arXiv subjects

Patrick Valageas

Publications and source records attributed to Patrick Valageas.

At least 55 records · Page 3Linked to original sources

arXiv2016

K-mouflage effects on clusters of galaxies

We investigate the effects of a K-mouflage modification of gravity on the dynamics of clusters of galaxies. We extend the description of K-mouflage to situations where the scalar field responsible for the modification of gravity is coupled to a perfect fluid with pressure. We describe the coupled system at both the background cosmology and cosmological perturbations levels, focusing on cases where the pressure emanates from small-scale nonlinear physics. We derive these properties in both the Einstein and Jordan frames, as these two frames already differ by a few percents at the background level for K-mouflage scenarios, and next compute cluster properties in the Jordan frame that is better suited to these observations. Galaxy clusters are not screened by the K-mouflage mechanism and therefore feel the modification of gravity in a maximal way. This implies that the halo mass function deviates from $Λ$-CDM by a factor of order one for masses $M\gtrsim 10^{14} \ h^{-1} M_\odot$. We then consider the hydrostatic equilibrium of gases embedded in galaxy clusters and the consequences of K-mouflage on the X-ray cluster luminosity, the gas temperature, and the Sunyaev-Zel'dovich effect. We find that the cluster temperature function, and more generally number counts, are largely affected by K-mouflage, mainly due to the increased cluster abundance in these models. Other scaling relations such as the mass-temperature and the temperature-luminosity relations are only modified at the percent level due to the constraints on K-mouflage from local Solar System tests.

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arXiv2016

Supersymmetric chameleons and ultra-local models

Super-chameleon models where all types of matter belong to three secluded sectors, i.e. the dark, supersymmetry breaking and matter sectors, are shown to be dynamically equivalent to ultra-local models of modified gravity. In the dark sector, comprising both dark matter and dark energy, the interaction range between the dark energy field and dark matter is constrained to be extremely short, i.e. shorter than the inverse gravitino mass set by supersymmetry breaking. This realises an extreme version of chameleon screening of the dark energy interaction. On the other hand, the baryonic matter sector decouples from the dark energy in a Damour-Polyakov way. These two mechanisms preclude the existence of any modification of gravity locally in the Solar System due to the presence of the super-chameleon field. On larger scales, the super-chameleon can have effects on the growth of structure and the number of dark matter halos. It can also affect the dynamics of galaxies where the fifth force interaction that it induces can have the same order of magnitude as Newton's interaction.

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arXiv2016

Ultra-local models of modified gravity without kinetic term

We present a class of modified-gravity theories which we call ultra-local models. We add a scalar field, with negligible kinetic terms, to the Einstein-Hilbert action. We also introduce a conformal coupling to matter. This gives rise to a new screening mechanism which is not entirely due to the non-linearity of the scalar field potential or the coupling function but to the absence of the kinetic term. As a result this removes any fifth force between isolated objects in vacuum. The predictions of these models only depend on a single free function, as the potential and the coupling function are degenerate, with an amplitude given by a parameter $α\lesssim 10^{-6}$, whose magnitude springs from requiring a small modification of Newton's potential astrophysically and cosmologically. This singles out a redshift $z_α \sim α^{-1/3} \gtrsim 100$ where the fifth force is the greatest. The cosmological background follows the $Λ$-CDM history within a $10^{-6}$ accuracy, while cosmological perturbations are significantly enhanced (or damped) on small scales, $k \gtrsim 2 h {\rm Mpc}^{-1}$ at $z=0$. The spherical collapse and the halo mass function are modified in the same manner. We find that the modifications of gravity are greater for galactic or sub-galactic structures. We also present a thermodynamic analysis of the non-linear and inhomogeneous fifth-force regime where we find that the Universe is not made more inhomogeneous before $z_α$ when the fifth force dominates, and does not lead to the existence of clumped matter on extra small scales inside halos for large masses while this possibility exists for masses $M\lesssim 10^{11} M_\odot$ where the phenomenology of ultra-local models would be most different from $Λ$-CDM.

