Probing Scalar Polarization with Next-Generation Gravitational-Wave Detectors: The Roles of Detector Geometry and Source Inclination
Abstract
Gravitational-wave observations of compact binaries offer a direct way to test whether gravity admits polarizations beyond the two tensor modes of general relativity. We perform a forecast to assess how well future multi-messenger events will constrain the presence of scalar polarizations in gravitational waves. Performing binary neutron star injections, we forecast next-generation constraints on three modified gravity parameters in the PPE formalism: α (tensor amplitude), αB (scalar amplitude) and β (tensor and scalar phase). Cosmic Explorer (CE) and Einstein Telescope (ET) each tighten every parameter by 1--2 orders of magnitude over an O5 LIGO-Virgo network, with ET's triangular geometry giving a persistent ≈2× improvement over 2×CE on the scalar-mode amplitude αB in both a GW170817-like and a second, representative BNS. We show that, if the sky position and luminosity distance are known from EM observations, adding an EM-based prior on the orbital inclination ι narrows the αB constraint by ≈56× relative to an isotropic inclination prior, for a GW170817-like event. We also assess how EM inclination priors as well as assumptions on the neutron star equation of state may affect the inference on the scalar amplitude αB. Overall, these results indicate that detector geometry and the availability of well-placed EM external information are both important levers --- alongside raw network sensitivity --- for next decade scalar-polarization tests.
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BibTeX
@misc{imam2026probing,
title = {{Probing Scalar Polarization with Next-Generation Gravitational-Wave Detectors: The Roles of Detector Geometry and Source Inclination}},
author = {Sk Md Adil Imam and Macarena Lagos},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.38956},
eprint = {2610.38956},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.38956},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Probing Scalar Polarization with Next-Generation Gravitational-Wave Detectors: The Roles of Detector Geometry and Source Inclination
AU - Sk Md Adil Imam
AU - Macarena Lagos
PY - 2026
DA - 2026/10/28
PB - arXiv
DO - 10.48550/arxiv.2610.38956
UR - https://arxiv.org/abs/2610.38956
AB - Gravitational-wave observations of compact binaries offer a direct way to test whether gravity admits polarizations beyond the two tensor modes of general relativity. We perform a forecast to assess how well future multi-messenger events will constrain the presence of scalar polarizations in gravitational waves. Performing binary neutron star injections, we forecast next-generation constraints on three modified gravity parameters in the PPE formalism: α (tensor amplitude), α_B (scalar amplitude) and β (tensor and scalar phase). Cosmic Explorer (CE) and Einstein Telescope (ET) each tighten every parameter by 1--2 orders of magnitude over an O5 LIGO-Virgo network, with ET's triangular geometry giving a persistent ≈2× improvement over 2×CE on the scalar-mode amplitude α_B in both a GW170817-like and a second, representative BNS. We show that, if the sky position and luminosity distance are known from EM observations, adding an EM-based prior on the orbital inclination ι narrows the α_B constraint by ≈56× relative to an isotropic inclination prior, for a GW170817-like event. We also assess how EM inclination priors as well as assumptions on the neutron star equation of state may affect the inference on the scalar amplitude α_B. Overall, these results indicate that detector geometry and the availability of well-placed EM external information are both important levers --- alongside raw network sensitivity --- for next decade scalar-polarization tests.
ER -