André Füzfa

Imprints of dark energy on cosmic structure formation: II) Non-Universality of the halo mass function

The universality of the halo mass function is investigated in the context of dark energy cosmologies. This widely used approximation assumes that the mass function can be expressed as a function of the matter density omega_m and the rms linear density fluctuation sigma only, with no explicit dependence on the properties of dark energy or redshift. In order to test this hypothesis we run a series of 15 high-resolution N-body simulations for different cosmological models. These consists of three LCDM cosmologies best fitting WMAP-1, 3 and 5 years data, and three toy-models characterized by a Ratra-Peebles quintessence potential with different slopes and amounts of dark energy density. These toy models have very different evolutionary histories at the background and linear level, but share the same sigma8 value. For each of these models we measure the mass function from catalogues of halos identified in the simulations using the Friend-of-Friend (FoF) algorithm. We find redshift dependent deviations from a universal behaviour, well above numerical uncertainties and of non-stochastic origin, which are correlated with the linear growth factor of the investigated cosmologies. Using the spherical collapse as guidance, we show that such deviations are caused by the cosmology dependence of the non-linear collapse and virialization process. For practical applications, we provide a fitting formula of the mass function accurate to 5 percents over the all range of investigated cosmologies. We also derive an empirical relation between the FoF linking parameter and the virial overdensity which can account for most of the deviations from an exact universal behavior. Overall these results suggest that the halo mass function contains unique cosmological information since it carries a fossil record of the past cosmic evolution.

Imprints of dark energy on cosmic structure formation - I. Realistic quintessence models and the non-linear matter power spectrum

Quintessence has been proposed to account for dark energy (DE) in the Universe. This component causes a typical modification of the background cosmic expansion, which, in addition to its clustering properties, can leave a potentially distinctive signature on large-scale structures. Many previous studies have investigated this topic, particularly in relation to the non-linear regime of structure formation. However, no careful pre-selection of viable quintessence models with high precision cosmological data was performed. Here we show that this has led to a misinterpretation (and underestimation) of the imprint of quintessence on the distribution of large-scale structures. To this purpose, we perform a likelihood analysis of the combined Supernova Ia UNION data set and Wilkinson Microwave Anisotropy Probe 5-yr data to identify realistic quintessence models. These are specified by different model parameter values, but still statistically indistinguishable from the vanilla Lambda cold dark matter (Lambda CDM). Differences are especially manifest in the predicted amplitude and shape of the linear matter power spectrum though these remain within the uncertainties of the Sloan Digital Sky Survey data. We use these models as a benchmark for studying the clustering properties of dark matter haloes by performing a series of high-resolution N-body simulations. In this first paper, we specifically focus on the non-linear matter power spectrum. We find that realistic quintessence models allow for relevant differences of the dark matter distribution with respect to the Lambda CDM scenario well into the non-linear regime, with deviations of up to 40 per cent in the non-linear power spectrum. Such differences are shown to depend on the nature of DE, as well as the scale and epoch considered. At small scales (k similar to 1-5 h Mpc-1, depending on the redshift), the structure formation process is about 20 per cent more efficient than in Lambda CDM. We show that these imprints are a specific record of the cosmic structure formation history in DE cosmologies and therefore cannot be accounted for in standard fitting functions of the non-linear matter power spectrum.

Radioscience simulations in general relativity and in alternative theories of gravity

This paper deals with tests of general relativity (GR) in the Solar System using tracking observables from planetary spacecraft. We present a new software that simulates the Range and Doppler signals resulting from a given spacetime metric. This flexible approach allows one to perform simulations in GR as well as in alternative metric theories of gravity. The outputs of this software provide templates of anomalous residuals that should show up in real data if the underlying theory of gravity is not GR. Those templates can be used to give a rough estimation of constraints on additional parameters entering alternative theory of gravity and also signatures that can be searched for in data from past or future space missions aiming at testing gravitational laws in the Solar System. As an application of the potentiality of this software, we present some simulations performed for Cassini-like mission in post-Einsteinian gravity and in the context of MOND external field effect. We derive signatures arising from these alternative theories of gravity and estimate expected amplitudes of the anomalous residuals.

