arXiv subjects

Marco Baldi

Publications and source records attributed to Marco Baldi.

At least 163 records · Page 9Linked to original sources

arXiv2013

Characterizing dark interactions with the halo mass accretion history and structural properties

We study the halo mass accretion history (MAH) and its correlation with the internal structural properties in coupled dark energy cosmologies (cDE). To accurately predict all the non-linear effects caused by dark interactions, we use the COupled Dark Energy Cosmological Simulations (CoDECS). We measure the halo concentration at z=0 and the number of substructures above a mass resolution threshold for each halo. Tracing the halo merging history trees back in time, following the mass of the main halo, we develope a MAH model that accurately reproduces the halo growth in term of M_{200} in the ΛCDM Universe; we then compare the MAH in different cosmological scenarios. For cDE models with a weak constant coupling, our MAH model can reproduce the simulation results, within 10% of accuracy, by suitably rescaling the normalization of the linear matter power spectrum at z=0, σ_8. However, this is not the case for more complex scenarios, like the "bouncing" cDE model, for which the numerical analysis shows a rapid growth of haloes at high redshifts, that cannot be reproduced by simply rescaling the value of σ_8. Moreover, at fixed value of σ_8, cold dark matter (CDM) haloes in these cDE scenarios tend to be more concentrated and have a larger amount of substructures with respect to ΛCDM predictions. Finally, we present an accurate model that relates the halo concentration to the time at which it assembles half or 4% of its mass. Combining this with our MAH model, we show how halo concentrations change while varying only σ_8 in a ΛCDM Universe, at fixed halo mass.

astro-ph.CO↗

arXiv2013

Improving the efficiency of the LDPC code-based McEliece cryptosystem through irregular codes

We consider the framework of the McEliece cryptosystem based on LDPC codes, which is a promising post-quantum alternative to classical public key cryptosystems. The use of LDPC codes in this context allows to achieve good security levels with very compact keys, which is an important advantage over the classical McEliece cryptosystem based on Goppa codes. However, only regular LDPC codes have been considered up to now, while some further improvement can be achieved by using irregular LDPC codes, which are known to achieve better error correction performance than regular LDPC codes. This is shown in this paper, for the first time at our knowledge. The possible use of irregular transformation matrices is also investigated, which further increases the efficiency of the system, especially in regard to the public key size.

cs.IT↗

arXiv2013

Supernova constraints on Multi-coupled Dark Energy

The persisting consistency of ever more accurate observational data with the predictions of the standard LCDM cosmological model puts severe constraints on possible alternative scenarios, but still does not shed any light on the fundamental nature of the cosmic dark sector.As large deviations from a LCDM cosmology are ruled out by data, the path to detect possible features of alternative models goes necessarily through the definition of cosmological scenarios that leave almost unaffected the background and -- to a lesser extent -- the linear perturbations evolution of the universe. In this context,the Multi-coupled DE (McDE) model was proposed by Baldi 2012 as a particular realization of an interacting Dark Energy field characterized by an effective screening mechanism capable of suppressing the effects of the coupling at the background and linear perturbation level. In the present paper, for the first time, we challenge the McDE scenario through a direct comparison with real data, in particular with the luminosity distance of Type Ia supernovae. By studying the existence and stability conditions of the critical points of the associated background dynamical system, we select only the cosmologically consistent solutions, and confront their background expansion history with data. Confirming previous qualitative results, the McDE scenario appears to be fully consistent with the adopted sample of Type Ia supernovae, even for coupling values corresponding to an associated scalar fifth-force about four orders of magnitude stronger than standard gravity. Our analysis demonstrates the effectiveness of the McDE background screening, and shows some new non-trivial asymptotic solutions for the future evolution of the universe. Our results show how the background expansion history might be highly insensitive to the fundamental nature and to the internal complexity of the dark sector. [Abridged]

astro-ph.CO↗

arXiv2013

Using LDGM Codes and Sparse Syndromes to Achieve Digital Signatures

In this paper, we address the problem of achieving efficient code-based digital signatures with small public keys. The solution we propose exploits sparse syndromes and randomly designed low-density generator matrix codes. Based on our evaluations, the proposed scheme is able to outperform existing solutions, permitting to achieve considerable security levels with very small public keys.

cs.CR↗

arXiv2013

Modified Gravity-GADGET: A new code for cosmological hydrodynamical simulations of modified gravity models

