14,532 research outputs found
Clarifying the effects of interacting dark energy on linear and non-linear structure formation processes
No full textWe present a
detailed numerical study of the impact that cosmological
models featuring a direct interaction between the dark energy component
that drives the accelerated expansion of the Universe and cold dark
matter can have on the linear and non-linear stages of structure
formation. By means of a series of collisionless N-body simulations, we
study the influence that each of the different effects characterizing
these cosmological models - which include among others a fifth force, a
time variation of particle masses and a velocity-dependent acceleration
- separately have on the growth of density perturbations and on a series
of observable quantities related to linear and non-linear cosmic
structures, as the matter power spectrum, the gravitational bias between
baryons and cold dark matter, the halo mass function and the halo
density profiles. We perform our analysis applying and comparing
different numerical approaches previously adopted in the literature, and
we address the partial discrepancies recently claimed in a similar study
by Li & Barrow with respect to the first outcomes of Baldi et al.,
which are found to be related to the specific numerical approach adopted
in the former work. Our results fully confirm the conclusions of Baldi
et al. and show that when linear and non-linear effects of the
interaction between dark energy and cold dark matter are properly
disentangled, the velocity-dependent acceleration is the leading effect
acting at non-linear scales and in particular is the most important
mechanism in lowering the concentration of cold dark matter haloes
The nonlinear evolution of large scale structures in Growing Neutrino cosmologies
No full textWe present the results of the first N-body simulations of the Growing
Neutrino scenario, as recently discussed in Baldi et al. (2011). Our results
have shown for the first time how neutrino lumps forming in the context of
Growing Neutrino cosmologies are expected to pulsate as a consequence of the
rapid oscillations of the dark energy scalar field. We have also computed for
the first time a realistic statistical distribution of neutrino halos and
determined their impact on the underlying Cold Dark Matter structures
Gian Vittorio Baldi
No full textUn profilo bio-filmografico del regista e produttore cinematografico Gian Vittorio Baldi
Time-dependent couplings in the dark sector: from background evolution to non-linear structure formation
No full textWe present
a complete numerical study of cosmological models with a
time-dependent coupling between the dark energy component driving the
present accelerated expansion of the Universe and the cold dark matter
(CDM) fluid. Depending on the functional form of the coupling strength,
these models show a range of possible intermediate behaviours between
the standard ΛCDM background evolution and the widely studied
case of interacting dark energy models with a constant coupling. These
different background evolutions play a crucial role in the growth of
cosmic structures and determine strikingly different effects of the
coupling on the internal dynamics of non-linear objects. By means of a
suitable modification of the cosmological N-body code GADGET-2, we have
performed a series of high-resolution N-body simulations of structure
formation in the context of interacting dark energy models with variable
couplings. Depending on the type of background evolution, the halo
density profiles are found to be either less or more concentrated with
respect to ΛCDM, contrarily to what happens for constant coupling
models where concentrations can only decrease. However, for some
specific choice of the interaction function, the reduction in halo
concentrations can be larger than in constant coupling scenarios. We
also find that different types of coupling evolution determine specific
features in the growth of large-scale structures, like peculiar
distortions of the matter power spectrum shape or different
time-evolutions of the halo mass function. Furthermore, also for
time-dependent couplings, baryons and CDM develop a bias already on
large scales, which is progressively enhanced for smaller and smaller
scales, and the effect can be significantly larger compared to constant
coupling scenarios. The same happens to the baryon fraction of haloes,
which can be more significantly reduced below its universal value in
variable coupling models with respect to constant coupling cosmologies.
