E. I. Guendelman, A. B. Kaganovich
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 turn into functions of the cold fermion density n. This n dependence becomes negligible in regular (laboratory) conditions but it has a crucial role in cosmology: the cosmological dynamics is governed by the scale factor dependent fermion density yielding the results similar to what quintessence does. In the 4D gravity model where the original action involves only the cosmological constant and massive fermions without selfinteraction, in the broad range of the parameter space, we find a rapid transition (at z\sim 1) from the cold fermionic matter dominated epoch with the equation of state (EoS) w=p/\rho\approx 0 to the accelerated epoch with w<-1/3 that is followed by a further asymptotic decrease towards w=-1. We argue that the fermions should be associated with neutrinos. Depending on the parameters, the late time evolution can proceed either with monotonic decreasing EoS to w=-1 or in the phantom-like regime. In the latter case, for a broad range of initial conditions, w crosses the phantom divide 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_{neutrino}= -\rho_{neutrino}. After passing their minima, w and rho increase asymptotically with cosmic time approaching from below w=-1 and \rho=const respectively and thus providing a pseudo-rip scenario.
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http://arxiv.org/abs/1208.2132
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