We present a new massively parallel code for N-body and cosmological hydrodynamical simulations of modified gravity models. The code employs a multigrid-accelerated Newton-Gauss-Seidel relaxation solver on an adaptive mesh to efficiently solve for perturbations in the scalar degree of freedom of the modified gravity model. As this new algorithm is implemented as a module for the P-Gadget3 code, it can at the same time follow the baryonic physics included in P-Gadget3, such as hydrodynamics, radiative cooling and star formation. We demonstrate that the code works reliably by applying it to simple test problems that can be solved analytically, as well as by comparing cosmological simulations to results from the literature. Using the new code, we perform the first non-radiative and radiative cosmological hydrodynamical simulations of an f(R)-gravity model. We also discuss the impact of AGN feedback on the matter power spectrum, as well as degeneracies between the influence of baryonic processes and modifications of gravity.

astro-ph.CO↗

arXiv2013

Cosmological models with multiple dark matter species and long-range scalar interactions

In this talk, I have discussed the implications of a multi-component nature of cosmic Dark Matter for the observational bounds on possible long-range fifth-forces mediated by a Dark Energy scalar field. By assuming a simple internal symmetry of the Dark Matter component associated to opposite coupling "charges" of two different particle species, the effects of Dark Energy interactions on both the background and linear perturbations evolution are strongly suppressed during the whole matter dominated phase, thereby relaxing present bounds on the coupling strength. The associated attractive and repulsive fifth-forces, however, might still have a very significant impact on the nonlinear dynamics of collapsed structures. I have also described how some of these nonlinear effects are identified through dedicated cosmological N-body simulations as i) a possible fragmentation of bound Dark Matter halos into smaller objects, and ii) a consequent suppression of the nonlinear matter power at small scales. Both effects are potentially observable and might allow to further constrain the model.

astro-ph.CO↗

arXiv2013

Optimization of the parity-check matrix density in QC-LDPC code-based McEliece cryptosystems

Low-density parity-check (LDPC) codes are one of the most promising families of codes to replace the Goppa codes originally used in the McEliece cryptosystem. In fact, it has been shown that by using quasi-cyclic low-density parity-check (QC-LDPC) codes in this system, drastic reductions in the public key size can be achieved, while maintaining fixed security levels. Recently, some proposals have appeared in the literature using codes with denser parity-check matrices, named moderate-density parity-check (MDPC) codes. However, the density of the parity-check matrices to be used in QC-LDPC code-based variants of the McEliece cryptosystem has never been optimized. This paper aims at filling such gap, by proposing a procedure for selecting the density of the private parity-check matrix, based on the security level and the decryption complexity. We provide some examples of the system parameters obtained through the proposed technique.

cs.IT↗

arXiv2013

Low-power Secret-key Agreement over OFDM

Information-theoretic secret-key agreement is perhaps the most practically feasible mechanism that provides unconditional security at the physical layer to date. In this paper, we consider the problem of secret-key agreement by sharing randomness at low power over an orthogonal frequency division multiplexing (OFDM) link, in the presence of an eavesdropper. The low power assumption greatly simplifies the design of the randomness sharing scheme, even in a fading channel scenario. We assess the performance of the proposed system in terms of secrecy key rate and show that a practical approach to key sharing is obtained by using low-density parity check (LDPC) codes for information reconciliation. Numerical results confirm the merits of the proposed approach as a feasible and practical solution. Moreover, the outage formulation allows to implement secret-key agreement even when only statistical knowledge of the eavesdropper channel is available.

cs.IT↗

arXiv2013

Security and complexity of the McEliece cryptosystem based on QC-LDPC codes

In the context of public key cryptography, the McEliece cryptosystem represents a very smart solution based on the hardness of the decoding problem, which is believed to be able to resist the advent of quantum computers. Despite this, the original McEliece cryptosystem, based on Goppa codes, has encountered limited interest in practical applications, partly because of some constraints imposed by this very special class of codes. We have recently introduced a variant of the McEliece cryptosystem including low-density parity-check codes, that are state-of-the-art codes, now used in many telecommunication standards and applications. In this paper, we discuss the possible use of a bit-flipping decoder in this context, which gives a significant advantage in terms of complexity. We also provide theoretical arguments and practical tools for estimating the trade-off between security and complexity, in such a way to give a simple procedure for the system design.

cs.CR↗

arXiv2012

A Physical Layer Secured Key Distribution Technique for IEEE 802.11g Wireless Networks

Key distribution and renewing in wireless local area networks is a crucial issue to guarantee that unauthorized users are prevented from accessing the network. In this paper, we propose a technique for allowing an automatic bootstrap and periodic renewing of the network key by exploiting physical layer security principles, that is, the inherent differences among transmission channels. The proposed technique is based on scrambling of groups of consecutive packets and does not need the use of an initial authentication nor automatic repeat request protocols. We present a modification of the scrambling circuits included in the IEEE 802.11g standard which allows for a suitable error propagation at the unauthorized receiver, thus achieving physical layer security.

cs.IT↗

arXiv2012

Exploiting the shift of baryonic acoustic oscillations as a dynamical probe for dark interactions