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arXiv2016

3D Weak Lensing: Modified Theories of Gravity

Weak lensing (WL) promises to be a particularly sensitive probe of both the growth of large scale structure (LSS) as well as the fundamental relation between matter density perturbations and metric perturbations, thus providing a powerful tool with which we may constrain modified theories of gravity (MG) on cosmological scales. Future deep, wide-field WL surveys will provide an unprecedented opportunity to constrain deviations from General Relativity (GR). Employing a three-dimensional (3D) analysis based on the spherical Fourier-Bessel (sFB) expansion, we investigate the extent to which MG theories will be constrained by a typical 3D WL survey configuration including noise from the intrinsic ellipticity distribution $σ_ε$ of source galaxies. Here we focus on two classes of screened theories of gravity: i) $f(R)$ chameleon models and ii) environmentally dependent dilaton models. We use one-loop perturbation theory combined with halo models in order to accurately model the evolution of matter power-spectrum with redshift in these theories. Using a Fisher information matrix based approach, we show that for an all-sky spectroscopic survey, the parameter $f_{R_0}$ can be constrained in the range $f_{R_0}< 5\times 10^{-6}(9\times 10^{-6})$ for $n=1(2)$ with a 3$σ$ confidence level. This can be achieved by using relatively low order angular harmonics $\ell<100$. Including higher order harmonics $\ell>100$ can further tighten the constraints, making them comparable to current solar-system constraints. We also employ a Principal Component Analysis (PCA) in order to study the parameter degeneracies in the MG parameters. Our results can trivially be extended to other MG theories, such as the K-mouflage models. The confusion from intrinsic ellipticity correlation and modification of the matter power-spectrum at small scale due to feedback mechanisms is briefly discussed.

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arXiv2015

Galaxy Clustering in 3D and Modified Gravity Theories

We study Modified Gravity (MG) theories by modelling the redshifted matter power spectrum in a spherical Fourier-Bessel (sFB) basis. We use a fully non-linear description of the real-space matter power-spectrum and include the lowest-order redshift-space correction (Kaiser effect), taking into account some additional non-linear contributions. Ignoring relativistic corrections, which are not expected to play an important role for a shallow survey, we analyse two different modified gravity scenarios, namely the generalised Dilaton scalar-tensor theories and the $f({R})$ models in the large curvature regime. We compute the 3D power spectrum ${\cal C}^s_{\ell}(k_1,k_2)$ for various such MG theories with and without redshift space distortions, assuming precise knowledge of background cosmological parameters. Using an all-sky spectroscopic survey with Gaussian selection function $φ(r)\propto \exp(-{r^2 / r^2_0})$, $r_0 = 150 \, h^{-1} \, {\textrm{Mpc}}$, and number density of galaxies $\bar {\textrm{N}} =10^{-4}\;{\textrm{Mpc}}^{-3}$, we use a $χ^2$ analysis, and find that the lower-order $(\ell \leq 25)$ multipoles of ${\cal C}^s_\ell(k,k')$ (with radial modes restricted to $k < 0.2 \, h \,{\textrm{Mpc}}^{-1}$) can constraint the parameter $f_{R_0}$ at a level of $2\times 10^{-5} (3\times 10^{-5})$ with $3 σ$ confidence for $n=1(2)$. Combining constraints from higher $\ell > 25$ modes can further reduce the error bars and thus in principle make cosmological gravity constraints competitive with solar system tests. However this will require an accurate modelling of non-linear redshift space distortions. Using a tomographic $β(a)$-$m(a)$ parameterization we also derive constraints on specific parameters describing the Dilaton models of modified gravity.

