arXiv subjects

Bahram Mashhoon

Publications and source records attributed to Bahram Mashhoon.

At least 19 recordsLinked to original sources

arXiv2026

Generic Peculiar Motions in FLRW Spacetimes

In the standard Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetime, we consider a local cosmic test mass that is boosted in some direction relative to the standard comoving observers. The geodesic (Fermi) normal coordinate system established around the world line of the boosted cosmic mass is constructed within an approximation scheme and the resulting spacetime metric is compared with the corresponding metric of the Fermi system established around the world line of a comoving observer. The circular gravitomagnetic field around the direction of motion of the boosted cosmic mass is studied.

gr-qc↗

arXiv2026

Modification of Aberration due to the Helicity-Rotation Coupling

We review the physical basis for the assumption of locality in relativistic physics and its connection with the aberration of starlight. As a consequence of the hypothesis of locality, the standard relativistic formulas for the Doppler effect and aberration are independent of the polarization of the incident electromagnetic or gravitational radiation. The modification of these formulas due to the helicity-rotation coupling are discussed. In connection with the aberration of polarized radiation, we note that the helicity of radiation incident on a rotating observer couples to its angular velocity of rotation resulting in a slight helicity-dependent modification of the standard aberration formula. We discuss the physical origin of this effect and estimate the magnitude of the helicity aberration.

gr-qc↗

arXiv2025

Schwarzschild Spacetime and the Local Limit of Nonlocal Gravity

We investigate static spherically symmetric solutions within the framework of the local limit of nonlocal gravity. This theory departs from Einstein's general relativity (GR) through the introduction of a scalar gravitational susceptibility function $S(x)$, $1+S > 0$, which is a new feature of the spacetime that vanishes in the GR limit. It is shown that the Schwarzschild spacetime constitutes an exact solution of the modified source-free gravitational field equations provided $S$ is a constant. The mass of the corresponding Schwarzschild solution is given by $M/(1+S)$, where $M$ denotes the mass of the solution in the GR limit. The interpretation of the solution in terms of a black hole is ruled out due to the divergence of the algebraic invariants of the Weitzenb\"ock torsion at the Schwarzschild horizon.

gr-qc↗

arXiv2025

Cosmic Jets in General Relativity

In certain general relativistic time-dependent gravitational fields, free test particles can asymptotically line up relative to fiducial static observers and produce a cosmic jet whose speed approaches the speed of light. Two scenarios for the formation of these purely gravitational jets have thus far been theoretically demonstrated: the double-jet collapse scenario and the wave scenario that involves both a cosmic jet in the general direction of wave propagation as well as a counterjet in the opposite direction. These scenarios are briefly reviewed in this paper. Moreover, to elucidate the process of jet formation in these scenarios, we study the propagation of scalar fields in related gravitational fields in connection with the correspondence between the scalar wave perturbations and the motion of free test particles and null rays.

gr-qc↗

arXiv2025

Inertia

Inertia of a particle is due to its mass as well as intrinsic spin. The latter is revealed via the coupling of intrinsic spin with rotation. The spin-rotation coupling and the concomitant spin-gravity coupling are discussed in connection with the nature of inertia. The spin-rotation-gravity coupling leads to a gravitomagnetic Stern-Gerlach type of force on the particle that is independent of the particle's mass and thus violates the universality of free fall. This effect is extremely small and its measurement is beyond present capabilities.

gr-qc↗

arXiv2025

Elliptically Polarized Plane Gravitational Waves

Exact plane gravitational radiation fields are presented within the framework of general relativity and their properties are described. The physics of nonlinear elliptically polarized plane gravitational waves is developed in close analogy with electromagnetic waves. The motion of free test particles in the dynamic gravitational fields of elliptically polarized plane waves is investigated. In particular, we demonstrate the cosmic jet property of these spacetimes, namely, most timelike geodesics tend to line up in the direction of wave propagation and produce a cosmic jet whose speed asymptotically approaches the speed of light.

