arXiv subjects

Keita Takizawa

Publications and source records attributed to Keita Takizawa.

6 recordsLinked to original sources

arXiv2023

Equivalence between definitions of the gravitational deflection angle of light for a stationary spacetime

The Gibbons-Werner-Ono-Ishihara-Asada method for gravitational lensing in a stationary spacetime has been recently reexamined [Huang and Cao, arXiv:2306.04145], in which the gravitational deflection angle of light based on the Gauss-Bonnet theorem can be rewritten as a line integral of two functions $H$ and $T$. The present paper proves that the Huang-Cao line integral definition and the Ono-Ishihara-Asada one [Phys. Rev. D 96, 104037 (2017)] are equivalent to each other, whatever asymptotic regions are. A remark is also made concerning the direction of a light ray in a practical use of these definitions.

gr-qc↗

arXiv2023

Gravitational lens on a static optical constant-curvature background: Its application to Weyl gravity model

This paper extends the de-Sitter/anti-de Sitter (dS/AdS) background method based on the optical metric for gravitational lens [Phys. Rev. D 105, 084022 (2022)] to a static optical constant-curvature (SOCC) background. It is shown that the exact lens equation on the SOCC background can be written in the same form as that for either Minkowski, dS or AdS background in terms of flat, spherical or hyperbolic trigonometry, depending on the Gaussian curvature of the equatorial plane in the SOCC background. To exemplify the SOCC method, we consider the gravitational lens in Mannheim-Kazanas (MK) solution of Weyl gravity, which includes Rindler and de Sitter terms. In the zero mass limit, the deflection angle of light for the MK solution in the literature diverges to infinity. This is because there is a self-contradiction in their perturbative approximations of the MK metric and the orbit equation. The SOCC method incorporates the long-distance curvature effect into the background. Thereby the SOCC expression for the deflection angle of light in the MK solution is finite also in the zero mass limit.

gr-qc↗

arXiv2022

Gravitational lens on de-Sitter background

Gravitational lenses are examined in de-Sitter (dS) background, for which the existence of the dS horizon is taken into account and hyperbolic trigonometry is used together with the hyperbolic angular diameter distance. Spherical trigonometry is used to discuss a gravitational lens in anti-de Sitter (AdS) background. The difference in the form among the dS/AdS lens equations and the exact lens equation in Minkowski background begins at the third order, when a small angle approximation is used in terms of lens and source planes. The angular separation of lensed images is decreased by the third-order deviation in the dS lens equation, while it is increased in AdS. In the present framework on the dS/AdS backgrounds, we discuss also the deflection angle of light, which does not include any term of purely the cosmological constant. Despite the different geometry, the deflection angle of light rays in hyperbolic and spherical geometry can take the same form. Through a coupling of the cosmological constant with lens mass, the separation angle of multiple images is larger (smaller) in dS (AdS) than in the flat case, for a given mass, source direction, and angular diameter distances among the lens, receiver and source.

gr-qc↗

arXiv2021

Iterative solutions for the gravitational lens equation in the strong deflection limit

Two exact lens equations have been recently shown to be equivalent to each other, being consistent with the gravitational deflection angle of light from a source to an observer, both of which can be within a finite distance from a lens object [Phys. Rev. D 102, 064060 (2020)]. We examine methods for iterative solutions of the gravitational lens equations in the strong deflection limit. It has been so far unclear whether a convergent series expansion can be provided by the gravitational lens approach based on the geometrical optics for obtaining approximate solutions in the strong deflection limit in terms of a small offset angle. By using the ratio of the lens mass to the lens distance, we discuss a slightly different method for iterative solutions and the behavior of the convergence. Finite distance effects begin at the third order in the iterative method. The iterative solutions in the strong deflection limit are estimated for Sgr $A^{*}$ and M87. These results suggest that only the linear order solution can be relevant with current observations, while the finite distance effects at the third order may be negligible in the Schwarzschild lens model for these astronomical objects.

gr-qc↗

arXiv2020

Gravitational lens without asymptotic flatness: Its application to the Weyl gravity

We discuss, without assuming asymptotic flatness, a gravitational lens for an observer and source that are within a finite distance from a lens object. The proposed lens equation is consistent with the deflection angle of light that is defined for nonasymptotic observer and source by Takizawa et al. [Phys. Rev. D 101, 104032 (2020)] based on the Gauss-Bonnet theorem with using the optical metric. This lens equation, though it is shown to be equivalent to the Bozza lens equation[Phys. Rev. D 78, 103005 (2008)], is linear in the deflection angle. Therefore, the proposed equation is more convenient for the purpose of doing an iterative analysis. As an explicit example of an asymptotically nonflat spacetime, we consider a static and spherically symmetric solution in Weyl conformal gravity, especially a case that $γ$ parameter in the Weyl gravity model is of the order of the inverse of the present Hubble radius. For this case, we examine iterative solutions for the finite-distance lens equation up to the third order. The effect of the Weyl gravity on the lensed image position begins at the third order and it is linear in the impact parameter of light. The deviation of the lensed image position from the general relativistic one is $\sim 10^{-2}$ microarcsecond for the lens and source with a separation angle of $\sim 1$ arcminute, where we consider a cluster of galaxies with $10^{14} M_{\odot}$ at $\sim 1$ Gpc for instance. The deviation becomes $\sim 10^{-1}$ microarcseconds, even if the separation angle is $\sim 10$ arcminutes. Therefore, effects of the Weyl gravity model are negligible in current and near-future observations of gravitational lensing. On the other hand, the general relativistic corrections at the third order $\sim 0.1$ milliarcseconds can be relevant with VLBI observations.

gr-qc↗

arXiv2020

Gravitational deflection angle of light: Definition by an observer and its application to an asymptotically nonflat spacetime

The gravitational deflection angle of light for an observer and source at finite distance from a lens object has been studied by Ishihara et al. [Phys. Rev. D, 94, 084015 (2016)], based on the Gauss-Bonnet theorem with using the optical metric. Their approach to finite-distance cases is limited within an asymptotically flat spacetime. By making several assumptions, we give an interpretation of their definition from the observer's viewpoint: The observer assumes the direction of a hypothetical light emission at the observer position and makes a comparison between the fiducial emission direction and the direction along the real light ray. The angle between the two directions at the observer location can be interpreted as the deflection angle by Ishihara et al. The present interpretation does not require the asymptotic flatness. Motivated by this, we avoid such asymptotic regions to discuss another integral form of the deflection angle of light. This form makes it clear that the proposed deflection angle can be used not only for asymptotically flat spacetimes but also for asymptotically nonflat ones. We examine the proposed deflection angle in two models for the latter case; Kottler (Schwarzschild-de Sitter) solution in general relativity and a spherical solution in Weyl conformal gravity. Effects of finite distance on the light deflection in Weyl conformal gravity result in an extra term in the deflection angle, which may be marginally observable in a certain parameter region. On the other hand, those in Kottler spacetime are beyond reach of the current technology.

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