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arXiv2015

K-mouflage gravity models that pass Solar System and cosmological constraints

We show that Solar System tests can place very strong constraints on K-mouflage models of gravity, which are coupled scalar field models with nontrivial kinetic terms that screen the fifth force in regions of large gravitational acceleration. In particular, the bounds on the anomalous perihelion of the Moon imposes stringent restrictions on the K-mouflage Lagrangian density, which can be met when the contributions of higher-order operators in the static regime are sufficiently small. The bound on the rate of change of the gravitational strength in the Solar System constrains the coupling strength $β$ to be smaller than $0.1$. These two bounds impose tighter constraints than the results from the Cassini satellite and Big Bang Nucleosynthesis. Despite the Solar System restrictions, we show that it is possible to construct viable models with interesting cosmological predictions. In particular, relative to $Λ$-CDM, such models predict percent-level deviations for the clustering of matter and the number density of dark matter haloes. This makes these models predictive and testable by forthcoming observational missions.

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arXiv2015

Testing Modified Gravity with Cosmic Shear

We use the cosmic shear data from the Canada-France-Hawaii Telescope Lensing Survey to place constraints on $f(R)$ and {\it Generalized Dilaton} models of modified gravity. This is highly complimentary to other probes since the constraints mainly come from the non-linear scales: maximal deviations with respects to the General-Relativity + $Λ$CDM scenario occurs at $k\sim1 h \mbox{Mpc}^{-1}$. At these scales, it becomes necessary to account for known degeneracies with baryon feedback and massive neutrinos, hence we place constraints jointly on these three physical effects. To achieve this, we formulate these modified gravity theories within a common tomographic parameterization, we compute their impact on the clustering properties relative to a GR universe, and propagate the observed modifications into the weak lensing $ξ_{\pm}$ quantity. Confronted against the cosmic shear data, we reject the $f(R)$ $\{ |f_{R_0}|=10^{-4}, n=1\}$ model with more than 99.9% confidence interval (CI) when assuming a $Λ$CDM dark matter only model. In the presence of baryonic feedback processes and massive neutrinos with total mass up to 0.2eV, the model is disfavoured with at least 94% CI in all different combinations studied. Constraints on the $\{ |f_{R_0}|=10^{-4}, n=2\}$ model are weaker, but nevertheless disfavoured with at least 89% CI. We identify several specific combinations of neutrino mass, baryon feedback and $f(R)$ or Dilaton gravity models that are excluded by the current cosmic shear data. Notably, universes with three massless neutrinos and no baryon feedback are strongly disfavoured in all modified gravity scenarios studied. These results indicate that competitive constraints may be achieved with future cosmic shear data.

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arXiv2015

Cosmological tests of modified gravity: constraints on $F(R)$ theories from the galaxy clustering ratio

The clustering ratio $η$, a large-scale structure observable originally designed to constrain the shape of the power spectrum of matter density fluctuations, is shown to provide a sensitive probe of the nature of gravity in the cosmological regime. We apply this analysis to $F(R)$ theories of gravity using the luminous red galaxy (LRG) sample extracted from the spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 and 10 catalogues. We find that General Relativity (GR), complemented with a Friedmann-Robertson-Walker (FRW) cosmological model with parameters fixed by the Planck satellite, describes extremely well the clustering of galaxies up to $z\sim 0.6$. On large cosmic scales, the absolute amplitude of deviations from GR, $|f_{R_0 }|$, is constrained to be smaller than $4.6 \times 10^{-5}$ at the $95\%$ confidence level. This bound makes cosmological probes of gravity almost competitive with the sensitivity of Solar System tests, although still one order of magnitude less effective than astrophysical tests. We also extrapolate our results to future large surveys like Euclid and show that the astrophysical bound will certainly remain out of reach for such a class of modified-gravity models that only differ from $Λ$CDM at low redshifts.

