Weak Gravitational Lensing in Fourth Order Gravity

TL;DR

This paper investigates gravitational lensing within a broad class of fourth order gravity theories, revealing that despite modifications to the gravitational potential, photon trajectories and lensing phenomena remain consistent with General Relativity.

Contribution

It demonstrates that in a general class of $f(R,R_{\alpha\beta}R^{\alpha\beta})$ gravity theories, lensing effects are equivalent to those in General Relativity despite additional Yukawa potential terms.

Findings

Yukawa terms modify the gravitational potential but not photon trajectories.

Deflection angles in these theories are smaller than in General Relativity.

Lensing phenomena remain identical to those predicted by General Relativity.

Abstract

For a general class of analytic $f(R,R_{\alpha\beta}R^{\alpha\beta},R_{\alpha\beta\gamma\delta}R^{\alpha\beta\gamma\delta})$ we discuss the gravitational lensing in the Newtonian Limit of theory. From the properties of Gauss Bonnet invariant it is successful to consider only two curvature invariants between the Ricci and Riemann tensor. Then we analyze the dynamics of photon embedded in a gravitational field of a generic $f(R,R_{\alpha\beta}R^{\alpha\beta})$-Gravity. The metric is time independent and spherically symmetric. The metric potentials are Schwarzschild-like, but there are two additional Yukawa terms linked to derivatives of $f$ with respect to two curvature invariants. Considering the case of a point-like lens, and after of a generic matter distribution of lens, we study the deflection angle and the images angular position. Though the additional Yukawa terms in the gravitational potential modifies dynamics with respect to General Relativity, the geodesic trajectory of photon is unaffected by the modification in the action by only $f(R)$. While we find different results (deflection angle smaller than one of General Relativity) only thank to introduction of a generic function of Ricci tensor square. Finally we can affirm the lensing phenomena for all $f(R)$-Gravities are equal to the ones known from General Relativity. We conclude the paper showing and comparing the deflection angle and image positions for $f(R,R_{\alpha\beta}R^{\alpha\beta})$-Gravity with respect to ones of General Relativity.