1,475 research outputs found
Fractal Structure of Loop Quantum Gravity
In this paper we have calculated the spectral dimension of loop quantum
gravity (LQG) using simple arguments coming from the area spectrum at different
length scales. We have obtained that the spectral dimension of the spatial
section runs from 2 to 3, across a 1.5 phase, when the energy of a probe scalar
field decrees from high to low energy. We have calculated the spectral
dimension of the space-time also using results from spin-foam models, obtaining
a 2-dimensional effective manifold at hight energy. Our result is consistent
with other two approach to non perturbative quantum gravity: causal dynamical
triangulation and asymptotic safety quantum gravity.Comment: 5 pages, 5 figure
Charged rotating noncommutative black holes
In this paper we complete the program of the noncomutative geometry inspired
black holes, providing the richest possible solution, endowed with mass, charge
and angular momentum. After providing a prescription for employing the
Newman-Janis algorithm in the case of nonvanishing stress tensors, we find
regular axisymmetric charged black holes in the presence of a minimal length.
We study also the new thermodynamics and we determine the corresponding
higher-dimensional solutions. As a conclusion we make some consideration about
possible applications.Comment: 13 pages, 3 figures, correction of a typesetting inattention, updated
reference list, version accepted for publication on Physical Review
Granulomatous mesenteric lymphadenitis after three doses of the COVID-19 vaccine
Lymphadenopathy related to vaccination has been reported as an adverse effect of mRNA-based COVID-19 vaccines. Most cases are regional lymph nodes near of injection site, with mild-moderate 18 F-fluorodeoxyglucose uptake on positron emission tomography. We report a middle-aged Brazilian man with mesenteric lymphadenitis manifested five days after the third dose of the Pfizer-BioNTech mRNA-based vaccine against COVID-19. The patient had no known risk factors and evolved with rapid clinical improvement. The imaging findings, laboratory determinations, histopathological and microbiological evaluations raised doubts about the hypothesis of an eventual adverse effect of the vaccine. The aim is to call attention to possible rare reactions of SARS-CoV-2 vaccinations
Spectral dimension of a quantum universe
In this paper, we calculate in a transparent way the spectral dimension of a
quantum spacetime, considering a diffusion process propagating on a fluctuating
manifold. To describe the erratic path of the diffusion, we implement a minimal
length by averaging the graininess of the quantum manifold in the flat space
case. As a result we obtain that, for large diffusion times, the quantum
spacetime behaves like a smooth differential manifold of discrete dimension. On
the other hand, for smaller diffusion times, the spacetime looks like a fractal
surface with a reduced effective dimension. For the specific case in which the
diffusion time has the size of the minimal length, the spacetime turns out to
have a spectral dimension equal to 2, suggesting a possible renormalizable
character of gravity in this regime. For smaller diffusion times, the spectral
dimension approaches zero, making any physical interpretation less reliable in
this extreme regime. We extend our result to the presence of a background field
and curvature. We show that in this case the spectral dimension has a more
complicated relation with the diffusion time, and conclusions about the
renormalizable character of gravity become less straightforward with respect to
what we found with the flat space analysis.Comment: 5 pages, 1 figure, references added, typos corrected, title changed,
final version published in Physical Review
Unattainable extended spacetime regions in conformal gravity
The Janis-Newman-Winicour metric is a solution of Einstein's gravity
minimally coupled to a real massless scalar field. The -metric is
instead a vacuum solution of Einstein's gravity. These spacetimes have no
horizon and possess a naked singularity at a finite value of the radial
coordinate, where curvature invariants diverge and the spacetimes are
geodetically incomplete. In this paper, we reconsider these solutions in the
framework of conformal gravity and we show that it is possible to solve the
spacetime singularities with a suitable choice of the conformal factor. Now
curvature invariants remain finite over the whole spacetime. Massive particles
never reach the previous singular surface and massless particles can never do
it with a finite value of their affine parameter. Our results support the
conjecture according to which conformal gravity can fix the singularity problem
that plagues Einstein's gravity.Comment: 1+10 pages, 2 figures. v2: refereed versio
False-positive HIV results and COVID-19 infection or vaccination?
Recently, there have been reports of false-positive HIV results associated with COVID-19 infections and vaccination, which require attention. The similarity between the spike proteins of HIV and SARS-CoV-2 may lead to cross-reactivity of antibodies, resulting in false-positive results on immunoassay screening tests. This hypothesis presents a serious diagnostic challenge. Patients presenting discordant COVID-19 and HIV results should undergo confirmation of the HIV chemiluminescent immunoassay due to the potential for analytical errors. It is essential to highlight the potential for false-positive HIV results related to SARS-CoV-2
Gravitational collapse in loop quantum gravity
In this paper we study the gravitational collapse in loop quantum gravity. We
consider the space-time region inside the Schwarzschild black hole event
horizon and we divide this region in two parts, the first one where the matter
(dust matter) is localized and the other (outside) where the metric is
Kantowski-Sachs type. We calculate the state solving Hamiltonian constraint and
we obtain a set of three difference equations that give a regular and natural
evolution beyond the classical singularity point in "r=0" localized.Comment: 16 pages, 2 figure
Self-completeness and spontaneous dimensional reduction
A viable quantum theory of gravity is one of the biggest challenges facing
physicists. We discuss the confluence of two highly expected features which
might be instrumental in the quest of a finite and renormalizable quantum
gravity -- spontaneous dimensional reduction and self-completeness. The former
suggests the spacetime background at the Planck scale may be effectively
two-dimensional, while the latter implies a condition of maximal compression of
matter by the formation of an event horizon for Planckian scattering. We
generalize such a result to an arbitrary number of dimensions, and show that
gravity in higher than four dimensions remains self-complete, but in lower
dimensions it is not. In such a way we established an "exclusive disjunction"
or "exclusive or" (XOR) between the occurrence of self-completeness and
dimensional reduction, with the goal of actually reducing the unknowns for the
scenario of the physics at the Planck scale. Potential phenomenological
implications of this result are considered by studying the case of a
two-dimensional dilaton gravity model resulting from dimensional reduction of
Einstein gravity.Comment: 12 pages, 3 figures; v3: final version in press on Eur. Phys. J. Plu
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