25,580 research outputs found
“Under the Blue Beret: A U.N. Peacekeeper in the Middle East (Book Review)” by Terry “Stoney” Burke
Review of Under the Blue Beret: A U.N. Peacekeeper in the Middle East by Terry “Stoney” Burk
Detecting and quantifying stellar magnetic fields -- Sparse Stokes profile approximation using orthogonal matching pursuit
In the recent years, we have seen a rapidly growing number of stellar
magnetic field detections for various types of stars. Many of these magnetic
fields are estimated from spectropolarimetric observations (Stokes V) by using
the so-called center-of-gravity (COG) method. Unfortunately, the accuracy of
this method rapidly deteriorates with increasing noise and thus calls for a
more robust procedure that combines signal detection and field estimation. We
introduce an estimation method that provides not only the effective or mean
longitudinal magnetic field from an observed Stokes V profile but also uses the
net absolute polarization of the profile to obtain an estimate of the apparent
(i.e., velocity resolved) absolute longitudinal magnetic field. By combining
the COG method with an orthogonal-matching-pursuit (OMP) approach, we were able
to decompose observed Stokes profiles with an overcomplete dictionary of
wavelet-basis functions to reliably reconstruct the observed Stokes profiles in
the presence of noise. The elementary wave functions of the sparse
reconstruction process were utilized to estimate the effective longitudinal
magnetic field and the apparent absolute longitudinal magnetic field. A
multiresolution analysis complements the OMP algorithm to provide a robust
detection and estimation method. An extensive Monte-Carlo simulation confirms
the reliability and accuracy of the magnetic OMP approach.Comment: A&A, in press, 15 pages, 14 figure
Gauss-Bonnet gravity, brane world models, and non-minimal coupling
We study the case of brane world models with an additional Gauss-Bonnet term
in the presence of a bulk scalar field which interacts non-minimally with
gravity, via a possible interaction term of the form . The
Einstein equations and the junction conditions on the brane are formulated, in
the case of the bulk scalar field. Static solutions of this model are obtained
by solving numerically the Einstein equations with the appropriate boundary
conditions on the brane. Finally, we present graphically and comment these
solutions for several values of the free parameters of the model.Comment: 13 pages,4 figures, published versio
Spot evolution on the red giant star XX Triangulum. A starspot-decay analysis based on time-series Doppler imaging
Solar spots appear to decay linearly proportional to their size. The decay
rate of solar spots is directly related to magnetic diffusivity, which itself
is a key quantity for the length of a magnetic-activity cycle. Is a linear spot
decay also seen on other stars, and is this in agreement with the large range
of solar and stellar activity cycle lengths? We investigate the evolution of
starspots on the rapidly-rotating ( 24 d) K0 giant XX
Tri, using consecutive time-series Doppler images. Our aim is to obtain a
well-sampled movie of the stellar surface over many years, and thereby detect
and quantify a starspot decay law for further comparison with the Sun. We
obtained continuous high-resolution and phase-resolved spectroscopy with the
1.2-m robotic STELLA telescope on Tenerife over six years. For each observing
season, we obtained between 5 to 7 independent Doppler images, one per stellar
rotation, making up a total of 36 maps. To quantify starspot area decay and
growth, we match the observed images with simplified spot models based on a
Monte Carlo approach. It is shown that the surface of XX Tri is covered with
large high-latitude and even polar spots and with occasional small equatorial
spots. Just over the course of six years, we see a systematically changing spot
distribution with various timescales and morphology, such as spot fragmentation
and spot merging as well as spot decay and formation. An average linear decay
of = 0.022 0.002 SH/day is inferred. We found evidence of an
active longitude in phase toward the (unseen) companion star. Furthermore, we
detect a weak solar-like differential rotation with a surface shear of
= 0.016 0.003. From the decay rate, we determine a turbulent diffusivity
of = (6.3 0.5) 10 cm/s and predict a
magnetic activity cycle of 26 6 years
A fast method for Stokes profile synthesis -- Radiative transfer modeling for ZDI and Stokes profile inversion
The major challenges for a fully polarized radiative transfer driven approach
to Zeeman-Doppler imaging are still the enormous computational requirements. In
every cycle of the iterative interplay between the forward process (spectral
synthesis) and the inverse process (derivative based optimization) the Stokes
profile synthesis requires several thousand evaluations of the polarized
radiative transfer equation for a given stellar surface model. To cope with
these computational demands and to allow for the incorporation of a full Stokes
profile synthesis into Doppler- and Zeeman-Doppler imaging applications as well
as into large scale solar Stokes profile inversions, we present a novel fast
and accurate synthesis method for calculating local Stokes profiles. Our
approach is based on artificial neural network models, which we use to
approximate the complex non-linear mapping between the most important
atmospheric parameters and the corresponding Stokes profiles. A number of
specialized artificial neural networks, are used to model the functional
relation between the model atmosphere, magnetic field strength, field
inclination, and field azimuth, on one hand and the individual components
(I,Q,U,V) of the Stokes profiles, on the other hand. We performed an extensive
statistical evaluation and show that our new approach yields accurate local as
well as disk-integrated Stokes profiles over a wide range of atmospheric
conditions. The mean rms errors for the Stokes I and V profiles are well below
0.2% compared to the exact numerical solution. Errors for Stokes Q and U are in
the range of 1%. Our approach does not only offer an accurate approximation to
the LTE polarized radiative transfer it, moreover, accelerates the synthesis by
a factor of more than 1000.Comment: A&A, in pres
How Decoherence Affects the Probability of Slow-Roll Eternal Inflation
Slow-roll inflation can become eternal if the quantum variance of the
inflaton field around its slowly rolling classical trajectory is converted into
a distribution of classical spacetimes inflating at different rates, and if the
variance is large enough compared to the rate of classical rolling that the
probability of an increased rate of expansion is sufficiently high. Both of
these criteria depend sensitively on whether and how perturbation modes of the
inflaton interact and decohere. Decoherence is inevitable as a result of
gravitationally-sourced interactions whose strength are proportional to the
slow-roll parameters. However, the weakness of these interactions means that
decoherence is typically delayed until several Hubble times after modes grow
beyond the Hubble scale. We present perturbative evidence that decoherence of
long-wavelength inflaton modes indeed leads to an ensemble of classical
spacetimes with differing cosmological evolutions. We introduce the notion of
per-branch observables---expectation values with respect to the different
decohered branches of the wave function---and show that the evolution of modes
on individual branches varies from branch to branch. Thus single-field
slow-roll inflation fulfills the quantum-mechanical criteria required for the
validity of the standard picture of eternal inflation. For a given potential,
the delayed decoherence can lead to slight quantitative adjustments to the
regime in which the inflaton undergoes eternal inflation.Comment: 27 pages, 3 figures; v2 reflects peer review process and has new
results in Section
De Sitter Space Without Dynamical Quantum Fluctuations
We argue that, under certain plausible assumptions, de Sitter space settles
into a quiescent vacuum in which there are no dynamical quantum fluctuations.
