Dark Matter Decaying into a Fermi Sea of Neutrinos

We study the possible decay of a coherently oscillating scalar field, interpreted as dark matter, into light fermions. Specifically, we consider a scalar field with sub-eV mass decaying into a Fermi sea of neutrinos. We recognize the similarity between our scenario and inflationary preheating where a coherently oscillating scalar field decays into standard model particles. Like the case of fermionic preheating, we find that Pauli blocking controls the dark matter decay into the neutrino sea. The radius of the Fermi sphere depends on the expansion of the universe leading to a time varying equation of state of dark matter. This makes the scenario very rich and we show that the decay rate might be different at different cosmological epochs. We categorize this in two interesting regimes and then study the cosmological perturbations to find the impact on structure formation. We find that the decay may help alleviating some of the standard problems related to cold dark matter.Comment: 8 pages, 3 figures, accepted for publication in Phys. Rev.

Dark energy properties from large future galaxy surveys

We perform a detailed forecast on how well a {\sc Euclid}-like survey will be able to constrain dark energy and neutrino parameters from a combination of its cosmic shear power spectrum, galaxy power spectrum, and cluster mass function measurements. We find that the combination of these three probes vastly improves the survey's potential to measure the time evolution of dark energy. In terms of a dark energy figure-of-merit defined as $(\sigma(w_{\mathrm p}) \sigma(w_a))^{-1}$, we find a value of 690 for {\sc Euclid}-like data combined with {\sc Planck}-like measurements of the cosmic microwave background (CMB) anisotropies in a 10-dimensional cosmological parameter space, assuming a $\Lambda$CDM fiducial cosmology. For the more commonly used 7-parameter model, we find a figure-of-merit of 1900 for the same data combination. We consider also the survey's potential to measure dark energy perturbations in models wherein the dark energy is parameterised as a fluid with a nonstandard non-adiabatic sound speed, and find that in an \emph{optimistic} scenario in which $w_0$ deviates by as much as is currently observationally allowed from $-1$, models with $\hat{c}_\mathrm{s}^2 = 10^{-6}$ and $\hat{c}_\mathrm{s}^2 = 1$ can be distinguished at more than $2\sigma$ significance. We emphasise that constraints on the dark energy sound speed from cluster measurements are strongly dependent on the modelling of the cluster mass function; significantly weaker sensitivities ensue if we modify our model to include fewer features of nonlinear dark energy clustering. Finally, we find that the sum of neutrino masses can be measured with a $1 \sigma$ precision of 0.015~eV, (abridged)Comment: 26 pages, 5 figures, matches JCAP versio

Phenomenology of Hybrid Scenarios of Neutrino Dark Energy

We study the phenomenology of hybrid scenarios of neutrino dark energy, where in addition to a so-called Mass Varying Neutrino (MaVaN) sector a cosmological constant (from a false vacuum) is driving the accelerated expansion of the universe today. For general power law potentials we calculate the effective equation of state parameter w_{eff}(z) in terms of the neutrino mass scale. Due to the interaction of the dark energy field (acceleron) with the neutrino sector, w_{eff}(z) is predicted to become smaller than -1 for z>0, which could be tested in future cosmological observations. For the considered scenarios, the neutrino mass scale additionally determines which fraction of the dark energy is dynamical, and which originates from the cosmological constant like vacuum energy of the false vacuum. On the other hand, the field value of the acceleron field today as well as the masses of the right-handed neutrinos, which appear in the seesaw-type mechanism for small neutrino masses, are not fixed. This, in principle, allows to realise hybrid scenarios of neutrino dark energy with a high-scale seesaw where the right-handed neutrino masses are close to the GUT scale. We also comment on how MaVaN Hybrid Scenarios with high-scale seesaw might help to resolve stability problems of dark energy models with non-relativistic neutrinos.Comment: 22 pages, 5 figures; references and comments added; version to be published in JCA

Coupling active and sterile neutrinos in the cosmon plus seesaw framework

The cosmological evolution of neutrino energy densities driven by cosmon-type field equations is introduced assuming that active and sterile neutrinos are intrinsically connected by cosmon fields through the {\em seesaw} mechanism. Interpreting sterile neutrinos as dark matter adiabatically coupled with dark energy results in a natural decoupling of (active) mass varying neutrino (MaVaN) equations. Identifying the dimensionless scale of the {\em seesaw} mechanism, $m/M$, with a power of the cosmological scale factor, $a$, allows for embedding the resulting masses into the generalized Chaplygin gas (GCG) scenario for the dark sector. Without additional assumptions, our findings establish a precise connection among three distinct frameworks: the cosmon field dynamics for MaVaN's, the {\em seesaw} mechanism for dynamical mass generation and the GCG scenario. Our results also corroborate with previous assertions that mass varying particles can be the right responsible for the stability issue and for the cosmic acceleration of the universe.Comment: 12 pages, 2 figure

