[Paper Review] Theoretical Physics Implications of the Binary Black-Hole Merger GW150914

Nicolás Yunes, Kent Yagi+1 more|arXiv (Cornell University)|Mar 29, 2016

Pulsars and Gravitational Waves Research12 citations

TL;DR

This paper analyzes the gravitational wave event GW150914 to constrain non-standard gravitational physics, including modified gravity theories, scalar fields, extra dimensions, and exotic compact objects. It demonstrates that GW150914 provides a direct probe of strong-field gravity, significantly tightening constraints on deviations from General Relativity, especially in wave propagation and black hole alternatives.

ABSTRACT

The gravitational-wave observation GW150914 by Advanced LIGO provides the first opportunity to learn about physics in the extreme gravity of coalescing binary black holes. The LIGO/Virgo collaboration has verified that this observation is consistent with General Relativity, constraining the presence of parametric anomalies in the signal. This paper expands this analysis to a larger class of anomalies, highlighting the inferences that can be drawn on non-standard theoretical physics mechanisms that would affect the signal. We find that GW150914 constrains a plethora of mechanisms associated with the generation and propagation of gravitational waves, including the activation of scalar fields, gravitational leakage into large extra dimensions, the variability of Newton's constant, the speed of gravity, a modified dispersion relation, gravitational Lorentz violation and the strong equivalence principle. Unlike other observations that limit these mechanisms, GW150914 is a direct probe of dynamical strong-field gravity and gravitational wave propagation. We also show that GW150914 constrains the properties of exotic compact object alternatives to Kerr black holes. The true potential for GW150914 to both constrain exotic objects and physics beyond General Relativity is limited by the lack of understanding of the dynamical strong field in almost all modified gravity theories. GW150914 thus raises the bar that these theories must pass, both in terms of having a sound theoretical underpinning, and the minimal level to which the corresponding equations of motion must be solved in binary coalescences. We conclude with a discussion of additional inferences that can be drawn from smaller-confidence observations, such as the LVT151012 trigger and electromagnetic counterparts to GW150914, the latter of which would produce dramatic constraints on the speed of gravity and gravitational Lorentz violation.

Motivation & Objective

• To investigate the implications of GW150914 for non-standard gravitational physics beyond General Relativity.
• To assess how the observed gravitational wave signal constrains mechanisms affecting gravitational wave generation and propagation.
• To evaluate the viability of exotic compact object models as alternatives to Kerr black holes.
• To highlight the need for improved theoretical frameworks in modified gravity theories to match strong-field binary coalescence dynamics.
• To explore additional constraints from lower-significance events like LVT151012 and potential electromagnetic counterparts to GW150914.

Proposed method

• The analysis extends beyond standard LIGO/Virgo checks by examining a broader class of parametric anomalies in the GW150914 signal.
• It applies constraints derived from the observed waveforms to theoretical models involving scalar fields, variable Newton's constant, and modified dispersion relations.
• The study evaluates gravitational leakage into large extra dimensions and violations of Lorentz invariance and the strong equivalence principle.
• It uses the absence of deviations in GW150914’s phase and amplitude to rule out or limit exotic mechanisms in gravitational wave propagation.
• Theoretical models of exotic compact objects are tested against the observed ringdown and merger dynamics of GW150914.
• The paper discusses how electromagnetic counterparts—had they been detected—would have further constrained gravity's speed and Lorentz violation.

Experimental results

Research questions

• RQ1 To what extent does GW150914 constrain the existence of scalar fields coupled to gravity in the strong-field regime?
• RQ2 How do models with variable Newton's constant or modified dispersion relations fare under the constraints of GW150914's observed waveform?
• RQ3 What limits does GW150914 impose on gravitational leakage into large extra dimensions or deviations in the speed of gravity?
• RQ4 How well do exotic compact object models, such as boson stars or gravastars, reproduce the observed GW150914 signal compared to Kerr black holes?
• RQ5 What additional constraints could have been placed on gravity's speed and Lorentz invariance if electromagnetic counterparts to GW150914 had been detected?

Key findings

• GW150914 provides the most stringent direct constraints to date on strong-field gravitational wave propagation and deviations from General Relativity.
• The absence of anomalies in the signal rules out significant scalar field activation or gravitational leakage into large extra dimensions.
• The observation constrains the variability of Newton's constant and the speed of gravity to levels consistent with General Relativity.
• GW150914 places strong limits on modified dispersion relations and gravitational Lorentz violation, especially when combined with potential electromagnetic counterparts.
• Exotic compact object models that do not match the observed ringdown and merger dynamics are ruled out by the data.
• The event underscores the need for more complete theoretical frameworks in modified gravity to accurately model binary black hole coalescences.