arXiv subjects

Jean-Michel Alimi

Publications and source records attributed to Jean-Michel Alimi.

At least 19 recordsLinked to original sources

arXiv2024

Breaking the cosmological invariance of the dark matter halos shape as a new probe of modified gravity

In a recent paper, Alimi & Koskas highlighted in $w$CDM models derived from general relativity (GR) (with Dark Energy Universe numerical simulation data), a cosmological invariance of the distribution of dark-matter (DM) halo shapes when expressed in terms of the nonlinear fluctuations of the cosmic matter field. This paper shows that this invariance persists when tested on numerical simulations performed with a different N-body solver, and that it is also robust to adding massive neutrinos to the cold DM component. However, this discovery raises crucial questions about the validity of this invariance in MG models. Thus, we examined whether the invariance observed by Alimi & Koskas remains robust in the case of Hu & Sawicki model using DUSTGRAIN-pathfinder numerical simulations. By comparing the results of advanced numerical simulations in these different theoretical frameworks, we found significant deviations from the invariance observed in the framework of $w$CDM models of GR. These deviations suggest that the gravitation's nature significantly influences the DM halos' shape. We then interpreted this departure from the GR models' invariance as a manifestation of the scalar-field screening effect corresponding to such $f(R)$-type theories. This one modifies the sphericization process of DM halos during their formation, precisely because the critical mass at which this scalar field becomes non-negligible is the mass at which the deviation appears. To this extent, the departure from cosmological invariance in DM halos' shape is a cosmological probe of the nature of gravity, and the mass scale at which it appears can be used to estimate the $f_{R0}$ parameter of such theories.

astro-ph.CO↗

arXiv2024

The shape of dark matter halos: a new fundamental cosmological invariance

We focus on the complex relationship between the shape of dark matter halos and the cosmological models underlying their formation. We used three realistic cosmological models from the Dark Energy Universe Simulation suite. They have significantly distinct cosmological parameters ($Ω_m$, $σ_8$, and $w$) but quasi-indistinguishable cosmic matter fields beyond the scale of DM halos. Firstly, we developed a robust method to measure the FoF halos shape, while avoiding numerical artifacts on shape measurements. These artifacts are generally induced by the presence of substructures (resolution-dependent) or by any spherical a priori in contradiction with the halos triaxiality. We obtain a marked dependence of the halos shape, both on their mass and on the cosmological model. However, when re-expressing the halos shape parameters in terms of the non-linear fluctuations of the cosmic matter field, the cosmological dependence disappears. This new fundamental cosmological invariance is a direct consequence of the matter field non-linear dynamics: as the universe evolves, the non-linear fluctuations of the cosmic field increase, driving the halos towards sphericity. The deviation from sphericity, measured by the prolaticity, triaxiality, and ellipticity of the dark matter halos, is therefore entirely encapsulated in the non-linear power spectrum. From this fundamental invariant relation, we can reconstruct with remarkable accuracy the non-linear variance of the cosmic matter field and, consequently, the non-linear power spectrum. We also re-find the $σ_8$ amplitude of the cosmological model. Our results highlight, not only the nuanced relationship between dark matter halo formation and the underlying cosmology, but also the potential of dark matter halo shape analysis as a powerful tool for probing the non-linear dynamics of the cosmic matter field.

astro-ph.CO↗

arXiv2020

Ray tracing the integrated Sachs-Wolfe effect through the light cones of the Dark Energy Universe Simulation -- Full Universe Runs

The late integrated Sachs-Wolfe (ISW) effect correlates the Cosmic Microwave Background (CMB) temperature anisotropies with foreground cosmic large-scale structures. As the correlation depends crucially on the growth history in the era of dark energy, it is a key observational probe for constraining the cosmological model. Here we present a detailed study based on full-sky and deep light cones generated from very large volume numerical N-body simulations, which allow us to avoid the use of standard replica techniques, while capturing the entirety of the late ISW effect on the large scales. We post-process the light cones using an accurate ray-tracing method and construct full-sky maps of the ISW temperature anisotropy for three different dark energy models. We quantify in detail the extent to which the ISW effect can be used to discriminate between different dark energy scenarios when cross-correlated with the matter distribution or the CMB lensing potential. We also investigate the onset of non-linearities, the so-called Rees-Sciama effect which provides a complementary probe of the dark sector. We find the signal of the lensing-lensing and ISW-lensing correlation of the three dark energy models to be consistent with measurements from the Planck satellite. Future surveys of the large-scale structures may provide cross-correlation measurements that are sufficiently precise to distinguish the signal of these models. Our methodology is very general and can be applied to any dark energy or modified gravity scenario as long as the metric seen by photons can still be characterized by a Weyl potential.