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arXiv2015

Redshift-space equal-time angular-averaged consistency relations of the gravitational dynamics

We present the redshift-space generalization of the equal-time angular-averaged consistency relations between $(\ell+n)$- and $n$-point polyspectra of the cosmological matter density field. Focusing on the case of $\ell=1$ large-scale mode and $n$ small-scale modes, we use an approximate symmetry of the gravitational dynamics to derive explicit expressions that hold beyond the perturbative regime, including both the large-scale Kaiser effect and the small-scale fingers-of-god effects. We explicitly check these relations, both perturbatively, for the lowest-order version that applies to the bispectrum, and nonperturbatively, for all orders but for the one-dimensional dynamics. Using a large ensemble of $N$-body simulations, we find that our squeezed bispectrum relation is valid to better than $20\%$ up to $1h$Mpc$^{-1}$, for both the monopole and quadrupole at $z=0.35$, in a $Λ$CDM cosmology. Additional simulations done for the Einstein-de Sitter background suggest that these discrepancies mainly come from the breakdown of the approximate symmetry of the gravitational dynamics. For practical applications, we introduce a simple ansatz to estimate the new derivative terms in the relation using only observables. Although the relation holds worse after using this ansatz, we can still recover it within $20\%$ up to $1h$Mpc$^{-1}$, at $z=0.35$ for the monopole. On larger scales, $k = 0.2 h\mathrm{Mpc}^{-1}$, it still holds within the statistical accuracy of idealized simulations of volume $\sim8h^{-3}\mathrm{Gpc}^3$ without shot-noise error.

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arXiv2015

Linear perturbations in K-mouflage cosmologies with massive neutrinos

We present a comprehensive derivation of linear perturbation equations for different matter species, including photons, baryons, cold dark matter, scalar fields, massless and massive neutrinos, in the presence of a generic conformal coupling. Starting from the Lagrangians, we show how the conformal transformation affects the dynamics. In particular, we discuss how to incorporate consistently the scalar coupling in the equations of the Boltzmann hierarchy for massive neutrinos and the subsequent fluid approximations. We use the recently proposed K-mouflage model as an example to demonstrate the numerical implementation of our linear perturbation equations. K-mouflage is a new mechanism to suppress the fifth force between matter particles induced by the scalar coupling, but in the linear regime the fifth force is unsuppressed and can change the clustering of different matter species in different ways. We show how the CMB, lensing potential and matter power spectra are affected by the fifth force, and find ranges of K-mouflage parameters whose effects could be seen observationally. We also find that the scalar coupling can have the nontrivial effect of shifting the amplitude of the power spectra of the lensing potential and density fluctuations in opposite directions, although both probe the overall clustering of matter. This paper can serve as a reference for those who work on generic coupled scalar field cosmology, or those who are interested in the cosmological behaviour of the K-mouflage model.

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arXiv2014

Kinematic consistency relations of large-scale structures

We describe how the kinematic consistency relations satisfied by density correlations of the large-scale structures of the Universe can be derived within the usual Newtonian framework. These relations express a kinematic effect and show how the $(\ell+n)$-density correlation factors in terms of the $n$-point correlation and $\ell$ linear power spectrum factors, in the limit where the $\ell$ soft wave numbers become linear and much smaller than the $n$ other wave numbers. We describe how these relations extend to multifluid cases. In the standard cosmology, these consistency relations derive from the equivalence principle. A detection of their violation would indicate non-Gaussian initial conditions, non-negligible decaying modes, or a modification of gravity that does not converge to General Relativity on large scales.

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arXiv2014

K-mouflage Cosmology: the Background Evolution

We study the cosmology of K-mouflage theories at the background level. We show that the effects of the scalar field are suppressed at high matter density in the early Universe and only play a role in the late time Universe where the deviations of the Hubble rate from its $Λ$-CDM counterpart can be of the order five percent for redshifts $1 \lesssim z \lesssim 5$. Similarly, we find that the equation of state can cross the phantom divide in the recent past and even diverge when the effective scalar energy density goes negative and subdominant compared to matter, preserving the positivity of the squared Hubble rate. These features are present in models for which Big Bang Nucleosynthesis is not affected. We analyze the fate of K-mouflage when the nonlinear kinetic terms give rise to ghosts, particle excitations with negative energy. In this case, we find that the K-mouflage theories can only be considered as an effective description of the Universe at low energy below $1$ keV. In the safe ghost-free models, we find that the equation of state always diverges in the past and changes significantly by a few percent since $z\lesssim 1$.

