arXiv subjects

Siddarth Ajith

Publications and source records attributed to Siddarth Ajith.

10 recordsLinked to original sources

arXiv2026

New Universal Relations for Magnetized Neutron Stars

Unlike the neutron-star mass--radius relation, which depends sensitively on the internal stellar structure through the equation of state of dense nuclear matter, certain neutron-star properties obey approximately universal relations that exhibit only weak dependence on the equation of state. In this paper, we explore new universal relations for magnetized neutron stars. We focus on a purely poloidal dipolar magnetic-field configuration and treat the effects of the magnetic field perturbatively to second order in the field strength. After appropriately normalizing the relevant quantities by the stellar mass and the magnetic-field strength at the pole, we first identify an exact universal relation between the magnetic dipole moment and stellar compactness. We then find approximate universal relations among the magnetic dipole moment, magnetically-induced stellar quadrupole moment, and ellipticity, with fractional variations due to the equation of state at the level of O(10%). We support these numerical findings with analytic estimates for Newtonian polytropes, which reproduce the observed behavior. Among the newly discovered relations, the one connecting the magnetic dipole moment and stellar quadrupole moment may be particularly useful for the analysis of gravitational-wave signals from binaries containing magnetized neutron stars.

gr-qc↗

arXiv2026

Higher-Derivative Corrections to Reissner--Nordstr\"om Black Holes from Worldline QFT

In this paper we derived the corrections to the Reissner-Nordstr\"om black hole when higher-derivative $RF^2$ terms (contractions of the Riemann tensor with the Maxwell field strength squared) are added to the Einstein-Maxwell action. Such terms arise naturally in the context of effective field theories. We used wordline QFT methods to obtain the leading order post-Minkowskian corrections. We verified these results by solving the modified Einstein-Maxwell field equations in closed form, to all orders in Newton's constant $G$. We discussed the first law and computed the entropy of the perturbed black holes. When fixing the mass to that of the extremal Reissner-Nordstr\"om black hole, the entropy shift is positive if the weak gravity conjecture holds. In fact, this condition is nothing but the requirement that the spacetime with the extremal Reissner-Nordstr\"om mass possesses an outer horizon in higher-derivative gravity. This condition also rules out Drummond-Hathrell theory.

hep-th↗

arXiv2026

Spacetime Quasi-normal Mode Oscillations of Anisotropic Neutron Stars

Neutron star asteroseismology offers a unique opportunity to probe nuclear physics through stellar oscillations. Although the pressure inside of a neutron star is typically assumed to be isotropic, pressure anisotropy can arise from various physical mechanisms, including elasticity, viscosity, and magnetic fields. Previous studies of nonradial stellar quasi-normal mode oscillations with anisotropic pressure have focused primarily on fluid modes. In this paper, we compute, for the first time, spacetime oscillation modes (so-called w-modes) of anisotropic neutron stars. Using a perturbative framework for stellar oscillations with pressure anisotropy, developed previously by some of the authors, together with a phenomenological anisotropy model, we find that both the real and imaginary parts of the w-mode frequencies decrease as the tangential pressure becomes dominant over the radial pressure. Although we do not find any unstable w-modes within the physically viable parameter space, unstable w-modes appear in an unphysical branch of solutions when the tangential pressure strongly dominates the radial one. We also find that the relation between the real part of the w-mode frequency and the stellar compactness is quasi-universal with respect to variations in the equation of state and the degree of pressure anisotropy. In contrast, the relation between the imaginary part of the w-mode frequency and the stellar compactness depends on the degree of anisotropy, but remains equation-of-state universal when the anisotropy is fixed. Finally, we discuss potential mode crossings and the validity of certain approximations that have been shown to work well for w-mode calculations in the isotropic case.

gr-qc↗

arXiv2025

Gravitational-Wave Constraints on Neutron-Star Pressure Anisotropy via Universal Relations