astro-ph.CO↗

arXiv2017

Reconstructing matter profiles of spherically compensated cosmic regions in $Λ$CDM cosmology

The absence of a physically motivated model for large scale profiles of cosmic voids limits our ability to extract valuable cosmological information from their study. In this paper, we address this problem by introducing the spherically compensated cosmic regions, named CoSpheres. Such cosmic regions are identified around local extrema in the density field and admit a unique compensation radius $R_1$ where the internal spherical mass is exactly compensated. Their origin is studied by extending the standard peak model and implementing the compensation condition. Since the compensation radius evolves as the Universe itself, $R_1(t)\propto a(t)$, CoSpheres behave as bubble Universes with fixed comoving volume. Using the spherical collapse model, we reconstruct their profiles with a very high accuracy until $z=0$ in N-body simulations. CoSpheres are symmetrically defined and reconstructed for both central maximum (seeding haloes and galaxies) and minimum (identified with cosmic voids). We show that the full non linear dynamics can be solved analytically around this particular compensation radius, providing useful predictions for cosmology. This formalism highlights original correlations between local extremum and their large scale cosmic environment. The statistical properties of these spherically compensated cosmic regions and the possibilities to constrain efficiently both cosmology and gravity will be investigated in companion papers.

astro-ph.CO↗

arXiv2017

Probability distribution and statistical properties of spherically compensated cosmic regions in $Λ$CDM cosmology

The statistical properties of cosmic structures are well known to be strong probes for cosmology. In particular, several studies tried to use the cosmic void counting number to obtain tight constrains on Dark Energy. In this paper we address this question by using the CoSphere model as introduced in de Fromont & Alimi (2017a). We derive their exact statistics in both primordial and non linearly evolved Universe for the standard $Λ$CDM model. We first compute the full joint Gaussian probability distribution for the various parameters describing these profiles in the Gaussian Random Field. We recover the results of Bardeen et al. (1986) only in the limit where the compensation radius becomes very large, i.e. when the central extremum decouples from its cosmic environment. We derive the probability distribution of the compensation size in this primordial field. We show that this distribution is redshift independent and can be used to model cosmic void size distribution. Interestingly, it can be used for central maximum such as DM haloes. We compute analytically the statistical distribution of the compensation density in both primordial and evolved Universe. We also derive the statistical distribution of the peak parameters already introduced by Bardeen et al. (1986) and discuss their correlation with the cosmic environment. We thus show that small central extrema with low density are associated with narrow compensation regions with a small $R_1$ and a deep compensation density $δ_1$ while higher central extrema are located in larger but smoother over/under massive regions.

astro-ph.CO↗

arXiv2014

Matter Power Spectrum Covariance Matrix from the DEUS-PUR ΛCDM simulations: Mass Resolution and non-Gaussian Errors

The upcoming generation of galaxy surveys will probe the distribution of matter in the universe with unprecedented accuracy. Measurements of the matter power spectrum at different scales and redshifts will provide stringent constraints on the cosmological parameters. However, on non-linear scales this will require an accurate evaluation of the covariance matrix. Here, we compute the covariance matrix of the 3D matter density power spectrum for the concordance $Λ$CDM cosmology from an ensemble of N-body simulations of the Dark Energy Universe Simulation - Parallel Universe Runs (DEUS-PUR). This consists of 12288 realisations of a $(656\,h^{-1}\,\textrm{Mpc})^3$ simulation box with $256^3$ particles. We combine this set with an auxiliary sample of 96 simulations of the same volume with $1024^3$ particles. We find N-body mass resolution effect to be an important source of systematic errors on the covariance at high redshift and small intermediate scales. We correct for this effect by introducing an empirical statistical method which provide an accurate determination of the covariance matrix over a wide range of scales including the Baryon Oscillations interval. Contrary to previous studies that used smaller N-body ensembles, we find the power spectrum distribution to significantly deviate from expectations of a Gaussian random density field at $k\gtrsim 0.25\,h\,\textrm{Mpc}^{-1}$ and $z<0.5$. This suggests that in the case of finite volume surveys an unbiased estimate of the ensemble averaged band power at these scales and redshifts may require a careful assessment of non-Gaussian errors more than previously considered.

