Linking Tests of Gravity On All Scales: from the Strong-Field Regime to Cosmology (1412.3455v3)

Abstract: The current effort to test General Relativity employs multiple disparate formalisms for different 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.

• The paper introduces a unified parameter space connecting diverse gravity tests from strong-field regimes to cosmological observations.
• It employs potential (ε) and curvature (ξ) metrics to bridge experiments with observations from gravitational waves to the CMB.
• The study identifies an unexplored 'curvature desert' that may signal deviations from General Relativity and offer insights into cosmic acceleration.

Linking Tests of Gravity Across All Scales

The paper by Baker et al., titled "Linking Tests of Gravity On All Scales: from the Strong-Field Regime to Cosmology," addresses the disjunction between different formal approaches to testing General Relativity (GR) across various scales—ranging from laboratory to cosmological scales. The authors propose a unified parameter space to facilitate the comparison of gravitational tests conducted under different regimes, such as those involving gravitational waves, large-scale cosmic structures, and the Cosmic Microwave Background (CMB).

Summary

Motivation

Current methodologies for testing GR involve a myriad of disparate formalisms tailored to specific observables, making it challenging to understand the links and distinctions between constraints arising from laboratory, astrophysical, and cosmological observations. This situation complicates the comprehensive evaluation of GR's robustness, especially given the multiplicity of potential modifications suggested to explain cosmic acceleration without invoking dark energy or other unknowns.

Parameter Space Framework

The authors develop a parameter space characterized by potential ($\epsilon$) and curvature ($\xi$) to comprehend various gravitational environments tested experimentally and astrophysically. This parameter space accommodates different gravitational scales and allows the representation of gravitational potential and the Kretschmann scalar, facilitating comparisons between disparate tests of GR.

This framework reveals an untested regime, termed the "curvature desert," lying between well-tested strong gravities (like those near black holes) and cosmologically large scales where GR's predictions coincide with dark matter and dark energy phenomena. The curvature desert, therefore, poses an intriguing domain for potential deviations from GR that remains unexplored by current experimental facilities.

Experimental and Observational Analysis

The paper contextualizes various observable systems within this framework:

• Stellar and Compact Objects: The framework maps the gravitational fields surrounding stars and compact binary systems, noting the excellent agreement with GR in strongly curved regimes characteristic of compact binaries.
• Galaxies and Clusters: Galaxies provide extensive testing grounds for GR within this space, bridging the potential and curvature spectrum. Here, dark matter plays a pivotal role in sustaining the predicted dynamics.
• Cosmic Microwave Background & Large-Scale Structures: The CMB and galaxy clustering observations probe lower curvature regimes, indirectly touching the poorly tested regime and indicating legitimate regions of potential new physics.

Implications and Future Directions

The work implies critical ramifications for gravity theories beyond GR. Specifically, the under-explored parameter space introduces an opportunity to predict deviations that could resolve the current cosmological tensions without resorting to unknown components like dark matter or energy. For instance, GR modifications could emerge ideally across the uncharted parameter zones, potentially explaining cosmic acceleration in novel theoretical terms.

The paper also stresses the need for further investigations in galactic and extragalactic scales. Upcoming observational platforms, such as enhanced gravitational wave detectors and precision telescopes, spotlight the weakly tested regions and promise to extend GR's testing to unexplored territories within the parameter space.

Conclusion

The authors propose an integration strategy for tests conducted across the gravitational spectrum, thus establishing a comprehensive matrix for evaluating GR's applicability. The outlined parameter space provides a foundational tool for exploring potential new gravitational physics. The paper encourages probing the curvature desert as a promising direction for uncovering violations and extensions to Einstein's theory—a task pertinent to the advancement of cosmological understanding.