Neutron stars may exhibit pressure anisotropy arising from various physical mechanisms, such as elasticity, magnetic fields, viscosity, and superfluidity. We compute the tidal deformability and the $f$-mode oscillation frequency of anisotropic neutron stars using a phenomenological quasi-local model characterized by a single dimensionless anisotropy parameter. We find that while the relation between the tidal deformability and the $f$-mode frequency depends on the degree of anisotropy, it remains largely insensitive to variations in the equation of state (the relation between radial pressure and energy density) for a fixed anisotropy parameter, similar to the isotropic case. Leveraging this anisotropy-dependent universal relation within a statistical framework, we place constraints on the anisotropy parameter using both the gravitational wave observation of GW170817 and simulated data for a GW170817-like event observed by a future network of detectors. We find that the anisotropy parameter can be constrained to order unity with current data, and the bounds remain comparable with future detector sensitivities. Importantly, these constraints are only weakly affected by uncertainties in the neutron-star equation of state.

astro-ph.HE↗

arXiv2024

Worldline Formalism, Eikonal Expansion and the Classical Limit of Scattering Amplitudes

We revisit the fundamentals of two different methods for calculating classical observables: the eikonal method, which is a scattering amplitude-based method, and the worldline quantum field theory (WQFT) method. The latter has been considered an extension of the worldline effective field theory. We show that the eikonal and WQFT methods are equivalent and that calculations can be translated freely between them. Concretely, we focus on 2-into-2 scattering processes mediated by massless force carriers. On the one hand, taking the classical limit of the QFT scattering amplitude leads to the eikonal method. On the other hand, since in the classical limit the scattering particles are almost on-shell throughout the scattering process, the worldline, a first quantized formalism, is the most efficient framework to study the scattering amplitude. This is an alternate but equivalent formalism to the quantum field theoretic (QFT) framework. By taking the classical limit of the scattering amplitude computed in the worldline, we can derive the WQFT rules of Mogull, Plefka and Steinhoff. In WQFT, the Feynman diagrams are reorganized into a new set of diagrams that facilitate the $\hbar$ expansion. Unlike the QFT eikonal method, which works recursively in identifying the eikonal phase, the worldline-based computation allows to target and systematically extract the classical contributions directly through a specific set of WQFT diagrams. In worldline formalism the perturbative expansion of the scattering amplitude is naturally organized in diagrams which factorize (reducible) and diagrams which are new to that order (irreducible), in a one-to-one map with the structure of the amplitude in the eikonal method. This opened up the possibility to investigate and prove the conjectured exponentiation of the eikonal phase in arXiv: 2409.12895.

hep-th↗

arXiv2024

Worldline Proof of Eikonal Exponentiation

In this paper, working in the eikonal approximation, we present a proof for the exponentiation of the 2-body eikonal phase to {\it all orders in the eikonal expansion}, for scalar particles interacting electromagnetically or gravitationally. The proof is based on the worldline formalism, which is an alternative, first quantized method to the standard QFT calculation of the scattering amplitude. We show that in the worldline formalism the 2-body scattering amplitude written in impact parameter space naturally factorizes at each loop order. This factorization is responsible for the exponentiation of the eikonal phase, a result which was anticipated in the work of Mogull, Plefka, and Steinhoff [2010.02865 [hep-th]].

hep-th↗

arXiv2024

Nonradial instabilities in anisotropic neutron stars

Non-radial oscillation modes of a neutron star possess valuable information about its internal structure and nuclear physics. Starting from the quadrupolar order, such modes under general relativity are known as quasi-normal modes since they dissipate energy through gravitational radiation and their frequencies are complex. The stability of these modes is governed by the sign of the imaginary part of the frequency, which determines whether the mode would decay or grow over time. In this Letter, we develop a fully consistent framework in general relativity to study quasi-normal modes of neutron stars with anisotropic pressure, whose motivation includes strong internal magnetic fields and non-vanishing shear or viscosity. We employ parametrized models for the anisotropy and solve the perturbed Einstein field equations numerically. We find that, unlike the case for isotropic neutron stars, the imaginary parts of some of the pressure ($p$-)modes flip signs as the degree of anisotropy deviates from zero, depicting a transition from stable modes to unstable modes. This finding indicates that some anisotropic neutron star models are unstable, potentially restricting the form of sustained anisotropy.

