Closed timelike curves in modified theories of gravity
We study closed timelike curves (CTCs) in three rotating spacetimes arising in modified gravity: the tidally charged braneworld black hole, the Kerr-Newman black hole in Rastall gravity, and the charged Johannsen-Psaltis spacetime. Using the sign of $g_{ϕϕ}$ on the equatorial plane, we show that an additional parameter of each geometry controls the chronology-violating region, and we obtain critical parameter values analytically. In the braneworld case, the positive $r$ CTC region exists only for positive tidal charge and disappears completely for $q\leq 0$. In Rastall gravity, the extent of the CTC region depends jointly on the Rastall coupling and the surrounding matter. For each matter type we find a finite range of the coupling for which the CTC region is absent. In the charged Johannsen-Psaltis spacetime, the deformation parameter can either increase or decrease $g_{ϕϕ}$ depending on the radial position, and we derive a window of its values for which the CTC region is removed from the radial domain considered. Thus, in all three examples, the additional parameters can be chosen to control the extent of equatorial CTCs, and even to eliminate them entirely.