Roberto Mainini

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.

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.

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.

Limits on coupling between dark components

DM–DE coupling can be a phenomenological indication of a common origin of the dark cosmic components. In this work we outline a new constraint to coupled-DE models: the coupling can partially or totally suppress the Meszaros effect, yielding transferred spectra with quite a soft bending above khor,eq. Models affected by this anomaly do not show major variation in the CMB anisotropy spectrum and it is therefore hard to reconcile them with both CMB and deep sample data, through the same value of the primeval spectral index.

Non-linear predictions from linear theories in models with Dark Energy

Abstract We study the cluster mass function and its evolution in different models with Dark Energy arising from a self-interacting scalar field, with Ratra and Peebles and SUGRA potentials. Computations are based on a Press and Schechter approximation. The mass functions we obtain are compared with results holding for open models or models with Dark Energy due to a cosmological constant. Evolution results, in some Dark Energy models, closely approach open models.

Dark matter and dark energy from a single scalar field and cosmic microwave background data

Axions are likely to be the dark matter (DM) that cosmological data require. They arise in the Peccei-Quinn solution of the strong-CP problem. In a previous work we showed that their model has a simple and natural generalization that also yields dark energy (DE), in fair proportions, without tuning any parameter: DM and DE arise from a single scalar field and are weakly coupled in the present era. In this paper we extend the analysis of this dual-axion cosmology and fit it to WMAP data, by using a Markov chain technique. We find that ΛCDM, dynamical DE with a SUGRA potential, DE with a SUGRA potential and a constant DE-DM coupling, and the dual-axion model with a SUGRA potential fit data with a similar accuracy. The best-fit parameters are, however, fairly different, although consistency is mostly recovered at the 2 σ level. A peculiarity of the dual-axion model with a SUGRA potential is to cause more stringent constraints on most parameters and to favor high values of the Hubble parameter.

Tracing the nature of dark energy with galaxy distribution

Dynamical Dark Energy (DE) is a viable alternative to the cosmological constant. Yet, constructing tests to discriminate between Lambda and dynamical DE models is difficult because the differences are not large. In this paper we explore tests based on the galaxy mass function, the void probability function (VPF), and the number of galaxy clusters. At high z the number density of clusters shows large differences between DE models, but geometrical factors reduce the differences substantially. We find that detecting a model dependence in the cluster redshift distribution is a hard challenge. We show that the galaxy redshift distribution is potentially a more sensitive characteristics. We do so by populating dark matter halos in Nbody simulations with galaxies using well-tested Halo Occupation Distribution (HOD). We also estimate the Void Probability Function and find that, in samples with the same angular surface density of galaxies in different models, the VPF is almost model independent and cannot be used as a test for DE. Once again, geometry and cosmic evolution compensate each other. By comparing VPFs for samples with fixed galaxy mass limits, we find measurable differences.

Strongly coupled dark energy cosmologies: Preserving ΛCDM success and easing low-scale problems - II. cosmological simulations

In this second paper we present the first Nbody cosmological simulations of strongly coupled Dark Energy models (SCDEW), a class of models that alleviates theoretical issues related to the nature of dark energy. SCDEW models assume a strong coupling between Dark Energy (DE) and an ancillary Cold Dark Matter (CDM) component together with the presence of an uncoupled Warm Dark Matter component. The strong coupling between CDM and DE allows us to preserve small scale fluctuations even if the warm particle is quite light (

Fluctuations in strongly coupled cosmologies

\approx 100

Dark matter-baryon segregation in the nonlinear evolution of coupled dark energy models

eV). Our large scale simulations show that, for