Hyperonic compact stars with vector portal dark matter

Deep inside the universe's most extreme environments, where gravity crushes matter into a state denser than any laboratory on Earth can create, a long-standing mystery has puzzled astrophysicists for decades. These environments are the cores of neutron stars, the collapsed remnants of massive stars that have exploded. In their centers, the pressure is so immense that the usual building blocks of matter, protons and neutrons, are thought to break down and transform into heavier, stranger cousins known as hyperons. For a long time, scientists believed that the appearance of these hyperons would act like a structural weakness, softening the star's internal pressure and causing it to collapse under its own weight before it could reach a mass of two times that of our Sun. Yet, telescopes have observed neutron stars that are exactly that heavy, creating a contradiction known as the "hyperon puzzle." How can these stars remain stable if their cores contain ingredients that should make them collapse?

A new study by researchers in India and Portugal offers a potential solution by looking beyond the visible universe. They propose that the dark matter, an invisible substance that makes up most of the universe's mass, might be accumulating inside these stars. Specifically, they investigated a scenario where this dark matter interacts with normal matter through a force-carrying particle, acting like a hidden spring that pushes back against gravity. By building a detailed computer model of a neutron star that includes both the strange hyperons and this specific type of dark matter, the researchers found that the invisible repulsive force from the dark sector could be strong enough to counteract the softening effect of the hyperons. Their calculations suggest that this hidden interaction allows the stars to support the massive weights observed in the sky, resolving the tension between theory and observation.

The researchers began their work by constructing a sophisticated model of the matter inside a neutron star. They treated the star's core not just as a soup of protons and neutrons, but as a system where these particles are made of smaller components called quarks. In their model, these quarks are confined within the particles by a specific type of force, and they interact with each other through fields that permeate the star. This approach allowed them to describe how the particles behave when squeezed to densities far beyond what exists in normal atomic nuclei. They then introduced the possibility that hyperons, which are heavier versions of protons and neutrons containing a strange quark, appear in the core once the pressure gets high enough. As expected, their model confirmed that adding these hyperons makes the star's internal pressure drop, which would normally limit the star's maximum mass to well below two solar masses.

To address this problem, the team introduced a second ingredient: fermionic dark matter. Unlike the dark matter that only interacts through gravity, this type of dark matter is proposed to interact with normal matter through a new, invisible force carried by a particle called a vector mediator. The researchers imagined a scenario where this dark matter accumulates in the star's core, forming a dense cloud alongside the hyperons. Crucially, the interaction between the dark matter and the normal matter is repulsive, meaning it pushes the particles apart rather than pulling them together. This repulsion generates an extra source of pressure that stiffens the star's internal structure. The team ran simulations with different amounts of this dark matter, represented by a parameter called the Fermi momentum, which essentially measures how densely packed the dark matter particles are.

The results of these simulations revealed a clear trend. When the researchers included a significant amount of this repulsive dark matter, the equation of state, which describes how the star's pressure changes with density, became much stiffer. This extra stiffness allowed the stars to support much higher masses without collapsing. In their simulations, a star with no dark matter and hyperons could only reach a maximum mass of about 1.96 times the mass of the Sun. However, as they increased the density of the dark matter, the maximum mass the star could support rose steadily. With the highest density of dark matter they tested, the star could support a mass of approximately 2.15 solar masses. This is a crucial finding because it brings the theoretical predictions in line with real-world observations of massive pulsars, such as PSR J0740 + 6620, which has a measured mass of about 2.08 solar masses.

Beyond just the mass, the study also looked at how these stars would behave if they were part of a binary system, where two stars orbit each other and eventually merge. When such stars spiral inward, they distort each other's shapes due to tidal forces, a phenomenon known as tidal deformability. The researchers calculated how the presence of dark matter and hyperons would change this deformation. They found that the dark matter interaction not only changed the maximum mass but also altered the size and internal structure of the stars in a way that affects how they respond to these tidal forces. Similarly, they calculated the moment of inertia, which describes how difficult it is to spin the star. They discovered that the dark matter changes the distribution of mass inside the star, leading to different rotational behaviors compared to stars without dark matter. These differences in size, shape, and spin provide unique signatures that could be detected by future gravitational-wave observatories.

The study also explored how the specific properties of the dark matter, such as the mass of the dark matter particle and the strength of its interaction, influence the star. The researchers used a relatively light dark matter particle and a light force-carrying mediator, a scenario that is motivated by other problems in cosmology regarding how galaxies form and evolve. They found that even with these specific parameters, the dark matter could have a profound effect on the star's structure. The repulsive force generated by the dark matter acts as a counterweight to the softening caused by the hyperons. Without this extra push, the hyperons would cause the star to collapse at lower masses. With it, the star remains stable at the high masses we see in the sky. This suggests that the "hyperon puzzle" might not be a failure of our understanding of nuclear physics, but rather a sign that we are missing a piece of the cosmic puzzle: the presence of dark matter inside these stellar remnants.

Ultimately, the work provides a concrete framework for testing the existence of dark matter in the cores of neutron stars. The researchers showed that if dark matter interacts with normal matter through this specific vector portal, it leaves a distinct imprint on the star's mass, radius, and tidal behavior. These imprints are not just theoretical curiosities; they are quantities that astronomers are actively measuring. By comparing the observed properties of massive neutron stars with the predictions of models that include this dark matter interaction, scientists can place strict limits on how dark matter behaves. If future observations of neutron star mergers reveal tidal deformabilities or masses that match the predictions of this model, it would provide strong evidence for the existence of this hidden sector of the universe. Conversely, if the observations continue to disagree with models that include dark matter, it would help rule out this specific type of interaction. The study thus bridges the gap between the microscopic world of particle physics and the macroscopic world of astrophysics, offering a new way to probe the nature of dark matter using the most extreme laboratories in the universe.