Neutron star mergers are a source of new physics signals, which could help determine the true nature of dark matter, as revealed by research from Washington University in St. Louis.
On August 17, 2017, the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and Virgo, a detector in Italy, detected gravitational waves from the collision of two neutron stars. For the first time, this astronomical event was visible in light by dozens of telescopes on the ground and in space.
Research on Axion-Like Particles
Physicist Bhupal Dev in Arts & Sciences used observations from this neutron star merger — an event identified in astronomical circles as GW170817 — to derive new constraints on axion-like particles. Although they haven’t been directly observed, these hypothetical particles appear in many extensions of the standard model of physics.
Axions and axion-like particles are leading candidates to compose part or all of the “missing” matter, or dark matter, of the universe that scientists have not been able to account for yet. At the very least, these feebly-interacting particles can serve as a kind of portal, connecting the visible sector that humans know much about to the unknown dark sector of the universe.
“We have good reason to suspect that new physics beyond the standard model might be lurking just around the corner,” said Dev, first author of the study in Physical Review Letters and a faculty fellow of the university’s McDonnell Center for the Space Sciences.
Insights From Neutron Star Mergers
Two neutron stars merging results in the formation of a hot, dense remnant for a brief amount of time. This remnant is an ideal breeding ground for the production of exotic particles, Dev said. “The remnant gets much hotter than the individual stars for about a second before settling down into a bigger neutron star or a black hole, depending on the initial masses,” he said.
The collision debris is quietly ejected by these new particles, who can decay into known particles, typically photons, far away from their source. Dev and his team — including WashU alum Steven Harris (now NP3M fellow at Indiana University), as well as Jean-Francois Fortin, Kuver Sinha, and Yongchao Zhang — showed that these escaped particles give rise to unique electromagnetic signals that can be detected by gamma-ray telescopes, such as NASA’s Fermi-LAT.
By analyzing spectral and temporal information from these electromagnetic signals, the research team was able to distinguish the signals from the known astrophysical background. Then they used Fermi-LAT data on GW170817 to derive new constraints on the axion-photon coupling as a function of the axion mass. The astrophysical constraints are in sync with those from laboratory experiments like the Axion Dark Matter eXperiment (ADMX), which investigates a different part of the axion parameter space.
Future Prospects in Particle Physics
In the future, scientists could use existing gamma-ray space telescopes, like the Fermi-LAT, or proposed gamma-ray missions, like the WashU-led Advanced Particle-astrophysics Telescope (APT), to take other measurements during neutron star collisions and help improve upon their understanding of axion-like particles.
“Extreme astrophysical environments, like neutron star mergers, provide a new window of opportunity in our quest for dark sector particles like axions, which might hold the key to understanding the missing 85% of all the matter in the universe,” Dev said.
Unlocking the Secrets of Axion-Like Particles
By analyzing the electromagnetic signals emanating from GW170817, Dev and his team distinguished these signals from known astrophysical backgrounds, focusing on the interaction between axions and photons based on the axion’s mass.
Their research, published in Physical Review Letters, not only provided new constraints on the axion-photon coupling but also highlighted the unique role of neutron star mergers in advancing our understanding of the dark sector of the universe. Using extreme astrophysical environments to probe dark matter is a new approach to the composition of the cosmos that complements traditional laboratory experiments.
Charting the Future of Dark Matter Research
The implications of this study go far beyond the constraints it places on axion-like particles. It highlights the potential of using the universe as a laboratory to explore fundamental physics questions that ground-based experiments may not be able to answer.
Moreover, the research shows how advances in gravitational wave astronomy and electromagnetic observation techniques can synergize to unravel the layers of mystery that shroud our understanding of dark matter. As we stand on the brink of a new era in astrophysics, studies like Dev’s not only enrich our knowledge of the universe but also inspire a sense of wonder and infinite possibility.