Theoretical physics implications of the binary black-hole mergers GW150914 and GW151226

Abstract

The gravitational wave observations GW150914 and GW151226 by Advanced LIGO provide the first opportunity to learn about physics in the extreme gravity environment of coalescing binary black holes. The LIGO Scientific Collaboration and the Virgo Collaboration have verified that this observation is consistent with Einstein's theory of General Relativity, constraining the presence of certain parametric anomalies in the signal. This paper expands their analysis to a larger class of anomalies, highlighting the inferences that can be drawn on non-standard theoretical physics mechanisms that could otherwise have affected the observed signals. We find that these gravitational wave events constrain 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. Though other observations limit many of these mechanisms already, GW150914 and GW151226 are unique in that they are direct probes of dynamical strong-field gravity and of gravitational wave propagation. We also show that GW150914 constrains inferred properties of exotic compact object alternatives to Kerr black holes. We argue, however, that the true potential for GW150914 to both rule out exotic objects and constrain physics beyond General Relativity is severely limited by the lack of understanding of the coalescence regime in almost all relevant modified gravity theories. This event thus significantly raises the bar that these theories have to pass, both in terms of having a sound theoretical underpinning, and reaching the minimal level of being able to solve the equations of motion for binary merger events. We conclude with a discussion of the additional inferences that can be drawn if the lower-confidence observation of an electromagnetic counterpart to GW150914 holds true, or such a coincidence is observed with future events; this would provide dramatic constraints on the speed of gravity and gravitational Lorentz violation.

Implications of the gravitational wave event GW150914

“…There has also been detailed analysis of the implications of GW150914 itself for gravitational physics [64,65]. Here we summarize some of the main points.…”

“…As emphasized by [65], there are multiple pillars of GR that could be tested by analysis of the gravitational waves from double black hole coalescences, including the strong equivalence principle, the no-hair theorem, the speed of gravity, the tensorial nature of gravity (i.e., without including scalar or vector components), and the four-dimensionality of spacetime. A double black hole detection can not, of course, test deviations from GR that appear only in the presence of matter.…”

“…Here we summarize some of the main points. We draw in particular on [65], which has excellent discussions of many details that we omit for brevity. We can, in some sense, divide tests into two categories: (1) tests of the correctness of GR, and (2) constraints on alternatives to black holes in GW150914, given the assumption that GR is correct.…”

“…The approach of [65] is to divide deviations from GR into generation mechanisms, which they define as deviations that operate close to the binary, and propagation mechanisms, which they define as deviations that accumulate with distance traveled. They emphasize that propagation mechanisms can typically be constrained far more tightly than generation mechanisms, because any process that leads to an accumulation of phase with distance (such as a frequencydependent propagation speed for gravity, which would emerge if the graviton were massive) has an enormous amount of time to accumulate (roughly a billion years for GW150914) compared to the ∼ 0.2 s duration of the event.…”

Black Hole Spectroscopy with Coherent Mode Stacking

“…The recent detection of gravitational waves (GWs) emitted during the coalescence of binary black holes [1, 2] marked the beginning of the era of gravitational wave astronomy, a feat that heralds a boom of scientific discoveries to come. GWs not only provide a new window to our universe, they also offer a unique opportunity to test General Relativity (GR) in the dynamical and highly non-linear gravitational regime [3][4][5][6][7]. One celebrated prediction of GR is the uniqueness, or "no-hair" property of vacuum black holes (BHs) [8][9][10][11][12]: all isolated BHs are described by the Kerr family of solutions, each uniquely characterized by only its mass and spin [69].…”