arXiv subjects

Silvio A. Bonometto

Publications and source records attributed to Silvio A. Bonometto.

At least 19 recordsLinked to original sources

arXiv2017

Growth and dissolution of spherical density enhancements in SCDEW cosmologies

Strongly Coupled Dark Energy plus Warm dark matter (SCDEW) cosmologies are based on the finding of a conformally invariant (CI) attractor solution during the early radiative expansion, requiring then the stationary presence of $\sim 1\, \%$ of coupled-DM and DE, since inflationary reheating. In these models, coupled-DM fluctuations, even in the early radiative expansion, grow up to non-linearity, as shown in a previous associated paper. Such early non-linear stages are modelized here through the evolution of a top-hat density enhancement. As expected, its radius $R$ increases up to a maximum and then starts to decrease. Virial balance is reached when the coupled-DM density contrast is just 25-26 and DM density enhancement is $\cal O$$(10\, \%)$ of total density. Moreover, we find that this is not an equilibrium configuration as, afterwards, coupling causes DM particle velocities to increase, so that the fluctuation gradually dissolves. We estimate the duration of the whole process, from horizon crossing to dissolution, and find $z_{horizon}/z_{erasing} \sim 3 \times 10^4$. Therefore, only fluctuations entering the horizon at $z \lesssim 10^9$-$10^{10}$ are able to accrete WDM with mass $\sim 100\, $eV -as soon as it becomes non-relativistic- so avoiding full disruption. Accordingly, SCDEW cosmologies, whose WDM has mass $\sim 100\, $eV, can preserve primeval fluctuations down to stellar mass scale.

astro-ph.CO↗

arXiv2017

Strongly Coupled Dark Energy with Warm dark matter vs. LCDM

Cosmologies including strongly Coupled (SC) Dark Energy (DE) and Warm dark matter (SCDEW) are based on a conformally invariant (CI) attractor solution modifying the early radiative expansion. Then, aside of radiation, a kinetic field $Φ$ and a DM component account for a stationary fraction, $\sim 1\, \%$, of the total energy. Most SCDEW predictions are hardly distinguishable from LCDM, while SCDEW alleviates quite a few LCDM conceptual problems, as well as its difficulties to meet data below the average galaxy scale. The CI expansion begins at the inflation end, when $Φ$ (future DE) possibly plays a role in reheating, and ends at the Higgs' scale. Afterwards, a number of viable options is open, allowing for the transition from the CI expansion to the present Universe. In this paper: (i) We show how the attractor is recovered when the spin degrees of freedom decreases. (ii) We perform a detailed comparison of CMB anisotropy and polarization spectra for SCDEW and LCDM, including tensor components, finding negligible discrepancies. (iii) Linear spectra exhibit a greater parameter dependence at large $k$'s, but are still consistent with data for suitable parameter choices. (iv) We also compare previous simulation results with fresh data on galaxy concentration. Finally, (v) we outline numerical difficulties at high $k$. This motivates a second related paper, where such problems are treated in a quantitative way.

astro-ph.CO↗

arXiv2017

Coupled DM heating in SCDEW cosmologies

Strongly Coupled Dark Energy plus Warm dark matter (SCDEW) cosmologies admit the stationary presence of $\sim 1\, \%$ of coupled-DM and DE, since inflationary reheating. Coupled-DM fluctuations therefore grow up to non-linearity even in the early radiative expansion. Such early non-linear stages are modelized here through the evolution of a top-hat density enhancement, reaching an early virial balance when the coupled-DM density contrast is just 25-26 and DM density enhancement is $ \sim 10\, \%$ of total density. During the time needed to settle in virial equilibium, the virial balance conditions however continue to modify, so that "virialized" lumps undergo a complete evaporation. Here we outline that DM particles processed by overdentities preserve a fraction of their virial momentum. Although fully non-relativistic, the resulting velocities (moderately) affect the fluctuation dynamics over greater scales, entering the horizon later on.

astro-ph.CO↗

arXiv2016

Baryon number transfer could delay Quark-Hadron transition in cosmology

In the early Universe, strongly interacting matter was a quark-gluon plasma. Both lattice computations and heavy ion collision experiments however tell us that, in the absence of chemical potentials, no plasma survives at $T <\sim 150$ MeV. The cosmological Quark-Hadron transition, however, seems to have been a crossover; cosmological consequences envisaged when it was believed to be a phase transition no longer hold. In this paper we discuss whether even a crossover transition can leave an imprint that cosmological observations can seek or, viceversa, there are questions cosmology should address to QCD specialists. In particular, we argue that it is still unclear how baryons (not hadrons) could form at the cosmological transition. A critical role should be played by diquark states, whose abundance in the early plasma needs to be accurately evaluated. We estimate that, if the number of quarks belonging to a diquark state, at the beginning of the cosmological transition, is $<\sim 1:10^6$, its dynamics could be modified by the process of B-transfer from plasma to hadrons. In turn, by assuming B-transfer to cause just mild perturbations and, in particular, no entropy input, we study the deviations from the tracking regime, in the frame of SCDEW models. We find that, in some cases, residual deviations could propagate down to primeval nuclesynthesis.

