Abstract
Pulsar timing and laser-interferometer gravitational-wave (GW) detectors are superb laboratories to study gravity theories in the strong-field regime. Here, we combine these tools to test the mono-scalar-tensor theory of Damour and Esposito-Farèse (DEF), which predicts nonperturbative scalarization phenomena for neutron stars (NSs). First, applying Markov-chain Monte Carlo techniques, we use the absence of dipolar radiation in the pulsar-timing observations of five binary systems composed of a NS and a white dwarf, and eleven equations of state (EOSs) for NSs, to derive the most stringent constraints on the two free parameters of the DEF scalar-tensor theory. Since the binary-pulsar bounds depend on the NS mass and the EOS, we find that current pulsar-timing observations leave scalarization windows, i.e., regions of parameter space where scalarization can still be prominent. Then, we investigate if these scalarization windows could be closed and if pulsar-timing constraints could be improved by laser-interferometer GW detectors, when spontaneous (or dynamical) scalarization sets in during the early (or late) stages of a binary NS (BNS) evolution. For the early inspiral of a BNS carrying constant scalar charge, we employ a Fisher-matrix analysis to show that Advanced LIGO can improve pulsar-timing constraints for some EOSs, and next-generation detectors, such as the Cosmic Explorer and Einstein Telescope, will be able to improve those bounds for all eleven EOSs. Using the late inspiral of a BNS, we estimate that for some of the EOSs under consideration, the onset of dynamical scalarization can happen early enough to improve the constraints on the DEF parameters obtained by combining the five binary pulsars. Thus, in the near future, the complementarity of pulsar timing and direct observations of GWs on the ground will be extremely valuable in probing gravity theories in the strong-field regime.
- Received 26 April 2017
DOI:https://doi.org/10.1103/PhysRevX.7.041025
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Einstein’s general theory of relativity has enjoyed a century of success as the central framework for understanding gravity. Some physicists are, however, interested in exploring alternatives. Modifications to general relativity could offer a step toward a quantum theory of gravity, for example, or an explanation of dark energy (the acceleration of the expansion of the Universe). Pulsar timing, which allows precise monitoring of the orbits of binary pulsars, and laser-interferometer gravitational-wave (GW) detectors, which look directly for ripples in spacetime from cosmic collisions, offer superb tools for testing alternative theories of gravity. We investigate whether combining these techniques could help rule out alternative theories, using, as an example, a class of theories that predict intriguing behavior in neutron stars.
Specifically, we explore the mono-scalar-tensor theories of Damour and Esposito-Farèse. We first propose a Bayesian method that combines timing measurements of five binary-pulsar systems, and we derive the hitherto best theoretical constraints with the state-of-the-art observations. Then, we address the question of whether future binary neutron-star observations from GW detectors can improve these constraints, and we find that they can for certain equations of state (equations that relate thermodynamic variables in the interior of a neutron star); next-generation observatories will be able to provide constraints for a broader range of possible equations of state.
Our results show that, depending on the specifics of binary systems and neutron-star equations of state, both pulsar timing and GW observations can provide complementary constraints on alternative theories of gravity, a timely realization as new instruments come online in upcoming years.
