V. Pettorino

Hydrodynamical N‐body simulations of coupled dark energy cosmologies

If the accelerated expansion of the Universe at the present epoch is driven by a dark energy scalar field, there may well be a non-trivial coupling between the dark energy and the cold dark matter (CDM) fluid. Such interactions give rise to new features in cosmological structure growth, like an additional long-range attractive force between CDM particles, or variations of the dark matter particle mass with time. We have implemented these effects in the N-body code GADGET-2 and present results of a series of high-resolution N-body simulations where the dark energy component is directly interacting with the CDM. As a consequence of the new physics, CDM and baryon distributions evolve differently both in the linear and in the non-linear regime of structure formation. Already on large scales, a linear bias develops between these two components, which is further enhanced by the non-linear evolution. We also find, in contrast with previous work, that the density profiles of CDM haloes are less concentrated in coupled dark energy cosmologies compared with ACDM, and that this feature does not depend on the initial conditions setup, but is a specific consequence of the extra physics induced by the coupling. Also, the baryon fraction in haloes in the coupled models is significantly reduced below the universal baryon fraction. These features alleviate tensions between observations and the ACDM model on small scales. Our methodology is ideally suited to explore the predictions of coupled dark energy models in the fully non-linear regime, which can provide powerful constraints for the viable parameter space of such scenarios.

Coupled and Extended Quintessence: theoretical differences and structure formation

The case of a coupling between dark energy and matter [coupled quintessence (CQ)] or gravity [extended quintessence (EQ)] has recently attracted a deep interest and has been widely investigated both in the Einstein and in the Jordan frames (EF, JF), within scalar-tensor theories. Focusing on the simplest models proposed so far, in this paper we study the relation existing between the two scenarios, isolating the Weyl scaling which allows one to express them in the EF and JF. Moreover, we perform a comparative study of the behavior of linear perturbations in both scenarios, which turn out to behave in a markedly different way. In particular, while the clustering is enhanced in the considered CQ models with respect to the corresponding quintessence ones where the coupling is absent and to the ordinary cosmologies with a cosmological constant and cold dark matter (

The darkness that shaped the void: dark energy and cosmic voids

\ensuremath{\Lambda}\mathrm{CDM}

Neutrino clustering in growing neutrino quintessence

), structures in EQ models may grow slower. This is likely to have direct consequences on the inner properties of nonlinear structures, like cluster concentration, as well as on the weak lensing shear on large scales. Finally, we specialize our study for interfacing linear dynamics and N-body simulations in these cosmologies, giving a recipe for the corrections to be included in N-body codes in order to take into account the modifications to the expansion rate, growth of structures, and strength of gravity.

Clarifying spherical collapse in coupled dark energy cosmologies

We assess the sensitivity of void shapes to the nature of dark energy that was pointed out in recent studies and also investigate whether or not void shapes are useable as an observational probe in galaxy redshift surveys. Our focus is on the evolution of the mean void ellipticity and its underlying physical cause. To this end, we analyse the morphological properties of voids in five sets of cosmological N-body simulations, each with a different nature of dark energy. To address the question of whether galaxy redshift surveys yield sufficiently accurate void morphologies, voids in the dark matter distribution are compared to those in the halo population. Voids are identified using the parameter-free Watershed Void Finder. The effect of redshift distortions is investigated as well. The main conclusions of this study are as follows: (i) the statistically significant sensitivity of voids in the dark matter distribution is confirmed; (ii) the level of clustering as measured by s8(z) is identified as the main cause of differences in the mean void shape ; and (iii) in the halo and/or galaxy distribution, it is practically unfeasible to distinguish at a statistically significant level between the various cosmologies due to the sparsity and spatial bias of the sample.

Constraints on coupled dark energy using CMB data from WMAP and SPT

Abstract A growing neutrino mass can stop the dynamical evolution of a dark energy scalar field, thus explaining the “why now” problem. We show that such models lead to a substantial neutrino clustering on the scales of superclusters. Non-linear neutrino lumps form at redshift z ≈ 1 and could partially drag the clustering of dark matter. If observed, large scale non-linear structures could be an indication for a new attractive “cosmon force” stronger than gravity.

Hydrodynamical simulations of galaxy clusters in dark energy cosmologies – I. General properties

The spherical collapse model is often used to follow the evolution of overdensities into the nonlinear regime. We describe the correct approach to be used in coupled dark energy cosmologies, where a fifth force, different from gravity and mediated by the dark energy scalar field, influences the collapse. We reformulate the spherical collapse description by deriving it directly from the set of nonlinear hydrodynamical Navier-Stokes equations. By comparing with the corresponding relativistic equations, we show how the fifth force should be taken into account within the spherical collapse picture and clarify the problems arising when an inhomogeneous scalar field is considered within a spherical collapse picture. We then apply our method to the case of coupled quintessence, where the fifth force acts among cold dark matter particles, and to growing neutrino quintessence, where the fifth force acts between neutrinos. Furthermore, we review this method within standard cosmologies and apply our analysis to minimally coupled quintessence. We also check past results for early dark energy parametrizations.

