arXiv subjects

Marco Baldi

Publications and source records attributed to Marco Baldi.

At least 127 records · Page 7Linked to original sources

arXiv2016

On the Error Detection Capability of Combined LDPC and CRC Codes for Space Telecommand Transmissions

We present a method for estimating the undetected error rate when a cyclic redundancy check (CRC) is performed on the output of the decoder of short low-density parity-check (LDPC) codes. This system is of interest for telecommand links, where new LDPC codes have been designed for updating the current standard. We show that these new LDPC codes combined with CRC are adequate for complying with the stringent requirements of this kind of transmissions in terms of error detection.

cs.IT↗

arXiv2016

The effect of interacting dark energy on local measurements of the Hubble constant

In the current state of cosmology, where cosmological parameters are being measured to percent accuracy, it is essential to understand all sources of error to high precision. In this paper we present the results of a study of the local variations in the Hubble constant measured at the distance scale of the Coma Cluster, and test the validity of correcting for the peculiar velocities predicted by gravitational instability theory. The study is based on N-body simulations, and includes models featuring a coupling between dark energy and dark matter, as well as two $Λ$CDM simulations with different values of $σ_8$. It is found that the variance in the local flows is significantly larger in the coupled models, which increases the uncertainty in the local measurements of the Hubble constant in these scenarios. By comparing the results from the different simulations, it is found that most of the effect is caused by the higher value of $σ_8$ in the coupled cosmologies, though this cannot account for all of the additional variance. Given the discrepancy between different estimates of the Hubble constant in the universe today, cosmological models causing a greater cosmic variance is something that we should be aware of.

astro-ph.CO↗

arXiv2016

Time-Invariant Spatially Coupled Low-Density Parity-Check Codes with Small Constraint Length

We consider a special family of SC-LDPC codes, that is, time-invariant LDPCC codes, which are known in the literature for a long time. Codes of this kind are usually designed by starting from QC block codes, and applying suitable unwrapping procedures. We show that, by directly designing the LDPCC code syndrome former matrix without the constraints of the underlying QC block code, it is possible to achieve smaller constraint lengths with respect to the best solutions available in the literature. We also find theoretical lower bounds on the syndrome former constraint length for codes with a specified minimum length of the local cycles in their Tanner graphs. For this purpose, we exploit a new approach based on a numerical representation of the syndrome former matrix, which generalizes over a technique we already used to study a special subclass of the codes here considered.

cs.IT↗

arXiv2016

Structure formation simulations with momentum exchange: alleviating tensions between high-redshift and low-redshift cosmological probes

Persisting tensions between the cosmological constraints derived from low-redshift probes and the ones obtained from temperature and polarisation anisotropies of the Cosmic Microwave Background -- although not yet providing compelling evidence against the $Λ$CDM model -- seem to consistently indicate a slower growth of density perturbations as compared to the predictions of the standard cosmological scenario. Such behavior is not easily accommodated by the simplest extensions of General Relativity, such as f(R) models, which generically predict an enhanced growth rate. In the present work we present the outcomes of a suite of large N-body simulations carried out in the context of a cosmological model featuring a non-vanishing scattering cross section between the dark matter and the dark energy fields, for two different parameterisations of the dark energy equation of state. Our results indicate that these Dark Scattering models have very mild effects on many observables related to large-scale structures formation and evolution, while providing a significant suppression of the amplitude of linear density perturbations and the abundance of massive clusters. Our simulations therefore confirm that these models offer a promising route to alleviate existing tensions between low-redshift measurements and those of the CMB.

astro-ph.CO↗

arXiv2016

The mass accretion rate of galaxy clusters: a measurable quantity

We explore the possibility of measuring the mass accretion rate (MAR) of galaxy clusters from their mass profiles beyond the virial radius $R_{200}$. We derive the accretion rate from the mass of a spherical shell whose inner radius is $2R_{200}$, whose thickness changes with redshift, and whose infall velocity is assumed to be equal to the mean infall velocity of the spherical shells of dark matter halos extracted from $N$-body simulations. This approximation is rather crude in hierarchical clustering scenarios where both smooth accretion and aggregation of smaller dark matter halos contribute to the mass accretion of clusters.Nevertheless, in the redshift range $z=[0,2]$, our prescription returns an average MAR within $20-40 \%$ of the average rate derived from the merger trees of dark matter halos extracted from $N$-body simulations. The MAR of galaxy clusters has been the topic of numerous detailed numerical and theoretical investigations, but so far it has remained inaccessible to measurements in the real universe. Since the measurement of the mass profile of clusters beyond their virial radius can be performed with the caustic technique applied to dense redshift surveys of the cluster outer regions, our result suggests that measuring the mean MAR of a sample of galaxy clusters is actually feasible. We thus provide a new potential observational test of the cosmological and structure formation models.