The baryonic acoustic peak in the correlation function of galaxies and galaxy clusters provides a standard ruler to probe the space-time geometry of the Universe, jointly constraining the angular diameter distance and the Hubble expansion rate. Moreover, non-linear effects can systematically shift the peak position, giving us the opportunity to exploit this clustering feature also as a dynamical probe. We investigate the possibility of detecting interactions in the dark sector through an accurate determination of the baryonic acoustic scale. Making use of the public halo catalogues extracted from the CoDECS simulations -- the largest suite of N-body simulations of interacting dark energy models to date -- we determine the position of the baryonic scale fitting a band-filtered correlation function, specifically designed to amplify the signal at the sound horizon. We analyze the shifts due to non-linear dynamics, redshift-space distortions and Gaussian redshift errors, in the range 0 < z < 2. Since the coupling between dark energy and dark matter affects in a particular way the clustering properties of haloes and, specifically, the amplitude and location of the baryonic acoustic oscillations, the cosmic evolution of the baryonic peak position might provide a direct way to discriminate interacting dark energy models from the standard ΛCDM framework. To maximize the efficiency of the baryonic peak as a dynamic probe, the correlation function has to be measured in redshift-space, where the baryonic acoustic shift due to non-linearities is amplified. The typical redshift errors of spectroscopic galaxy surveys do not significantly impact these results.

astro-ph.CO↗

arXiv2012

Dark Energy Simulations

(Abridged) The growing role played by numerical N-body simulations in cosmological studies as a fundamental connection between theoretical modeling and direct observations has led to impressive advancements also in the development and application of specific algorithms designed to probe a wide range of Dark Energy scenarios. Over the last decade, a large number of independent and complementary investigations have been carried out in the field of Dark Energy N-body simulations, starting from the simplest case of homogeneous Dark Energy models up to the recent development of highly sophisticated iterative solvers for a variety of Modified Gravity theories. In this Review - which is meant to be complementary to the general Review by Kuhlen et al. published in this Volume - I will discuss the range of scenarios for the cosmic acceleration that have been successfully investigated by means of dedicated N-body simulations, and I will provide a broad summary of the main results that have been obtained in this rather new research field. I will focus the discussion on a few selected studies that have led to particularly significant advancements in the field, and I will provide a comprehensive list of references for a larger number of related works. Due to the vastness of the topic, the discussion will not enter into the finest details of the different implementations and will mainly focus on the outcomes of the various simulations studies. Although quite recent, the field of Dark Energy simulations has witnessed huge developments in the last few years, and presently stands as a reliable approach to the investigation of the fundamental nature of Dark Energy.

astro-ph.CO↗

arXiv2012

Progressive Differences Convolutional Low-Density Parity-Check Codes

We present a new family of low-density parity-check (LDPC) convolutional codes that can be designed using ordered sets of progressive differences. We study their properties and define a subset of codes in this class that have some desirable features, such as fixed minimum distance and Tanner graphs without short cycles. The design approach we propose ensures that these properties are guaranteed independently of the code rate. This makes these codes of interest in many practical applications, particularly when high rate codes are needed for saving bandwidth. We provide some examples of coded transmission schemes exploiting this new class of codes.

cs.IT↗

arXiv2012

Weak lensing predictions for coupled dark energy cosmologies at non-linear scales

We present non-linear weak lensing predictions for coupled dark energy models using the CoDECS simulations. We calculate the shear correlation function and error covariance expected for these models, for forthcoming ground-based (such as DES) and space-based (Euclid) weak lensing surveys. We obtain predictions for the discriminatory power of a ground-based survey similar to DES and a space-based survey such as Euclid in distinguishing between $Λ$CDM and coupled dark energy models; we show that using the non-linear lensing signal we could discriminate between $Λ$CDM and exponential constant coupling models with $β_0\geq0.1$ at $4σ$ confidence level with a DES-like survey, and $β_0\geq0.05$ at $5σ$ confidence level with Euclid. We also demonstrate that estimating the coupled dark energy models' non-linear power spectrum, using the $Λ$CDM Halofit fitting formula, results in biases in the shear correlation function that exceed the survey errors.

astro-ph.CO↗

arXiv2012

Structure formation in Multiple Dark Matter cosmologies with long-range scalar interactions