Testing modified gravity with Planck: the case of coupled dark energy

We consider the case of a coupling in the dark cosmological sector, where a dark energy scalar field modifies the gravitational attraction between dark matter particles. We find that the strength of the coupling {\beta} is constrained using current Cosmic Microwave Background (CMB) data, including WMAP7 and SPT, to be less than 0.063 (0.11) at 68% (95%) confidence level. Further, we consider the additional effect of the CMB-lensing amplitude, curvature, effective number of relativistic species and massive neutrinos and show that the bound from current data on {\beta} is already strong enough to be rather stable with respect to any of these variables. The strongest effect is obtained when we allow for massive neutrinos, in which case the bound becomes slightly weaker, {\beta} < 0.084(0.14). A larger value of the effective number of relativistic degrees of freedom favors larger couplings between dark matter and dark energy as well as values of the spectral index closer to 1. Adding the present constraints on the Hubble constant, as well as from baryon acoustic oscillations and supernovae Ia, we find {\beta} < 0.050(0.074). In this case we also find an interesting likelihood peak for {\beta} = 0.041 (still compatible with 0 at 1{\sigma}). This peak comes mostly from a slight difference between the Hubble parameter HST result and the WMAP7+SPT best fit. Finally, we show that forecasts of Planck+SPT mock data can pin down the coupling to a precision of better than 1% and detect whether the marginal peak we find at small non zero coupling is a real effect.

Nonlinear matter spectra in coupled quintessence

We investigate the influence of dark energy on structure form ation, within five different cos- mological models, namely a concordanceCDM model, two models with dynamical dark en- ergy, viewed as a quintessence scalar field (using a RP and a SU GRA potential form) and two extended quintessence models (EQp and EQn) where the quintessence scalar field interacts non-minimally with gravity (scalar-tensor theories). We adopted for all models the normaliza- tion of the matter power spectrum σ8 to match the CMB data. In the models with dynamical dark energy and quintessence, we describe the equation of state with w0 � 0.9, still within the range allowed by observations. For each model, we have performed hydrodynamical sim- ulations in a cosmological box of (300 Mpc/h) 3 including baryons and allowing for cooling and star formation. The contemporary presence of evolving dark energy and baryon physics allows us to investigate the interplay between the differen t background cosmology and the evolution of the luminous matter. Since cluster baryon fraction can be used to constrain other cosmological parameters such as m, we also analyse how dark energy influences the baryon content of galaxy clusters. We find that in models with dynami cal dark energy, the evolving cosmological background leads to different star formation rates and different formation histo- ries of galaxy clusters, but the baryon physics is not affect ed in a relevant way. We investigate several proxies of the cluster mass function based on X-ray observables like temperature, lu- minosity, Mgas, and Ygas. We conclude that the X-ray temperature and Mgas functions are better diagnostic to disentangle the growth of structures a mong different dark energy mod- els. We also evaluate the cosmological volumes needed to distinguish the dark energy models here investigated using the cluster number counts (in terms of the mass function and the X- ray luminosity and temperature functions). Relaxed, massive clusters, when studied in regions sufficiently far from from the centre, are built up in a very si milar way despite the different dark energy models here considered. We confirm that, the over all baryon fraction is almost independent of the dark energy models at a few percent level. The same is true for the gas frac- tion. This evidence reinforces the use of galaxy clusters as cosmological probe of the matter and energy content of the Universe.

A comparison of structure formation in minimally and non-minimally coupled quintessence models

The Planck collaboration has recently published maps of the Cosmic Microwave Background (CMB) radiation, in good agreement with a LCDM model, a fit especially valid for multipoles l > 40. We explore here the possibility that dark energy is dynamical and gravitational attraction between dark matter particles is effectively different from the standard one in General Relativity: this is the case of coupled dark energy models, where dark matter particles feel the presence of a fifth force, larger than gravity by a factor beta^2. We investigate constraints on the strength of the coupling beta in view of Planck data. Interestingly, we show that a non-zero coupling is compatible with data and find a likelihood peak at beta = 0.036 \pm 0.016 (Planck + WP + BAO) (compatible with zero at 2sigma). The significance of the peak increases to beta = 0.066 \pm 0.018 (Planck + WP + HST) (around 3.6sigma) when Planck is combined to Hubble Space Telescope data. This peak comes mostly from the small difference between the Hubble parameter determined with CMB measurements and the one coming from astrophysics measurements. In this sense, future observations and further tests of current observations are needed to determine whether the discrepancy is due to systematics in any of the datasets. Our aim here is not to claim new physics but rather to show how Planck data can be used to provide information on dynamical dark energy and modified gravity, allowing us to test the strength of an effective fifth force between dark matter particles with precision smaller than 2%.