arXiv subjects

Valerio Marra

Publications and source records attributed to Valerio Marra.

At least 73 records · Page 4Linked to original sources

arXiv2018

Null tests of the standard model using the linear model formalism

We test both the FLRW geometry and $Λ$CDM cosmology in a model independent way by reconstructing the Hubble function $H(z)$, the comoving distance $D(z)$ and the growth of structure $fσ_8(z)$ using the most recent data available. We use the linear model formalism in order to optimally reconstruct the latter cosmological functions, together with their derivatives and integrals. We then evaluate four of the null tests available in literature: $Om_{1}$ by Sahni et al., $Om_{2}$ by Zunckel \& Clarkson, $Ok$ by Clarkson et al., and $ns$ by Nesseris \& Sapone. For all the four tests we find agreement, within the errors, with the standard cosmological model.

astro-ph.CO↗

arXiv2017

Clustering dark energy and halo abundances

Within the standard paradigm, dark energy is taken as a homogeneous fluid that drives the accelerated expansion of the universe and does not contribute to the mass of collapsed objects such as galaxies and galaxy clusters. The abundance of galaxy clusters -- measured through a variety of channels -- has been extensively used to constrain the normalization of the power spectrum: it is an important probe as it allows us to test if the standard $Λ$CDM model can indeed accurately describe the evolution of structures across billions of years. It is then quite significant that the Planck satellite has detected, via the Sunyaev-Zel'dovich effect, less clusters than expected according to the primary CMB anisotropies. One of the simplest generalizations that could reconcile these observations is to consider models in which dark energy is allowed to cluster, i.e., allowing its sound speed to vary. In this case, however, the standard methods to compute the abundance of galaxy clusters need to be adapted to account for the contributions of dark energy. In particular, we examine the case of clustering dark energy -- a dark energy fluid with negligible sound speed -- with a redshift-dependent equation of state. We carefully study how the halo mass function is modified in this scenario, highlighting corrections that have not been considered before in the literature. We address modifications in the growth function, collapse threshold, virialization densities and also changes in the comoving scale of collapse and mass function normalization. Our results show that clustering dark energy can impact halo abundances at the level of 10\%--30\%, depending on the halo mass, and that cluster counts are modified by about 30\% at a redshift of unity.

astro-ph.CO↗

arXiv2017

Evidences against cuspy dark matter halos in large galaxies

We develop and apply new techniques in order to uncover galaxy rotation curves (RC) systematics. Considering that an ideal dark matter (DM) profile should yield RCs that have no bias towards any particular radius, we find that the Burkert DM profile satisfies the test, while the Navarro-Frenk-While (NFW) profile has a tendency of better fitting the region between one and two disc scale lengths than the inner disc scale length region. Our sample indicates that this behaviour happens to more than 75% of the galaxies fitted with an NFW halo. Also, this tendency does not weaken by considering "large" galaxies, for instance those with $M_*\gtrsim 10^{10} M_\odot$. Besides the tests on the homogeneity of the fits, we also use a sample of 62 galaxies of diverse types to perform tests on the quality of the overall fit of each galaxy, and to search for correlations with stellar mass, gas mass and the disc scale length. In particular, we find that only 13 galaxies are better fitted by the NFW halo; and that even for the galaxies with $M_* \gtrsim 10^{10} M_\odot$ the Burkert profile either fits as good as, or better than, the NFW profile. This result is relevant since different baryonic effects important for the smaller galaxies, like supernova feedback and dynamical friction from baryonic clumps, indicate that at such large stellar masses the NFW profile should be preferred over the Burkert profile. Hence, our results either suggest a new baryonic effect or a change of the dark matter physics.

astro-ph.GA↗

arXiv2016

Cosmological constraints on the radiation released during structure formation

During the process of structure formation in the universe matter is converted into radiation through a variety of processes such as light from stars, infrared radiation from cosmic dust and gravitational waves from binary black holes/neutron stars and supernova explosions. The production of this astrophysical radiation background (ARB) could affect the expansion rate of the universe and the growth of perturbations. Here, we aim at understanding to which level one can constraint the ARB using future cosmological observations. We model the energy transfer from matter to radiation through an effective interaction between matter and astrophysical radiation. Using future supernova data from LSST and growth-rate data from Euclid we find that the ARB density parameter is constrained, at the 95% confidence level, to be $Ω_{ar_0}<0.008$. Estimates of the energy density produced by well-known astrophysical processes give roughly $Ω_{ar_0}\sim 10^{-5}$. Therefore, we conclude that cosmological observations will only be able to constrain exotic or not-well understood sources of radiation.

