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Influence of Dark Matter on Light Propagation in Solar System

We investigated the influence of dark matter on light propagation in the solar system. We assumed the spherical symmetry of spacetime and derived the approximate solution of the Einstein equation, which consists of the gravitational attractions caused by the central celestial body, i.e. the Sun, and the dark matter surrounding it. We expressed the dark matter density in the solar system in the following simple power-law form, $\varrho(t, r) = ρ(t)(\ell/r)^k$, where $t$ is the coordinate time; $r$, the radius from the central body; $\ell$, the normalizing factor; $k$, the exponent characterizing $r$-dependence of dark matter density; and $ρ(t)$, the arbitrary function of time $t$. On the basis of the derived approximate solution, we focused on light propagation and obtained the additional corrections of the gravitational time delay and the relative frequency shift caused by the dark matter. As an application of our results, we considered the secular increase in the astronomical unit reported by Krasinsky and Brumberg (2004) and found that it was difficult to provide an explanation for the observed $d{\rm AU}/dt = 15 \pm 4 ~[{\rm m/century}]$.