24,095 research outputs found
Quasi-local energy for cosmological models
First we briefly review our covariant Hamiltonian approach to quasi-local
energy, noting that the Hamiltonian-boundary-term quasi-local energy
expressions depend on the chosen boundary conditions and reference
configuration. Then we present the quasi-local energy values resulting from the
formalism applied to homogeneous Bianchi cosmologies. Finally we consider the
quasi-local energies of the FRW cosmologies. Our results do not agree with
certain widely accepted quasi-local criteria.Comment: Contributed to International Symposium on Cosmology and Particle
Astrophysics (CosPA 2006), Taipei, Taiwan, 15-17 Nov 200
Quasi-local energy and the choice of reference
A quasi-local energy for Einstein's general relativity is defined by the
value of the preferred boundary term in the covariant Hamiltonian formalism.
The boundary term depends upon a choice of reference and a time-like
displacement vector field (which can be associated with an observer) on the
boundary of the region. Here we analyze the spherical symmetric cases. For the
obvious analytic choice of reference based on the metric components, we find
that this technique gives the same quasi-local energy values using several
standard coordinate systems and yet can give different values in some other
coordinate systems. For the homogeneous-isotropic cosmologies, the energy can
be non-positive, and one case which is actually flat space has a negative
energy. As an alternative, we introduce a way to determine the choice of both
the reference and displacement by extremizing the energy. This procedure gives
the same value for the energy in different coordinate systems for the
Schwarzschild space, and a non-negative value for the cosmological models, with
zero energy for the dynamic cosmology which is actually Minkowski space. The
timelike displacement vector comes out to be the dual mean curvature vector of
the two-boundary.Comment: 21 pages; revised version to appear in CQ
Multiple-Level Power Allocation Strategy for Secondary Users in Cognitive Radio Networks
In this paper, we propose a multiple-level power allocation strategy for the
secondary user (SU) in cognitive radio (CR) networks. Different from the
conventional strategies, where SU either stays silent or transmit with a
constant/binary power depending on the busy/idle status of the primary user
(PU), the proposed strategy allows SU to choose different power levels
according to a carefully designed function of the receiving energy. The way of
the power level selection is optimized to maximize the achievable rate of SU
under the constraints of average transmit power at SU and average interference
power at PU. Simulation results demonstrate that the proposed strategy can
significantly improve the performance of SU compared to the conventional
strategies.Comment: 12 page
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Computational Inferences of Mutations Driving Mesenchymal Differentiation in Glioblastoma
This dissertation reviews the development and implementation of integrative, systems biology methods designed to parse driver mutations from high- throughput array data derived from human patients. The analysis of vast amounts of genomic and genetic data in the context of complex human genetic diseases such as Glioblastoma is a daunting task. Mutations exist by the hundreds, if not thousands, and only an unknown handful will contribute to the disease in a significant way. The goal of this project was to develop novel computational methods to identify candidate mutations from these data that drive the molecular differentiation of glioblastoma into the mesenchymal subtype, the most aggressive, poorest-prognosis tumors associated with glioblastoma
Image segmentation using fuzzy LVQ clustering networks
In this note we formulate image segmentation as a clustering problem. Feature vectors extracted from a raw image are clustered into subregions, thereby segmenting the image. A fuzzy generalization of a Kohonen learning vector quantization (LVQ) which integrates the Fuzzy c-Means (FCM) model with the learning rate and updating strategies of the LVQ is used for this task. This network, which segments images in an unsupervised manner, is thus related to the FCM optimization problem. Numerical examples on photographic and magnetic resonance images are given to illustrate this approach to image segmentation
Leptons from Dark Matter Annihilation in Milky Way Subhalos
Numerical simulations of dark matter collapse and structure formation show
that in addition to a large halo surrounding the baryonic component of our
galaxy, there also exists a significant number of subhalos that extend hundreds
of kiloparsecs beyond the edge of the observable Milky Way. We find that for
dark matter (DM) annihilation models, galactic subhalos can significantly
modify the spectrum of electrons and positrons as measured at our galactic
position. Using data from the recent Via Lactea II simulation we include the
subhalo contribution of electrons and positrons as boundary source terms for
simulations of high energy cosmic ray propagation with a modified version of
the publicly available GALPROP code. Focusing on the DM DM -> 4e annihilation
channel, we show that including subhalos leads to a better fit to both the
Fermi and PAMELA data. The best fit gives a dark matter particle mass of 1.2
TeV, for boost factors of 90 in the main halo and 1950-3800 in the subhalos
(depending on assumptions about the background), in contrast to the 0.85 TeV
mass that gives the best fit in the main halo-only scenario. These fits suggest
that at least a third of the observed electron cosmic rays from DM annihilation
could come from subhalos, opening up the possibility of a relaxation of recent
stringent constraints from inverse Compton gamma rays originating from the
high-energy leptons.Comment: 8 pages, 13 figures; added referenc
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