astro-ph.CO↗

arXiv2016

Constraining the halo mass function with observations

The abundances of dark matter halos in the universe are described by the halo mass function (HMF). It enters most cosmological analyses and parametrizes how the linear growth of primordial perturbations is connected to these abundances. Interestingly, this connection can be made approximately cosmology independent. This made it possible to map in detail its near-universal behavior through large-scale simulations. However, such simulations may suffer from systematic effects, especially if baryonic physics is included. In this paper we ask how well observations can constrain directly the HMF. The observables we consider are galaxy cluster number counts, galaxy cluster power spectrum and lensing of type Ia supernovae. Our results show that DES is capable of putting the first meaningful constraints on the HMF, while both Euclid and J-PAS can give stronger constraints, comparable to the ones from state-of-the-art simulations. We also find that an independent measurement of cluster masses is even more important for measuring the HMF than for constraining the cosmological parameters, and can vastly improve the determination of the halo mass function. Measuring the HMF could thus be used to cross-check simulations and their implementation of baryon physics. It could even, if deviations cannot be accounted for, hint at new physics.

astro-ph.CO↗

arXiv2016

Cosmology and Fundamental Physics with the Euclid Satellite

Euclid is a European Space Agency medium class mission selected for launch in 2020 within the Cosmic Vision 2015 2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

astro-ph.CO↗

arXiv2016

Coupling dark energy to dark matter inhomogeneities

We propose that dark energy in the form of a scalar field could effectively couple to dark matter inhomogeneities. Through this coupling energy could be transferred to/from the scalar field, which could possibly enter an accelerated regime. Though phenomenological, this scenario is interesting as it provides a natural trigger for the onset of the acceleration of the universe, since dark energy starts driving the expansion of the universe when matter inhomogeneities become sufficiently strong. Here we study a possible realization of this idea by coupling dark energy to dark matter via the linear growth function of matter perturbations. The numerical results show that it is indeed possible to obtain a viable cosmology with the expected series of radiation, matter and dark-energy dominated eras. In particular, the current density of dark energy is given by the value of the coupling parameters rather than by very special initial conditions for the scalar field. In other words, this model - unlike standard models of cosmic late acceleration - does not suffer from the so-called "coincidence problem" and its related fine tuning of initial conditions.

astro-ph.CO↗

arXiv2016

Perturbed Newtonian description of the Lemaître model with non-negligible pressure

We study the validity of the Newtonian description of cosmological perturbations using the Lemaitre model, an exact spherically symmetric solution of Einstein's equation. This problem has been investigated in the past for the case of a dust fluid. Here, we extend the previous analysis to the more general case of a fluid with non-negligible pressure, and, for the numerical examples, we consider the case of radiation (P=ρ/3). We find that, even when the density contrast has a nonlinear amplitude, the Newtonian description of the cosmological perturbations using the gravitational potential ψand the curvature potential ϕis valid as long as we consider sub-horizon inhomogeneities. However, the relation ψ+ϕ={\cal O}(ϕ^2), which holds for the case of a dust fluid, is not valid for a relativistic fluid and effective anisotropic stress is generated. This demonstrates the usefulness of the Lemaitre model which allows us to study in an exact nonlinear fashion the onset of anisotropic stress in fluids with non-negligible pressure. We show that this happens when the characteristic scale of the inhomogeneity is smaller than the sound horizon and that the deviation is caused by the nonlinear effect of the fluid's fast motion. We also find that ψ+ϕ= \max[{\cal O}(ϕ^2),{\cal O}(c_s^2ϕ\, δ)] for an inhomogeneity with density contrast δwhose characteristic scale is smaller than the sound horizon, unless w is close to -1, where w and c_s are the equation of state parameter and the sound speed of the fluid, respectively. On the other hand, we expect ψ+ϕ={\cal O}(ϕ^2) to hold for an inhomogeneity whose characteristic scale is larger than the sound horizon, unless the amplitude of the inhomogeneity is large and w is close to -1.

