Non-local hybrid metric-Palatini gravity generically contains ghosts, but a degenerate subclass with metric f(R) gravity plus Palatini non-local terms is ghost-free and can drive slow-roll inflation resembling Starobinsky inflation with a spectator field.
Alleviating $H_0$ tension in scalar-tensor and bi-scalar-tensor theories
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abstract
We investigate scalar-tensor and bi-scalar-tensor modified theories of gravity that can alleviate the $H_0$ tension. In the first class of theories we show that choosing particular models with shift-symmetric friction term we are able to alleviate the tension by obtaining smaller effective Newton's constant at intermediate times, a feature that cannot be easily obtained in modified gravity. In the second class of theories, which involve two extra propagating degrees of freedom, we show that the $H_0$ tension can be alleviated, and the mechanism behind it is the phantom behavior of the effective dark-energy equation-of-state parameter. Hence, scalar-tensor and bi-scalar-tensor theories have the capability of alleviating $H_0$ tension with both known sufficient late-time mechanisms.
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Inflation in non-local hybrid metric-Palatini gravity
Non-local hybrid metric-Palatini gravity generically contains ghosts, but a degenerate subclass with metric f(R) gravity plus Palatini non-local terms is ghost-free and can drive slow-roll inflation resembling Starobinsky inflation with a spectator field.