astro-ph.CO↗

arXiv2015

The Imprint of f(R) Gravity on Non-Linear Structure Formation

We test the imprint of f(R) modified gravity on the halo mass function, using N-body simulations and a theoretical model developed in (Kopp et al. 2013). We find a very good agreement between theory and simulations. We extend the theoretical model to the conditional mass function and apply it to the prediction of the linear halo bias in f(R) gravity. Using the halo model we obtain a prediction for the non-linear matter power spectrum accurate to ~10% at z=0 and up to k=2h/Mpc. We also study halo profiles for the f(R) models and find a deviation from the standard general relativity result up to 40%, depending on the halo masses and redshift. This has not been pointed out in previous analysis. Finally we study the number density and profiles of voids identified in these f(R) N-body simulations. We underline the effect of the bias and the sampling to identify voids. We find significant deviation from GR when measuring the f(R) void profiles with fR0<-10^{-6}.

astro-ph.CO↗

arXiv2015

Cosmology and fundamental physics with the Euclid satellite

Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. 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↗

arXiv2015

Modified Gravity N-body Code Comparison Project

Self-consistent ${\it N}$-body simulations of modified gravity models are a key ingredient to obtain rigorous constraints on deviations from General Relativity using large-scale structure observations. This paper provides the first detailed comparison of the results of different ${\it N}$-body codes for the $f(R)$, DGP, and Symmetron models, starting from the same initial conditions. We find that the fractional deviation of the matter power spectrum from $Λ$CDM agrees to better than $1\%$ up to $k \sim 5-10~h/{\rm Mpc}$ between the different codes. These codes are thus able to meet the stringent accuracy requirements of upcoming observational surveys. All codes are also in good agreement in their results for the velocity divergence power spectrum, halo abundances and halo profiles. We also test the quasi-static limit, which is employed in most modified gravity ${\it N}$-body codes, for the Symmetron model for which the most significant non-static effects among the models considered are expected. We conclude that this limit is a very good approximation for all of the observables considered here.

astro-ph.CO↗

arXiv2015

Identification of galaxy cluster substructures with the Caustic method

We investigate the power of the caustic technique for identifying substructures of galaxy clusters from optical redshift data alone. The caustic technique is designed to estimate the mass profile of galaxy clusters to radii well beyond the virial radius, where dynamical equilibrium does not hold. Two by-products of this technique are the identification of the cluster members and the identification of the cluster substructures. We test the caustic technique as a substructure detector on two samples of 150 mock redshift surveys of clusters; the clusters are extracted from a large cosmological $N$-body simulation of a $Λ$CDM model and have masses of $M_{200} \sim 10^{14} h^{-1} M_{\odot}$ and $M_{200} \sim 10^{15} h^{-1} M_{\odot}$ in the two samples. We limit our analysis to substructures identified in the simulation with masses larger than $10^{13} h^{-1} M_{\odot}$. With mock redshift surveys with 200 galaxies within $3R_{200}$, (1) the caustic technique recovers $\sim 30-50$\% of the real substructures, and (2) $\sim 15-20$\% of the substructures identified by the caustic technique correspond to real substructures of the central cluster, the remaining fraction being low-mass substructures, groups or substructures of clusters in the surrounding region, or chance alignments of unrelated galaxies. These encouraging results show that the caustic technique is a promising approach for investigating the complex dynamics of galaxy clusters.

astro-ph.CO↗

arXiv2015

Disentangling dark sector models using weak lensing statistics

We perform multi-plane ray-tracing using the GLAMER gravitational lensing code within high-resolution light-cones extracted from the CoDECS simulations: a suite of cosmological runs featuring a coupling between Dark Energy and Cold Dark Matter. We show that the presence of the coupling is evident not only in the redshift evolution of the normalisation of the convergence power spectrum, but also in differences in non-linear structure formation with respect to ΛCDM. Using a tomographic approach under the assumption of a ΛCDM cosmology, we demonstrate that weak lensing measurements would result in a σ8 value that changes with the source redshift if the true underlying cosmology is a coupled Dark Energy one. This provides a generic null test for these types of models. We also find that different models of coupled Dark Energy can show either an enhanced or a suppressed correlation between convergence maps with differing source redshifts as compared to ΛCDM. This would provide a direct way to discriminate between different possible realisations of the coupled Dark Energy scenario. Finally, we discuss the impact of the coupling on several lensing observables for different source redshifts and angular scales with realistic source redshift distributions for current ground-based and future space-based lensing surveys.

