Silvio A. Bonometto

Halo Properties in Models with Dynamical Dark Energy

We study the properties of dark matter (DM) halos in several models in which we have included dark energy (DE). We consider both dynamical DE, due to a scalar field self-interacting through Ratra-Peebles or supergravity potentials, and DE with constant negative w = p/? > -1. We find that at zero redshift, both the nonlinear power spectrum of DM and the mass function of halos do not depend appreciably on the state equation of DE, which implies that both statistics are almost indistinguishable from those of ?-dominated cold dark matter (?CDM). This result is consistent with the nonlinear treatment in the accompanying paper and is also a welcome feature, because ?CDM fits a large variety of data. On the other hand, DE halos differ from ?CDM halos in that they are denser in their central parts, because DE halos collapsed earlier. Nevertheless, such differences are not so large. For example, the density at 10 kpc of a ~1013 M? DE halo is only 50% denser than the ?CDM halo. This means that DE does not ease the problem with cuspy DM profiles. Addressing another cosmological problem, the abundance of subhalos, we find that the number of satellites of halos in various DE models does not change with respect to ?CDM when normalized to the same circular velocity as the parent halo. Most of the above similarities are related to choosing for all models the same normalization factor ?8 at z = 0. At high redshifts, different DE and ?CDM models have different amplitudes of fluctuations, which causes substantial deviations of halo properties to occur. Therefore, the way to find which DE equation of state gives the best fit to the observed universe is to look at the evolution of halo properties. For example, the abundance of galaxy groups with mass larger than 1013 h-1 M? at z 2 can be used to discriminate between the models and thus to constrain the nature of DE.

A path-integral approach to large-scale matter distribution originated by non-Gaussian fluctuations

The possibility that, in the framework of a biased theory of galaxy clustering, the underlying matter distribution be non-Gaussian itself, because of the very mechanisms generating its present status, is explored. It is shown that a number of contradictory results, seemingly present in large-scale data, in principle can recover full coherence, once the requirement that the underlying matter distribution be Gaussian is dropped. For example, in the present framework, the requirement that the two-point correlation functions vanish at the same scale (for different kinds of objects) is overcome. A general formula, showing the effects of a non-Gaussian background on the expression of three-point correlations in terms of two-point correlations, is given. 28 references.

Modeling dynamical dark energy

Cosmological models with different types of dark energy are becoming viable alternatives for standard models with the cosmological constant, yet such models are more difficult to analyze and to simulate. We present analytical approximations and discuss ways of making simulations for two families of models, which cover a wide range of possibilities and include models with both slow- and fast-changing ratio w = p/ρ. More specifically, we give analytical expressions for the evolution of the matter density parameter Ωm(z) and the virial density contrast Δc at any redshift z. The latter is used to identify halos and to find their virial masses. We also provide an approximation for the linear growth factor of linear fluctuations between redshift z = 40 and 0. This is needed to set the normalization of the spectrum of fluctuations. Finally, we discuss the expected behavior of the halo mass function and its time evolution.

Do WMAP data favor neutrino mass and a coupling between Cold Dark Matter and Dark Energy

We fit WMAP5 and related data by allowing for a CDM-DE coupling and non-zero neutrino masses, simultaneously. We find a significant correlation between these parameters, so that simultaneous higher coupling and {nu}-masses are allowed. Furthermore, models with a significant coupling and {nu}-mass are statistically favoured in respect to a cosmology with no coupling and negligible neutrino mass (our best fits are: C{approx}1/2 m{sub p}, m{sub {nu}{approx}0}.12 eV per flavor). We use a standard Monte Carlo Markov Chain approach, by assuming DE to be a scalar field self-interacting through Ratra-Peebles or SUGRA potentials.

High-accuracy power spectra including baryonic physics in dynamical Dark Energy models

The next generation mass probes will obtain information on non-linear power spectra P(k, z) and their evolution, allowing us to investigate the nature of Dark Energy. To exploit such data we need high-precision simulations, extending at least up to scales of k ≃ 10h Mpc -1 , where the effects of baryons can no longer be neglected. In this paper, we present a series of large scale hydrodynamical simulations for ACDM and dynamical Dark Energy (dDE) models, in which the equation of state parameter is z dependent. The simulations include gas cooling, star formation and Supernovae feedback. They closely approximate the observed star formation rate and the observationally derived star/Dark Matter mass ratio in collapsed systems. Baryon dynamics cause spectral shifts exceeding 1 per cent at k > 2-3 h Mpc -1 compared to pure N-body simulations in the ACDM simulations. This agrees with previous studies, although we find a smaller effect (~50 per cent) on the power spectrum amplitude at higher k values. dDE exhibits similar behaviour, even though the dDE simulations produce ~20 per cent less stars than the analogous ACDM cosmologies. Finally, we show that the technique introduced in Casarini et al. to obtain spectra for any w(z) cosmology from constant-w models at any redshift still holds when gas physics is taken into account. While this relieves the need to explore the entire functional space of DE state equations, we illustrate a severe risk that future data analysis could lead to misinterpretation of the DE state equation.

