Falsifying Paradigms for Cosmic Acceleration

Abstract

Consistency relations between growth of structure and expansion history observables exist for any physical explanation of cosmic acceleration, be it a cosmological constant, scalar field quintessence, or a general component of dark energy that is smooth relative to dark matter on small scales. The high-quality supernova sample anticipated from an experiment like SNAP and CMB data expected from Planck thus make strong predictions for growth and expansion observables that additional observations can test and potentially falsify. We perform an MCMC likelihood exploration of the strength of these consistency relations based on a complete parametrization of dark energy behavior by principal components. For LCDM, future SN and CMB data make percent level predictions for growth and expansion observables. For quintessence, many of the predictions are still at a level of a few percent with most of the additional freedom coming from curvature and early dark energy. While such freedom is limited for quintessence where phantom equations of state are forbidden, it is larger in the smooth dark energy class. Nevertheless, even in this general class predictions relating growth measurements at different redshifts remain robust, although predictions for the instantaneous growth rate do not. Finally, if observations falsify the whole smooth dark energy class, new paradigms for cosmic acceleration such as modified gravity or interacting dark matter and dark energy would be required.