Categories
Science

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Categories
General Relativity and Quantum Cosmology Science

Cosmic no-hair theorems for viscous contracting Universes. (arXiv:2008.02286v1 [gr-qc])

A cosmic no-hair theorem for all initially contracting, spatially
homogeneous, orthogonal Bianchi Cosmologies is derived – which shows that all
such Universes asymptote to a spatially flat, isotropic Universe with the
inclusion of a shear viscous stress. This establishes a new mechanism of
isotropisation in a contracting Universe, which does not take recourse to an
ekpyrosis-like mechanism using an effective ultra-stiff equation of state
fluid, that is, one in which the pressure is much greater than the energy
density.

Categories
Science Statistics

The Athena Class of Risk-Limiting Ballot Polling Audits. (arXiv:2008.02315v1 [cs.CR])

Existing ballot polling election tabulation audits are designed for stopping
decisions that are made ballot-by-ballot; we refer to these as B2 audits. Real
ballot polling audits are not B2, but round-by-round (R2): many ballots are
drawn in a single round before a stopping decision is made. We show that B2
BRAVO – the main risk-limiting stopping rule in use today – results in
significant inefficiency when directly applied to real R2 audits. We present
the ATHENA class of risk-limiting R2 stopping rules. We prove that each rule is
more efficient than the corresponding B2 BRAVO stopping rule applied at the end
of the round. We have software libraries implementing most of our results in
both python and MATLAB.

As an illustrative example, we show that the first round size for a 90%
stopping probability for ATHENA is about half that for B2 BRAVO (stopping rule
applied at the end of the round) for all state margins in the 2016 US
Presidential election. Further, the first round sizes for the ATHENA class of
audits are 15-29% smaller when compared to the B2 BRAVO stopping rule applied
on ballots in selection order, with the best percentage improvements for larger
round sizes. This implies that keeping track of the order when drawing ballots
R2 is not beneficial, and is, in fact, information-theoretically wasteful,
because ATHENA is more efficient even without information on selection order.
These results are significant because current approaches to real ballot polling
election audits use the B2 BRAVO rules, requiring about twice as much work on
the part of election officials. Many states performing risk-limiting audits for
the first time plan to use ballot polling audits in November 2020 for the US
Presidential election and could substantially benefit from improvements.

Categories
Science Statistics

Machine learning for faster and smarter fluorescence lifetime imaging microscopy. (arXiv:2008.02320v1 [eess.IV])

Fluorescence lifetime imaging microscopy (FLIM) is a powerful technique in
biomedical research that uses the fluorophore decay rate to provide additional
contrast in fluorescence microscopy. However, at present, the calculation,
analysis, and interpretation of FLIM is a complex, slow, and computationally
expensive process. Machine learning (ML) techniques are well suited to extract
and interpret measurements from multi-dimensional FLIM data sets with
substantial improvement in speed over conventional methods. In this topical
review, we first discuss the basics of FILM and ML. Second, we provide a
summary of lifetime extraction strategies using ML and its applications in
classifying and segmenting FILM images with higher accuracy compared to
conventional methods. Finally, we discuss two potential directions to improve
FLIM with ML with proof of concept demonstrations.

Categories
Science Statistics

Spatiotemporal dynamic of COVID-19 mortality in the city of Sao Paulo, Brazil: shifting the high risk from the best to the worst socio-economic conditions. (arXiv:2008.02322v1 [stat.AP])

Currently, Brazil has one of the fastest increasing COVID-19 epidemics in the
world, that has caused at least 94 thousand confirmed deaths until now. The
city of Sao Paulo is particularly vulnerable because it is the most populous in
the country. Analyzing the spatiotemporal dynamics of COVID-19 is important to
help the urgent need to integrate better actions to face the pandemic. Thus,
this study aimed to analyze the COVID-19 mortality, from March to July 2020,
considering the spatio-time architectures, the socio-economic context of the
population, and using a fine granular level, in the most populous city in
Brazil. For this, we conducted an ecological study, using secondary public data
from the mortality information system. We describe mortality rates for each
epidemiological week and the entire period by sex and age. We modelled the
deaths using spatiotemporal and spatial architectures and Poisson probability
distributions in a latent Gaussian Bayesian model approach. We obtained the
relative risks for temporal and spatiotemporal trends and socio-economic
conditions. To reduce possible sub notification, we considered the confirmed
and suspected deaths. Our findings showed an apparent stabilization of the
temporal trend, at the end of the period, but that may change in the future.
Mortality rate increased with increasing age and was higher in men. The risk of
death was greater in areas with the worst social conditions throughout the
study period. However, this was not a uniform pattern over time, since we
identified a shift from the high risk in the areas with best socio-economic
conditions to the worst ones. Our study contributed by emphasizing the
importance of geographic screening in areas with a higher risk of death, and,
currently, worse socio-economic contexts, as a crucial aspect to reducing
disease mortality and health inequities, through integrated public health
actions.

Categories
High Energy Physics Phenomenology Science

The Cosmological Tension of Ultralight Axion Dark Matter and its Solutions. (arXiv:2008.02279v1 [hep-ph])

A number of proposed and ongoing experiments search for axion dark matter
with a mass nearing the limit set by small scale structure (${\cal O} ( 10 ^{ –
21 } {\rm eV} ) $). We consider the late universe cosmology of these models,
showing that requiring the axion to have a matter-power spectrum that matches
that of cold dark matter constrains the magnitude of the axion couplings to the
visible sector. Comparing these limits to current and future experimental
efforts, we find that many searches require axions with an abnormally large
coupling to Standard Model fields, independently of how the axion was populated
in the early universe. We survey mechanisms that can alleviate the bounds,
namely, the introduction of large charges, various forms of kinetic mixing, a
clockwork structure, and imposing a discrete symmetry. We provide an explicit
model for each case and explore their phenomenology and viability to produce
detectable ultralight axion dark matter.

