Neutrino generated dynamical dark energy with no dark energy field

Abstract

Dynamical dark energy (DE) phenomenon emerges as a geometrical effect accompanying the cosmological expansion of nonrelativistic fermionic matter. This occurs without the need for any fluid, like e.g. dynamical scalar field (quintessence, cosmon, etc.), and with conventional form of the Einstein equations in contrast to other known geometrical DE models. The phenomenon results from first principles in the framework of the two measures field theory where, in the Einstein frame, both fermion masses and the cosmological constant (CC) turn into functions of the cold fermion density n. This n dependence becomes negligible in regular (laboratory) conditions but it may have an important role in cosmology. In the 4D gravity model where the original action involves only CC and massive fermions without selfinteraction, for different (but wide) regions in the parameter space we have found two possible classes of scenarios for the late universe starting from the cold matter domination era. We argue that the fermions which drive the variable CC should be associated with cold neutrinos disposed in voids and supervoids. The cosmological dynamics of the first class practically coincides with that of the LambdaCDM model, while the dynamics of the second class is of the phantom-like regime with a pseudo-rip scenario. Crossing the phantom divide happens due to a new type of the neutrino DE effect where neutrinos pass through the state with zero mass and with the vacuum-like EoS P_{\nu}=-\rho_{\nu}.