The effects of mass-varying neutrinos on cosmic microwave background (CMB) anisotropies and large scale structures (LSS) are studied. In these models, dark energy and neutrinos are coupled such that the neutrino masses are functions of the scalar field playing the role of dark energy. We begin by describing the cosmological background evolution of such a system. It is pointed out that, similar to models with a dark matter/dark energy interaction, the apparent equation of state measured with SNIa can be smaller than -1. We then discuss the effect of mass-varying neutrinos on the CMB anisotropies and the matter power spectrum. A suppression of power in the CMB power spectrum at large angular scales is usually observed. We give an explanation for this behaviour and discuss different couplings and quintessence potentials to show the generality of the results obtained. We perform a likelihood analysis using wide-ranging SNIa, CMB and LSS observations to assess whether such theories are viable. Treating the neutrino mass as a free parameter we find that the constraints on the coupling are weak, since CMB and LSS surveys give only upper bounds on the neutrino mass. However, fixing a priori the neutrino masses, we find that there is some evidence that the existence of such a coupling is actually preferred by current cosmological data over the standard ΛCDM cosmology.

The effects of mass-varying neutrinos on cosmic microwave background (CMB) anisotropies and large scale structures (LSS) are studied. In these models, dark energy and neutrinos are coupled such that the neutrino masses are functions of the scalar field playing the role of dark energy. We begin by describing the cosmological background evolution of such a system. It is pointed out that, similar to models with a dark matter/dark energy interaction, the apparent equation of state measured with SNIa can be smaller than -1. We then discuss the effect of mass-varying neutrinos on the CMB anisotropies and the matter power spectrum. A suppression of power in the CMB power spectrum at large angular scales is usually observed. We give an explanation for this behaviour and discuss different couplings and quintessence potentials to show the generality of the results obtained. We perform a likelihood analysis using wide-ranging SNIa, CMB and LSS observations to assess whether such theories are viable. Treating the neutrino mass as a free parameter we find that the constraints on the coupling are weak, since CMB and LSS surveys give only upper bounds on the neutrino mass. However, fixing a priori the neutrino masses, we find that there is some evidence that the existence of such a coupling is actually preferred by current cosmological data over the standard ΛCDM cosmology.

Cosmological consequences of a coupling between massive neutrinos and dark energy are investigated. In such models, the neutrino mass is a function of a scalar field, which plays the role of dark energy. The evolution of the background and cosmological perturbations are discussed. We find that mass-varying neutrinos can leave a significant imprint on the anisotropies in the CMB and even lead to a reduction of power on large angular scales.

A coupling between a light scalar field and neutrinos has been widely discussed as a mechanism for linking (time varying) neutrino masses and the present energy density and equation of state of dark energy. However, it has been pointed out that the viability of this scenario in the non-relativistic neutrino regime is threatened by the strong growth of hydrodynamic perturbations associated with a negative adiabatic sound speed squared. In this paper we revisit the stability issue in the framework of linear perturbation theory in a model independent way. The criterion for the stability of a model is translated into a constraint on the scalar-neutrino coupling, which depends on the ratio of the energy densities in neutrinos and cold dark matter. We illustrate our results by providing meaningful examples both for stable and unstable models.

We consider a modified gravity theory, f (R) = R − a/R n + bR m , in the metric formulation, which has been suggested to produce late time acceleration in the Universe, whilst satisfying local fifth-force constraints. We investigate the parameter range for this theory, considering the regimes of early and late-time acceleration, Big Bang Nucleosynthesis and fifth-force constraints. We conclude that it is difficult to find a unique range of parameters for consistency of this theory. I. INTRODUCTIONVarious cosmological observations suggest that the universe is pervaded by a new form of energy, dubbed dark energy, giving rise to accelerated expansion at the present epoch [1,2,3]. These observations are a challenge for fundamental physics, since a well-motivated candidate for dark energy has to be found. The usual candidates include scalar fields or extra dimensions.Recently, it has been suggested that instead of a new matter form, the cosmic accelerated expansion could be attributed to a modification of General Relativity itself, see e.g.