In general, we find that time-dependent interactions between dark energy
and CDM can in some cases determine stronger effects on structure
formation as compared to the constant coupling case, with a
significantly weaker impact on the background evolution of the universe,
and might therefore provide a more viable possibility to alleviate the
tensions between observations and the ΛCDM model on small scales
than the constant coupling scenario
Cosmological models with multiple dark matter species and long-range scalar interactions
No full textIn 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
The codecs project: a publicly available suite of cosmological N-body simulations for interacting dark energy models★
No full textWe present the largest set of N-body and
hydrodynamical simulations to
date for cosmological models featuring a direct interaction between the
dark energy (DE) scalar field, responsible for the observed cosmic
acceleration, and the cold dark matter (CDM) fluid. With respect to
previous works, our simulations considerably extend the statistical
significance of the simulated volume and cover a wider range of
different realizations of the interacting DE scenario, including the
recently proposed bouncing coupled DE model. Furthermore, all the
simulations are normalized in order to be consistent with the present
bounds on the amplitude of density perturbations at last scattering,
thereby providing the first realistic determination of the effects of a
DE coupling for cosmological growth histories fully compatible with the
latest cosmic microwave background data. As a first basic analysis, we
have studied the impact of the coupling on the non-linear matter power
spectrum and on the bias between the CDM and baryon distributions, as a
function of redshift and scale. For the former, we have addressed the
issue of the degeneracy between the effects of the coupling and other
standard cosmological parameters, e.g. σ8, showing how
the redshift evolution of the linear amplitude or the scale dependence
of the non-linear power spectrum might provide a way to break the
degeneracy. For the latter, instead, we have computed the redshift and
scale dependence of the bias in all our different models showing how a
growing coupling or a bouncing coupled DE scenario provides much
stronger effects with respect to constant coupling models. Furthermore,
we discuss the main features imprinted by the DE interactions on the
halo and subhalo mass functions. We refer to this vast numerical
initiative as the COupled Dark Energy Cosmological Simulations (CoDECS)
project, and release all the CoDECS outputs for public use through a
dedicated web data base, providing information on how to access and
interpret the data
Dark Energy simulations
No full textAbstractCosmology is presently facing the deep mystery of the origin of the observed accelerated expansion of the Universe. Be it a cosmological constant, a homogeneous scalar field, or a more complex inhomogeneous field possibly inducing effective modifications of the laws of gravity, such elusive physical entity is indicated with the general term of “Dark Energy”. 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. (2012) [1] 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
High-z massive clusters as a test for dynamical coupled dark energy
No full textThe recent detection by Jee et al. of the massive cluster XMMU
J2235.3-2557 at a redshift z≈ 1.4, with an estimated mass
M324= (6.4 ± 1.2) × 1014 M&sun;, has been claimed to be a possible challenge to the standard ΛCDM cosmological model. More specifically, the probability to detect such a cluster has been estimated to be ̃0.005 if a ΛCDM model with Gaussian initial conditions is assumed, resulting in a 3σ discrepancy from the standard cosmological model. In this Letter we propose to use high-redshift clusters as the one detected in Jee et al. to compare the cosmological constant scenario with interacting dark energy models. We show that coupled dark energy models, where an interaction is present between dark energy and cold dark matter, can significantly enhance the probability to observe very massive clusters at high redshift
QC-LDPC code-based cryptography
No full textThis book describes the fundamentals of cryptographic primitives based on quasi-cyclic low-density parity-check (QC-LDPC) codes, with a special focus on the use of these codes in public-key cryptosystems derived from the McEliece and Niederreiter schemes. In the first part of the book, the main characteristics of QC-LDPC codes are reviewed, and several techniques for their design are presented, while tools for assessing the error correction performance of these codes are also described. Some families of QC-LDPC codes that are best suited for use in cryptography are also presented. The second part of the book focuses on the McEliece and Niederreiter cryptosystems, both in their original forms and in some subsequent variants. The applicability of QC-LDPC codes in these frameworks is investigated by means of theoretical analyses and numerical tools, in order to assess their benefits and drawbacks in terms of system efficiency and security. Several examples of QC-LDPC code-based public key cryptosystems are presented, and their advantages over classical solutions are highlighted. The possibility of also using QC-LDPC codes in symmetric encryption schemes and digital signature algorithms is also briefly examined
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