arXiv subjects

Kjell Tangen

Publications and source records attributed to Kjell Tangen.

5 recordsLinked to original sources

arXiv2025

Observables for the Effect of Gravity on Electromagnetic Polarization

Does gravity affect the polarization of electromagnetic radiation in an observable way? The effect of gravity on the observed polarization of a ray of electromagnetic radiation is investigated for an arbitrary 4-dimensional spacetime and radiation with a frequency spectrum within the geometric optics limit and with arbitrary state of polarization. Focusing on effects observable by a single inertial observer, we show how the presence of curvature along the null geodesic of polarized electromagnetic radiation may induce observable changes in the state of polarization. We find a set of scalars that quantify the effect and derive their transport equations. Two of these scalars, the polarization degree and the circular polarization degree, are polarization state observables that are conserved along the radiation geodesic. Four observables that quantify time rate of change of the observed state of polarization are identified. These observables and their corresponding transport equations provide a complete representation of how gravity affects the observed state of polarization of electromagnetic radiation with frequencies above the geometric optics limit. Polarization wiggling is sourced by curvature twist, which is a scalar derived from the Riemann tensor. Curvature twist is closely related to the magnetic part of the Weyl tensor, the second Weyl scalar as well as the rotation of the rest frame geodesic congruence. The results of this paper are valid for any metric theory of gravity.

gr-qc↗

arXiv2024

Sensing Gravity with Polarized Electromagnetic Radiation

Polarization wiggling is an observational effect of a gravitational field on the polarization of electromagnetic radiation traversing it. We find that in linear gravity, the polarization wiggle rate contributions from scalar, vector and tensor perturbations are independent and gauge invariant. While vector and tensor perturbations do induce polarization wiggling, scalar perturbations do not. This poses two natural questions: Can polarized electromagnetic radiation be used to measure vectorial and tensorial components of gravitational fields directly? And if so, how? Polarization wiggling is studied for an arbitrary vector perturbation to the spacetime metric. In a stationary spacetime, the polarization wiggle rate is proportional to the difference in frame dragging rate around the direction of propagation between radiation emission and measurement events. We show how this can be used to measure the angular momentum of a gravitational source if the emitter orbits the gravitational source on a known orbit. Finally, the polarization wiggling effect induced by a gravitational tensor mode with arbitrary polarization is analyzed. The effect is demonstrated for two cases: A spacetime with a flat Minkowski background and an expanding cosmology with a conformally flat background. In both cases, the polarization wiggling frequency equals the frequency of the gravitational tensor mode, while the other state parameters of the gravitational tensor mode are encoded in the polarization wiggling amplitude and phase of the polarized radiation. We show that measurements of polarization wiggling frequency, amplitude and phase of polarized electromagnetic radiation emitted by multiple sources at known positions from different directions enables all state parameters of a gravitational tensor mode to be determined.

gr-qc↗

arXiv2010

CMB signature of a super-Hubble inhomogeneity in the gravitational field enclosing the present Hubble volume

Repeated studies of the CMB based on WMAP data have revealed an apparent asymmetry in the distribution of temperature fluctuations over the celestial sphere. The studies indicate that the amplitudes of temperature fluctuations are higher in one hemisphere than in the other. We consider whether this asymmetry could originate from a large scale inhomogeneity in the gravitational field enclosing the present Hubble volume. We examine what effect the presence of an inhomogeneity in the gravitational field of size larger than the present Hubble radius would have on the temperature distribution of the CMB and start eliciting its observational signature in the CMB power spectrum. The covariance function contains, in addition to the diagonal entries of the conventional CMB temperature anisostropy power spectrum, non-diagonal entries. We find that specific non-diagonal entries of the covariance function are sensitive to the strength of the inhomogeneity, while the diagonal entries are not. These non-diagonal entries, which are not present in the case of a homogeneous background geometry, are observational signatures of a large-scale inhomogeneity in the background geometry of the universe. Furthermore, we find that an inhomogeneity in the gravitational potential of super-Hubble size would yield a power asymmetry in the CMB with maximal asymmetry at an angle of 90 degrees to the CMB dipole axis. The axis of the CMB power asymmetry was recently estimated by Eriksen et. al. to be located at angles between 83 and 96 degrees to the CMB dipole axis, which is consistent with the prediction of our model. This implies that the location of the observed power asymmetry in the CMB sky could be accounted for by a large-scale inhomogeneity in the gravitational field enclosing the present Hubble volume.

astro-ph.CO↗

arXiv2007

Could the Pioneer anomaly have a gravitational origin?

If the Pioneer anomaly has a gravitational origin, it would, according to the equivalence principle, distort the motions of the planets in the Solar System. Since no anomalous motion of the planets has been detected, it is generally believed that the Pioneer anomaly can not originate from a gravitational source in the Solar System. However, this conclusion becomes less obvious when considering models that either imply modifications to gravity at long range or gravitational sources localized to the outer Solar System, given the uncertainty in the orbital parameters of the outer planets. Following the general assumption that the Pioneer spacecraft move geodesically in a spherically symmetric spacetime metric, we derive the metric disturbance that is needed in order to account for the Pioneer anomaly. We then analyze the residual effects on the astronomical observables of the three outer planets that would arise from this metric disturbance, given an arbitrary metric theory of gravity. Providing a method for comparing the computed residuals with actual residuals, our results imply that the presence of a perturbation to the gravitational field necessary to induce the Pioneer anomaly is in conflict with available data for the planets Uranus and Pluto, but not for Neptune. We therefore conclude that the motion of the Pioneer spacecraft must be non-geodesic. Since our results are model independent within the class of metric theories of gravity, they can be applied to rule out any model of the Pioneer anomaly that implies that the Pioneer spacecraft move geodesically in a perturbed spacetime metric, regardless of the origin of this metric disturbance.

gr-qc↗

arXiv2007

Generating Minimally Coupled Einstein-Scalar Field Solutions from Vacuum Solutions with Arbitrary Cosmological Constant

This paper generalizes two previously known techniques for generating minimally coupled Einstein-scalar field solutions in 4 dimensions; the Buchdahl and Fonarev transformations. By applying this solution generation technique, minimally coupled Einstein-scalar field solutions can be generated from vacuum solutions with arbitrary cosmological constant in arbitrary dimension. The only requirement to a seed solution is that it posesses a hypersurface-orthogonal Killing vector field. The generalization that allows us to use seed solutions with arbitrary cosmological constant uncovers a new class of Einstein-scalar field solutions that has previously not been studied. We apply the new solution transformation to the (A)dS4 vacuum solution. Transforming the resulting Einstein-scalar field solution to the conformal frame, a two-parameter family of spatially finite, expanding and accelerating cosmological solutions are found that are conformally isometric to the Einstein static universe RxS^3. We study null geodesics and find that for any observer, the solution has a cosmological horizon at an angular distance of pi/2 away from the observer. We find that a subset of these solutions can be naturally interpreted as expanding cosmologies in which a scalar black hole is formed at late times. The conformally coupled scalar field satisfies the weak energy condition as long as the energy density is positive, while the strong energy condition is generally violated.

gr-qc↗