Dark Matter and Dark Energy from the solution of the strong CP problem

The Peccei-Quinn (PQ) solution of the strong CP problem requires the existence of axions, which are viable candidates for dark matter. If the Nambu-Goldstone potential of the PQ model is replaced by a potential V(|Phi|) admitting a tracker solution, the scalar field |Phi| can account for dark energy, while the phase of Phi yields axion dark matter. If V is a supergravity (SUGRA) potential, the model essentially depends on a single parameter, the energy scale Lambda. Once we set Lambda approximately equal to 10(10) GeV at the quark-hadron transition, |Phi| naturally passes through values suitable to solve the strong CP problem, later growing to values providing fair amounts of dark matter and dark energy.

The Sky Polarization Observatory

Abstract The Sky Polarization Observatory (SPOrt) is an ASI-funded experiment specifically designed to measure the sky polarization at 22, 32 and 90 GHz, which was selected in 1997 by ESA to be flown on the International Space Station. Starting in 2006 and for at least 18 months, it will be taking direct and simultaneous measurements of the Stokes parameters Q and U at 660 sky pixels, with FWHM=7°. Due to development efforts over the past few years, the design specifications have been significantly improved with respect to the first proposal. Here we present an up-to-date description of the instrument, which now warrants a pixel sensitivity of 1.7 μK for the polarization of the cosmic background radiation, assuming two years of observations. We discuss SPOrt scientific goals in the light of WMAP results, in particular in connection with the emerging double-reionization cosmological scenario.

Mixed dark matter from axino distribution

We study the possibility of mixed dark matter obtained through the phase space distribution of a single particle. An example is offered in the context of SUSY models with a Peccei-Quinn symmetry. Axinos in the 100 keV range can naturally have both thermal and nonthermal components. The latter one arises from the lightest neutralino decays and derelativizes at [ital z][similar to]10[sup 4].

Coupling between cold dark matter and dark energy from neutrino mass experiments

Abstract We consider cosmological models with dynamical dark energy (dDE) coupled to cold dark matter (CDM), while simultaneously allowing neutrinos to be massive. Using a MCMC approach, we compare these models with a wide range of cosmological data sets. We find a strong correlation between this coupling strength and the neutrino mass. This correlation persists when BAO data are included in the analysis. We add then priors on ν mass from particle experiments. The claimed detection of ν mass from the Heidelberg–Moscow neutrinoless double- β decay experiment would imply a 7– 8 σ detection of CDM–DE coupling. Similarly, the detection of ν mass from coming KATRIN tritium β decay experiment will imply a safe detection of a coupling in the dark sector. Previous attempts to accommodate cosmic phenomenology with such possible ν mass data made recourse to a w - 1 eoS. We compare such an option with the coupling option and find that the latter allows a drastic improvement.

Sizes of voids as a test for dark matter models

We use the void probability statistics to study the redshift-space galaxy distribution as described by a volume-limited subsample of the Perseus-Pisces survey. We compare the results with the same analysis realized on artificial samples, extracted from high-resolution N-body simulations by reproducing the observational biases of the real data set. Simulations are run for the Cold+HotDM model (CHDM) and for unbiased and biased (b=1.5) CDM models in a 50 Mpc/h box. We identify galaxies as residing in peaks of the evolved density field. We fragment overmerged structures into individual galaxies so as to reproduce both the correct luminosity function (after assuming M/ L values for the resulting galaxy groups) and the two-point correlation function. Our main result is that a void-probability function (VPF) from the standard CHDM model with fractions 60% cold, 30% hot, 10% barions, exceeds the observational VPF with a high confidence level. CDM models produce smaller VPF independent of the biasing parameter. We verify the robustness of this result against changing the observer position in the simulations and the galaxy identification in the evolved density field.We use the void probability statistics to study the redshift-space galaxy distribution as described by a volume-limited subsample of the Perseus-Pisces survey. We compare the results with the same analysis realized on artificial samples, extracted from high-resolution N-body simulations by reproducing the observational biases of the real data set. Simulations are run for the Cold+HotDM model (CHDM) and for unbiased and biased (b=1.5) CDM models in a 50 Mpc/h box. We identify galaxies as residing in peaks of the evolved density field. We fragment overmerged structures into individual galaxies so as to reproduce both the correct luminosity function (after assuming M/ L values for the resulting galaxy groups) and the two-point correlation function. Our main result is that a void-probability function (VPF) from the standard CHDM model with fractions 60% cold, 30% hot, 10% barions, exceeds the observational VPF with a high confidence level. CDM models produce smaller VPF independent of the biasing parameter. We verify the robustness of this result against changing the observer position in the simulations and the galaxy identification in the evolved density field.