Category Archives: Highlights on physics papers

A fractal model can explain Dark Energy

Highlights on physics papers, ,

La cosa davvero particolare del modello cosmologico omogeneo attuale (quello che richiede che il 95% della materia dell’universo sia oscura e di un tipo mai osservato né in laboratorio né altrove, ma solo dedotto per fare la differenza tra teoria e osservazioni) è che è basato sulla assunzione che l’unica soluzione esatta che siamo capaci di trovare alle equazioni teoriche che descrivono il sistema (cioè le equazioni di Einstein della Relatività Generale) sia la soluzione che si applica perfettamente alla realtà.

Nessun’altra teoria fisica gode di questa proprietà: in genere le soluzioni esatte si trovano per avere una guida teorica poi bisogna trovare la maniera di adattarle alla realtà. Forse è questa situazione che rende necessari l’introduzione di tutta quella materia oscura ?

L’idea che abbiamo sviluppato con i modelli disomogenei è quella di interpretare l’accelerazione derivata dalle osservazioni delle supernovae come un effetto apparente che si verifica in una soluzione troppo semplificata delle equazioni di campo di Einstein cioè quelle derivate con una densità e una pressione costanti nello spazio. Questo nuova idea ha generato un dibattito nel campo e alcuni hanno mostrato che non è vero. Noi invece mostriamo che si sono sbagliati loro. Aspettiamo la nuova puntata

Link all’articolo

La massa della Via Lattea

Highlights on physics papers, Science Diffusion

La massa di un sistema auto-gravitante come una galassia si può stimare misurando la velocità “tipica” e assumendo che il sistema sia in equilibrio. Nel caso di un disco rotante, come la Via Lattea, la forza centrifuga dovuta alla rotazione deve essere bilanciata dalla forza centripeta dovuta alla gravità. La forza centrifuga dipende dalla velocità di rotazione (che si misura dai nuovi dati delle missione GAIA) e la forza centripeta dipende dalla massa. In questo modo misurando la velocità di rotazione si può stimare la massa.

Continue reading La massa della Via Lattea →

Mapping the Milky Way Disk with GAIA DR3: 3D extended kinematic maps and rotation curve to ≈30 kpc

Highlights on physics papers,

Il satellite GAIA sta fornendo delle misure di incredibile precisione della posizione spaziale e della velocità di più di 30 milioni di stelle nella nostra galassia: sappiamo cioè le tre coordinate spaziali e le tre coordinate della velocità una quantità di informazioni incredibile. Per dare un’idea trenta anni fa si arrivava qualche decina di stelle!

E’ chiaro che una quantità così enorme di dati sta cambiando la nostra idea di come è fatta la nostra galassia e di come si sia formata. Con alcuni colleghi abbia esteso le mappe cinematiche tridimensionali fino ad una distanza 30% più grande di quelle ufficiali di GAIA, cioè da 20 kpc a 30 kpc. Questo è stato possibile utilizzando un metodo statistico di deconvoluzione degli errori di misura. Troviamo così tante cose interessanti, dalla presenza di moti non circolari in tutte le direzioni alla natura turbolenta della cinematica della galassia.

Ma la cosa più interessante è la curva di rotazione, misurata con una accuratezza migliore che mai, che, invece di essere piatta – cioè velocità costante- mostra un chiaro decadimento. Questo fatto ha implicazioni sulla quantità di materia oscura che era usata proprio per spiegare una curva di rotazione piatta. La stima della massa della Via Lattea nel prossimo articolo 🙂

Link all’articolo (arXives)

Link to Astrophysical Journal

Prioritizing the first doses of SARS-CoV-2 vaccine to save the elderly: the case study of Italy.

Highlights on physics papers

SARS-CoV-2 is currently causing hundreds of deaths everyday in European countries, mostly in not yet vaccinated elderly.
Vaccine shortage poses relevant challenges to health authorities, called to act in a timely manner with scarcity of data.
We modelled the mortality reduction of the elderly according to a schedule of mRNA SARS-CoV-2 vaccine that prioritized first dose administration. For the case study of Italy, we show an increase in protected individuals up to 53.4% and a decrease of deaths up to 19.8% in the cohort of over 80s compared to the standard vaccine recalls after 3 or 4 weeks.
Our model supports the adoption of vaccination campaigns that prioritize administration of the first doses in the elderly.

