3 research outputs found
Axionic instabilities and new black hole solutions
The coupling between scalar and vector fields has a long and interesting
history. Axions are one key possibility to solve the strong CP problem and
axion-like particles could be one solution to the dark matter puzzle. Given the
nature of the coupling, and the universality of free fall, nontrivial important
effects are expected in regions where gravity is strong. Here, we show that i.
A background EM field induces an axionic instability in flat space, for large
enough electric fields. Conversely, a homogeneous harmonic axion field induces
an instability in the Maxwell sector. When carried over to curved spacetime,
this phenomena translates into generic instabilities of charged black holes
(BHs). ii. In the presence of charge, BH uniqueness results are lost. We find
solutions which are small deformations of the Kerr-Newman geometry and hairy
stationary solutions without angular momentum, which are `dragged' by the
axion. Axion fields must exist around spinning BHs if these are immersed in
external magnetic fields. The axion profile can be obtained perturbatively from
the electro-vacuum solution derived by Wald. iii. Ultralight axions trigger
superradiant instabilities of spinning BHs and form an axionic cloud in the
exterior geometry. The superradiant growth can be interrupted or suppressed
through axionic or scalar couplings to EM. These couplings lead to periodic
bursts of light, which occur throughout the history of energy extraction from
the BH. We provide numerical and simple analytical estimates for the rates of
these processes. iv. Finally, we discuss how plasma effects can affect the
evolution of superradiant instabilities.Comment: 28 pages, RevTeX4. v2: overall improvements, typos corrected; version
to appear in Physical Review
Global uncertainty in the diagnosis of neurological complications of SARS-CoV-2 infection by both neurologists and non-neurologists: An international inter-observer variability study
Introduction: Uniform case definitions are required to ensure harmonised reporting of neurological syndromes associated with SARS-CoV-2. Moreover, it is unclear how clinicians perceive the relative importance of SARSCoV-2 in neurological syndromes, which risks under- or over-reporting. Methods: We invited clinicians through global networks, including the World Federation of Neurology, to assess ten anonymised vignettes of SARS-CoV-2 neurological syndromes. Using standardised case definitions, clinicians assigned a diagnosis and ranked association with SARS-CoV-2. We compared diagnostic accuracy and assigned association ranks between different settings and specialties and calculated inter-rater agreement for case definitions as "poor" (kappa <= 0.4), "moderate" or "good" (kappa > 0.6). Results: 1265 diagnoses were assigned by 146 participants from 45 countries on six continents. The highest correct proportion were cerebral venous sinus thrombosis (CVST, 95.8%), Guillain-Barre ' syndrome (GBS, 92.4%) and headache (91.6%) and the lowest encephalitis (72.8%), psychosis (53.8%) and encephalopathy (43.2%). Diagnostic accuracy was similar between neurologists and non-neurologists (median score 8 vs. 7/10, p = 0.1). settings with few neurologists. However, encephalopathy, encephalitis, and psychosis were often misdiagnosed