Can modified gravity theories host time travel geometries?

Certain exotic rotating spacetimes — ones that technically permit closed timelike curves, or paths through spacetime that loop back in time — are exact solutions in standard general relativity but fail entirely when plugged into a modified gravity framework that adds a scalar field and its kinetic energy to the gravitational action. Testing two classic geometries (Petrov type-N and type-III spacetimes), the authors show the field equations become inconsistent. This suggests the modified theory is more restrictive about what causal structures it can support, which matters for assessing whether such theories are physically viable.