gr-qc↗

arXiv2015

The evolving perception of controversial movies

Polarization of opinion is an important feature of public debate on political, social and cultural topics. The availability of large internet databases of users' ratings has permitted quantitative analysis of polarization trends-for instance, previous studies have included analyses of controversial topics on Wikipedia, as well as the relationship between online reviews and a product's perceived quality. Here, we study the dynamics of polarization in the movie ratings collected by the Internet Movie database (IMDb) website in relation to films produced over the period 1915-2015. We define two statistical indexes, dubbed hard and soft controversiality, which quantify polarized and uniform rating distributions, respectively. We find that controversy decreases with popularity and that hard controversy is relatively rare. Our findings also suggest that more recent movies are more controversial than older ones and we detect a trend of "convergence to the mainstream" with a time scale of roughly 40-50 years. This phenomenon appears qualitatively different from trends observed in both online reviews of commercial products and in political debate, and we speculate that it may be connected with the absence of long-lived "echo chambers" in the cultural domain. This hypothesis can and should be tested by extending our analysis to other forms of cultural expression and/or to databases with different demographic user bases.

physics.soc-ph↗

arXiv2015

Constraining the growth of perturbations with lensing of supernovae

A recently proposed technique allows one to constrain both the background and perturbation cosmological parameters through the distribution function of supernova Ia apparent magnitudes. Here we extend this technique to alternative cosmological scenarios, in which the growth of structure does not follow the $Λ$CDM prescription. We apply the method first to the supernova data provided by the JLA catalog combined with all the current independent redshift distortion data and with low-redshift cluster data from Chandra and show that although the supernovae alone are not very constraining, they help in reducing the confidence regions. Then we apply our method to future data from LSST and from a survey that approximates the Euclid satellite mission. In this case we show that the combined data are nicely complementary and can constrain the normalization $σ_8$ and the growth rate index $γ$ to within $0.6\%$ and $7\%$, respectively. In particular, the LSST supernova catalog is forecast to give the constraint $γ(σ_8/0.83)^{6.7} = 0.55 \pm 0.1$. We also report on constraints relative to a step-wise parametrization of the growth rate of structures. These results show that supernova lensing serves as a good cross-check on the measurement of perturbation parameters from more standard techniques.

astro-ph.CO↗

arXiv2014

Searching for bias and correlations in a Bayesian way

A range of Bayesian tools has become widely used in cosmological data treatment and parameter inference (see Kunz, Bassett & Hlozek (2007), Trotta (2008), Amendola, Marra & Quartin (2013)). With increasingly big datasets and higher precision, tools that enable us to further enhance the accuracy of our measurements gain importance. Here we present an approach based on internal robustness, introduced in Amendola, Marra & Quartin (2013) and adopted in Heneka, Marra & Amendola (2014), to identify biased subsets of data and hidden correlation in a model independent way.

astro-ph.CO↗

arXiv2014

Intrinsic uncertainty on the nature of dark energy

We argue that there is an intrinsic noise on measurements of the equation of state parameter $w=p/ρ$ from large-scale structure around us. The presence of the large-scale structure leads to an ambiguity in the definition of the background universe and thus there is a maximal precision with which we can determine the equation of state of dark energy. To study the uncertainty due to local structure, we model density perturbations stemming from a standard inflationary power spectrum by means of the exact Lemaître-Tolman-Bondi solution of Einstein's equation, and show that the usual distribution of matter inhomogeneities in a $Λ$CDM cosmology causes a variation of $w$ -- as inferred from distance measures -- of several percent. As we observe only one universe, or equivalently because of the cosmic variance, this uncertainty is systematic in nature.

astro-ph.CO↗

arXiv2014

Linear Perturbation constraints on Multi-coupled Dark Energy

The Multi-coupled Dark Energy (McDE) scenario has been recently proposed as a specific example of a cosmological model characterized by a non-standard physics of the dark sector of the universe that nevertheless gives an expansion history which does not significantly differ from the one of the standard $Λ$CDM model. In this work, we present the first constraints on the McDE scenario obtained by comparing the predicted evolution of linear density perturbations with a large compilation of recent data sets for the growth rate $fσ_{8}$, including 6dFGS, LRG, BOSS, WiggleZ and VIPERS. Confirming qualitative expectations, growth rate data provide much tighter bounds on the model parameters as compared to the extremely loose bounds that can be obtained when only the background expansion history is considered. In particular, the $95\%$ confidence level on the coupling strength $|β|$ is reduced from $|β|\leq 83$ (background constraints only) to $|β|\leq 0.88$ (background and linear perturbation constraints). We also investigate how these constraints further improve when using data from future wide-field surveys such as supernova data from LSST and growth rate data from Euclid-type missions. In this case the $95\%$ confidence level on the coupling further reduce to $|β|\leq 0.85$. Such constraints are in any case still consistent with a scalar fifth-force of gravitational strength, and we foresee that tighter bounds might be possibly obtained from the investigation of nonlinear structure formation in McDE cosmologies.[Abridged]