(Abridged) An interaction between Cold Dark Matter (CDM) and a classical scalar field playing the role of the cosmic dark energy (DE) might provide long-range dark interactions without conflicting with solar system bounds. Although presently available observations allow to constrain such interactions to a few percent of the gravitational strength, some recent studies have shown that if CDM is composed by two different particle species having opposite couplings to the DE field, such tight constraints can be considerably relaxed, allowing for long-range scalar forces of order gravity without significantly affecting observations both at the background and at the linear perturbations level. In the present work, we extend the investigation of such Multiple Dark Matter scenarios to the nonlinear regime of structure formation, by presenting the first N-body simulations ever performed for these cosmologies. Our results highlight some characteristic footprints of long-range scalar forces that arise only in the nonlinear regime for specific models that would be otherwise practically indistinguishable from the standard LCDM scenario both in the background and in the growth of linear density perturbations. Among these effects, the formation of "mirror" cosmic structures in the two CDM species, the suppression of the nonlinear matter power spectrum at k > 1 h/Mpc, and the fragmentation of collapsed halos, represent peculiar features that might provide a direct way to constrain this class of cosmological models.

astro-ph.CO↗

arXiv2012

The halo mass function in interacting Dark Energy models

We present a detailed investigation of the effects that a direct interaction between Dark Energy (DE) and Cold Dark Matter (CDM) particles imprints on the Halo Mass Function (HMF) of groups and clusters of galaxies. Making use of the public halo catalogs of the {\small CoDECS} simulations, we derive the HMF for several different types of coupled DE scenarios both based on the FoF algorithm and on the SO halo identification for different values of the overdensity threshold $Δ_{c}$. We compare the computed HMFs for coupled DE cosmologies with $Λ$CDM as well as with the predictions of the standard analytic fitting functions. Our results show that the standard fitting functions still reproduce reasonably well both the FoF and the SO HMFs of interacting DE cosmologies at intermediate masses and at low redshifts, once rescaled to the characteristic amplitude of linear density perturbations of each specific model as given by $σ_{8}$. However, we also find that such apparent degeneracy with $σ_{8}$ is broken both by the high-mass tail and by the redshift evolution of our HMFs, with deviations beyond $\sim 10%$ for most of the models under investigation. Furthermore, the discrepancy with respect to the predictions of standard fitting functions rescaled with the characteristic value of $σ_{8}$ shows -- for some models -- a strong dependence on the spherical overdensity threshold $Δ_{c}$ used for the halo identification. We find that such effect is due to a significant increase of halo concentration at low redshifts in these models, that is however absent in the majority of the cosmological scenarios considered in this work. We can therefore conclude that the universality of the HMF is violated by cosmological models that feature a direct interaction between DE and CDM.

astro-ph.CO↗

arXiv2012

Multiple Dark Matter as a self-regulating mechanism for dark sector interactions

(Abridged) Present cosmological constraints and the absence of a direct detection and identification of any dark matter particle candidate leave room to the possibility that the dark sector of the Universe be actually more complex than it is normally assumed. In particular, more than one new fundamental particle could be responsible for the observed dark matter density in the Universe, and possible new interactions between dark energy and dark matter might characterize the dark sector. In the present work, we investigate the possibility that two dark matter particles exist in nature, with identical physical properties except for the sign of their coupling constant to dark energy. Extending previous works on similar scenarios, we study the evolution of the background cosmology as well as the growth of linear density perturbations for a wide range of parameters of such model. Interestingly, our results show how the simple assumption that dark matter particles carry a "charge" with respect to their interaction with the dark energy field allows for new long-range scalar forces of gravitational strength in the dark sector without conflicting with present observations both at the background and linear levels. Our scenario does not introduce new parameters with respect to the case of a single dark matter species for which such strong dark interactions have been already ruled out.

astro-ph.CO↗

arXiv2012

Constraints on interacting dark energy models from galaxy Rotation Curves

[Abridged] High-resolution N-body simulations have recently shown that the structural properties of highly nonlinear cosmic structures, as e.g. their average concentration at a given mass, could be significantly modified in the presence of an interaction between Dark Energy and Dark Matter. While a constant interaction strength leads to less concentrated density profiles, a steep growth in time of the coupling function has been shown to determine a large increase of halo concentrations over a wide range of masses, including the typical halos hosting luminous spiral galaxies. This determines a substantial worsening of the "cusp-core" tension arising in the standard $Λ$CDM model and provides a direct way to constrain the form of the Dark Energy interaction. In the present paper we make use of the outcomes of some high-resolution N-body simulations of a specific class of interacting Dark Energy models to compare the predicted rotation curves of luminous spiral galaxies forming in these cosmologies against real observational data. Our results show how some specific interacting Dark Energy scenarios featuring a steep growth in time of the coupling function -- which are virtually indistinguishable from LCDM in the background -- cannot fit the observed rotation curves of luminous spiral galaxies and can therefore be ruled out only on the basis of dynamical properties of small-scale structures. Our study is a pilot investigation of the effects of a Dark Energy interaction at small scales, and demonstrates how the dynamical properties of visible galaxies can in some cases provide direct constraints on the nature of Dark Energy.

astro-ph.CO↗