gr-qc↗

arXiv2024

Local Peculiar Motions in McVittie and LTB Spacetimes

We consider two inhomogeneous cosmological models, namely, the flat McVittie spacetime and a simple specific LTB spacetime. Relative to the world line of a reference comoving observer that remains spatially at rest, we study the local deviations of the world lines of free test particles. These local peculiar motions can be invariantly characterized within the framework of a quasi-inertial Fermi normal coordinate system established along the world line of the reference comoving observer. Tidal dynamics in the McVittie model involves the sum of the curvature due to the inhomogeneity, the curvature due to the background FLRW spacetime and a mixed term, while tidal dynamics in the particular LTB model turns out to be qualitatively the same as in the Einstein-de Sitter universe. Peculiar motions in the two cosmological models are briefly compared and contrasted.

gr-qc↗

arXiv2024

Spin-of-Light Gyroscope and the Spin-Rotation Coupling

We discuss the coupling of photon spin with rotation in connection with a recent proposal of Fedderke et al. [arXiv:2406.16178 [physics.optics]] regarding a precision gyroscope based on the intrinsic spin of light. To this end, we analyze the propagation of electromagnetic radiation in a physical system that uniformly rotates about the direction of wave propagation in the presence of an ambient medium. Finally, we consider the possibility of using this type of spin-of-light gyroscope to measure gravitomagnetic fields.

quant-ph↗

arXiv2024

McVittie-Plummer Spacetime: Plummer Sphere Immersed in the FLRW Universe

The McVittie-Plummer spacetime is a spherically symmetric inhomogeneous cosmological model that represents a spherical star system embedded in a standard FLRW cosmological model. We study the main physical properties of this gravitational field. Regarding the interplay between the physics of the local system and the expanding background, we employ the Misner-Sharp mass-energy function to show that there is a relatively weak time-dependent general relativistic coupling between the astrophysical system and the background FLRW cosmological model. The coupling term is proportional to the inverse of the scale factor and decreases as the universe expands.

gr-qc↗

arXiv2024

Local Limit of Nonlocal Gravity: A Teleparallel Extension of General Relativity

We describe a general constitutive framework for a teleparallel extension of the general theory of relativity. This approach goes beyond the teleparallel equivalent of general relativity (TEGR) by broadening the analogy with the electrodynamics of media. In particular, the main purpose of this paper is to investigate in detail a local constitutive extension of TEGR that is the local limit of nonlocal gravity (NLG). Within this framework, we study the modified FLRW cosmological models. Of these, the most cogent turns out to be the modified Cartesian flat model which is shown to be inconsistent with the existence of a positive cosmological constant. Moreover, dynamic dark energy and other components of the modified Cartesian flat model evolve differently with the expansion of the universe as compared to the standard flat cosmological model. The observational consequences of the modified Cartesian flat model are briefly explored and it is shown that the model is capable of resolving the H_0 tension.

gr-qc↗

arXiv2024

Rotational Doppler Effect and Spin-Rotation Coupling

The Rotational Doppler Effect (RDE) involves both the orbital angular momentum of electromagnetic radiation as well as its helicity. The RDE phenomena associated with photon helicity go beyond the standard theory of relativity. The purpose of this paper is to elucidate the theoretical basis of the helicity-dependent RDE in terms of the general phenomenon of spin-rotation coupling. The physical implications of this coupling are briefly pointed out and the nonlocal theory of rotating observers is described. For an observer rotating uniformly about the direction of incidence of a plane wave of definite helicity, the nonlocally measured amplitude of the wave is larger (smaller) if the observer rotates in the same (opposite) sense as the electromagnetic field. The implications of the nonlocal helicity dependence of the measured amplitude of the radiation are briefly discussed.

