Dark Energy in light of Multi-Messenger Gravitational-Wave astronomy (1807.09241v2)

Abstract: Gravitational waves (GWs) provide a new tool to probe the nature of dark energy (DE) and the fundamental properties of gravity. We review the different ways in which GWs can be used to test gravity and models for late-time cosmic acceleration. Lagrangian-based gravitational theories beyond general relativity (GR) are classified into those breaking fundamental assumptions, containing additional fields and massive graviton(s). In addition to Lagrangian based theories we present the effective theory of DE and the $\mu$-$\Sigma$ parametrization as general descriptions of cosmological gravity. Multi-messenger GW detections can be used to measure the cosmological expansion (standard sirens), providing an independent test of the DE equation of state and measuring the Hubble parameter. Several key tests of gravity involve the cosmological propagation of GWs, including anomalous GW speed, massive graviton excitations, Lorentz violating dispersion relation, modified GW luminosity distance and additional polarizations, which may also induce GW oscillations. We summarize present constraints and their impact on DE models, including those arising from the binary neutron star merger GW170817. Upgrades of LIGO-Virgo detectors to design sensitivity and the next generation facilities such as LISA or Einstein Telescope will significantly improve these constraints in the next two decades.

Dark Energy and Gravitational Waves: Exploring Multi-Messenger Astronomy

The paper "Dark Energy in Light of Multi-Messenger Gravitational-Wave Astronomy" focuses on leveraging the advancements in gravitational-wave (GW) astronomy to paper dark energy (DE) and modified theories of gravity. This research exploits observational data from GW detections, particularly those with electromagnetic counterparts, to refine constraints on DE models and inform our understanding of gravity's fundamental properties.

Overview

Gravitational-wave astronomy has poised itself as a revolutionary tool in probing the universe. The detection of gravitational waves from astrophysical sources provides a novel mechanism to measure cosmological parameters and test theories of gravity beyond General Relativity (GR). The standard model of cosmology, ΛCDM, which includes a cosmological constant as dark energy (DE) to drive late-time cosmic acceleration, is an incomplete picture that has led scientists to explore alternatives through modified-gravity frameworks and DE scalar field models.

Implications for Dark Energy Models

The propagation of GWs through the cosmos offers potential insights into the equation of state of DE and challenges the conventional understanding grounded in GR. Key aspects examined in this paper include:

1. Cosmological Tests Using GW Speed: The propagation speed of GWs compared to light provides constraints on alternative theories of gravity, as deviations could signal modifications from GR due to extra fields or massive gravitons, typically associated with DE models.
2. GW Luminosity Distance: The disparity between GW and electromagnetic (EM) luminosity distances can reveal insights about DE models, particularly in scenarios where GWs are damped differently from EM waves, suggesting additional forces at play in the regime modifying gravity.
3. Additional GW Polarizations: Beyond GR predicts extra polarizations, such as scalar or vector modes, which can be tracked through GW observatories when cross-referenced with EM signals, providing a venue to falsify or validate various modified-gravity theories.
4. GW Oscillations and Mixing: Some theories propose a mixing of GW states leading to oscillations analogous to neutrino physics, presenting another frontier in the empirical testing of multi-field gravity embodiments.

Practical and Theoretical Implications

Multi-messenger GW astronomy enhances the precision in measuring cosmic expansion rates and can resolve specific tensions in ΛCDM, such as the Hubble constant discrepancy. The combination of GW data with EM observations significantly constrains the viable parameter space for DE models, demanding theories comply with stringent propagation characteristics (e.g., same speed for GWs and light).

Future Prospects and Developments: Upcoming advancements in GW detectors (e.g., LISA and Einstein Telescope) promise sensitivity to broader frequency ranges and distant astrophysical sources, potentially expanding the scope of tests for DE and gravity theories. This progression underscores the necessity for collaborative international frameworks in GW and EM observations, aiming to capture more comprehensive data and enhance cross-validation.

Conclusion

This research marks a pivotal junction where theoretical predictions in cosmology and quantum gravity intersect with empirical data provided by GW astronomy. Crucially, it advocates for ongoing exploration in alternative theories of gravity, stimulated by observational validations, especially with DE's enigmatic role in cosmic acceleration. The synergy between theory, detector technology, and multi-messenger observational campaigns will continue to shape our cosmological paradigm in the decades to come.