Such fluctuations require either an evolving microstate, or time-dependent
histories of out-of-equilibrium recording devices, which we argue are absent in
stationary states. For a massive scalar field in a fixed de Sitter background,
the cosmic no-hair theorem implies that the state of the patch approaches the
vacuum, where there are no fluctuations. We argue that an analogous conclusion
holds whenever a patch of de Sitter is embedded in a larger theory with an
infinite-dimensional Hilbert space, including semiclassical quantum gravity
with false vacua or complementarity in theories with at least one Minkowski
vacuum. This reasoning provides an escape from the Boltzmann brain problem in
such theories. It also implies that vacuum states do not uptunnel to
higher-energy vacua and that perturbations do not decohere while slow-roll
inflation occurs, suggesting that eternal inflation is much less common than
often supposed. On the other hand, if a de Sitter patch is a closed system with
a finite-dimensional Hilbert space, there will be Poincare recurrences and
dynamical Boltzmann fluctuations into lower-entropy states. Our analysis does
not alter the conventional understanding of the origin of density fluctuations
from primordial inflation, since reheating naturally generates a high-entropy
environment and leads to decoherence, nor does it affect the existence of
non-dynamical vacuum fluctuations such as those that give rise to the Casimir
effect.Comment: version accepted for publication in Foundations of Physic
Why Boltzmann Brains Don't Fluctuate Into Existence From the De Sitter Vacuum
Many modern cosmological scenarios feature large volumes of spacetime in a de
Sitter vacuum phase. Such models are said to be faced with a "Boltzmann Brain
problem" - the overwhelming majority of observers with fixed local conditions
are random fluctuations in the de Sitter vacuum, rather than arising via
thermodynamically sensible evolution from a low-entropy past. We argue that
this worry can be straightforwardly avoided in the Many-Worlds (Everett)
approach to quantum mechanics, as long as the underlying Hilbert space is
infinite-dimensional. In that case, de Sitter settles into a truly stationary
quantum vacuum state. While there would be a nonzero probability for observing
Boltzmann-Brain-like fluctuations in such a state, "observation" refers to a
specific kind of dynamical process that does not occur in the vacuum (which is,
after all, time-independent). Observers are necessarily out-of-equilibrium
physical systems, which are absent in the vacuum. Hence, the fact that
projection operators corresponding to states with observers in them do not
annihilate the vacuum does not imply that such observers actually come into
existence. The Boltzmann Brain problem is therefore much less generic than has
been supposed.Comment: Based on a talk given by SMC at, and to appear in the proceedings of,
the Philosophy of Cosmology conference in Tenerife, September 201
Extremal black holes, gravitational entropy and nonstationary metric fields
We show that extremal black holes have zero entropy by pointing out a simple
fact: they are time-independent throughout the spacetime and correspond to a
single classical microstate. We show that non-extremal black holes, including
the Schwarzschild black hole, contain a region hidden behind the event horizon
where all their Killing vectors are spacelike. This region is nonstationary and
the time labels a continuous set of classical microstates, the phase space
, where is a three-metric induced on a
spacelike hypersurface and is its momentum conjugate. We
determine explicitly the phase space in the interior region of the
Schwarzschild black hole. We identify its entropy as a measure of an outside
observer's ignorance of the classical microstates in the interior since the
parameter which labels the states lies anywhere between 0 and 2M. We
provide numerical evidence from recent simulations of gravitational collapse in
isotropic coordinates that the entropy of the Schwarzschild black hole stems
from the region inside and near the event horizon where the metric fields are
nonstationary; the rest of the spacetime, which is static, makes no
contribution. Extremal black holes have an event horizon but in contrast to
non-extremal black holes, their extended spacetimes do not possess a bifurcate
Killing horizon. This is consistent with the fact that extremal black holes are
time-independent and therefore have no distinct time-reverse.Comment: 12 pages, 2 figures. To appear in Class. and Quant. Gravity. Based on
an essay selected for honorable mention in the 2010 gravity research
foundation essay competitio
- …