gr-qc↗

arXiv2024

Dynamic Dark Energy from the Local Limit of Nonlocal Gravity

Nonlocal gravity (NLG), a classical extension of Einstein's theory of gravitation, has been studied mainly in linearized form. In particular, nonlinearities have thus far prevented the treatment of cosmological models in NLG. In this essay, we discuss the local limit of NLG and apply this limit to the expanding homogenous and isotropic universe. The theory only allows spatially flat cosmological models; furthermore, de Sitter spacetime is forbidden. The components of the model will have different dynamics with respect to cosmic time as compared to the standard $Λ$CDM model; specifically, instead of the cosmological constant, the modified flat model of cosmology involves a dynamic dark energy component in order to account for the accelerated phase of the expansion of the universe.

gr-qc↗

arXiv2024

Local Limit of Nonlocal Gravity: Cosmological Perturbations

We explore the cosmological implications of the local limit of nonlocal gravity, which is a classical generalization of Einstein's theory of gravitation within the framework of teleparallelism. An appropriate solution of this theory is the modified Cartesian flat cosmological model. The main purpose of this paper is to study linear perturbations about the orthonormal tetrad frame field adapted to the standard comoving observers in this model. The observational viability of the perturbed model is examined using all available data regarding the cosmic microwave background. The implications of the linearly perturbed modified Cartesian flat model are examined and it is shown that the model is capable of alleviating the $H_0$ tension.

gr-qc↗

arXiv2023

Review of the Gravitomagnetic Clock Effect

The general relativistic gravitomagnetic clock effect, in its simplest form, consists of the non-vanishing difference in the orbital periods of two counter-orbiting objects moving in opposite directions along circular orbits lying in the equatorial plane of a central rotating source. We briefly review both the theoretical and observational aspects of such an intriguing consequence of Einstein's theory of gravitation.

gr-qc↗

arXiv2023

Anisotropic Cosmology in the Local Limit of Nonlocal Gravity

Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case and some special solutions are discussed in detail. This modified gravity theory contains a susceptibility function S(x) such that general relativity (GR) is recovered for S = 0. In the modified anisotropic cosmological models, we explore the contribution of S(t) and its temporal derivative to the local anisotropic cosmic acceleration. The implications of our results for observational cosmology are briefly discussed.

gr-qc↗

arXiv2023

Spin-Gravity Coupling in a Rotating Universe

The coupling of intrinsic spin with the nonlinear gravitomagnetic fields of Goedel-type spacetimes is studied. We work with Goedel-type universes in order to show that the main features of spin-gravity coupling are independent of causality problems of the Goedel universe. The connection between the spin-gravitomagnetic field coupling and Mathisson's spin-curvature force is demonstrated in the Goedel-type universe. That is, the gravitomagnetic Stern--Gerlach force due to the coupling of spin with the gravitomagnetic field reduces in the appropriate correspondence limit to the classical Mathisson spin-curvature force.

gr-qc↗

arXiv2022

Properties and Patterns of Polarized Gravitational Waves

We discuss polarization of gravitational radiation within the standard framework of linearized general relativity. The recent experimental discovery of gravitational waves provides the impetus to revisit the implications of the spin-rotation-gravity coupling for polarized gravitational radiation; therefore, we consider the coupling of helicity of gravitational waves to the rotation of an observer or the gravitomagnetic field of a rotating astronomical source. Observational possibilities regarding polarization-dependent effects in connection with future gravitational wave detectors are briefly explored.

gr-qc↗

arXiv2022

Nonlocal Gravity: Fundamental Tetrads and Constitutive Relations

Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein's theory of gravitation based on a certain analogy with the nonlocal electrodynamics of media. The history dependence enters NLG through a constitutive relation involving a causal kernel that should ultimately be determined via observational data. The purpose of this paper is to reformulate nonlocal gravity such that the nonlocal aspect of the constitutive relation directly connects measurable quantities as in the nonlocal electrodynamics of media. The resulting constitutive relation turns out to coincide with the recent suggestion of Puetzfeld, Obukhov, and Hehl [25]. With the new constitutive relation of NLG, it is possible to show that de Sitter spacetime is not a solution of NLG.

gr-qc↗