astro-ph.CO↗

arXiv2015

Cosmic voids in coupled dark energy cosmologies: the impact of halo bias

In this work we analyse the properties of cosmic voids in standard and coupled dark energy cosmologies. Using large numerical simulations, we investigate the effects produced by the dark energy coupling on three statistics: the filling factor, the size distribution and the stacked profiles of cosmic voids. We find that the bias of the tracers of the density field used to identify the voids strongly influences the properties of the void catalogues, and, consequently, the possibility of using the identified voids as a probe to distinguish coupled dark energy models from the standard $Λ$CDM cosmology. In fact, on one hand coupled dark energy models are characterised by an excess of large voids in the cold dark matter distribution as compared to the reference standard cosmology, due to their higher normalisation of linear perturbations at low redshifts. Specifically, these models present an excess of large voids with $R_{eff}>20, 15, 12$ Mpc h^{-1}, at $z=0, 0.55, 1$, respectively. On the other hand, we do not find any significant difference in the properties of the void detected in the distribution of collapsed dark matter halos. These results imply that the tracer bias has a significant impact on the possibility of using cosmic void catalogues to probe cosmology.

astro-ph.CO↗

arXiv2015

Performance assessment and design of finite length LDPC codes for the Gaussian wiretap channel

In this work we study the reliability and secrecy performance achievable by practical LDPC codes over the Gaussian wiretap channel. While several works have already addressed this problem in asymptotic conditions, i.e., under the hypothesis of codewords of infinite length, only a few approaches exist for the finite length regime. We propose an approach to measure the performance of practical codes and compare it with that achievable in asymptotic conditions. Moreover, based on the secrecy metrics we adopt to achieve this target, we propose a code optimization algorithm which allows to design irregular LDPC codes able to approach the ultimate performance limits even at moderately small codeword lengths (in the order of 10000 bits).

cs.IT↗

arXiv2015

Semi-analytic galaxy formation in coupled dark energy cosmologies

Among the possible alternatives to the standard cosmological model ($Λ$CDM), coupled Dark Energy models postulate that Dark Energy (DE), seen as a dynamical scalar field, may interact with Dark Matter (DM), giving rise to a "fifth-force", felt by DM particles only. In this paper, we study the impact of these cosmologies on the statistical properties of galaxy populations by combining high-resolution numerical simulations with semi-analytic models (SAM) of galaxy formation and evolution. New features have been implemented in the reference SAM in order to have it run self-consistently and calibrated on these cosmological simulations. They include an appropriate modification of the mass temperature relation and of the baryon fraction in DM haloes, due to the different virial scalings and to the gravitational bias, respectively. Our results show that the predictions of our coupled-DE SAM do not differ significantly from theoretical predictions obtained with standard SAMs applied to a reference $Λ$CDM simulation, implying that the statistical properties of galaxies provide only a weak probe for these alternative cosmological models. On the other hand, we show that both galaxy bias and the galaxy pairwise velocity distribution are sensitive to coupled DE models: this implies that these probes might be successfully applied to disentangle among quintessence, $f(R)$-Gravity and coupled DE models.

astro-ph.CO↗

arXiv2015

Effects of Coupled Dark Energy on the Milky Way and its Satellites

We present the first numerical simulations in coupled dark energy cosmologies with high enough resolution to investigate the effects of the coupling on galactic and sub-galactic scales. We choose two constant couplings and a time-varying coupling function and we run simulations of three Milky-Way-size halos ($\sim$10$^{12}$M$_{\odot}$), a lower mass halo (6$\times$10$^{11}$M$_{\odot}$) and a dwarf galaxy halo (5$\times$10$^{9}$M$_{\odot}$). We resolve each halo with several millions dark matter particles. On all scales the coupling causes lower halo concentrations and a reduced number of substructures with respect to LCDM. We show that the reduced concentrations are not due to different formation times, but they are related to the extra terms that appear in the equations describing the gravitational dynamics. On the scale of the Milky Way satellites, we show that the lower concentrations can help in reconciling observed and simulated rotation curves, but the coupling values necessary to have a significant difference from LCDM are outside the current observational constraints. On the other hand, if other modifications to the standard model allowing a higher coupling (e.g. massive neutrinos) are considered, coupled dark energy can become an interesting scenario to alleviate the small-scale issues of the LCDM model.