Categories
High Energy Physics Phenomenology Science

Non-vanishing normal density in cold holographic superfluids. (arXiv:2008.02289v1 [hep-th])

The low energy and finite temperature excitations of a $d+1$-dimensional
system exhibiting superfluidity are well described by a hydrodynamic model with
two fluid flows: a normal flow and a superfluid flow. In the vicinity of a
quantum critical point, thermodynamics and transport in the system are expected
to be controlled by the critical exponents and by the spectrum of irrelevant
deformations away from the quantum critical point. Here, using gauge-gravity
duality, we present the low temperature dependence of thermodynamic and charge
transport coefficients at first order in the hydrodynamic derivative expansion
in terms of the critical exponents. Special attention will be paid to the
behavior of the charge density of the normal flow in systems with emergent
infrared conformal and Lifshitz symmetries, parameterized by a Lifshitz
dynamical exponent $z>1$. When $1\leq z<d+2$, we recover ($z=1$) and extend
($z>1$) previous results obtained by relativistic effective field theory
techniques. Instead, when $z>d+2$, we show that the normal charge density
becomes non-vanishing at zero temperature. An extended appendix generalizes
these results to systems that violate hyperscaling as well as systems with
generalized photon masses. Our results clarify previous work in the holographic
literature and have relevance to recent experimental measurements of the
superfluid density on cuprate superconductors.

Categories
High Energy Physics Science Theory

GLSMs for exotic Grassmannians. (arXiv:2008.02281v1 [hep-th])

In this paper we explore nonabelian gauged linear sigma models (GLSMs) for
symplectic and orthogonal Grassmannians and flag manifolds, checking e.g.
global symmetries, Witten indices, and Calabi-Yau conditions, following up a
proposal in the math community. For symplectic Grassmannians, we check that the
Coulomb branch vacua of the GLSM are consistent with ordinary and equivariant
quantum cohomology of the space.

Categories
High Energy Physics Phenomenology Science

Analog reheating of the early universe in the laboratory. (arXiv:2008.02290v1 [cond-mat.quant-gas])

Cosmic reheating describes the transition of the post-inflationary universe
to a hot and thermal state. In order to shed light on the nature of this
process, we propose a quantum simulation of cosmic reheating in an ultracold
Bose gas. In our model, the inflaton field dynamics is mapped onto that of an
atomic Bose-Einstein condensate whose excitations are identified with the
particles produced by the decaying inflaton field. The expansion of the
universe as well as the oscillations of the inflaton field are encoded in the
time-dependence of the atomic interactions, which can be tuned experimentally
via Feshbach resonances. As we illustrate by means of classical-statistical
simulations for the case of two spatial dimensions, the dynamics of the atomic
system exhibits the characteristic stages of far-from-equilibrium reheating,
including the amplification of fluctuations via parametric instabilities and
the subsequent turbulent transport of energy towards higher momenta. The
transport is governed by a non-thermal fixed point showing universal
self-similar time evolution as well as a transient regime of prescaling with
time-dependent scaling exponents. While the classical-statistical simulations
can only capture the earlier stages of the dynamics for weak couplings, the
proposed experimental implementation provides a protocol for the quantum
simulation of the entire evolution even beyond the weak coupling regime.

Categories
High Energy Physics Science Theory

Cosmic no-hair theorems for viscous contracting Universes. (arXiv:2008.02286v1 [gr-qc])

A cosmic no-hair theorem for all initially contracting, spatially
homogeneous, orthogonal Bianchi Cosmologies is derived – which shows that all
such Universes asymptote to a spatially flat, isotropic Universe with the
inclusion of a shear viscous stress. This establishes a new mechanism of
isotropisation in a contracting Universe, which does not take recourse to an
ekpyrosis-like mechanism using an effective ultra-stiff equation of state
fluid, that is, one in which the pressure is much greater than the energy
density.

Categories
High Energy Physics Science Theory

Non-vanishing normal density in cold holographic superfluids. (arXiv:2008.02289v1 [hep-th])

The low energy and finite temperature excitations of a $d+1$-dimensional
system exhibiting superfluidity are well described by a hydrodynamic model with
two fluid flows: a normal flow and a superfluid flow. In the vicinity of a
quantum critical point, thermodynamics and transport in the system are expected
to be controlled by the critical exponents and by the spectrum of irrelevant
deformations away from the quantum critical point. Here, using gauge-gravity
duality, we present the low temperature dependence of thermodynamic and charge
transport coefficients at first order in the hydrodynamic derivative expansion
in terms of the critical exponents. Special attention will be paid to the
behavior of the charge density of the normal flow in systems with emergent
infrared conformal and Lifshitz symmetries, parameterized by a Lifshitz
dynamical exponent $z>1$. When $1\leq z<d+2$, we recover ($z=1$) and extend
($z>1$) previous results obtained by relativistic effective field theory
techniques. Instead, when $z>d+2$, we show that the normal charge density
becomes non-vanishing at zero temperature. An extended appendix generalizes
these results to systems that violate hyperscaling as well as systems with
generalized photon masses. Our results clarify previous work in the holographic
literature and have relevance to recent experimental measurements of the
superfluid density on cuprate superconductors.