astro-ph.CO↗

arXiv2015

Strongly Coupled Dark Energy Cosmologies: preserving LCDM success and easing low scale problems I - Linear theory revisited

In this first paper we discuss the linear theory and the background evolution of a new class of models we dub SCDEW: Strongly Coupled DE, plus WDM. In these models, WDM dominates today's matter density; like baryons, WDM is uncoupled. Dark Energy is a scalar field $Φ$; its coupling to ancillary CDM, whose today's density is $\ll 1\, \%$, is an essential model feature. Such coupling, in fact, allows the formation of cosmic structures, in spite of very low WDM particle masses ($\sim 100$ eV). SCDEW models yields Cosmic Microwave Background and linear Large Scale features substantially undistinguishable from $Λ$CDM, but thanks to the very low WDM masses they strongly alleviate $Λ$CDM issues on small scales, as confirmed via numerical simulations in the II associated paper. Moreover SCDEW cosmologies significantly ease the coincidence and fine tuning problems of $Λ$CDM and, by using a field theory approach, we also outline possible links with inflationary models. We also discuss a possible fading of the coupling at low redshifts which prevents non linearities on the CDM component to cause computational problems. The (possible) low-$z$ coupling suppression, its mechanism, and its consequences are however still open questions -not necessarily problems- for SCDEW models. The coupling intensity and the WDM particle mass, although being extra parameters in respect to $Λ$CDM, are found to be substantially constrained a priori so that, if SCDEW is the underlying cosmology, we expect most data to fit also $Λ$CDM predictions.

astro-ph.CO↗

arXiv2015

Strongly Coupled Dark Energy Cosmologies: preserving LCDM 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 ($\approx 100$ eV). Our large scale simulations show that, for $10^{11}<M/M_\odot<10^{14}$, SCDEW haloes exhibit a number density and distribution similar to a standard Lambda Cold Dark Matter (LCDM) model, even though they have lower concentration parameters. High resolution simulation of a galactic halo ($M\sim 10^{12} M_{\odot} $) shows $\sim 60\%$ less substructures than its LCDM counterpart, but the same cuspy density profile. On the scale of galactic satellites ($M\sim 10^{9} M_{\odot}$) SCDEW haloes dramatically differ from LCDM. Due to the high thermal velocities of the WDM component they are almost devoid of any substructures and present strongly cored dark matter density profiles. These density cores extend for several hundreds of parsecs, in very good agreement with Milky Way satellites observations. Strongly coupled models, thanks to their ability to match observations on both large and small scales might represent a valid alternative to a simple LCDM model.

astro-ph.CO↗

arXiv2014

Fluctuations in strongly coupled cosmologies

In the early Universe, a dual component made of coupled CDM and a scalar field $Φ$, if their coupling $β> \sqrt{3}/2$, owns an attractor solution, making them a stationary fraction of cosmic energy during the radiation dominated era. Along the attractor, both such components expand $\propto a^{-4}$ and have early density parameters $Ω_{d} = 1/ (4β^2)$ and $Ω_c= 2, Ω_d$ (field and CDM, respectively). In a previous paper it was shown that, if a further component, expanding $\propto a^{-3}$, breaks such stationary expansion at $z \sim 3$--$5 \times 10^3$, cosmic components gradually acquire densities consistent with observations. This paper, first of all, considers the case that this component is warm. However, its main topic is the analysis of fluctuation evolution: out of horizon modes are then determined; their entry into horizon is numerically evaluated as well as the dependence of Meszaros effect on the coupling $β$; finally, we compute: (i) transfer function and linear spectral function; (ii) CMB $C_l$ spectra. Both are close to standard $Λ$CDM models; in particular, the former one can be so down to a scale smaller than Milky Way, in spite of its main DM component being made of particles of mass $<1$ keV. The previously coupled CDM component, whose present density parameter is $\cal O$$(10^{-3})$, exhibits wider fluctuations $δρ/ρ$, but approximately $β$-independent $δρ$ values. We discuss how lower scale features of these cosmologies might ease quite a few problems that $Λ$CDM does not easily solve.

