arXiv subjects

Delaram Mirfendereski

Publications and source records attributed to Delaram Mirfendereski.

7 recordsLinked to original sources

arXiv2026

Scalar-Field Wave Dynamics and Quasinormal Modes of the Teo Rotating Wormhole

We investigate scalar field perturbations of the rotating Teo wormhole. We also compute the quasinormal mode (QNM) spectrum using first order WKB approximation. After separation of variables, we obtain a Schroedinger type radial equation and a smooth barrier potential which is shaped by the localized frame-dragging effects of the wormhole throat. This barrier potential provides damped oscillatory modes for the range of spins that were examined. The QNM spectrum shows a coherent and monotonic dependence on rotation. As the spin increases, both the oscillating frequency of the scalar wave and its damping rate decrease, which indicates progressively longer lived modes in the absence of absorption due to a horizon. We have verified the correspondence in the Eikonal limit, by obtaining the radius of the photon ring, its orbital frequency, and the Lyaponov exponent. Next, we compared the Teo wormhole QNM with that of the Kerr black hole QNM to find that the Kerr QNM is dictated by absorption at the horizon and they also exhibit symmetric pro-grade retrograde mode splitting, whereas the Teo wormhole QNM shows a stronger, and spatially confined response to spin. The Teo wormhole also exhibit partial reflection at the throat and a very distinct one-sided mode splitting which rapidly saturates as the spin increases. Additionally, the rotating Teo wormhole allows an ergoregion with the possibility of frequency kinematics compatible with superradiance. Due to the absence of an event horizon or a dissipative boundary, there is no evidence of classical superradiant amplification that was seen in Kerr. The results we obtained clearly demonstrates how rotation and boundary conditions jointly shape wave propagation in horizonless compact objects. They also provide certain characteristic spectral signatures that can be used to distinguish rotating wormhole spacetimes from rotating black hole spacetimes.

gr-qc↗

arXiv2026

Gauge-Invariant Gravitational Wave Polarization in Metric f(R) Gravity with Cosmological Implications

We develop a fully gauge invariant analysis of gravitational wave polarizations in metric f(R) gravity with a particular focus on the modified Starobinsky model, whose constant curvature solution provides a natural deSitter background for both early and late time cosmology. Linearizing the field equations around this background, we derive the Klein Gordon equation for the curvature perturbation and show that the scalar propagating mode acquires a mass, highlighting how the same scalar degree of freedom governs inflationary dynamics at high curvature and the propagation of gravitational waves in the current accelerating Universe. Using the scalar vector tensor (SVT) decomposition and a decomposition of the perturbed Ricci tensor, we obtain a set of fully gauge invariant propagation equations that isolate the contributions of the scalar, vector, and tensor modes in the presence of matter. We find that the tensor sector retains the two transverse traceless polarizations of General Relativity, while the scalar sector supports a massive breathing-longitudinal mode determined by the massive scalar propagating mode. Through the geodesic deviation equation, computed both in a local Minkowski patch and in fully covariant de Sitter form, we independently recover the same polarization content and identify its tidal signatures. The resulting framework connects the extra scalar polarization to cosmological observables, providing a unified, gauge invariant link between gravitational wave phenomenology and the cosmological implications of metric f(R) gravity.

gr-qc↗

arXiv2024

A Review of Stable, Traversable Wormholes in f (R) Gravity Theories

It has been proven that in standard Einstein gravity, exotic matter is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in any quantum field. Even reasonable classical scalar fields violate the energy conditions. In the case of the Casimir effect and squeezed vacuum states, these violations have been experimentally proven. It is advantageous to investigate methods to minimize the use of exotic matter. One such area of interest is extended theories of Einstein gravity. It has been claimed that in some extended theories, stable traversable wormholes solutions can be found without the use of exotic matter. There are many extended theories of gravity, and in this review paper, we first explore modified gravity theories and then explore some wormhole solutions in such theories, including Lovelock gravity and Einstein Dilaton Gauss Bonnet gravity. For completeness, we have also reviewed other wormholes such as Casimir wormholes, dark matter halo wormholes, thin-shell wormholes, and Nonlocal Gravity wormholes, where alternative techniques are used to either avoid or reduce the amount of exotic matter that is required.

gr-qc↗

arXiv2022

The geometry of gauged (super)conformal mechanics

Motivated by recently explored examples, we undertake a systematic study of conformal invariance in one-dimensional sigma models where an isometry group has been gauged. Perhaps surprisingly, we uncover classes of sigma models which are only scale invariant in their ungauged form and become fully conformally invariant only after gauging. In these cases the target space of the gauged sigma model satisfies a deformation of the well-known conformal geometry constraints. We consider bosonic models as well as their $\mathcal{N} = 1,2,4$ supersymmetric extensions. We solve the quantum ordering ambiguities in implementing (super-) conformal symmetry on the physical Hilbert space. Examples of our general results are furnished by the $D(2,1;0)$-invariant Coulomb branch quiver models relevant for black hole physics.

hep-th↗

arXiv2021

Lectures on the Swampland Program in String Compactifications

The Swampland program aims to determine the constraints that an effective field theory must satisfy to be consistent with a UV embedding in a quantum gravity theory. Different proposals have been formulated in the form of Swampland conjectures. In these lecture notes, we provide a pedagogical introduction to the most important Swampland conjectures, their connections and their realization in string theory compactifications. The notes are based on the series of lectures given by Irene Valenzuela at the online QFT and Geometry summer school in July 2020.

hep-th↗

arXiv2020

Superconformal mechanics of AdS$_2$ D-brane boundstates

We explicitly construct a family of ${\cal N}=4$ superconformal mechanics of dyonic particles, generalizing the work of Anninos et al.\,\cite{Anninos:2013nra} to an arbitrary number of particles. These mechanics are obtained from a scaling limit of the effective Coulomb branch description of ${\mathcal N}=4$ quiver quantum mechanics describing D-branes in type II Calabi-Yau compactifications. In the supergravity description of these D-branes this limit changes the asymptotics to AdS$_2\times $S$^2\times$CY$_3$. We exhibit the $D(1,2;0)$ superconformal symmetry and conserved charges of the mechanics in detail. In addition we present an alternative formulation as a sigma model on a hyperkähler manifold with torsion.

hep-th↗

arXiv2018

A twist on multicenter AdS$_2$ solutions

The multicenter solutions of 4d ${\cal N}=2$ supergravity contain a subset of scaling solutions with vanishing total angular momentum. In a near limit those solutions are asymptotically locally AdS$_2\times$ S$^2$, but we show that a higher moment of angular momentum contributes a subtle twist, rotating the S$^2$ with time. This provides some potential hair distinguishing the asymptotics of these scaling solutions from the near horizon geometry of an extremal BPS black hole.

hep-th↗