A new study published in Physical Review D has systematically calculated the probability of detecting gravitational wave signals from primordial black holes (PBHs) interacting with neutron stars in the Galactic center, concluding that current observatories face extremely long odds. Despite the theoretical appeal of PBH-neutron star encounters as a window into dark matter, the probability of LIGO-Virgo-KAGRA spotting one within a decade is less than one in 100 million.
Primordial black holes are hypothetical objects thought to have formed from extreme density fluctuations in the first moments after the Big Bang, long before stars and galaxies existed. Unlike stellar-mass black holes born from collapsing supernovae, PBHs could have masses comparable to asteroids or small planets, making them fundamentally different from the black holes routinely detected by gravitational wave observatories. They are also considered a leading candidate for dark matter, the invisible substance that makes up approximately 27 percent of the universe’s mass-energy content.
The study, led by Nicolas Esser of the Université Libre de Bruxelles together with Juan García-Bellido (Universidad Autónoma de Madrid) and Peter Tinyakov (Université Libre de Bruxelles), focused specifically on gravitational wave signals generated when planetary-mass PBHs pass near or through neutron stars, the ultra-dense remnants of supernova explosions that cram up to two solar masses into a city-sized sphere.
Two types of encounters
The team examined two distinct scenarios. The first involves PBHs captured into bound orbits around neutron stars, forming eccentric binary systems. Each close passage, or periastron passage, produces a gravitational wave burst. A single bound pair can generate thousands of correlated bursts over time, creating a repeating signal pattern that might seem easier to detect.
The second scenario involves random unbound encounters, where PBHs simply fly past neutron stars without being gravitationally captured. These are one-off events but occur much more frequently across the galaxy.
Counterintuitively, the researchers found that the total number of signals from unbound encounters dominates over those from bound systems, even accounting for the repeated bursts produced by bound pairs. “Despite the enhancement from the large number of bursts produced by a single PBH-NS pair, the total number of signals produced in this way remains subdominant to those due to random unbound encounters,” the authors write.
Grim numbers for detection
The punchline of the paper is stark: over a 10-year observation period with the current LIGO-Virgo-KAGRA network, the probability of detecting any PBH-neutron star gravitational wave signal (bound or unbound) is less than 10⁻⁸, or roughly one in 100 million.
The calculation assumes all observable neutron stars in the Galactic center region, which has the highest density of neutron stars in the Milky Way. Even with this optimistic targeting, the expected event rate is vanishingly small.
This does not mean PBHs do not exist or that they never interact with neutron stars. Rather, it reflects the extreme rarity of such encounters within the volume that current detectors can probe. Neutron stars are compact objects with radii of only about 10 kilometers, making direct collisions or close flybys exceedingly rare events even over cosmic timescales.
Context in the PBH debate
The findings arrive amid a surge of interest in primordial black holes following a series of unusual gravitational wave signals detected by LIGO. In November 2025, the observatory captured a signal designated S251112cm involving objects with masses between 0.1 and 0.87 solar masses, well below the typical minimum mass of stellar black holes. Several research groups have since argued that these sub-solar mass events could represent the first direct evidence of primordial black holes.
The Esser et al. study does not directly address those detections (which involve PBH-PBH mergers rather than PBH-neutron star interactions), but it underscores a broader point: if PBH are abundant enough to constitute dark matter, their interactions with neutron stars might still be too rare for existing instruments to catch.
Future observatories with greater sensitivity, such as the space-based Laser Interferometer Space Antenna (LISA) or next-generation ground-based detectors like the Einstein Telescope, could shift these odds significantly. The authors note that their framework applies to any future detector network and could be updated once those instruments come online.