1,721,189 research outputs found
How to reduce epidemic peaks keeping under control the time-span of the epidemic
No full textOne of the main challenges of the measures against the COVID-19 epidemic is to reduce the amplitude of the epidemic peak without increasing without control its timescale. We investigate this problem using the SIR model for the epidemic dynamics, for which reduction of the epidemic peak IP can be achieved only at the price of increasing the time tP of its occurrence and its entire time-span tE. By means of a time reparametrization we linearize the equations for the SIR dynamics. This allows us to solve exactly the dynamics in the time domain and to derive the scaling behaviour of the size, the timescale and the speed of the epidemics, by reducing the infection rate α and by increasing the removal rate β by a factor of λ. We show that for a given value of the size (IP, the total, IE and average I^P number of infected), its occurrence time tP and entire time-span tE can be reduced by a factor 1/λ if the reduction of I is achieved by increasing the removal rate instead of reducing the infection rate. Thus, epidemic containment measures based on tracing, early detection followed by prompt isolation of infected individuals are more efficient than those based on social distancing. We apply our results to the COVID-19 epidemic in Northern Italy. We show that the peak time tP and the entire time span tE could have been reduced by a factor 0.9 ≤ 1/λ ≤ 0.34 with containment measures focused on increasing β instead of reducing α
Galactic dynamics and long-range quantum gravity
No full textWe explore in a systematic way the possibility that long-range quantum gravity effects could play a role at galactic scales and could be responsible for the phenomenology commonly attributed to dark matter. We argue that the presence of baryonic matter breaks the scale symmetry of the de Sitter (dS) spacetime generating an IR scale r(0), corresponding to the scale at which the typical dark matter effects we observe in galaxies arise. It also generates a huge number of bosonic excitations with wavelength larger than the size of the cosmological horizon and in thermal equilibrium with dS spacetime. We show that for r greater than or similar to r(0) these excitations produce a new component for the radial acceleration of stars in galaxies which leads to the result found by McGaugh et al. by fitting a large amount of observational data and with the MOND theory. We also propose a generalized thermal equivalence principle and use it to give another independent derivation of our result. Finally, we show that our result can be also derived as the weak field limit of Einstein's general relativity sourced by an anisotropic fluid
Unified description of galactic dynamics and the cosmological constant
Full text linkWe explore the phenomenology of a two-fluid cosmological model, where the field equations of general relativity are sourced by baryonic and cold dark matter. We find that the model allows for a unified description of small and large scale, late-time cosmological dynamics. Specifically, in the static regime we recover the flattening of galactic rotation curves by requiring the matter density profile to scale as 1/r2. The same behavior describes matter inhomogeneities distribution at small cosmological scales. This traces galactic dynamics back to structure formation. At large cosmological scales, we focus on back reaction effects of the spacetime geometry to the presence of matter inhomogeneities. We find that a cosmological constant (CC) with the observed order of magnitude, emerges by averaging the back reaction term on spatial scales of order 100 Mpc and it is related in a natural way to matter distribution. This provides a resolution to both the CC and the coincidence problems and shows the existence of an intriguing link between the small and large scale behavior in cosmology
Emergence of a cosmological constant in anisotropic fluid cosmology
Full text linkIn this paper, we investigate anisotropic fluid cosmology in a situation where the space-time metric back-reacts in a local, time-dependent way to the presence of inhomogeneities. We derive exact solutions to the Einstein field equations describing Friedmann-Lemaítre-Robertson-Walker (FLRW) large-scale cosmological evolution in the presence of local inhomogeneities and time-dependent backreaction. We use our derivation to tackle the cosmological constant problem. A cosmological constant emerges by averaging the backreaction term on spatial scales of the order of 100 Mpc, at which our universe begins to appear homogeneous and isotropic. We find that the order of magnitude of the "emerged"cosmological constant agrees with astrophysical observations and is related in a natural way to baryonic matter density. Thus, there is no coincidence problem in our framework
Unitarity and Page Curve for Evaporation of 2D AdS Black Holes