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arXiv2014

K-mouflage Cosmology: Formation of Large-Scale Structures

We study structure formation in K-mouflage cosmology whose main feature is the absence of screening effect on quasilinear scales. We show that the growth of structure at the linear level is affected by both a new time dependent Newton constant and a friction term which depend on the background evolution. These combine with the modified background evolution to change the growth rate by up to ten percent since $z\sim 2$. At the one loop level, we find that the nonlinearities of the K-mouflage models are mostly due to the matter dynamics and that the scalar perturbations can be treated at tree level. We also study the spherical collapse in K-mouflage models and show that the critical density contrast deviates from its $Λ$-CDM value and that, as a result, the halo mass function is modified for large masses by an order one factor. Finally we consider the deviation of the matter spectrum from $Λ$-CDM on nonlinear scales where a halo model is utilized. We find that the discrepancy peaks around $1\ h{\rm Mpc}^{-1}$ with a relative difference which can reach fifty percent. Importantly, these features are still true at larger redshifts, contrary to models of the chameleon-$f(R)$ and Galileon types.

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arXiv2014

Small-scale Nonlinear Dynamics of K-mouflage Theories

We investigate the small-scale static configurations of K-mouflage models defined by a general function $K(χ)$ of the kinetic terms. The fifth force is screened by the nonlinear K-mouflage mechanism if $K'(χ)$ grows sufficiently fast for large negative $χ$. In the general non-spherically symmetric case, the fifth force is not aligned with the Newtonian force. For spherically symmetric static matter density profiles, the results depend on the potential function $W_{-}(y) = y K'(-y^2/2)$, which must be monotonically increasing to $+\infty$ for $y \geq 0$ to guarantee the existence of a single solution throughout space for any matter density profile. Small radial perturbations around these static profiles propagate as traveling waves with a velocity greater than the speed of light. Starting from vanishing initial conditions for the scalar field and for a time-dependent matter density corresponding to the formation of an overdensity, we numerically check that the scalar field converges to the static solution. If $W_{-}$ is bounded, for high-density objects there are no static solutions throughout space, but one can still define a static solution restricted to large radii. Our dynamical study shows that the scalar field relaxes to this static solution at large radii, whereas spatial gradients keep growing with time at smaller radii. If $W_{-}$ is not bounded but non-monotonic, there is an infinite number of discontinuous static solutions but they are not physical and these models are not theoretically sound. Such K-mouflage scenarios provide an example of theories that can appear viable at the cosmological level, for the cosmological background and perturbative analysis, while being meaningless at a nonlinear level for small-scale configurations. This shows the importance of small-scale nonlinear analysis of screening models.

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arXiv2014

Testing the equal-time angular-averaged consistency relation of the gravitational dynamics in N-body simulations

We explicitly test the equal-time consistency relation between the angular-averaged bispectrum and the power spectrum of the matter density field, employing a large suite of cosmological $N$-body simulations. This is the lowest-order version of the relations between $(\ell+n)$-point and $n$-point polyspectra, where one averages over the angles of $\ell$ soft modes. This relation depends on two wave numbers, $k'$ in the soft domain and $k$ in the hard domain. We show that it holds up to a good accuracy, when $k'/k\ll 1$ and $k'$ is in the linear regime, while the hard mode $k$ goes from linear ($0.1\,h\mathrm{Mpc}^{-1}$) to nonlinear ($1.0\,h\mathrm{Mpc}^{-1}$) scales. On scales $k\lesssim 0.4\,h\mathrm{Mpc}^{-1}$, we confirm the relation within the statistical error of the simulations (typically a few percent depending on the wave number), even though the bispectrum can already deviate from leading-order perturbation theory by more than $30\%$. We further examine the relation on smaller scales with higher resolution simulations. We find that the relation holds within the statistical error of the simulations at $z=1$, whereas we find deviations as large as $\sim 7\%$ at $k \sim 1.0\,h\mathrm{Mpc}^{-1}$ at $z=0.35$. We show that this can be explained partly by the breakdown of the approximation $Ω_\mathrm{m}/f^2\simeq1$ with supplemental simulations done in the Einstein-de Sitter background cosmology. We also estimate the impact of this approximation on the power spectrum and bispectrum.

