We propose a cosmological model that makes a significant step toward solving the coincidence problem of the near similarity at the present of the dark energy and dark matter components. Our cosmology has the following properties: a) among flat and homogeneous spaces, the present universe is a global attractor: all the possible initial conditions lead to the observed proportion of dark energy and dark matter; once reached, it remains fixed forever; b) the expansion is accelerated at the present; c) the model is consistent with the large-scale structure and microwave background data; d) the dark energy and the dark matter densities scale similarly after equivalence and are close to within two orders of magnitude.Since the introduction of inflationary models the notion of attractor cosmological solutions has been regarded as a desirable property of any successful model. Unfortunately, inflation itself has never completely solved the problem of the initial conditions, since the subsequent decelerated era is no longer an attractor, and any fluctuation away from flatness will be amplified in the future, unless a new accelerated era prevents it.The search for cosmological attractors has been revived by the recent findings ( [1] [2]) according to which the dominant component of the universe medium is in a form of energy density possessing peculiar characteristics: negative pressure and weak clustering. This energy, dubbed dark energy or quintessence ( [3] [4] [5] [6]), should fill roughly 70% of the critical energy density and, along with another 30% in ordinary dark matter (and a minor component of baryons), explains the SNIa observations, is consistent with the CMB data (see e.g. [7]), and other evidences like the cluster masses. The fact that the energy densities of the dark energy and the dark matter are comparable at the present time is indeed an enigma, since we have no reason to expect that the dark energy and the dark matter components, which have always given a very different contribute to the total density in the past and will again give a different one in the future, are almost equal right now. In terms of the phase-space view of the cosmological equations, the problem is that the mixture of dark energy and dark matter we observe today is not a global attractor; a different initial condition or, equivalently, a different instant of observation, gives a different sharing of the total density. The problem lies in the fact that the two energy forms scale differently with time because they are assumed to be completely unrelated. To explain the coincidence we propose to couple dark energy to dark matter.The model we propose in this paper, denoted stationary dark energy, is based on a non-linear coupling of dark energy to dark matter. The resulting cosmological solution has the following properties: a) among flat and homogeneous spaces, the present universe is a global attractor: all the possible initial conditions lead to the observed percentages of dark energy and dark matter; once reached, they remain fixed fore...

In most models of dark energy the structure formation stops when the accelerated expansion begins. In contrast, we show that the coupling of dark energy to dark matter may induce the growth of perturbations even in the accelerated regime. In particular, we show that this occurs in the models proposed to solve the cosmic coincidence problem, in which the ratio of dark energy to dark matter is constant. Depending on the parameters, the growth may be much faster than in a standard matter-dominated era. Moreover, if the dark energy couples only to dark matter and not to baryons, as requested by the constraints imposed by local gravity measurements, the baryon fluctuations develop a constant, scale-independent, large-scale bias which is in principle directly observable. We find that a lower limit to the baryon bias b>0.5 requires the total effective parameter of state w_e=1+p/rho to be larger than 0.6 while a limit b>0.73 would rule out the model.Comment: 5 pages, submitted to Phys. Rev. v2: Minor change

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.

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We develop a new approach toward a high resolution non-parametric reconstruction of the primordial power spectrum using WMAP cosmic microwave background temperature anisotropies that we confront with SDSS large-scale structure data in the range k~0.01-0.1 h/Mpc. We utilise the standard LambdaCDM cosmological model but we allow the baryon fraction to vary. In particular, for the concordance baryon fraction, we compare indications of a possible feature at k~0.05 h/Mpc in WMAP data with suggestions of similar features in large scale structure surveys.Comment: revised version, conclusions unchanged, 7 figures, accepted for publication in MNRA