Full text linkWe explore the Hawking evaporation of two-dimensional anti-de Sitter (AdS2 ), dilatonic black hole coupled with conformal matter, and derive the Page curve for the entanglement entropy of radiation. We first work in a semiclassical approximation with backreaction. We show that the end-point of the evaporation process is AdS2 with a vanishing dilaton, i.e., a regular, singularity-free, zero-entropy state. We explicitly compute the entanglement entropies of the black hole and the radiation as functions of the horizon radius, using the conformal field theory (CFT) dual to AdS2 gravity. We use a simplified toy model, in which evaporation is described by the forming and growing of a negative mass configuration in the positive-mass black hole interior. This is similar to the “islands” proposal, recently put forward to explain the Page curve for evaporating black holes. The resulting Page curve for AdS2 black holes is in agreement with unitary evolution. The entanglement entropy of the radiation initially grows, closely following a thermal behavior, reaches a maximum at half-way of the evaporation process, and then goes down to zero, following the Bekenstein–Hawking entropy of the black hole. Consistency of our simplified model requires a non-trivial identification of the central charge of the CFT describing AdS2 gravity with the number of species of fields describing Hawking radiation
Long-range quantum gravity
Full text linkIt is a tantalising possibility that quantum gravity (QG) states remaining coherent at astrophysical, galactic and cosmological scales could exist and that they could play a crucial role in understanding macroscopic gravitational effects. We explore, using only general principles of General Relativity, quantum and statistical mechanics, the possibility of using long-range QG states to describe black holes. In particular, we discuss in a critical way the interplay between various aspects of long-range quantum gravity, such as the holographic bound, classical and quantum criticality and the recently proposed quantum thermal generalisation of Einstein's equivalence principle. We also show how black hole thermodynamics can be easily explained in this framework
Quasi-normal modes and microscopic description of 2D black holes
Full text linkWe investigate the possibility of using quasi-normal modes (QNMs) to probe the microscopic structure of two-dimensional (2D) anti-de Sitter (AdS2) dilatonic black holes. We first extend previous results on the QNMs spectrum, found for external massless scalar perturbations, to the case of massive scalar perturbations. We find that the quasi-normal frequencies are purely imaginary and scale linearly with the overtone number. Motivated by this and extending previous results regarding Schwarzschild black holes, we propose a microscopic description of the 2D black hole in terms of a coherent state of N massless particles quantized on a circle, with occupation numbers sharply peaked on the characteristic QNMs frequency ω̂. We further model the black hole as a statistical ensemble of N decoupled quantum oscillators of frequency ω̂. This allows us to recover the Bekenstein-Hawking (BH) entropy S of the hole as the leading contribution to the Gibbs entropy for the set of oscillators, in the high-temperature regime, and to show that S = N. Additionally, we find sub-leading logarithmic corrections to the BH entropy. We further corroborate this microscopic description by outlining a holographic correspondence between QNMs in the AdS2 bulk and the de Alfaro-Fubini-Furlan conformally invariant quantum mechanics. Our results strongly suggest that modelling a black hole as a coherent state of particles and as a statistical ensemble of decoupled harmonic oscillators is always a good approximation in the large black-hole mass, large overtone number limit
Anisotropic fluid cosmology: An alternative to dark matter?
Full text linkWe use anisotropic fluid cosmology to describe the present, dark energy-dominated Universe without assuming the presence of dark matter. The resulting anisotropic fluid spacetime naturally generates inhomogeneities at small scales, triggered by an anisotropic stress, that could therefore be responsible for structure formation at these scales. We show that the dynamics of the scale factor a is described by the usual Friedmann-Lemaître-Robertson-Walker cosmology and decouples completely from that describing inhomogeneities. Assuming that the fluid inherits the equation of state from galactic dynamics, we show that, in the large scale regime, it can be described as a generalized Chaplygin gas. We find that our model fits well the distance modulus experimental data of type Ia supernovae, thus correctly modeling the observed accelerated expansion of the Universe. Conversely, in the small scale regime, we use cosmological perturbation theory to derive the power spectrum P(k) for mass density distribution. At short wavelengths, we find a 1/k4 behavior, in good accordance with the observed correlation function for matter distribution at small scales
Iconic Methods for Multimodal Face Recognition: a Comparative Study
No full textWhen dealing with face recognition, multimodal algorithms, with their potential to capture complementary characteristics
from the 2D and 3D data channels, can reach high
level of efficiency and robustness. In this paper, we explore different combinations of iconic descriptors coupled with a shape descriptor and propose a fully automatic, multimodal,face recognition paradigm. Two iconic features extractors, the Scale Invariant Feature Transform (SIFT) and the Speeded-Up
Robust Features (SURF), are used, in turn, to extract salient points from the images of the faces. The corresponding points
on the scans are validated with Joint Differential Invariants, a 3D characterisation method based on local and global shape
information. SIFT and SURF are then combined at feature level
and the 3D Joint Differential Invariants used to validate them on the shape channel. The proposed method has been tested on the FRGCv2 database. Experimental results highlight the complementarity of the feature points extracted by SIFT and SURF and the effectiveness of their 3D validation
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