Link to the original paper

Highlights on physics papers

Gravitational collapse from cold uniform asymmetric initial conditions

Francesco Sylos Labini, Michael Joyce

Using controlled numerical N-body experiments, we show how, in the collapse dynamics of an initially cold and uniform distribution of particles with a generic asymmetric shape, finite N fluctuations and perturbations induced by the anisotropic gravitational field compete to determine the physical properties of the asymptotic quasi-stationary state. When finite N fluctuations dominate the dynamics, the particle energy distribution changes greatly and the final density profile {decays outside its core} as r^(-4) with an N-dependent amplitude. On the other hand, in the limit where the anisotropic perturbations dominate, the collapse is softer and the density profile shows a decay as r^(−3), as is typical of halos in cosmological simulations. However, even in this limit, convergence with N of the macroscopic properties of the virialized system, such as the particle energy distributions, the bound mass, and the density profile, is very slow and not clearly established, including for our largest simulations (with N∼10^6). Our results illustrate the challenges of accurately simulating the first collapsing structures in standard-type cosmological models

Comments:8 pages, 6 figures. Accepted for publication in Astronomy and Astrophysics
Subjects:Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as:arXiv:2106.02388 [astro-ph.CO]
 (or arXiv:2106.02388v1 [astro-ph.CO] for this version)

Zipf’s law for cosmic structures: how large are the greatest structures in the universe?

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The statistical characterization of the distribution of visible matter in the universe is a central problem inmodern cosmology. In this respect, a crucial question still lacking a definitive answer concerns how large are the greateststructures in the universe. This point is closely related to whether or not such a distribution can be approximated as beinghomogeneous on large enough scales. Here we assess this problem by considering the size distribution of superclustersof galaxies and by leveraging on the properties of Zipf-Mandelbrot law, providing a novel approach which complementsstandard analysis based on the correlation functions. We find that galaxy superclusters are well described by a pureZipf’s law with no deviations and this implies that all the catalogs currently available are not sufficiently large to spot atruncation in the power-law behavior. This finding provides evidence that structures larger than the greatest superclustersalready observed are expected to be found when deeper redshift surveys will be completed. As a consequence the scalebeyond which galaxy distribution crossovers toward homogeneity, if any, should increase accordingly

Link to the paper

Prioritizing the first doses of SARS-CoV-2 vaccine to save the elderly: the case study of Italy

Highlights on physics papers

Many countries are currently facing high mortality caused by the circulation of SARS-CoV-2 among the elderly not yet vaccinated. Vaccine shortage poses relevant challenges to health authorities, called to act in a timely manner, and with scarcity of vaccine, and data. We have developed a model for estimating of the impact of vaccination on the mortality of the elderly following a schedule of mRNA SARS-CoV-2 vaccine that prioritize first dose administration, as alternative to the standard schedule of two doses administered 3 to 4 weeks apart. We studied the Italian scenario, considering it representative of other Countries facing similar conditions in terms of virus circulation, mortality, and vaccine shortage, in the period from February 10 to April 14 2021. Under different conditions of quantity of vaccine administration, the schedule prioritizing first doses showed always significant increase of protected individuals, and a decrease of deaths, up to 19.8% less than the standard schedule. These findings support the vaccination option of prioritizing the first dose in the elderly until vaccine supplies are adequate.

LINK

PRE Editor’s suggetion: “Formation of disks with long-lived spiral arms from violent gravitational dynamics”

Highlights on physics papers


Formation of disks with long-lived spiral arms from violent
gravitational dynamics
by Francesco Sylos Labini, Luis Diego Pinto, and Roberto
Capuzzo-Dolcetta

Dear Dr. Sylos-Labini,

We are pleased to inform you that your paper has been selected by the
editors of Physical Review E to be an Editors’ Suggestion.

Continue reading PRE Editor’s suggetion: “Formation of disks with long-lived spiral arms from violent gravitational dynamics” →

Properties of self gravitating quasi-stationary states

Highlights on physics papers,

On a possibile solution of the core-cusp problem and the fundamental properties of self gravitating quasi-stationary states.

One of the crucial problems for the galaxy formation theory is to explain the cusp-core paradox: while the simulations of the theoretical models give rise to a diverging density profile at a small distance (1 / r) from the center,  observations of galaxy profiles show that these  are constant.

To understand the origin of these profiles it is necessary to understand the fundamental properties of the quasi-equilibrium states of gravitational systems and in particular what kind of dynamical path they followed during their formation.

We have found that, depending on the initial conditions, these stationary states can exhibit a small-scale divergent profile or a flat profile. This difference corresponds to a different dynamical nature underlying their origin.

Coming back to  galaxies, the profiles indicate that the dynamic that gave them origin is a violent type of relaxation. That is, a dynamical process different from the “standard” one of cold dark matter models.

arXiv:2009.11624

15 pages, 24 figures, Accepted for publication in Astronomy and Astrophysics

by Francesco Sylos Labini, Roberto Capuzzo-Dolcetta

Continue reading Properties of self gravitating quasi-stationary states →

Formation of disk galaxies with long-lived and non-stationary spiral arms

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This work represents a development of the study of the collapses of purely self-gravitating systems  (see https://physics.aps.org/articles/v12/s19) to the case in which a dissipational gas component is also present. These latter systems show much richer morphological and kinematical structures that may have important observational implications to understand the kinematic and dynamics of the Milky Way as revealed by ongoing surveys such as the Gaia Mission. 

 by Francesco Sylos Labini, Luis Diego Pinto, Roberto Capuzzo-Dolcetta 

Continue reading Formation of disk galaxies with long-lived and non-stationary spiral arms →