Primordial black holes, believed to have formed during the early stages of the universe, remain elusive and unconfirmed. However, if they do exist, these ancient cosmic objects could be numerous, drifting almost invisibly through space, and possibly accounting for dark matter. One intriguing theory suggests that low-mass primordial black holes might be captured by neutron stars, becoming trapped inside and gradually consuming them from within. If such events occur, it could explain the scarcity of neutron stars, particularly around regions like the Galactic center where black holes are expected to be abundant.
The Fascination with Black Holes and Their Potential Origins
Black holes captivate the imagination, not only because of their mysterious nature but also because of their potential origins. Some black holes form from the remnants of supermassive stars, while others, still theoretical, may have emerged from regions of higher density during the universe’s infancy. These primordial black holes could potentially explain a portion—or even all—of the dark matter, which makes up approximately 27% of the universe’s mass-energy content. Discovering these primordial black holes would not only shed light on the dark matter mystery but also provide insights into various cosmic phenomena, such as microlensing events and correlations in X-ray and cosmic infrared background fluctuations.

New Research on Primordial Black Holes and Neutron Stars
In a recent study by Roberto Caiozzo, Gianfranco Bertone, and Florian Kuhnel, the researchers delve into the abundance of sub-stellar mass primordial black holes and explore the possibility that these black holes could be captured by neutron stars. Once captured, the black holes might sink to the core of the neutron star, leading to its eventual destruction. This concept isn’t entirely new, with previous research by Y. G. Genolini, P. D. Serpico, and P. Tinyakov also investigating the capture of primordial black holes by neutron stars.
Caiozzo and his team focused their attention on the innermost region of the Milky Way galaxy, known as the Galactic center. This area was chosen due to its expected high density of dark matter and a predicted population of pulsars—rapidly rotating neutron stars that emit regular pulses of radiation. While pulsars are believed to exist in large numbers in the Galactic bulge, very few have been observed in the innermost region, a mystery that the team speculates might be due to the disruptive influence of primordial black holes.

The Role of Millisecond Pulsars in the Galactic Center
Millisecond pulsars, which have the highest spin rates, are prime candidates for study. They are relatively easy to detect and are expected to be plentiful in the Galactic center. Previous models have predicted around 67 long-lived neutron star–X-ray binaries in this region, which are thought to be progenitors of millisecond pulsars. This suggests that a significant population of pulsars should be present in the Galactic center.
The primary goal of Caiozzo and his team was to refine existing calculations and provide a more accurate prediction of the rate at which primordial black holes might be captured by neutron stars in the Galactic center. They also investigated the likelihood of neutron star collapse due to these black holes. However, after improving upon previous models, the team concluded that the capture of primordial black holes is unlikely to account for the observed scarcity of pulsars and neutron stars in this region. The disruption of these stars by black holes is simply not probable within their projected lifetimes.
Conclusion
While the capture and destruction of neutron stars by primordial black holes is a fascinating theory, current research suggests that it may not be the reason for the missing pulsars around the Galactic center. The ongoing search for primordial black holes continues, as their discovery could unlock many mysteries of the universe, including the nature of dark matter and the evolution of cosmic structures. Until then, the question of what disrupts neutron stars in certain regions remains an open and intriguing mystery.
Source : Revisiting Primordial Black Hole Capture by Neutron Stars