15,334 research outputs found
Efficient Higher Order Derivatives of Objective Functions Composed of Matrix Operations
This paper is concerned with the efficient evaluation of higher-order
derivatives of functions that are composed of matrix operations. I.e., we
want to compute the -th derivative tensor , where is given as an algorithm that
consists of many matrix operations. We propose a method that is a combination
of two well-known techniques from Algorithmic Differentiation (AD): univariate
Taylor propagation on scalars (UTPS) and first-order forward and reverse on
matrices. The combination leads to a technique that we would like to call
univariate Taylor propagation on matrices (UTPM). The method inherits many
desirable properties: It is easy to implement, it is very efficient and it
returns not only but yields in the process also the derivatives
for . As performance test we compute the gradient
% and the Hessian by a combination of forward
and reverse mode of f(X) = \trace (X^{-1}) in the reverse mode of AD for . We observe a speedup of about 100 compared to
UTPS. Due to the nature of the method, the memory footprint is also small and
therefore can be used to differentiate functions that are not accessible by
standard methods due to limited physical memory
Dust reverberation mapping in the era of big optical surveys and its cosmological application
The time lag between optical and near-infrared (IR) flux variability can be
taken as a means to determine the sublimation radius of the dusty "torus"
around supermassive black holes in active galactic nuclei (AGN). I will show
that data from big optical survey telescopes, e.g. the Large Synoptic Survey
Telescope (LSST), can be used to measure dust sublimation radii as well. The
method makes use of the fact that the Wien tail of the hot dust emission
reaches into the optical and can be reliably recovered with high-quality
photometry. Simulations show that dust sublimation radii for a large sample of
AGN can be reliably established out to redshift z ~ 0.1-0.2 with the LSST.
Owing to the ubiquitous presence of AGN up to high redshifts, they have been
studies as cosmological probes. Here, I discuss how optically-determined dust
time lags fit into the suggestion of using the dust sublimation radius as a
"standard candle" and propose and extension of the dust time lags as "standard
rulers" in combination with IR interferometry.Comment: 6 pages, 4 figures, 1 table; accepted by ApJL (Feb 18, 2014
Non-Singular Bounces Catalysed by Dark Energy
We investigate classically non-singular bounces caused by dark energy. In the
presence of positive spatial curvature, vacuum energy, either in the form of a
cosmological constant or a scalar field potential, allows for an open set of
initial conditions leading to non-singular bounces, without any violation of
the null energy condition. We study anisotropic Bianchi IX cosmologies, and
demonstrate that they can even have multiple bounces, accompanied by intricate
evolutions of the anisotropies that provide a non-singular analogue of
mixmaster crunches. The relation of these solutions to more complete
cosmological models, as well as to the recently proposed swampland criteria,
are briefly discussed.Comment: 30 pages, 25 figure files; v2: references adde
Sum-frequency ionic Raman scattering
In a recent report sum-frequency excitation of a Raman-active phonon was
experimentally demonstrated for the first time. This mechanism is the sibling
of impulsive stimulated Raman scattering, in which difference-frequency
components of a light field excite a Raman-active mode. Here we propose that
ionic Raman scattering analogously has a sum-frequency counterpart. We compare
the four Raman mechanisms, photonic and ionic difference- and sum-frequency
excitation, for three different example materials using a generalized
oscillator model for which we calculate the parameters with density functional
theory. Sum-frequency ionic Raman scattering completes the toolkit for
controlling materials properties by means of selective excitation of lattice
vibrations
The dust sublimation radius as an outer envelope to the bulk of the narrow Fe Kalpha line emission in Type 1 AGN
The Fe Kalpha emission line is the most ubiquitous feature in the X-ray
spectra of active galactic nuclei (AGN), but the origin of its narrow core
remains uncertain. Here, we investigate the connection between the sizes of the
Fe core emission regions and the measured sizes of the dusty tori in 13 local
Type 1 AGN. The observed Fe K emission radii (R_fe) are determined from
spectrally resolved line widths in X-ray grating spectra, and the dust
sublimation radii (R_dust) are measured either from optical/near-infrared
reverberation time lags or from resolved near-infrared interferometric data.
This direct comparison shows, on an object-by-object basis, that the dust
sublimation radius forms an outer envelope to the bulk of the Fe K emission.
R_fe matches R_dust well in the AGN with the best constrained line widths
currently. In a significant fraction of objects without a clear narrow line
core, R_fe is similar to, or smaller than the radius of the optical broad line
region. These facts place important constraints on the torus geometries for our
sample. Extended tori in which the solid angle of fluorescing gas peaks at well
beyond the dust sublimation radius can be ruled out. We also test for
luminosity scalings of R_fe, finding that Eddington ratio is not a prime driver
in determining the line location in our sample. We discuss in detail potential
caveats due to data analysis and instrumental limitations, simplistic line
modeling, uncertain black hole masses, as well as sample selection, showing
that none of these is likely to bias our core result. The calorimeter on board
Astro-H will soon vastly increase the parameter space over which line
measurements can be made, overcoming many of these limitations.Comment: ApJ in press. Community comments greatly appreciated. 13 pages, 4
figures and 2 tables including an appendi
Constraining Generalized Non-local Cosmology from Noether Symmetries
We study a generalized nonlocal theory of gravity which, in specific limits,
can become either the curvature non-local or teleparallel non-local theory.
Using the Noether Symmetry Approach, we find that the coupling functions coming
from the non-local terms are constrained to be either exponential or linear in
form. It is well known that in some non-local theories, a certain kind of
exponential non-local couplings are needed in order to achieve a renormalizable
theory. In this paper, we explicitly show that this kind of coupling does not
need to by introduced by hand, instead, it appears naturally from the
symmetries of the Lagrangian in flat Friedmann-Robertson-Walker cosmology.
Finally, we find de-Sitter and power law cosmological solutions for different
nonlocal theories. The symmetries for the generalized non-local theory is also
found and some cosmological solutions are also achieved under the full theory.Comment: 15 pages, to be published in Eur.Phys.J.
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