6,810 research outputs found
AdS Black Holes from Duality in Gauged Supergravity
We study and utilize duality transformations in a particular STU-model of
four dimensional gauged supergravity. This model is a truncation of the de
Wit-Nicolai N=8 theory and as such has a lift to eleven-dimensional
supergravity on the seven-sphere. Our duality group is and while it
can be applied to any solution of this theory, we consider the known
asymptotically AdS, supersymmetric black holes and focus on duality
transformations which preserve supersymmetry. For static black holes we
generalize the supersymmetric solutions of Cacciatori and Klemm from three
magnetic charges to include two additional electric charges and argue that this
is co-dimension one in the full space of supersymmetric static black holes in
the STU-model. These new static black holes have nontrivial profiles for
axions. For rotating black holes, we generalize the known two-parameter
supersymmetric solution to include an additional parameter which represents
scalar hair. When lifted to M-theory, these black holes correspond to the near
horizon geometry of a stack of BPS rotating M2-branes, spinning on an
which is fibered non-trivially over a Riemann surface.Comment: 21 page
The role of temperature dependent string-inspired CPT violating backgrounds in leptogenesis and the chiral magnetic effect
In a temperature dependent CPT-Violating (CPTV) axial time-like background
(induced by the Kalb-Ramond tensor field of string theory) we discuss
leptogenesis by solving the Boltzmann equation.The current work non-trivially
modifies the framework of a previous phenomenological approach by the authors
where the CPTV axial background was considered to be a constant (with no
microscopic justification). The constant background approximation though is
shown to capture the main phenomenological features of leptogenesis. On
comparing our analysis to the related chiral magnetic effect for axial current
condensates, we conclude that the Kalb-Ramond field does not play the role of
the chiral chemical potential needed for that effect.Comment: 17 pages revte
Oat variety characteristics for suppressing weeds
Oats are a valuable food source and useful in the crop rotation both in organic and conventional farming systems, partly because of their excellent weed suppression ability. Thomas Döring, Louisa Winkler and Nick Fradgley report new results that show how plant breeding can make oats even better
Graviton propagation within the context of the D-material universe
Motivated by the recent breakthrough of the detection of Gravitational Waves
(GW) from coalescent black holes by the aLIGO interferometers, we study the
propagation of GW in the {\sl D-material universe}, which we have recently
shown to be compatible with large-scale structure and inflationary
phenomenology. The medium of D-particles induces an effective mass for the
graviton, as a consequence of the formation of recoil-velocity field
condensates due to the underlying Born-Infeld dynamics. There is a competing
effect, due to a super-luminal refractive index, as a result of the
gravitational energy of D-particles acting as a dark matter component, with
which propagating gravitons interact. We examine conditions for the condensate
under which the latter effect is sub-leading. We argue that if quantum
fluctuations of the recoil velocity are relatively strong, which can happen in
the current era of the universe, then the condensate, and hence the induced
mass of the graviton, can be several orders of magnitude larger than the
magnitude of the cosmological constant today. Hence, we constrain the graviton
mass using aLIGO and pulsar timing observations (which give the most stringent
bounds at present). In such a sub-luminal graviton case, there is also a
gravitational Cherenkov effect for ordinary high energy cosmic matter, which is
further constrained by means of ultra-high-energy cosmic ray observations.
Assuming cosmic rays of extragalactic origin, the bounds on the quantum
condensate strength, based on the gravitational Cherenkov effect, are of the
same order as those from aLIGO measurements, in contrast to the case where a
galactic origin of the cosmic rays is assumed, in which case the corresponding
bounds are much weaker.Comment: 21 pages, JCAP style, no figure
Leptogenesis from Heavy Right-Handed Neutrinos in CPT Violating Backgrounds
We discuss leptogenesis in a model with heavy right-handed Majorana neutrinos
propagating in a constant but otherwise generic CPT-violating axial time-like
background (which could be motivated by string theory considerations). At
temperatures much higher than the temperature of the electroweak phase
transition we solve analytically but approximately (using Pade approximants)
the corresponding Boltzmann equations, which describe lepton asymmetry
generation due to the tree-level decays of the heavy neutrinos into standard
model leptons. These leptons are effectively massless at such temperatures. The
current work completes in a rigorous way a preliminary treatment of the same
system, by some of the present authors. In this earlier work, lepton asymmetry
was crudely estimated considering the decay of a right-handed neutrino at rest.
Our present analysis includes thermal momentum modes for the heavy neutrino and
this leads to a total lepton asymmetry which is bigger by a factor of two as
compared to the previous estimate. Nevertheless, our current and preliminary
results for the freezeout are found to be in agreement (within a 12.5%
uncertainty). Our analysis depends on a novel use of Pade approximants to solve
the Boltzmann equations and may be more widely useful in cosmology.Comment: 33 pages latex, two figures incorporated; text overlap with
arXive:1412.707
Physics-informed Neural Networks for Solving Inverse Problems of Nonlinear Biot's Equations: Batch Training
In biomedical engineering, earthquake prediction, and underground energy
harvesting, it is crucial to indirectly estimate the physical properties of
porous media since the direct measurement of those are usually
impractical/prohibitive. Here we apply the physics-informed neural networks to
solve the inverse problem with regard to the nonlinear Biot's equations.
Specifically, we consider batch training and explore the effect of different
batch sizes. The results show that training with small batch sizes, i.e., a few
examples per batch, provides better approximations (lower percentage error) of
the physical parameters than using large batches or the full batch. The
increased accuracy of the physical parameters, comes at the cost of longer
training time. Specifically, we find the size should not be too small since a
very small batch size requires a very long training time without a
corresponding improvement in estimation accuracy. We find that a batch size of
8 or 32 is a good compromise, which is also robust to additive noise in the
data. The learning rate also plays an important role and should be used as a
hyperparameter.Comment: arXiv admin note: text overlap with arXiv:2002.0823
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