arXiv subjects

Rakshak Adhikari

Publications and source records attributed to Rakshak Adhikari.

7 recordsLinked to original sources

arXiv2026

Further Results on Null and Force-free Electromagnetic Fields

The theory of Force-Free Electrodynamics (FFE) provides a robust framework for modeling the magnetospheres of compact objects, where the electromagnetic field's energy density dominates the surrounding plasma. Central to this theory is the existence of two-dimensional integral submanifolds, or field sheets, which foliate the spacetime. While it is established that every null force-free field possesses an associated 2-D null geodesic foliation, the converse, identifying which null geodesic congruences can support a force-free solution, remains a non-trivial computational challenge. In this paper, we extend the foliation-based approach to null FFE by addressing two primary obstacles to the existence of a solution: the equipartition of null mean curvature and the involutivity of the field sheet distribution. We prove a general existence theorem demonstrating that for any given null geodesic congruence, there always exists a local rotation of a 2-D basis transverse to the geodesic congruence that satisfies the equipartition condition. Furthermore, we establish that a shear-free null geodesic congruence is sufficient to guaranty the existence of an arbitrary function of three variables such that any choice of such a function will generate a null field sheet foliation. Additionally, each unique foliation will be associated with a null force-free field that further contains an arbitrary function of two variables. These results are formally linked to the vanishing of the shear tensor, providing a coordinate-independent geometric criterion for the existence of null FFE solutions. We illustrate these theorems with explicit examples in Schwarzschild and Kerr geometries and present new, non-trivial exact null solutions in flat spacetime and for the C-metric.

gr-qc↗

arXiv2025

Force-free Electrodynamics in Arbitrary Geometries

The dynamics of highly magnetized plasmas in extreme astrophysical environments are effectively modeled by Force-Free Electrodynamics (FFE), a framework essential for studying objects like neutron stars and accreting black holes. The inherently nonlinear nature of the FFE equations makes finding exact solutions a challenging task. This paper explores an innovative approach to solving these equations by foliating spacetime into two-dimensional surfaces, specifically tailored to the geometry of electromagnetic fields. The foliation approach exploits the fact that the kernel of the force-free field defines an involutive distribution, naturally lending itself to a geometric decomposition. This method has previously been applied with great success in Kerr and FLRW spacetimes. By extending this formalism, we develop new exact solutions to the FFE equations in several distinct spacetimes, including cases where the metric remains partially undetermined. Additionally, we present a novel class of solutions that smoothly transition between magnetically dominated and electrically dominated regimes over time. Finally, we construct a pair of vacuum degenerate fields in arbitrary axisymmetric spacetimes, further demonstrating the utility of the foliation approach. These results offer a new perspective on the dynamical evolution of electromagnetic fields and their geometric underpinnings in various spacetime geometries, broadening the applicability of the framework for understanding the dynamics of highly magnetized plasmas in general relativistic astrophysical contexts.

gr-qc↗

arXiv2025

Structure Formation under Inelastic Two-Component Dark Matter: Halo Statistics and Matter Power Spectra in the High-$z$ Universe

We present hydrodynamic simulations of a flavour-mixed two-component dark matter (2cDM) model that utilize IllustrisTNG baryonic physics. The model parameters are explored for two sets of power laws of the velocity-dependent cross sections, favoured on the basis of previous studies. The model is shown to suppress the formation of structures at scales $k\gtrsim 10^2\ h\text{ Mpc}^{-1}$ up to 40\% compared to cold dark matter (CDM) at redshifts $z\sim5-2$. We compare our results to structure enhancement and suppression due to cosmological and astrophysical parameters presented in the literature and find that 2cDM effects remain relevant at galactic and subgalactic scales. The results indicate the robustness of the role of nongravitational dark matter interactions in structure formation and the absence of putative degeneracies introduced by baryonic feedback at high $z$. The predictions made can be further tested with future Ly-$α$ forest observations.

astro-ph.CO↗

arXiv2024

Endothermic self-interacting dark matter in Milky Way-like dark matter haloes

Self-interacting dark matter (SIDM) offers the potential to mitigate some of the discrepancies between simulated cold dark matter (CDM) and observed galactic properties. We introduce a physically motivated SIDM model to understand the effects of self interactions on the properties of Milky Way and dwarf galaxy sized haloes. This model consists of dark matter with a nearly degenerate excited state, which allows for both elastic and inelastic scattering. In particular, the model includes a significant probability for particles to up-scatter from the ground state to the excited state. We simulate a suite of zoom-in Milky Way-sized N-body haloes with six models with different scattering cross sections to study the effects of up-scattering in SIDM models. We find that the up-scattering reaction greatly increases the central densities of the main halo through the loss of kinetic energy. However, the physical model still results in significant coring due to the presence of elastic scattering and down-scattering. These effects are not as apparent in the subhalo population compared to the main halo, but the number of subhaloes is reduced compared to CDM.

astro-ph.GA↗

arXiv2024

Force-free Electromagnetic Configurations in FLRW Geometry

Force-free electrodynamics is the theoretical paradigm used to describe electromagnetic fields in a region where the inertia of plasma is negligible compared to the strength of the electromagnetic field. While these fields are studied extensively around accreting black holes in an attempt to describe energy extraction, force-free fields also routinely appear in the study of cosmological magnetic fields. Despite this, there are no systematic studies of exact force-free fields in an expanding universe. In this paper, we use geometric methods to find a wide variety of force-free solutions in a fixed Friedmann-Lemaitre-Robertson-Walker metric background. The method we use can be directly generalized to any arbitrary electrically neutral space-time, and hence, provides a powerful tool to study force-free fields in general.

gr-qc↗

arXiv2023

Non-null and Force-Free Electromagnetic Configurations in Kerr Geometry

Force-Free Electrodynamics (FFE) is known to describe the highly magnetized plasma around pulsars and astrophysical black holes. The equations describing FFE are highly non-linear and the task of finding a well-defined analytical solution has been unyielding. However, FFE can also be understood in terms of 2-dimensional foliations of the ambient 4-dimensional spacetime. The study of the foliations can provide significant insights into the structure of the force-free fields and such foliations can be exploited to generate new null and non-null solutions. In this paper, we present several nonnull solutions to the FFE equations in the Kerr spacetime obtained by applying the aforementioned methods of non-null foliations.

gr-qc↗

arXiv2023

Inferring Warm Dark Matter Masses with Deep Learning

We present a new suite of over 1,500 cosmological N-body simulations with varied Warm Dark Matter (WDM) models ranging from 2.5 to 30 keV. We use these simulations to train Convolutional Neural Networks (CNNs) to infer WDM particle masses from images of DM field data. Our fiducial setup can make accurate predictions of the WDM particle mass up to 7.5 keV at a 95% confidence level from small maps that cover an area of (25 h$^{-1}$ Mpc)$^2$. We vary the image resolution, simulation resolution, redshift, and cosmology of our fiducial setup to better understand how our model is making predictions. Using these variations, we find that our models are most dependent on simulation resolution, minimally dependent on image resolution, not systematically dependent on redshift, and robust to varied cosmologies. We also find that an important feature to distinguish between WDM models is present with a linear size between 100 and 200 h$^{-1}$ kpc. We compare our fiducial model to one trained on the power spectrum alone and find that our field-level model can make 2x more precise predictions and can make accurate predictions to 2x as massive WDM particle masses when used on the same data. Overall, we find that the field-level data can be used to accurately differentiate between WDM models and contain more information than is captured by the power spectrum. This technique can be extended to more complex DM models and opens up new opportunities to explore alternative DM models in a cosmological environment.

astro-ph.CO↗