760 research outputs found
Bounce and cyclic cosmology in extended nonlinear massive gravity
We investigate non-singular bounce and cyclic cosmological evolutions in a
universe governed by the extended nonlinear massive gravity, in which the
graviton mass is promoted to a scalar-field potential. The extra freedom of the
theory can lead to certain energy conditions violations and drive cyclicity
with two different mechanisms: either with a suitably chosen scalar-field
potential under a given Stuckelberg-scalar function, or with a suitably chosen
Stuckelberg-scalar function under a given scalar-field potential. Our analysis
shows that extended nonlinear massive gravity can alter significantly the
evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA
Phase-Space analysis of Teleparallel Dark Energy
We perform a detailed dynamical analysis of the teleparallel dark energy
scenario, which is based on the teleparallel equivalent of General Relativity,
in which one adds a canonical scalar field, allowing also for a nonminimal
coupling with gravity. We find that the universe can result in the
quintessence-like, dark-energy-dominated solution, or to the stiff dark-energy
late-time attractor, similarly to standard quintessence. However, teleparallel
dark energy possesses an additional late-time solution, in which dark energy
behaves like a cosmological constant, independently of the specific values of
the model parameters. Finally, during the evolution the dark energy
equation-of-state parameter can be either above or below -1, offering a good
description for its observed dynamical behavior and its stabilization close to
the cosmological-constant value.Comment: 23 pages, 4 figures, 5 tables, version published at JCA
Aspects of Horava-Lifshitz cosmology
We review some general aspects of Horava-Lifshitz cosmology. Formulating it
in its basic version, we extract the cosmological equations and we use
observational data in order to constrain the parameters of the theory. Through
a phase-space analysis we extract the late-time stable solutions, and we show
that eternal expansion, and bouncing and cyclic behavior can arise naturally.
Concerning the effective dark energy sector we show that it can describe the
phantom phase without the use of a phantom field. However, performing a
detailed perturbation analysis, we see that Horava-Lifshitz gravity in its
basic version suffers from instabilities. Therefore, suitable generalizations
are required in order for this novel theory to be a candidate for the
description of nature.Comment: 10 pages, 4 figures, invited talk given at the 2nd International
Workshop on Dark Matter, Dark Energy and Matter-Antimatter Assymetry,
National Tsing Hua University, Hsinchu, Taiwan, November 5-6, 201
A note on Friedmann equation of FRW universe in deformed Horava-Lifshitz gravity from entropic force
With entropic interpretation of gravity proposed by Verlinde, we obtain the
Friedmann equation of the Friedmann-Robertson-Walker universe for the deformed
Ho\v{r}ava-Lifshitz gravity. It is shown that, when the parameter of
Ho\v{r}ava-Lifshitz gravity , the modified Friedmann
equation will go back to the one in Einstein gravity. This results may imply
that the entropic interpretation of gravity is effective for the deformed
Ho\v{r}ava-Lifshitz gravity.Comment: 9 pages, no figure
Notes on Matter in Horava-Lifshitz Gravity
We investigate the dynamics of a scalar field governed by the Lifshitz-type
action which should appear naturally in Horava-Lifshitz gravity. The wave of
the scalar field may propagate with any speed without an upper bound. To
preserve the causality, the action cannot have a generic form. Due to the
superluminal propagation, a formation of a singularity may cause the breakdown
of the predictability of the theory. To check whether such a catastrophe could
occur in Horava-Lifshitz gravity, we investigate the dynamics of a dust. It
turns out that the dust does not collapse completely to form a singularity in a
generic situation, but expands again after it attains a maximum energy density.Comment: 14 pages, references adde
A Unified Single-stage Learning Model for Estimating Fiber Orientation Distribution Functions on Heterogeneous Multi-shell Diffusion-weighted MRI
Diffusion-weighted (DW) MRI measures the direction and scale of the local
diffusion process in every voxel through its spectrum in q-space, typically
acquired in one or more shells. Recent developments in micro-structure imaging
and multi-tissue decomposition have sparked renewed attention to the radial
b-value dependence of the signal. Applications in tissue classification and
micro-architecture estimation, therefore, require a signal representation that
extends over the radial as well as angular domain. Multiple approaches have
been proposed that can model the non-linear relationship between the DW-MRI
