1,721,191 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
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
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
Static Analysis for GDPR Compliance
Full text linkEvolving business models, computing paradigms, and management practices are rapidly re-shaping the usage models of ICT infrastructures, and demanding for more flexibility and dynamicity in enterprise security, beyond the traditional “security perimeter” approach. Since valuable ICT assets cannot be easily enclosed within a trusted physical sandbox any more, there is an increasing need for a new generation of pervasive and capillary cyber- security paradigms over distributed and geographically-scattered systems. Following the generalized trend towards virtualization, automation, software-definition, and hardware/software disaggregation, in this paper we elaborate on a multi-tier architec- ture made of a common, programmable, and pervasive data-plane and a powerful set of multi-vendor detection and analysis algorithms. Our approach leverages the growing level of programmability of ICT infrastructures to create a common and unified framework that could be used to monitor and protect distributed heterogeneous environments, including legacy enterprise networks, IoT installations, and virtual resources deployed in the cloud
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Oscillating non-linear large-scale structures in growing neutrino quintessence
No full textGrowing neutrino quintessence
describes a form of dynamical dark energy
that could explain why dark energy dominates the universe only in recent
cosmological times. This scenario predicts the formation of large-scale
neutrino lumps which could allow for observational tests. We perform for
the first time N-body simulations of the non-linear growth of structures
for cold dark matter (CDM) and neutrino fluids in the context of growing
neutrino cosmologies. Our analysis shows a pulsation increase and
subsequent decrease in the neutrino density contrast. This could lead to
interesting observational signatures, as an enhanced bulk flow in a
situation where the DM density contrast differs only very mildly from
the standard ΛCDM scenario. We also determine for the first time
the statistical distribution of neutrino lumps as a function of mass at
different redshifts. Such determination provides an essential ingredient
for a realistic estimate of the observational signatures of growing
neutrino cosmologies. Due to a breakdown of the non-relativistic
Newtonian approximation, our results are limited to redshifts z≥ 1
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Secrecy Transmission on Parallel Channels: Theoretical Limits and Performance of Practical Codes
Full text linkWe consider a system where an agent (Alice) aims at transmitting a message to a second agent (Bob) over a set of parallel channels, while keeping it secret from a third agent (Eve) by using physical layer security techniques. We assume that Alice perfectly knows the set of channels with respect to Bob, but she has only a statistical knowledge of the channels with respect to Eve. We derive bounds on the achievable outage secrecy rates, by considering coding either within each channel or across all parallel channels. Transmit power is adapted to the channel conditions, with a constraint on the average power over the whole transmission. We also focus on the maximum cumulative outage secrecy rate that can be achieved. Moreover, in order to assess the performance in a real life scenario, we consider the use of practical error correcting codes. We extend the definitions of security gap and equivocation rate, previously applied to the single additive white Gaussian noise channel, to Rayleigh distributed parallel channels, on the basis of the error rate targets and the outage probability. Bounds on these metrics are also derived, considering the statistics of the parallel channels. Numerical results are provided, that confirm the feasibility of the considered physical layer security techniques
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