astro-ph.CO↗

arXiv2013

Constraints on Dark Energy state equation with varying pivoting redshift

We assume the DE state equations w(a) = w_0+w_a(a_p-a), and study the dependence of the constraints on w_0 and w_a coefficients on the pivoting redshift 1+z_p=1/a_p. Coefficients are fitted to data including WMAP7, SNIa (Union 2.1), BAO's (including WiggleZ and SDSS results) and H_0 constraints. The fitting algorithm is CosmoMC. We find specific differences between the cases when neutrino mass is allowed or disregarded. More in detail: i) The z_p value yielding uncorrelated constraints on w_0 and w_a is different in the two cases, holding ~0.25 and ~0.35, respectively. (ii) If we consider the intervals allowed to w_0, we find that they shift when z_p increases, in opposite directions for vanishing or allowed neutrino mass. This leads to no overlap between 1sigma intervals already at z_p >~0.4. (iii) The known effect that a more negative state parameter is required to allow for neutrino mass displays its effects on w_a, rather than on w_0. (iv) The w_0-w_a constraints found by using any pivot z_p can be translated into constraints holding at a specific z_p value (0 or the z_p where errors are uncorrelated). When we do so, error ellipses exhibit a satisfactory overlap.

astro-ph.CO↗

arXiv2013

Recovery of fluctuation spectrum evolution from tomographic shear spectra

Forthcoming large angle surveys are planned to obtain high precision tomographic shear data. In principle, they will allow us to recover the spectra of matter density fluctuation, at various redshift, through the inversion of the expressions yielding shear from fluctuation spectra. This was discussed in previous work, where SVD techniques for matrix inversion were also shown to be the optimal tool to this aim. Here we show the significant improvements obtainable by using a 7 bin tomography, as allowed by future Euclid data, as well as the question of error propagation from shear to fluctuation spectra. We find that the technique is a promising tool, namely for the analysis of baryon physics throug high-l shear spectra and to test the consistency between expansion rate and fluctuation growth.

astro-ph.CO↗

arXiv2012

Dark energy from dark radiation in strongly coupled cosmologies with no fine tuning

A dual component made of non-relativistic particles and a scalar field, exchanging energy, naturally falls onto an attractor solution, making them a (sub)dominant part of the cosmic energy during the radiation dominated era, provided that the constant β, measuring the coupling, is strong enough. The density parameters of both components are then constant, as they expand as a^{-4}. If the field energy is then prevalently kinetic, as is expected, its energy is exactly half of the pressureless component; the dual component as a whole, then, has a density parameter Ω_{cd} = 3/4β^2 (e.g., for β~2.5, Ω_{cd}~0.1, in accordance with Dark Radiation expectations). The stationary evolution can only be broken by the rising of other component(s), expanding as a^{-3}. In a realistic scenario, this happens when z~3-5x10^3. When such extra component(s) become(s) dominant, the densities of the dual components also rise above radiation. The scalar field behavior can be easily tuned to fit Dark Energy data, while the coupled DM density parameter becomes O(10^{-3}). This model however requires that, at present, two different DM components exist. The one responsible for the break of the stationary regime could be made, e.g., by thermally distributed particles with mass even >>1$-2 keV (or non-thermal particles with analogous average speed) so accounting for the size of observed galactic cores; in fact, a fair amount of small scale objects is however produced by fluctuation re-generated by the coupled DM component, in spite of its small density parameter, after the warm component has become non-relativistic.

astro-ph.CO↗

arXiv2012

Tomographic weak lensing shear spectra from large N-body and hydrodynamical simulations

Forthcoming experiments will enable us to determine tomographic shear spectra at a high precision level. Most predictions about them have until now been biased on algorithms yielding the expected linear and non-linear spectrum of density fluctuations. Even when simulations have been used, so-called Halofit (Smith et al 2003) predictions on fairly large scales have been needed. We wish to go beyond this limitation. We perform N-body and hydrodynamical simulations within a sufficiently large cosmological volume to allow a direct connection between simulations and linear spectra. While covering large length-scales, the simulation resolution is good enough to allow us to explore the high-l harmonics of the cosmic shear (up to l ~ 50000), well into the domain where baryon physics becomes important. We then compare shear spectra in the absence and in presence of various kinds of baryon physics, such as radiative cooling, star formation, and supernova feedback in the form of galactic winds. We distinguish several typical properties of matter fluctuation spectra in the different simulations and test their impact on shear spectra. We compare our outputs with those obtainable using approximate expressions for non--linear spectra, and identify substantial discrepancies even between our results and those of purely N-body results. Our simulations and the treatment of their outputs however enable us, for the first time, to obtain shear results taht are fully independent of any approximate expression, also in the high-l range, where we need to incorporate a non-linear power spectrum of density perturbations, and the effects of baryon physics. This will allow us to fully exploit the cosmological information contained in future high--sensitivity cosmic shear surveys, exploring the physics of cosmic shears via weak lensing measurements.

