Bayesian exploration of weakly interacting models of dark matter admixed neutron stars based on multi-density constraints
This study presents a unified Bayesian analysis of Higgs-portal, neutron-decay, and axion-like-particle dark matter models in neutron stars using multi-density constraints, revealing that while the models yield overlapping mass-radius predictions, the neutron-decay scenario is moderately favored by current data despite the inability to definitively distinguish between dark-sector mechanisms and hadronic equation-of-state uncertainties.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Neutron stars are the dense, dead cores left behind after massive stars explode. They are so heavy that a single teaspoon of their material would weigh billions of tons on Earth. Inside these stars, matter is squeezed to a point where it behaves in ways we cannot recreate in any laboratory. For decades, physicists have tried to understand the rules that govern this extreme matter, known as the equation of state. This set of rules describes how the pressure inside the star changes as the density increases, determining the star's size, its maximum possible weight, and how it reacts when squeezed by a partner star. At the same time, astronomers know that the universe is filled with invisible dark matter, which makes up most of the mass in the cosmos but does not emit light. Because neutron stars have such intense gravity, they act as natural traps that could capture and hold this dark matter. If dark matter exists inside a neutron star, it would change the star's internal structure, potentially making it smaller or altering how it vibrates. The big question is whether we can look at the stars we see today and tell if they are hiding a secret dark core, and if so, what kind of dark matter it might be.
A team of researchers has now tackled this question by running a massive statistical analysis that combines our best knowledge of nuclear physics with the latest observations from space. They focused on three different theories about how dark matter might interact with the normal matter inside a neutron star. In the first scenario, dark matter particles talk to normal matter through the Higgs field, the same field that gives particles their mass. In the second, dark matter is created when neutrons inside the star decay into a dark particle and a light boson. In the third, a light particle called an axion-like particle acts as a messenger between the dark and normal worlds. The researchers did not just look at one theory at a time; they built a unified framework to test all three against the same set of real-world data. This data included measurements of the size and mass of four specific pulsars taken by the NICER space telescope, information from the collision of two neutron stars detected by gravitational wave observatories, and constraints from experiments with heavy atomic nuclei on Earth.
The study found that while the data is good at pinning down the behavior of the normal matter inside the star, it struggles to identify exactly which type of dark matter is present. The researchers were able to narrow down the properties of the nuclear matter, such as how stiff or soft it is under pressure, but the specific numbers describing the dark matter remained broad and uncertain. This is because the different dark matter theories can produce very similar results for the overall size and weight of the star. For example, all three models predicted that the heaviest possible neutron star would weigh about 2.1 times the mass of our Sun, and the stars would have radii that overlap significantly. The data could not clearly rule out any of the three scenarios, nor could it confirm that dark matter is definitely present. The researchers found that the presence of dark matter tends to make the star slightly more compact, but the effect is subtle enough that it can be mimicked by changes in the normal matter's properties.
However, the study did reveal a distinct difference in how the stars would vibrate if they were struck, a phenomenon known as asteroseismology. The researchers calculated the frequency of the star's fundamental vibration mode, which is like the pitch of a bell, and how quickly that sound would fade away. They found that stars containing dark matter from the Higgs-portal model would vibrate at a slightly higher pitch and fade faster than those containing dark matter from the other two models. While the difference is small and the ranges of possible values still overlap, this suggests that future gravitational wave detectors, which are sensitive enough to hear these vibrations, might eventually be able to tell the theories apart. Currently, the data is not precise enough to make that distinction, but the path forward is clear.
When the researchers compared the three single-fluid models against each other and against other theories where dark matter and normal matter form separate layers, they found a slight statistical preference for the neutron-decay scenario. This model was the most consistent with the combined data, followed closely by the axion-like particle model. The Higgs-portal model and the theories where the two types of matter stay separate were slightly less favored, though the differences were not large enough to be considered a definitive victory for any single theory. The study concludes that while we have made progress in understanding the limits of these models, we are not yet ready to declare a winner. The uncertainty in the behavior of normal nuclear matter is still the biggest hurdle. To solve the mystery of dark matter in neutron stars, scientists will need better measurements of the stars' sizes and masses, more precise data from nuclear experiments, and the ability to listen for the faint, high-frequency vibrations of these cosmic giants. Until then, the dark heart of the neutron star remains a compelling possibility, but one that the current data cannot yet fully reveal.
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