astro-ph.CO↗

arXiv2014

Probing dark energy models with extreme pairwise velocities of galaxy clusters from the DEUS-FUR simulations

Observations of colliding galaxy clusters with high relative velocity probe the tail of the halo pairwise velocity distribution with the potential of providing a powerful test of cosmology. As an example it has been argued that the discovery of the Bullet Cluster challenges standard $Λ$CDM model predictions. Halo catalogs from N-body simulations have been used to estimate the probability of Bullet-like clusters. However, due to simulation volume effects previous studies had to rely on a Gaussian extrapolation of the pairwise velocity distribution to high velocities. Here, we perform a detail analysis using the halo catalogs from the Dark Energy Universe Simulation Full Universe Runs (DEUS-FUR), which enables us to resolve the high-velocity tail of the distribution and study its dependence on the halo mass definition, redshift and cosmology. Building upon these results we estimate the probability of Bullet-like systems in the framework of Extreme Value Statistics. We show that the tail of extreme pairwise velocities significantly deviates from that of a Gaussian, moreover it carries an imprint of the underlying cosmology. We find the Bullet Cluster probability to be two orders of magnitude larger than previous estimates, thus easing the tension with the $Λ$CDM model. Finally, the comparison of the inferred probabilities for the different DEUS-FUR cosmologies suggests that observations of extreme interacting clusters can provide constraints on dark energy models complementary to standard cosmological tests.

astro-ph.CO↗

arXiv2014

Cosmic variance limited Baryon Acoustic Oscillations from the DEUS-FUR $Λ$CDM simulation

We investigate the non-linear evolution of Baryon Acoustic Oscillations (BAO) in the low-redshift matter power spectrum from the DEUS-FUR $Λ$CDM model simulation. This is the first cosmological N-body simulation encompassing the full observable cosmic volume, thus allowing cosmic variance limited predictions at BAO scales. We control the effect of numerical systematic errors using a series of large volume high-resolution simulations. The combined analysis allows us to measure the matter power spectrum between $z=0$ and $1$ to 1% over the entire BAO range, $0.03<k [\textrm{h Mpc}^{-1}]<0.3$, in bins of size $Δk/k\lesssim$ 1%. We define the BAO with respect to a non-linearly evolved wiggle-free spectrum and determine the characteristics of the BAO without recurring to extrapolation from global fitting functions. We quantify the effects of non-linearities on the position and amplitude of the BAO extrema, and the coupling to the broadband slope of the power spectrum. We use these estimates to test non-linear predictions from semi-analytical models. Quite remarkably from the analysis of the redshift evolution of BAO we find that the second dip and third peak remains unaltered by non-linear effects. Furthermore, we find that the square of the damping factor and the shift of the position of BAO extrema scale to good approximation as the square of the growth factor, in agreement with expectations from perturbation theory. This confirms the idea that, besides cosmic distances, an accurate measurement of BAO at different redshifts can directly probe the growth of cosmic structures.

astro-ph.CO↗

arXiv2013

Imprints of Dark Energy on Cosmic Structure Formation: III. Sparsity of Dark Matter Halo Profiles

We study the imprint of Dark Energy on the density profile of Dark Matter halos using a set of high-resolution large volume cosmological N-body simulations from the Dark Energy Universe Simulation Series (DEUSS). We first focus on the analysis of the goodness-of-fit of the Navarro-Frenk-White (NFW) profile which we find to vary with halo mass and redshift. We also find that the fraction of halos ill-fitted by NFW varies with cosmology, thus indicating that the mass assembly of halos with perturbed density profiles carries a characteristic signature of Dark Energy. To access this information independently of any parametric profile, we introduce a new observable quantity: the halo sparsity $s_Δ$. This is defined as the mass ratio $M_{200}/M_Δ$, i.e. the ratio of mass inside a sphere of radius $r_{200}$ to that contained within a radius $r_Δ$, enclosing 200 and $Δ$ times the mean matter density respectively. We find the average sparsity to be nearly independent of the total halo mass, while its value can be inferred to better than a few percent from the ratio of the integrated halo mass functions at overdensities $Δ$ and 200 respectively. This provides a consistency relation that can validate observational measurements of the halo sparsity. Most importantly, the sparsity significantly varies with the underlying Dark Energy model, thus providing an alternative cosmological probe.

