Disformal gravity theories: a Jordan frame analysis

The Jordan frame action for general disformal theories is presented and studied for the first time, motivated by several unresolved mysteries that arise when working in the Einstein frame. We present the Friedmann equations and, specializing to exponential functions, study the late-time cosmology using both dynamical systems methods and by finding approximate solutions. Our analysis reveals that either the disformal effects are irrelevant or the Universe evolves towards a phantom phase where the equation of state of dark energy is -3. There is a marginal case where the asymptotic state of the Universe depends on the model parameters and de Sitter solutions can be obtained. Our findings indicate that the metric singularity found using the Einstein frame construction corresponds to phantom behavior in the Jordan frame and we argue that this is the case for general disformal theories.