Gravitational waves could reveal dark matter secrets

The study, published in Physical Review Letters, was led by Sulagna Bhattacharya, a graduate student at the Tata Institute of Fundamental Research (TIFR) in Mumbai. She explained that dark matter particles could interact with normal matter through forces other than gravity and get trapped inside neutron stars over time. If the dark matter particle is heavy enough and has no antiparticle, it could form a dense core inside the neutron star that would collapse into a tiny black hole. The black hole would then devour the neutron star from within and turn it into a black hole of similar mass.

This scenario would create low-mass black holes smaller than the maximum possible mass of neutron stars, which is about 2.5 times the mass of the Sun, according to current theories of stellar evolution. Anupam Ray, who co-led the work, said that “for dark matter parameters that are not yet ruled out by any other experiment, old binary neutron star systems in dense regions of the galaxy ought to have evolved into binary black hole systems. If we do not see any anomalously low-mass mergers, it puts new constraints on dark matter.”

New frontier in the quest to decode dark matter

Interestingly, some of the gravitational wave events detected by LIGO, such as GW190814 and GW190425, seem to involve at least one low-mass compact object. One possibility is that these objects are primordial black holes, which are ancient black holes formed from density fluctuations in the early universe, as Stephen Hawking and Yakov Zeldovich suggested in the 1960s. The LIGO collaboration has been looking for such exotic objects and has set some limits on their abundance. The new study by Bhattacharya and colleagues shows that the same nondetection of low-mass mergers by LIGO also puts stringent limits on particle dark matter. The study explores a range of dark matter particle masses beyond the reach of current terrestrial experiments like XENON1T, PANDA, LUX-ZEPLIN, especially for heavy dark matter particles.

The researchers also predict that future gravitational wave detectors like Advanced LIGO, Cosmic Explorer, and the Einstein Telescope will be able to probe even weaker interactions of heavy dark matter, well below the so-called “neutrino floor” where conventional dark matter detectors face background noise from astrophysical neutrinos. Alternatively, if low-mass black holes are discovered in the future, it could provide a valuable clue about the nature of dark matter.

The research opens a new frontier in the quest to decode dark matter, emphasizing the potential of gravitational waves as not just a scientific curiosity but a powerful tool for fundamental discoveries in the field of physics as a whole.

Concluding on a hopeful note, the authors observe, "Gravitational wave detectors, already instrumental in confirming the existence of black holes and Einstein-predicted gravitational waves, have the potential to emerge as a robust instrument for probing dark matter theories."

The study was published in Physical Review Letters

Study abstract:

Dark matter (DM) from the galactic halo can accumulate in neutron stars and transmute them into sub-2.5M⊙ black holes if the dark matter particles are heavy, stable, and have interactions with nucleons. We show that the nondetection of gravitational waves from mergers of such low-mass black holes can constrain the interactions of nonannihilating dark matter particles with nucleons. We find benchmark constraints with LIGO O3 data, viz., σχn≥O(10−47)  cm2 for bosonic DM with mχ∼PeV (or mχ∼GeV if they can Bose-condense) and ≥O(10−46)  cm2 for fermionic DM with mχ∼103  PeV. These bounds depend on the priors on DM parameters and on the currently uncertain binary neutron star merger rate density. However, with increased exposure by the end of this decade, LIGO will probe cross sections that are many orders of magnitude below the neutrino floor and completely test the dark matter solution to missing pulsars in the Galactic center, demonstrating a windfall science case for gravitational wave detectors as probes of particle dark matter.