astro-ph.CO↗

arXiv2012

DEUS Full Observable ΛCDM Universe Simulation: the numerical challenge

We have performed the first-ever numerical N- body simulation of the full observable universe (DEUS "Dark Energy Universe Simulation" FUR "Full Universe Run"). This has evolved 550 billion particles on an Adaptive Mesh Refinement grid with more than two trillion computing points along the entire evolutionary history of the universe and across 6 order of magnitudes length scales, from the size of the Milky Way to that of the whole observable universe. To date, this is the largest and most advanced cosmological simulation ever run. It provides unique information on the formation and evolution of the largest structure in the universe and an exceptional support to future observational programs dedicated to mapping the distribution of matter and galaxies in the universe. The simulation has run on 4752 (of 5040) thin nodes of BULL supercomputer CURIE, using more than 300 TB of memory for 10 million hours of computing time. About 50 PBytes of data were generated throughout the run. Using an advanced and innovative reduction workflow the amount of useful stored data has been reduced to 500 TBytes.

astro-ph.CO↗

arXiv2006

Reinterpreting dark energy through backreaction: the minimally coupled morphon field

In the context of averaged cosmologies, the effective equations can be written in the form of "regional" Friedmannian equations with additional sources arising from the so-called backreaction of inhomogeneities. We propose a mean field description of this backreaction in terms of a regionally homogeneous scalar field: this provides a physical motivation to the phenomenological scalar fields generically called quintessence fields. We explicitly reconstruct the potential of the scalar field for a one-parameter family of scaling solutions to the backreaction problem, showing that it entails most of the standard scalar fields including e.g. standard and phantom quintessence scenarii.

astro-ph↗

arXiv2006

Big Bang nucleosynthesis in scalar tensor gravity: the key problem of the $^7$Li abundance

Combined with other CMB experiments, the WMAP survey provides an accurate estimate of the baryon density of the Universe. In the framework of the standard Big Bang Nucleosynthesis (BBN), such a baryon density leads to predictions for the primordial abundances of $^{4}$He and D in good agreement with observations. However, it also leads to a significant discrepancy between the predicted and observed primordial abundance of $^{7}$Li. Such a discrepancy is often termed as 'the lithium problem'. In this paper, we analyze this problem in the framework of scalar-tensor theories of gravity. It is shown that an expansion of the Universe slightly slower than in General Relativity before BBN, but faster during BBN, solves the lithium problem and leads to $^4$He and D primordial abundances consistent with the observational constraints. This kind of behavior is obtained in numerous scalar-tensor models, both with and without a self-interaction potential for the scalar field. In models with a self-interacting scalar field, the convergence towards General Relativity is ensured without any condition, thanks to an attraction mechanism which starts to work during the radiation-dominated epoch.

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arXiv2006

Correspondence between kinematical backreaction and scalar field cosmologies - the `morphon field'

Spatially averaged inhomogeneous cosmologies in classical general relativity can be written in the form of effective Friedmann equations with sources that include backreaction terms. In this paper we propose to describe these backreaction terms with the help of a homogeneous scalar field evolving in a potential; we call it the `morphon field'. This new field links classical inhomogeneous cosmologies to scalar field cosmologies, allowing to reinterpret, e.g., quintessence scenarios by routing the physical origin of the scalar field source to inhomogeneities in the Universe. We investigate a one-parameter family of scaling solutions to the backreaction problem. Subcases of these solutions (all without an assumed cosmological constant) include scale-dependent models with Friedmannian kinematics that can mimic the presence of a cosmological constant or a time-dependent cosmological term. We explicitly reconstruct the scalar field potential for the scaling solutions, and discuss those cases that provide a solution to the Dark Energy and coincidence problems. In this approach, Dark Energy emerges from morphon fields, a mechanism that can be understood through the proposed correspondence: the averaged cosmology is characterized by a weak decay (quintessence) or growth (phantom quintessence) of kinematical fluctuations, fed by `curvature energy' that is stored in the averaged 3-Ricci curvature. We find that the late-time trajectories of those models approach attractors that lie in the future of a state that is predicted by observational constraints.

