Conformal and kinetic couplings as two Jordan frames of the same theory

Abstract

Non-minimal scalar-tensor (ST) theories may admit an Einstein frame representation, where gravity is described by the Einstein-Hilbert action plus the scalar sector. Some STs become just {\em minimal} Einstein-scalar (MES) theories, notable examples are Brans-Dicke and $R\phi^2$. ST theories with {\em derivative} coupling can also be reduced to an Einstein frame by disformal transformations, but, as a rule, their scalar sector will contain higher derivative terms. Here we draw attention to a new Palatini kinetically coupled theory which can be reduced to pure MES by an invertible disformal transformation. This theory can then be converted into the Jordan frame of another ST, $R\phi^2$, which also admits an invertible transformation to MES. Two theories, each of which is dual to MES, will be {\em sequentially dual} to each other and can be considered as two different Jordan frames of the same theory. Both chosen theories violate null energy condition. Transforming the same singular MES solutions, we find the desingularization signs in both Jordan frames, but these are more pronouned in the framework of kinetic theory, leading, in the cosmological case, to Genesis-type behavior.