789 research outputs found
A Way to Dynamically Overcome the Cosmological Constant Problem
The Cosmological Constant problem can be solved once we require that the full
standard Einstein Hilbert lagrangian, gravity plus matter, is multiplied by a
total derivative. We analyze such a picture writing the total derivative as the
covariant gradient of a new vector field (b_mu). The dynamics of this b_mu
field can play a key role in the explanation of the present cosmological
acceleration of the Universe.Comment: 5 page
The regular cosmic string in Born-Infeld gravity
It is shown that Born-Infeld gravity --a high energy deformation of Einstein
gravity-- removes the singularities of a cosmic string. The respective vacuum
solution results to be free of conical singularity and closed timelike curves.
The space ends at a minimal circle where the curvature invariants vanish; but
this circle cannot be reached in a finite proper time.Comment: 4 pages, submitted to Proceedings of Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Particle Currents in a Space-Time dependent and CP-violating Higgs Background: a Field Theory Approach
Motivated by cosmological applications like electroweak baryogenesis, we
develop a field theoretic approach to the computation of particle currents on a
space-time dependent and CP-violating Higgs background. We consider the
Standard Model model with two Higgs doublets and CP violation in the scalar
sector, and compute both fermionic and Higgs currents by means of an expansion
in the background fields. We discuss the gauge dependence of the results and
the renormalization of the current operators, showing that in the limit of
local equilibrium, no extra renormalization conditions are needed in order to
specify the system completely.Comment: 21 pages, LaTeX file, uses epsf.sty. 4 figures available as a
compressed .ep
Towards a Nonequilibrium Quantum Field Theory Approach to Electroweak Baryogenesis
We propose a general method to compute -violating observables from
extensions of the standard model in the context of electroweak baryogenesis. It
is alternative to the one recently developed by Huet and Nelson and relies on a
nonequilibrium quantum field theory approach. The method is valid for all
shapes and sizes of the bubble wall expanding in the thermal bath during a
first-order electroweak phase transition. The quantum physics of -violation
and its suppression coming from the incoherent nature of thermal processes are
also made explicit.Comment: 19 pages, 1 figure available upon e-mail reques
Hartree-Fock theory with a self-generated magnetic field
We prove the existence of a ground state within the Hartree-Fock theory for atoms and molecules, in the presence of self-generated magnetic fields, with and without direct spin coupling. The ground state exists provided that the total charge Z of the K nuclei exceeds N, where N is the number of electrons, and, in the spin-polarized case, provided in addition that the nuclear charge is not too high
Electroweak Evolution Equations
Enlarging a previous analysis, where only fermions and transverse gauge
bosons were taken into account, we write down infrared-collinear evolution
equations for the Standard Model of electroweak interactions computing the full
set of splitting functions. Due to the presence of double logs which are
characteristic of electroweak interactions (Bloch-Nordsieck violation), new
infrared singular splitting functions have to be introduced. We also include
corrections related to the third generation Yukawa couplings.Comment: 15 pages, 3 figure
Cosmological perturbations of self-accelerating universe in nonlinear massive gravity
We study cosmological perturbations of self-accelerating universe solutions
in the recently proposed nonlinear theory of massive gravity, with general
matter content. While the broken diffeomorphism invariance implies that there
generically are 2 tensor, 2 vector and 2 scalar degrees of freedom in the
gravity sector, we find that the scalar and vector degrees have vanishing
kinetic terms and nonzero mass terms. Depending on their nonlinear behavior,
this indicates either nondynamical nature of these degrees or strong couplings.
Assuming the former, we integrate out the 2 vector and 2 scalar degrees of
freedom. We then find that in the scalar and vector sectors, gauge-invariant
variables constructed from metric and matter perturbations have exactly the
same quadratic action as in general relativity. The difference from general
relativity arises only in the tensor sector, where the graviton mass modifies
the dispersion relation of gravitational waves, with a time-dependent effective
mass. This may lead to modification of stochastic gravitational wave spectrum.Comment: 32 pages, 1 figure; v2: minor update to match the published versio
Fast Gibbs sampling for high-dimensional Bayesian inversion
Solving ill-posed inverse problems by Bayesian inference has recently
attracted considerable attention. Compared to deterministic approaches, the
probabilistic representation of the solution by the posterior distribution can
be exploited to explore and quantify its uncertainties. In applications where
the inverse solution is subject to further analysis procedures, this can be a
significant advantage. Alongside theoretical progress, various new
computational techniques allow to sample very high dimensional posterior
distributions: In [Lucka2012], a Markov chain Monte Carlo (MCMC) posterior
sampler was developed for linear inverse problems with -type priors. In
this article, we extend this single component Gibbs-type sampler to a wide
range of priors used in Bayesian inversion, such as general priors
with additional hard constraints. Besides a fast computation of the
conditional, single component densities in an explicit, parameterized form, a
fast, robust and exact sampling from these one-dimensional densities is key to
obtain an efficient algorithm. We demonstrate that a generalization of slice
sampling can utilize their specific structure for this task and illustrate the
performance of the resulting slice-within-Gibbs samplers by different computed
examples. These new samplers allow us to perform sample-based Bayesian
inference in high-dimensional scenarios with certain priors for the first time,
including the inversion of computed tomography (CT) data with the popular
isotropic total variation (TV) prior.Comment: submitted to "Inverse Problems
Determinant-Gravity: Cosmological implications
We analyze the action as a possible alternative or addition to the Einstein gravity.
Choosing a particular form of we can restore the
Einstein gravity and, if , we obtain the cosmological constant
term. Taking and expanding the action in , we obtain as a leading term the Einstein Lagrangian with a cosmological
constant proportional to and a series of higher order operators. In
general case of non-vanishing and new cosmological
solutions for the Robertson-Walker metric are obtained.Comment: revtex format, 5 pages,8 figures,references adde
Flux vacua in DBI type Einstein-Maxwell theory
We study compactification of extra dimensions in a theory of
Dirac-Born-Infeld (DBI) type gravity. We investigate the solution for Minkowski
spacetime with an extra space as well as that for de Sitter spacetime
() with an extra space. They are derived by the effective potential
method in the presence of the magnetic flux on the extra sphere. We also
consider the higher dimensional generalization of the solutions. We find that,
in a certain model, the radius of the extra space has a minimum value
independent of the higher-dimensional Newton constant in weak-field limit.Comment: 13 pages, no figure. A reference added, typos fixe
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