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arXiv2014

Angular averaged consistency relations of large-scale structures

The cosmological dynamics of gravitational clustering satisfies an approximate invariance with respect to the cosmological parameters that is often used to simplify analytical computations. We describe how this approximate symmetry gives rise to angular averaged consistency relations for the matter density correlations. This allows one to write the $(\ell+n)$ density correlation, with $\ell$ large-scale linear wave numbers that are integrated over angles, and $n$ fixed small-scale nonlinear wave numbers, in terms of the small-scale $n$-point density correlation and $\ell$ prefactors that involve the linear power spectra at the large-scale wave numbers. These relations, which do not vanish for equal-time statistics, go beyond the already known kinematic consistency relations. They could be used to detect primordial non-Gaussianities, modifications of gravity, limitations of galaxy biasing schemes, or to help designing analytical models of gravitational clustering.

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arXiv2014

Source-lens clustering and intrinsic-alignment bias of weak-lensing estimators

We estimate the amplitude of the source-lens clustering bias and of the intrinsic-alignment bias of weak lensing estimators of the two-point and three-point convergence and cosmic-shear correlation functions. We use a linear galaxy bias model for the galaxy-density correlations, as well as a linear intrinsic-alignment model. For the three-point and four-point density correlations, we use analytical or semi-analytical models, based on a hierarchical ansatz or a combination of one-loop perturbation theory with a halo model. For two-point statistics, we find that the source-lens clustering bias is typically several orders of magnitude below the weak lensing signal, except when we correlate a very low-redshift galaxy ($z_2 \la 0.05$) with a higher redshift galaxy ($z_1 \ga 0.5$), where it can reach $10\%$ of the signal for the shear. For three-point statistics, the source-lens clustering bias is typically of order $10\%$ of the signal, as soon as the three galaxy source redshifts are not identical. The intrinsic-alignment bias is typically about $10\%$ of the signal for both two-point and three-point statistics. Thus, both source-lens clustering bias and intrinsic-alignment bias must be taken into account for three-point estimators aiming at a better than $10\%$ accuracy.

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arXiv2014

Density fields and halo mass functions in the Geometrical Adhesion toy Model

In dimension 2 and above, the Burgers dynamics, the so-called "adhesion model" in cosmology, can actually give rise to several dynamics in the inviscid limit. We investigate here the statistical properties of the density field when it is defined by a "geometrical model" associated with this Burgers velocity field and where the matter distribution is fully determined, at each time step, by geometrical constructions. Our investigations are based on a set of numerical experiments that make use of an improved algorithm, for which the geometrical constructions are efficient and robust. In this work we focus on Gaussian initial conditions with power-law power spectra of slope $n$ in the range $-3<n<1$, where a self-similar evolution develops, and we compute the behavior of power spectra, density probability distributions and mass functions. As expected for such dynamics, the density power spectra show universal high-$k$ tails that are governed by the formation of pointlike masses. The two other statistical indicators however show the same qualitative properties as those observed for 3D gravitational clustering. In particular, the mass functions obey a Press-Schechter like scaling up to a very good accuracy in 1D, and to a lesser extent in 2D. Our results suggest that the "geometrical adhesion model", whose solution is fully known at all times, provides a precious tool to understand some of the statistical constructions frequently used to study the development of mass halos in gravitational clustering.

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