Dynamical neutrino masses in the generalized Chaplygin gas scenario with mass varying CDM

Neutrinos coupled to an underlying scalar field in the scenario for unification of mass varying dark matter and cosmon-{\em like} dark energy is examined. In the presence of a tiny component of mass varying neutrinos, the conditions for the present cosmic acceleration and for the stability issue are reproduced. It is assumed that {\em sterile} neutrinos behave like mass varying dark matter coupled to mass varying {\em active} neutrinos through the {\em seesaw} mechanism, in a kind of {\em mixed} dark matter sector. The crucial point is that the dark matter mass may also exhibit a dynamical behavior driven by the scalar field. The scalar field mediates the nontrivial coupling between the mixed dark matter and the dark energy responsible for the accelerated expansion of the universe. The equation of state of perturbations reproduce the generalized Chaplygin gas (GCG) cosmology so that all the effective results from the GCG paradigm are maintained, being perturbatively modified by neutrinos.Comment: 19 pages, 5 figure

Cosmology when living near the Great Attractor

If we live in the vicinity of the hypothesized Great Attractor, the age of the universe as inferred from the local expansion rate can be off by three per cent. We study the effect that living inside or near a massive overdensity has on cosmological parameters induced from observations of supernovae, the Hubble parameter and the Cosmic Microwave Background. We compare the results to those for an observer in a perfectly homogeneous LCDM universe. We find that for instance the inferred value for the global Hubble parameter changes by around three per cent if we happen to live inside a massive overdensity such as the hypothesized Great Attractor. Taking into account the effect of such structures on our perception of the universe makes cosmology perhaps less precise, but more accurate.Comment: 8 pages, 6 figures, Submitted to MNRA

Mass-varying neutrino in light of cosmic microwave background and weak lensing

We aim to constrain mass-varying neutrino models using large scale structure observations and produce forecast for the Euclid survey. We investigate two models with different scalar field potential and both positive and negative coupling parameters \beta. These parameters correspond to growing or decreasing neutrino mass, respectively. We explore couplings up to |\beta|<5. In the case of the exponential potential, we find an upper limit on $\Omega_\nu h^2$<0.004 at 2-$\sigma$ level. In the case of the inverse power law potential the null coupling can be excluded with more than 2-\sigma significance; the limits on the coupling are \beta>3 for the growing neutrino mass and \beta<-1.5 for the decreasing mass case. This is a clear sign for a preference of higher couplings. When including a prior on the present neutrino mass the upper limit on the coupling becomes |\beta|<3 at 2-$\sigma$ level for the exponential potential. Finally, we present a Fisher forecast using the tomographic weak lensing from an Euclid-like experiment and we also consider the combination with the cosmic microwave background (CMB) temperature and polarisation spectra from a Planck-like mission. If considered alone, lensing data is more efficient in constraining $\Omega_\nu$ with respect to CMB data alone. There is, however, a strong degeneracy in the \beta-$\Omega_\nu h^2$ plane. When the two data sets are combined, the latter degeneracy remains, but the errors are reduced by a factor ~2 for both parameters.Comment: 5 pages, 6 figures. Now published in A&A 500, 657-665 (2009

Neutrino Condensate as Origin of Dark Energy

We propose a new solution to the origin of dark energy. We suggest that it was created dynamically from the condensate of a singlet neutrino at a late epoch of the early Universe through its effective self interaction. This singlet neutrino is also the Dirac partner of one of the three observed neutrinos, hence dark energy is related to neutrino mass. The onset of this condensate formation in the early Universe is also related to matter density and offers an explanation of the coincidence problem of why dark energy (70%) and total matter (30%) are comparable at the present time. We demonstrate this idea in a model of neutrino mass with (right-handed) singlet neutrinos and a singlet scalar.Comment: 5 pages, no figure

Oscillating nonlinear large scale structure in growing neutrino quintessence

Growing Neutrino quintessence describes a form of dynamical dark energy that could explain why dark energy dominates the universe only in recent cosmological times. This scenario predicts the formation of large scale neutrino lumps which could allow for observational tests. We perform for the first time N-body simulations of the nonlinear growth of structures for cold dark matter and neutrino fluids in the context of Growing Neutrino cosmologies. Our analysis shows a pulsation - increase and subsequent decrease - of the neutrino density contrast. This could lead to interesting observational signatures, as an enhanced bulk flow in a situation where the dark matter density contrast only differs very mildly from the standard LCDM scenario. We also determine for the first time the statistical distribution of neutrino lumps as a function of mass at different redshifts. Such determination provides an essential ingredient for a realistic estimate of the observational signatures of Growing Neutrino cosmologies. Due to a breakdown of the non-relativistic Newtonian approximation our results are limited to redshifts z > 1.Comment: 17 pages, 1 table, 10 figures; MNRAS in pres