Details & Abstract
In this Student Talk the speaker presented his research on the topic using a two-fluid TOV framework. The talk explored dark matter candidates, neutron star structure and the role of gravitational waves and tidal deformability in probing dark matter physics.
What Happened
The Fall Semester of 2025 began with the third version of Singularity’s Student Talk Series, where our own students illuminate their peers about the scientific work that they have been working on lately on their internships or research projects.
Eklavya Kukrety, a BS-MS student presented his work on Dark Matter Capture in Neutron Stars, which he did at IISER Bhopal, talking about the theoretical modeling of dark matter capture using a two-fluid Tolman–Oppenheimer–Volkoff (TOV) framework and its astrophysical consequences. The speaker discussed the methods used in the past century to validate the existence of dark matter, them being-
> Velocity Dispersion in Galaxy Clusters: Fritz Zwicky’s study of the Coma Cluster revealed that galaxy velocities were far higher than expected from visible mass alone. Application of the virial theorem indicated the presence of substantial unseen mass.
> Galactic Rotation Curves: According to Newtonian Formulations, the theoretically computed rotation curves of galaxies should have indicated reducing velocities with the distance from the galactic center, but observed rotation curves indicated that velocities remained flat at large radii, indicating a far-spread dark matter halo.
> Gravitational Lensing: Suppose there is a star we want to see. Let us call it our SOURCE star. But it is so far that the light we get from it is not strong enough. Now if there is a star just between us and our source star (we can call this the LENS star), then General Theory of Relativity states that the mass of the lens star acts like a mass-affecting lens causing the source star’s light to bend and converge within a small range, instead of just diverging away as it usually would have. This would enable the light coming from the distant source star to be much more detectable. The more massive the lens star (OR structure) is, the more pronounced is the lensing effect seen. Gravitational lensing measurements, particularly in systems like galaxy clusters, show bending of light that exceeds predictions based on visible matter alone. This directly traces the total mass distribution, including dark matter.
The speaker discussed potential candidates for dark matter that have been proposed till now, including Standard Model Neutrinos (initially considered due to weak interaction properties but ruled out because they are too light and relativistic), Supersymmetric Particles (SUSY: that predict stable, neutral particles such as neutralinos; widely studied but not experimentally confirmed), Axions (Hypothetical light particles arising from solutions to the strong CP problem; still undetected), and WIMPs (Weakly Interacting Massive Particles: leading candidates due to their natural consistency with relic abundance and weak-scale interactions). Baryonic candidates like MACHOs (Massive Astrophysical Compact Halo Objects) fail to substantiate the required mass density. Observations from microlensing surveys and cosmic microwave background measurements indicate that baryonic matter contributes only a small fraction of the total matter content.
The speaker then proceeded to explore the premise of neutron stars: compact stellar remnants with masses of ~1–2 solar masses and radii of ~10–20 km. They are supported by neutron degeneracy pressure and strong nuclear forces, and their extreme density and strong gravitational fields make them effective laboratories for dark matter studies:
> They can gravitationally capture dark matter particles.
> Accumulated dark matter can alter observable properties such as mass-radius relations and thermal evolution.
> They provide a controlled environment where dark matter interacts primarily through gravity.
The speaker laid the theoretical foundation of his research question with the formal description of the structure of neutron stars: The Tolman-Oppenheimer-Volkoff Equations that are derived from General Relativistic corrections of hydrostatic equilibrium (the condition in which the inward gravitational force is exactly balanced by the outward pressure gradient at every point inside the star) in spherically symmetric systems.
To model dark matter-admixed neutron stars (DANSs), a two-fluid approach was adopted in this pursuit:
> One fluid represents nuclear matter.
> The second fluid represents dark matter.
The two components interact only through gravity. Each fluid has its own equation of state (EoS), relating pressure and energy density. The coupled differential equations describe pressure gradients and mass distribution for both fluids under gravitational equilibrium. The total mass depends on the combined energy densities of both components.
Results:
The two-fluid TOV model reproduces standard single-fluid neutron star results when the dark matter fraction is set to zero, confirming consistency with standard model results.
Further, some points were noted by the speaker:
Mass-Radius Relations:
Increasing dark matter fraction leads to:
> Reduction in maximum stable mass.
> Modification of the mass-radius curve.
> Emergence of extended dark matter halos at higher fractions.
Internal Structure:
> Pressure and mass distribution profiles show:
> Formation of a dark matter core or halo depending on fraction.
> Redistribution of mass within the star.
> Distinct separation between nuclear and dark matter regions at higher fractions.
The study’s stability analysis indicates that increasing dark matter content reduces the maximum mass threshold for stable neutron stars, implying observable constraints on dark matter accumulation.
The speaker also discussed the effect that tidal deformability might have on the properties and structure of a neutron star.
> Tidal deformability quantifies how a neutron star deforms under an external gravitational field, such as in a binary system.
> It is directly linked to the internal structure and equation of state.
> It can be measured through gravitational wave signals from neutron star mergers.
> It provides an indirect probe of dense matter physics and dark matter effects.
> Both single-fluid and two-fluid formulations are considered, with the latter incorporating contributions from dark matter to the tidal response.
The speaker wrapped up by touching on the key takeaways that this study gave demonstrates:
> Dark matter capture in neutron stars produces measurable changes in astrophysical observables.
> Mass-radius relations and tidal deformability can serve as indirect probes of dark matter properties.
> Two-fluid modeling provides a consistent framework to study mixed matter systems under general relativity.
> Observations from gravitational wave astronomy offer a viable pathway to constrain dark matter models.
Speaker / Author
Eklavya Kukrety is a highly motivated and enthusiastic BS-MS student at IISER Kolkata, majoring in Physics with a strong academic record. He is passionate about the fundamental principles of quantum mechanics and the theory of relativity, with a keen interest in exploring intersecting fields such as Quantum Gravity and Quantum Field Theory.
Beyond academics, he is a dedicated team player and leader, holding the position of Secretary for the IISER Kolkata Table Tennis club and representing the institute in national inter-college sports meets.