Dark mountains of neutron stars: gravitational waves probe dark matter ✦
Context
The hunt for continuous gravitational waves from spinning neutron stars, often seen as pulsars, is a cornerstone of modern astrophysics. Since Jocelyn Bell Burnell's 1967 discovery, these objects have been extreme-physics laboratories. Despite decades of effort, faint signals remain elusive, but even their absence sets strict limits on compact-object structure. If dark matter lurks inside a neutron star, its mass and density profile can alter the emitted gravitational waves, offering a unique stellar-scale probe of the dark sector.
Methods
Researchers modeled a two-fluid neutron star with baryonic matter and fermionic self-interacting dark matter, captured during the progenitor's collapse in a supernova or over the star's lifetime. They computed how dark matter shifts the moment of inertia and how anisotropic distributions spawn 'dark mountains'—quadrupole deformations radiating continuous gravitational waves. Theoretical predictions were then confronted with upper limits from LIGO's third observing run (O3). This approach, rooted in the pioneering work of Rainer Weiss and others, provided the first constraints on dark matter self-interactions.
Results
O3 data showed that for dark matter with rest masses of 0.1–10 GeV and coupling constants above roughly 10^−5.5 (in the most optimistic scenario—ellipticity 10^−7 at 1 kpc), such neutron stars would have been detected, so these parameter regions are excluded. Under less favorable assumptions (lower ellipticity, larger distances), only couplings stronger than 10^−4 are ruled out. The maximum allowed ellipticity from gravitational-wave observations is far tighter for dark-matter-laden stars than for ordinary ones, due to the enhanced moment of inertia.
Implications
This work is the first to show that continuous gravitational-wave searches can directly test dark matter hypotheses on single-neutron star scales, complementing classic astrophysical constraints like galaxy cluster observations pioneered by Fritz Zwicky in the 1930s.
Future development
With next-generation detectors like the Einstein Telescope and Cosmic Explorer, probing dark matter via continuous waves will gain orders of magnitude in reach. Projected sensitivities will test models with coupling constants down to 10^−6 for neutron stars up to ~10 kpc away, vastly expanding the parameter space.
Impact
The findings will influence dark matter particle physics, nuclear astrophysics of neutron stars, and gravitational-wave astronomy.
Next steps
Immediate next steps include folding in data from the fourth observing run of LIGO–Virgo–KAGRA and refining models of dark matter capture in neutron stars with realistic equations of state.
Key open problems
This study addresses fundamental questions about dark matter's nature, the neutron star equation of state, and the formation of quadrupolar deformations. It helps crack the long-standing non-detection puzzle of continuous gravitational waves.
🎯 If a 'dark mountain' on a neutron star were visible, it would be a bump less than a millimeter high but packing a mass comparable to the Himalayas.
Key numbers
- LIGO O3 minimum upper strain limit: 1.1×10⁻²⁵ at 111.5 Hz
- excluded coupling constant (optimistic scenario): g > 10⁻⁵·⁵
- dark matter mass range: 0.1–10 GeV
- distances: 1–10 kpc
- ellipticities: 10⁻⁹–10⁻⁷