Dark energy as a born-infeld gauge interaction violating the equivalence principle

We investigate the possibility that dark energy does not couple to gravitation in the same way as ordinary matter, yielding a violation of the weak and strong equivalence principles on cosmological scales. We build a transient mechanism in which gravitation is pushed away from general relativity by a Born-Infeld gauge interaction acting as an abnormally weighting (dark) energy. This mechanism accounts for the Hubble diagram of far-away supernovae by cosmic acceleration and time variation of the gravitational constant while accounting naturally for the present tests on general relativity.

Combined cosmological and solar system constraints on chameleon mechanism

The chameleon mechanism appearing in the massive tensor-scalar theory of gravity can effectively reduce locally the nonminimal coupling between the scalar field and matter. This mechanism is invoked to reconcile large-scale departures from general relativity, supposedly accounting for cosmic acceleration, to small-scale stringent constraints on general relativity. In this paper, we carefully investigate this framework on cosmological and solar system scales to derive combined constraints on model parameters, notably by performing a nonambiguous derivation of observables like luminosity distance and local post-Newtonian parameters. The likelihood analysis of type Ia supernovae data and of an admissible domain for the parametrized-post-Newtonian parameters clearly demonstrates that the chameleon mechanism cannot occur in the same region of parameter space as the one necessary to account for cosmic acceleration with the assumed Ratra-Peebles potential and exponential coupling function.

Non-Abelian Einstein-Born-Infeld-Dilaton cosmology

The non-Abelian Einstein-Born-Infeld-dilaton theory, which rules the dynamics of tensor-scalar gravitation coupled to a

Some examples of exponentially harmonic maps

su(2)

The Jungle Universe: coupled cosmological models in a Lotka–Volterra framework

-valued gauge field ruled by Born-Infeld Lagrangian, is studied in a cosmological framework. The microscopic energy exchange between the gauge field and the dilaton which results from a nonuniversality of the coupling to gravity modifies the usual behavior of tensor-scalar theories coupled to matter fluids. General cosmological evolutions are derived for different couplings to gravitation and a comparison to universal coupling is highlighted. Evidences of cosmic acceleration are presented when the evolution is interpreted in the Jordan physical frame of a matter respecting the weak equivalence principle. The importance for the mechanism of cosmic acceleration of the dynamics of the Born-Infeld gauge field, the attraction role of the matter fluid, and the nonuniversality of the gravitational couplings are briefly outlined.

The Lemaitre-Schwarzschild problem revisited

The aim of this paper is to study some examples of exponentially harmonic maps. We study such maps first on flat Euclidean and Minkowski spaces and then on Friedmann-Lemaitre universes. We also consider some new models of exponentially harmonic maps which are coupled with gravity which happen to be based on a generalization of the Lagrangian for bosonic strings coupled with dilatonic field.

Probing Modified Gravity with Atom-Interferometry: a Numerical Approach

In this paper, we exploit the fact that the dynamics of homogeneous and isotropic Friedmann–Lemaître universes is a special case of generalized Lotka–Volterra system where the competitive species are the barotropic fluids filling the Universe. Without coupling between those fluids, Lotka–Volterra formulation offers a pedagogical and simple way to interpret usual Friedmann–Lemaître cosmological dynamics. A natural and physical coupling between cosmological fluids is proposed which preserves the structure of the dynamical equations. Using the standard tools of Lotka–Volterra dynamics, we obtain the general Lyapunov function of the system when one of the fluids is coupled to dark energy. This provides in a rigorous form a generic asymptotic behavior for cosmic expansion in presence of coupled species, beyond the standard de Sitter, Einstein-de Sitter and Milne cosmologies. Finally, we conjecture that chaos can appear for at least four interacting fluids.