astro-ph.CO↗

arXiv2012

Working out density fluctuation spectra from shear spectra

Forthcoming experiments will enable us to determine high precision tomographic shear spectra. Matter density fluctuation spectra, at various $z$, should then be worked out of them, in order to constrain the model and determine the DE state equation. Available analytical expressions, however, do the opposite, enabling us to derive shear spectra from fluctuation spectra. Here we find the inverse expression, yielding density fluctuation spectra from observational tomographic shear spectra. The procedure involves SVD techniques for matrix inversion. We show in detail how the approach works and provide a few examples.

astro-ph.CO↗

arXiv2011

Non-linear weak lensing forecasts

We investigate the impact of non-linear corrections on dark energy parameter estimation from weak lensing probes. We find that using halofit expressions, suited to LCDM models, implies substantial discrepancies with respect to results directly obtained from N-body simulations, when w(z)\neq-1. Discrepancies appear strong when using models with w'(z=0)>0, as fiducial models; they are however significant even in the neighborhood of LCDM, where neglecting the degrees of freedom associated with the DE state equation can lead to a misestimate of the matter density parameter Ω_m.

astro-ph.CO↗

arXiv2011

Mildly mixed coupled models vs. WMAP7 data

Mildly mixed coupled models include massive neutrinos and CDM--DE coupling. We present new tests of their likelihood vs. recent data including WMAP7, confirming it to exceed LCDM, although at ~2σ's. We then show the impact on the physics of the dark components of neutrino mass detection in tritium beta decay or neutrinoless double beta decay experiments.

astro-ph.CO↗

arXiv2010

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 10 h/Mpc, where the effects of baryons can no longer be neglected. In this paper, we present a series of large scale hydrodynamical simulations for LCDM 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% at k > 2-3 h/Mpc compared to pure n-body simulations in the LCDM simulations. This agrees with previous studies, although we find a smaller effect (~50%) on the power spectrum amplitude at higher k's. dDE exhibits similar behavior, even though the dDE simulations produce ~20% less stars than the analogous LCDM 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 dark energy state equations, we illustrate a severe risk that future data analysis could lead to misinterpretation of the DE state equation.

astro-ph.CO↗

arXiv2009

The Quark-Hadron Transition in the Early Universe

We use recent lattice QCD outputs to work out the expansion law of the Universe during the cosmological quark--hadron transition. To do so, a suitable technique to exploit both pressure and energy density data, with the related error bars, is introduced. We also implement suitable techniques to relate the T range where lattice outputs are available with lower and higher T's, for which we test suitable expressions. We finally compare the cosmological behavior found using lattice data with the one obtainable in the case the transition were first order, although not so far from the crossover transition we studied. Differences are small to be tested with cosmological data, but the coming of the era of precision cosmology might open a channel to inspect the QCD transition through them.

hep-lat↗

arXiv2009

Dynamical Dark Energy simulations: high accuracy Power Spectra at high redshift

Accurate predictions on non--linear power spectra, at various redshift z, will be a basic tool to interpret cosmological data from next generation mass probes, so obtaining key information on Dark Energy nature. This calls for high precision simulations, covering the whole functional space of w(z) state equations and taking also into account the admitted ranges of other cosmological parameters; surely a difficult task. A procedure was however suggested, able to match the spectra at z=0, up to k~3, hMpc^{-1}, in cosmologies with an (almost) arbitrary w(z), by making recourse to the results of N-body simulations with w = const. In this paper we extend such procedure to high redshift and test our approach through a series of N-body gravitational simulations of various models, including a model closely fitting WMAP5 and complementary data. Our approach detects w= const. models, whose spectra meet the requirement within 1% at z=0 and perform even better at higher redshift, where they are close to a permil precision. Available Halofit expressions, extended to (constant) w \neq -1 are unfortunately unsuitable to fit the spectra of the physical models considered here. Their extension to cover the desired range should be however feasible, and this will enable us to match spectra from any DE state equation.

astro-ph↗

arXiv2008

Dark Matter - Dark Energy coupling biasing parameter estimates from CMB data

When CMB data are used to derive cosmological parameters, their very choice does matter: some parameter values can be biased if the parameter space does not cover the "true" model. This is a problem, because of the difficulty to parametrize Dark Energy (DE) physics. We test this risk through numerical experiments. We create artificial data for dynamical or coupled DE models and then use MCMC techniques to recover model parameters, by assuming a constant DE state parameter w and no DM--DE coupling. For the DE potential considered, no serious bias arises when coupling is absent. On the contrary, ω_{o,c}, and thence H_o and Ω_{o,m}, suffer a serious bias when the "true" cosmology includes even just a mild DM--DE coupling. Until the dark components keep an unknown nature, therefore, it can be important to allow for a degree of freedom accounting for DM--DE coupling, even more than increasing the number of parameters accounting for the w(a) behavior.

astro-ph↗