astro-ph.CO↗

arXiv2015

Multiple Lensing of the Cosmic Microwave Background anisotropies

We study the gravitational lensing effect on the Cosmic Microwave Background (CMB) anisotropies performing a ray-tracing of the primordial CMB photons through intervening large-scale structures (LSS) distribution predicted by N-Body numerical simulations with a particular focus on the precise recovery of the lens-induced polarized counterpart of the source plane. We apply both a multiple plane ray-tracing and an effective deflection approach based on the Born approximation to deflect the CMB photons trajectories through the simulated lightcone. We discuss the results obtained with both these methods together with the impact of LSS non-linear evolution on the CMB temperature and polarization power spectra. We compare our results with semi-analytical approximations implemented in Boltzmann codes like, e.g., CAMB. We show that, with our current N-body setup, the predicted lensing power is recovered with good accuracy in a wide range of multipoles while excess power with respect to semi-analytic prescriptions is observed in the lensing potential on scales $\ell \gtrsim 3000$. We quantify the impact of the numerical effects connected to the resolution in the N-Body simulation together with the resolution and band-limit chosen to synthesise the CMB source plane. We found these quantities to be particularly important for the simulation of B-mode polarization power spectrum.

astro-ph.CO↗

arXiv2015

Cosmic voids detection without density measurements

Cosmic voids are effective cosmological probes to discriminate among competing world models. Their identification is generally based on density or geometry criteria that, because of their very nature, are prone to shot noise. We propose two void finders that are based on dynamical criterion to select voids in Lagrangian coordinates and minimise the impact of sparse sampling. The first approach exploits the Zel'dovich approximation to trace back in time the orbits of galaxies located in voids and their surroundings, the second uses the observed galaxy-galaxy correlation function to relax the objects' spatial distribution to homogeneity and isotropy. In both cases voids are defined as regions of the negative velocity divergence, that can be regarded as sinks of the back-in-time streamlines of the mass tracers. To assess the performance of our methods we used a dark matter halo mock catalogue CoDECS, and compared the results with those obtained with the ZOBOV void finder. We find that the void divergence profiles are less scattered than the density ones and, therefore, their stacking constitutes a more accurate cosmological probe. The significance of the divergence signal in the central part of voids obtained from both our finders is 60% higher than for overdensity profiles in the ZOBOV case. The ellipticity of the stacked void measured in the divergence field is closer to unity, as expected, than what is found when using halo positions. Therefore our void finders are complementary to the existing methods, that should contribute to improve the accuracy of void-based cosmological tests.

astro-ph.CO↗

arXiv2014

Raytracing simulations of coupled dark energy models

Dark matter and dark energy are usually assumed to couple only gravitationally. An extension to this picture is to model dark energy as a scalar field coupled directly to cold dark matter. This coupling leads to new physical effects, such as a fifth-force and a time-dependent dark matter particle mass. In this work we examine the impact that coupling has on weak lensing statistics by constructing realistic simulated weak-lensing maps using raytracing techniques through N-body cosmological simulations. We construct maps for different lensing quantities, covering a range of scales from a few arcminutes to several degrees. The concordance $Λ$CDM model is compared to different coupled dark energy models, described either by an exponential scalar field potential (standard coupled dark energy scenario) or by a SUGRA potential (bouncing model). We analyse several statistical quantities and our results, with sources at low redshifts are largely consistent with previous work on CMB lensing by Carbone et al., 2013. The most significant differences from the $Λ$CDM model are due to the enhanced growth of the perturbations and to the effective friction term in non-linear dynamics. For the most extreme models, we see differences in the power spectra up to 40% compared to the $Λ$CDM model. The different time evolution of the linear matter overdensity can account for most of the differences, but when controlling for this using a $Λ$CDM model having the same normalization, the overall signal is smaller due to the effect of the friction term appearing in the equation of motion for dark matter particles.

astro-ph.CO↗

arXiv2014

Simulating Momentum Exchange in the Dark Sector

Low energy interactions between particles are often characterised by elastic scattering. Just as electrons undergo Thomson scattering with photons, dark matter particles may experience an analogous form of momentum exchange with dark energy. We investigate the influence such an interaction has on the formation of linear and nonlinear cosmic structure, by running for the first time a suite of N-body simulations with different dark energy equations of state and scattering cross sections. In models where the linear matter power spectrum is suppressed by the scattering, we find that on nonlinear scales the power spectrum is strongly enhanced. This is due to the friction term increasing the efficiency of gravitational collapse, which also leads to a scale-independent amplification of the concentration and mass functions of halos. The opposite trend is found for models characterised by an increase of the linear matter power spectrum normalisation. More quantitatively, we find that power spectrum deviations at nonlinear scales ($k \approx 10\, h/$Mpc) are roughly ten times larger than their linear counterparts, exceeding $100%$ for the largest value of the scattering cross section considered in the present work. Similarly, the concentration-mass relation and the halo mass function show deviations up to $100%$ and $20%$, respectively, over a wide range of masses. Therefore, we conclude that nonlinear probes of structure formation might provide much tighter constraints on the scattering cross section between dark energy and dark matter as compared to the present bounds based on linear observables.

astro-ph.CO↗