Abstract
The observation of gravitational waves from a binary neutron star merger by LIGO/VIRGO and the associated electromagnetic counterpart provides a high precision test of orbital dynamics, and therefore a new and sensitive probe of extra forces and new radiative degrees of freedom. Axions are one particularly well-motivated class of extensions to the Standard Model leading to new forces and sources of radiation, which we focus on in this paper. Using an effective field theory (EFT) approach, we calculate the first post-Newtonian corrections to the orbital dynamics, radiated power, and gravitational waveform for binary neutron star mergers in the presence of an axion. This result is applicable to many theories which add an extra massive scalar degree of freedom to General Relativity. We then perform a detailed forecast of the potential for Advanced LIGO to constrain the free parameters of the EFT, and map these to the mass $m_a$ and decay constant $f_a$ of the axion. At design sensitivity, we find that Advanced LIGO can potentially exclude axions with $m_a \lesssim 10^{-11} \ {\rm eV}$ and $f_a \sim (10^{14} - 10^{17}) \ {\rm GeV}$. There are a variety of complementary observational probes over this region of parameter space, including the orbital decay of binary pulsars, black hole superradiance, and laboratory searches. We comment on the synergies between these various observables.
Highlights
The importance of the recent direct detection of gravitational waves from black hole and neutron star mergers can hardly be overemphasized [1,2,3,4]
New scalar forces that arise only in a strong gravity or high density environment, are unconstrained and can be looked for with Advanced LIGO. We focus on this last category, building upon previous work [31,32] suggesting that binary neutron star (NS-NS) and neutron star-black hole (NS-BH) mergers can provide powerful new probes of light scalar force mediators
We have examined the exciting possibility that Advanced LIGO could detect new light scalar particles through their influence on the gravitational waveform produced in NS-NS and NS-BH binary mergers
Summary
The importance of the recent direct detection of gravitational waves from black hole and neutron star mergers can hardly be overemphasized [1,2,3,4]. We extend the effective field theory of gravity for binary systems to include couplings to an axion, and calculate at next-to-leading order (e.g., to 1PN order) the axion forces between neutron stars as well as axion radiation, both of which are crucial for computing the GW waveform. We find that Advanced LIGO is a very sensitive probe of the scalar charges of neutron stars and the range of the scalar force (or equivalently, the mass of the axion) Translating this into constraints on ffa; mag for axions, we find that a GW170817-like event could look for axions in a large region of the theoretically interesting parameter space. We use the conventions: m2Pl 1⁄4 1=32πG, ħ 1⁄4 c 1⁄4 1 and ημν 1⁄4 ð1; −1; −1; −1Þ
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