Cosmological Tests of Gravity (1004.3294v1)

Abstract: Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the universe. We review recent developments in modified gravity theories, focusing on higher dimensional approaches and chameleon/f(R) theories. We classify these models in terms of the screening mechanisms that enable such theories to approach general relativity on small scales (and thus satisfy solar system constraints). We describe general features of the modified Friedman equation in such theories. The second half of this review describes experimental tests of gravity in light of the new theoretical approaches. We summarize the high precision tests of gravity on laboratory and solar system scales. We describe in some detail tests on astrophysical scales ranging from ~kpc (galaxy scales) to ~Gpc (large-scale structure). These tests rely on the growth and inter-relationship of perturbations in the metric potentials, density and velocity fields which can be measured using gravitational lensing, galaxy cluster abundances, galaxy clustering and the Integrated Sachs-Wolfe effect. A robust way to interpret observations is by constraining effective parameters, such as the ratio of the two metric potentials. Currently tests of gravity on astrophysical scales are in the early stages --- we summarize these tests and discuss the interesting prospects for new tests in the coming decade.

• The paper presents a comprehensive review of modified gravity models that adjust General Relativity using screening mechanisms like chameleon and f(R) theories.
• It details observational tests ranging from the CMB to large-scale structure surveys to constrain deviations from standard gravity.
• The study outlines future directions for empirical research, highlighting upcoming experiments that could refine our understanding of cosmic acceleration.

Overview of "Cosmological Tests of Gravity"

The paper "Cosmological Tests of Gravity" by Bhuvnesh Jain and Justin Khoury provides an encompassing review of modified gravity theories and their cosmological implications. This document is particularly focused on alternative explanations to the mysterious concept of dark energy, specifically involving possible modifications to Einstein's theory of General Relativity (GR). The core emphasis of the paper rests on higher-dimensional theories and chameleon/f(R) field theories that propose different screening mechanisms for maintaining consistency with solar system observations.

The impetus for this body of research is grounded in the cosmological problem known as the "energy budget" of the universe. Standard models only account for a small fraction of the universe's total energy density, while the nature and role of dark energy and dark matter remain elusive. This paper explores whether new gravitational degrees of freedom could replace dark energy, providing an explanation for the universe's observed acceleration.

Modified Gravity Theories

The paper categorizes modified gravity theories based on their screening mechanisms, which are essential for recovering GR's predictions on smaller scales while allowing for deviations on cosmic scales:

1. Chameleon/f(R) Field Theories: These theories introduce a scalar field that changes its mass with local density, thus hiding its effects on solar system scales. Crucial mechanisms include chameleon fields which modify the metric in high-density regions, and f(R) theories, which replace the Einstein-Hilbert action with a function of the Ricci scalar.
2. Symmetron Fields: These fields involve a mechanism where the symmetry is spontaneously broken in low-density regions, enabling a coupling to matter in such environments but not in high-density contexts.
3. Massive Gravity and its Offshoots: The Fiertz-Pauli formulation, Dvali-Gabadadze-Porrati (DGP) gravity, and cascading gravity are addressed, the latter extending to higher dimensions. All naturalize the cosmological scale modification of gravity by either explicitly adding a graviton mass or using extra-dimensional frameworks.

Tests from Astronomical Observations

The latter parts of the document delve into astrophysical tests of these theories, emphasizing various scales from kiloparsec to gigaparsec:

• Tests range from examining the effects on the Cosmic Microwave Background (CMB) and Big Bang Nucleosynthesis (BBN) to large-scale structures in the universe. The CMB provides foundational constraints, whereas ongoing and upcoming astronomical surveys offer high-resolution data critical for refining models.
• Metric perturbations and the evolution of cosmic structures in modified gravity scenarios are subject to careful examination. Differentiating growth factors and scales is crucial, as is the impact of non-linear gravitational clustering.

Current Constraints and Future Directions

Contemporary observational data provide constraints on the parameter space of modified gravity models. Experimental advancements hold promise for further constraining, or potentially falsifying, these theories. Upcoming surveys such as LSST and EUCLID, as well as improved redshift space and weak lensing measurements, are poised to deliver significant contributions.

Implications

The implications of this research are manifold: theoretically, it provides different paradigms for understanding cosmic acceleration outside the standard GR framework. Practically, it influences the design and focus of observational cosmology projects.

In conclusion, "Cosmological Tests of Gravity" synthesizes considerable developments in theoretical physics and lays out a comprehensive framework for future empirical testing. The synthesis of theoretical modeling with observational constraints represents a significant forward step in cosmology, offering potential pathways to resolve some of the most profound questions about the universe's structure and dynamics.