signal and biological microstructure. In the past few years, many deep
learning-based methods have been developed towards faster inference speed and
higher inter-scan consistency compared with traditional model-based methods
(e.g., multi-shell multi-tissue constrained spherical deconvolution). However,
a multi-stage learning strategy is typically required since the learning
process relied on various middle representations, such as simple harmonic
oscillator reconstruction (SHORE) representation. In this work, we present a
unified dynamic network with a single-stage spherical convolutional neural
network, which allows efficient fiber orientation distribution function (fODF)
estimation through heterogeneous multi-shell diffusion MRI sequences. We study
the Human Connectome Project (HCP) young adults with test-retest scans. From
the experimental results, the proposed single-stage method outperforms prior
multi-stage approaches in repeated fODF estimation with shell dropoff and
single-shell DW-MRI sequences
Particle Kinematics in Horava-Lifshitz Gravity
We study the deformed kinematics of point particles in the Horava theory of
gravity. This is achieved by considering particles as the optical limit of
fields with a generalized Klein-Gordon action. We derive the deformed geodesic
equation and study in detail the cases of flat and spherically symmetric
(Schwarzschild-like) spacetimes. As the theory is not invariant under local
Lorenz transformations, deviations from standard kinematics become evident even
for flat manifolds, supporting superluminal as well as massive luminal
particles. These deviations from standard behavior could be used for
experimental tests of this modified theory of gravity.Comment: Added references, corrected a typing erro
Fluctuations in a Ho\v{r}ava-Lifshitz Bouncing Cosmology
Ho\v{r}ava-Lifshitz gravity is a potentially UV complete theory with
important implications for the very early universe. In particular, in the
presence of spatial curvature it is possible to obtain a non-singular bouncing
cosmology. The bounce is realized as a consequence of higher order spatial
curvature terms in the gravitational action. Here, we extend the study of
linear cosmological perturbations in Ho\v{r}ava-Lifshitz gravity coupled to
matter in the case when spatial curvature is present. As in the case without
spatial curvature, we find that there is no extra dynamical degree of freedom
for scalar metric perturbations. We study the evolution of fluctuations through
the bounce and show that the solutions remain non-singular throughout. If we
start with quantum vacuum fluctuations on sub-Hubble scales in the contracting
phase, and if the contracting phase is dominated by pressure-less matter, then
for and in the infrared limit the perturbations at late times are
scale invariant. Thus, Ho\v{r}ava-Lifshitz gravity can provide a realization of
the ``matter bounce'' scenario of structure formation.Comment: 19 page
Scaling Up 3D Kernels with Bayesian Frequency Re-parameterization for Medical Image Segmentation
With the inspiration of vision transformers, the concept of depth-wise
convolution revisits to provide a large Effective Receptive Field (ERF) using
Large Kernel (LK) sizes for medical image segmentation. However, the
segmentation performance might be saturated and even degraded as the kernel
sizes scaled up (e.g., ) in a Convolutional Neural
Network (CNN). We hypothesize that convolution with LK sizes is limited to
maintain an optimal convergence for locality learning. While Structural
Re-parameterization (SR) enhances the local convergence with small kernels in
parallel, optimal small kernel branches may hinder the computational efficiency
for training. In this work, we propose RepUX-Net, a pure CNN architecture with
a simple large kernel block design, which competes favorably with current
network state-of-the-art (SOTA) (e.g., 3D UX-Net, SwinUNETR) using 6
challenging public datasets. We derive an equivalency between kernel
re-parameterization and the branch-wise variation in kernel convergence.
Inspired by the spatial frequency in the human visual system, we extend to vary
the kernel convergence into element-wise setting and model the spatial
frequency as a Bayesian prior to re-parameterize convolutional weights during
training. Specifically, a reciprocal function is leveraged to estimate a
frequency-weighted value, which rescales the corresponding kernel element for
stochastic gradient descent. From the experimental results, RepUX-Net
consistently outperforms 3D SOTA benchmarks with internal validation (FLARE:
0.929 to 0.944), external validation (MSD: 0.901 to 0.932, KiTS: 0.815 to
0.847, LiTS: 0.933 to 0.949, TCIA: 0.736 to 0.779) and transfer learning (AMOS:
0.880 to 0.911) scenarios in Dice Score.Comment: Both codes and pretrained models are available at:
https://github.com/MASILab/RepUX-Ne
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