaller scales we explore the physics of gravitational screening and how it might manifest itself in galaxies, clusters and, more generally, in the cosmic web. We then analyze the current constraints from large scale structure and conclude by discussing the future prospects of the field in light of the plethora of surveys currently being planned.

• The paper introduces an innovative framework to test gravity on cosmological scales by rigorously analyzing deviations from General Relativity.
• It employs a detailed cosmological perturbation formalism, leveraging the Horndeski action to simplify comparisons between GR and alternative theories.
• The study examines gravitational screening mechanisms and anticipates future surveys like Euclid, LSST, and SKA to decisively constrain modified gravity models.

Insights from "Cosmological Tests of Gravity"

The paper "Cosmological Tests of Gravity" by Pedro G. Ferreira explores the critical examination of gravity theories on cosmological scales, highlighting the precision cosmological observations required to challenge General Relativity (GR), the cornerstone of modern gravitational physics. Ferreira's paper offers a comprehensive framework for extending GR and systematically evaluating its modifications regarding the expanding Universe's large-scale structures.

The paper delineates several important findings regarding alternative gravity theories:

• Alternative Theories and Constraints: A plethora of alternative gravity theories exist, constrained primarily by fundamental physical principles. In particular, scalar-tensor theories, vector-tensor alternatives, and massive gravity models are thoroughly examined concerning their implications across cosmic scales.
• Cosmological Perturbation Formalism: A robust formalism now exists for articulating cosmological perturbations under general gravitational theories, extending the utility of cosmological surveys in verifying the consistency of GR. These theoretical developments leverage the structure of the Horndeski action, simplifying the characterization of deviations from GR through a reduced number of functional parameters, thereby facilitating a more precise data-theory comparison.
• Gravitational Screening Mechanisms: The phenomenon of gravitational screening—a process shielding shorter scales from exhibiting modified gravity effects—is investigated, particularly focusing on its manifestations in galaxies, clusters, and the cosmic web. Screening effects could obscure modifications on non-linear scales but leave fingerprints in the large-scale structure.
• Systematics and Constraints: Current constraints are discussed, highlighting systematic uncertainties that limit the exclusive determination of GR as the sole viable theory at cosmological scales. These uncertainties necessitate advanced statistical analysis and rigorous modeling of potential biases when interpreting large-scale structure data.
• Prospects of Upcoming Surveys: Emphasizing future cosmological surveys like Euclid, LSST, and SKA, the paper anticipates a significant advance in tightening constraints on GR by several orders of magnitude. These advancements are expected to provide critical insights into the veracity of GR or its potential deviations on cosmic scales.

Implications and Theoretical Impact

The theoretical implications of this research lie in its potential to substantively challenge or reinforce the GR framework. Extending GR with cosmological observations opens avenues for addressing the Universe's "dark sector," comprising dark matter and dark energy, within a modified gravity context. By offering a systematic method to assess these extensions, Ferreira's work lays foundational ground for reconciling standard cosmological models with potential deviations in gravitational physics.

Moreover, testing gravitational theories on cosmological scales can illuminate aspects of GR otherwise inaccessible through terrestrial or solar system tests, capturing phenomena uniquely indicative of alternative models. Screening mechanisms further complicate this endeavor, suggesting that distinctive gravitational effects could remain undetectable in high-curvature or high-density environments, where standard GR predictions hold strong.

Speculation on Future Developments

With upcoming cosmological observations poised to tighten the precision of gravitational tests dramatically, the landscape of theoretical physics may soon witness paradigmatic shifts. The propounding of accurate models accommodating or refuting GR modifications could reshape our understanding of cosmic evolution and the fundamental forces governing it.

Ferreira's work, situated at this crossroads of observational and theoretical physics, underscores a renewed opportunity to explore gravitational physics beyond empirical confines of singularity and strong-field dynamics, venturing into the cosmological regime where subtle deviations may reveal unprecedented aspects of spacetime's fabric. Such inquiries are pivotal in either reinforcing GR’s supremacy or heralding alternative frameworks that better accommodate emerging observational anomalies.