astro-ph.CO↗

arXiv2014

Accurate Weak Lensing of Standard Candles. II. Measuring sigma8 with Supernovae

Soon the number of type Ia supernova (SN) measurements should exceed 100,000. Understanding the effect of weak lensing by matter structures on the supernova brightness will then be more important than ever. Although SN lensing is usually seen as a source of systematic noise, we will show that it can be in fact turned into signal. More precisely, the non-Gaussianity introduced by lensing in the SN Hubble diagram dispersion depends rather sensitively on the amplitude sigma8 of the matter power spectrum. By exploiting this relation, we are able to predict constraints on sigma8 of 7% (3%) for a catalog of 100,000 (500,000) SNe of average magnitude error 0.12 without having to assume that such intrinsic dispersion is known a priori. The intrinsic dispersion has been assumed to be Gaussian; possible intrinsic non-Gaussianities in the dataset (due to the SN themselves and/or to other transients) could be potentially dealt with by means of additional nuisance parameters describing higher moments of the intrinsic dispersion distribution function. This method is independent of and complementary to the standard methods based on CMB, cosmic shear or cluster abundance observables.

astro-ph.CO↗

arXiv2014

Extensive search for bias in SNIa data

The use of advanced statistical analysis tools is crucial in order to improve cosmological parameter estimates via removal of systematic errors and identification of previously unaccounted for cosmological signals. Here we demonstrate the application of a new fully-Bayesian method, the internal robustness formalism, to scan for systematics and new signals in the recent supernova Ia Union compilations. Our analysis is tailored to maximize chances of detecting the anomalous subsets by means of a variety of sorting algorithms. We analyse supernova Ia distance moduli for effects depending on angular separation, redshift, surveys and hemispherical directions. The data have proven to be robust within 2 sigma, giving an independent confirmation of successful removal of systematics-contaminated supernovae. Hints of new cosmology, as for example the anisotropies reported by Planck, do not seem to be reflected in the supernova Ia data.

astro-ph.CO↗

arXiv2013

Testing the Copernican principle by constraining spatial homogeneity

We present a new programme for placing constraints on radial inhomogeneity in a dark-energy dominated universe. We introduce a new measure to quantify violations of the Copernican principle. Any violation of this principle would interfere with our interpretation of any dark-energy evolution. In particular, we find that current observations place reasonably tight constraints on possible late-time violations of the Copernican principle: the allowed area in the parameter space of amplitude and scale of a spherical inhomogeneity around the observer has to be reduced by a factor of three so as to confirm the Copernican principle. Then, by marginalizing over possible radial inhomogeneity we provide the first constraints on the cosmological constant which are free of the homogeneity prior prevalent in cosmology.

astro-ph.CO↗

arXiv2013

Seeding supermassive black holes with a non-vortical dark-matter subcomponent

A perfect irrotational fluid with the equation of state of dust, Irrotational Dark Matter (IDM), is incapable of virializing and instead forms a cosmoskeleton of filaments with supermassive black holes at the joints. This stark difference from the standard cold dark matter (CDM) scenario arises because IDM must exhibit potential flow at all times, preventing shell-crossing from occurring. This scenario is applicable to general non-oscillating scalar-field theories with a small sound speed. Our model of combined IDM and CDM components thereby provides a solution to the problem of forming the observed billion-solar-mass black holes at redshifts of six and higher. In particular, as a result of the reduced vortical flow, the growth of the black holes is expected to be more rapid at later times as compared to the standard scenario.

astro-ph.CO↗

arXiv2013

Accurate weak lensing of standard candles. I. Flexible cosmological fits

With the availability of thousands of type Ia supernovae in the near future the magnitude scatter induced by lensing will become a major issue as it affects parameter estimation. Current N-body simulations are too time consuming to be integrated in the likelihood analyses used for estimating the cosmological parameters. In this paper we show that in the weak lensing regime a statistical numerical approximation produces accurate results orders of magnitude faster. We write down simple fits to the second, third and fourth central moments of the lensing magnification probability distribution as a function of redshift, of the power spectrum normalization and of the present-day matter density. We also improve upon existing models of lensing variance and show that a shifted lognormal distribution fits well the numerical one. These fits can be easily employed in cosmological likelihood analyses. Moreover, our theoretical predictions make it possible to invert the problem and begin using supernovae lensing to constrain the cosmological parameters.

astro-ph.CO↗