Neutron stars as dark matter probes

Compact stars, due to their enormous gravitational field, can accumulate a sizable amount of dark matter in their interior. Depending on its nature, accumulated dark matter may affect the properties of neutron stars in quite different ways. I will give an overview of the impact of dark matter on various observable properties of neutron stars, i.e., the mass-radius relation, tidal deformability, merger dynamics, gravitational waveform, thermal evolution, etc. For two scenarios, asymmetric fermionic and bosonic dark matter, the conditions under which dark matter particles tend to condense in the core of the star or create an extended halo will be presented.

I will show how dark matter condensed in a core tends to decrease the total gravitational mass and tidal deformability compared to a pure baryonic star, which appears as an effective softening of the equation of state. On the other hand, the presence of a dark matter halo has the opposite effect, causing an increase in those observable quantities. Thus, observational data on compact stars could be affected by accumulated dark matter and, consequently, constraints we put on the strongly interacting matter at high densities.

While neutron stars provide a compelling testing ground for gravity, nuclear physics, and physics beyond the Standard Model, the possible degeneracy between the effect of dark matter or gravity beyond GR and dense matter properties could lead to misleading conclusions while analyzing the observational data.

We will discuss how the joint efforts of multi-messenger observations of neutron stars, along with experimental and theoretical subatomic physics, are pivotal for breaking a possible degeneracy and shedding light on the neutron star internal composition. In addition, I will review the effect of dark matter on binary neutron star mergers and emitted gravitational wave signals. I will present the numerical-relativity simulations of compact stars admixed with the dark matter component and discuss how the present and next-generation gravitational wave telescopes could shed light on dark matter-admixed compact stars and constrain the dark matter properties.