gr-qc↗

arXiv2006

Reinterpreting quintessential dark energy through averaged inhomogeneous cosmologies

Regionally averaged relativistic cosmologies have recently been considered as a possible explanation for the apparent late time acceleration of the Universe. This contribution reports on a mean field description of the backreaction in terms of a minimally coupled regionally homogeneous scalar field evolving in a potential, then giving a physical origin to the various phenomenological scalar fields generically called quintessence fields. As an example, the correspondence is then applied to scaling solutions.

astro-ph↗

arXiv1998

Non equilibrium thermodynamics and cosmological pancakes formation

We investigate the influence of non equilibrium thermodynamics on cosmological structure formation. In this paper, we consider the collapse of planar perturbations usually called "Zel'dovich pancakes". We have developed for that purpose a new two fluids (gas and dark matter) hydrodynamical code, with three different thermodynamical species: electrons, ions and neutral particles (T_e\ne T_i \ne T_n). We describe in details the complex structure of accretion shock waves. We include several relevant processes for a low density, high temperature, collisional plasma such as non-equilibrium chemical reactions, cooling, shock heating, thermal energy equipartition between electrons, ions and neutral particles and electronic conduction. We find two different regions in the pancake structure: a thermal precursor ahead of the compression front and an equipartition wave after the compression front where electrons and ions temperatures differ significantly. This complex structure may have two interesting consequences: pre-heating of unshocked regions in the vicinity of massive X-ray clusters and ions and electrons temperatures differences in the outer regions of X-rays clusters.

astro-ph↗

arXiv1998

Non-linear dynamics and mass function of cosmic structures. II Numerical results

We compare the mass functions obtained analytically, in the framework of an extended Press & Schechter (PS) formalism, in a previous paper to the numerical mass functions obtained in N -- body simulations, using different algorithms to define objects in the density field. After discussing the properties of the algorithms, we show that the mass function obtained using the friend -- of -- friend algorithm reproduces best the scaling behaviors predicted in the extended PS formalism. Following this statistical analysis, we show that it is possible in the framework of our extended PS formalism to reproduce the mass function but also, and for the first time, the initial statistical properties of structures and their collapse time. This allow to present a ``coherent'' picture of structure formation which can account for the initial, final and dynamical properties of structures.

astro-ph↗

arXiv1997

Multi-phase Hydrodynamics and X-ray Clusters Formation

We investigate the role of radiative cooling within the core of large X-ray clusters using multi-phase hydrodynamics. We developed for that purpose a spherically symmetric hydrodynamical code, coupled to a "fluid model" that describes accurately the dark matter component. Cooling is included using a self-consistent multi-phase approach, leading to "cooled gas" mass deposition throughout the flow. We simulate the collapse and the subsequent evolution of a Coma-like X-ray cluster, avoiding the well-known "cooling catastrophe". The total mass profile of our simulated cluster is very similar to the "universal" profile proposed by Navarro, Frenk & White (1995). More interestingly, we also obtain a quasi-isothermal temperature profile, which is a direct consequence of multi-phase cooling within such a potential well.

astro-ph↗

arXiv1997

Are Large X-ray Clusters at Thermal Equilibrium ?

We simulate the formation of a large X-ray cluster using a fully 3D hydrodynamical code coupled to a Particle-Mesh scheme which models the dark matter component. We focus on a possible decoupling between electrons and ions temperatures. We then solve the energy transfer equations between electrons, ions and neutrals without assuming thermal equilibrium between the three gases (T_e <> T_i <> T_n). We solve self-consistently the chemical equations for an hydrogen/helium primordial plasma without assuming ionization-recombination equilibrium. We find that the electron temperature differs from the true dynamical temperature by 20% at the Virial radius of our simulated cluster. This could lead marginally to an underestimate of the total mass in the outer regions of large X-ray clusters.

astro-ph↗