gr-qc↗

arXiv2023

I-Love-Q in Einstein-aether Theory

Although Lorentz symmetry is a staple of General Relativity (GR), there are several reasons to believe it may not hold in a more advanced theory of gravity, such as quantum gravity. Einstein-aether theory is a modified theory of gravity that breaks Lorentz symmetry by introducing a dynamical vector field called the aether. The theory has four coupling constants that characterize deviations from GR and that must be determined through observations. Although three of the four parameters have been constrained by various empirical observations and stability requirements, one, called $c_ω$, remains essentially unconstrained. The aim of this work is to see if a constraint on $c_ω$ can be derived from the I-Love-Q universal relations for neutron stars, which connect the neutron star moment of inertia (I), the tidal Love number (Love), and the quadrupole moment (Q) in a way that is insensitive to uncertainties in the neutron star equation-of-state. To understand if the theory can be constrained through such relations, we model slowly-rotating or weakly tidally-deformed neutron stars in Einstein-aether theory, derive their I-Love-Q relations, and study how they depend on $c_ω$. We find that the I-Love-Q relations in Einstein-aether theory are insensitive to $c_ω$ and that they are close to the relations in GR. This means that the I-Love-Q relations in Einstein-aether theory remain universal but cannot be used to constrain the theory. These results indicate that to constrain the theory with neutron stars, it is necessary to investigate relations involving other observables.

gr-qc↗

arXiv2022

I-Love-Q in Hořava-Lifshitz Gravity

Hořava-Lifshitz gravity is an alternative theory to general relativity which breaks Lorentz invariance in order to achieve an ultraviolet complete and power-counting renormalizable theory of gravity. In the low energy limit, Hořava-Lifshitz gravity coincides with a vector-tensor theory known as khronometric gravity. The deviation of khronometric gravity from general relativity can be parametrized by three coupling constants: $α$, $β$, and $λ$. Solar system experiments and gravitational wave observations impose stringent bounds on $α$ and $β$, while $λ$ is still relatively unconstrained ($λ\lesssim 0.01$). In this paper, we study whether one can constrain this remaining parameter with neutron star observations through the universal I-Love-Q relations between the moment of inertia (I), the tidal Love number (Love), and the quadrupole moment (Q), which are insensitive to details in the nuclear matter equation of state. To do so, we perturbatively construct slowly-rotating and weakly tidally-deformed neutron stars in khronometric gravity. We find that the I-Love-Q relations are independent of $λ$ in the limit $(α,β) \to 0$. Although some components of the field equations depend on $λ$, we show through induction and a post-Minkowskian analysis that slowly-rotating neutron stars do not depend on $λ$ at all. Tidally deformed neutron stars, on the other hand, are modified in khronometric gravity (though the usual Love number is not modified, as mentioned earlier), and there are potentially new, non-GR Love numbers, though their observability is unclear. These findings indicate that it may be difficult to constrain $λ$ with rotating/tidally-deformed neutron stars.

gr-qc↗

arXiv2020

Rotating black holes in valid vector-tensor theories after GW170817

Vector-tensor theories beyond General Relativity have widely been studied in the context of ultraviolet completion of gravity, endowing a mass to the graviton and explaining dark energy phenomena. We here construct rotating black hole solutions in vector-tensor theories valid after the binary neutron star merger event GW170817 that placed very stringent bound on the propagation speed of gravitational waves away from the speed of light. Such valid vector-tensor theories are constructed by performing a generic conformal transformation to Einstein-Maxwell theory, and the new rotating black hole solutions are constructed by applying the same conformal transformation to the Kerr-Newman solution. These theories fall outside of beyond generalized Proca theories but are within an extended class of vector-tensor theories that satisfy a degenerate condition to eliminate instability modes and are thus healthy. We find that such conformal Kerr-Newman solutions preserve the location of the singularities, event horizons and ergoregion boundary from Kerr-Newman, as well as the multipole moments and the Petrov type. On the other hand, the Hamilton-Jacobi equation is no longer separable, suggesting that the Carter-like constant does not exist in this solution. The standard Newman-Janis algorithm also does not work to construct the new solutions. We also compute the epicyclic frequencies, the location of the innermost stable circular orbits, and the Schwarzschild precession and apply the latter to the recent GRAVITY measurement to place bounds on the deviations away from Kerr-Newman for Sgr A$^*$.

gr-qc↗