28 research outputs found
Periaqueductal grey EP3 receptors facilitate spinal nociception in arthritic secondary hypersensitivity
Descending controls on spinal nociceptive processing play a pivotal role in shaping the pain experience following tissue injury. Secondary hypersensitivity develops within undamaged tissue adjacent, and distant to, damaged sites. Spinal neuronal pools innervating regions of secondary hypersensitivity are dominated by descending facilitation that amplifies spinal inputs from un-sensitized peripheral nociceptors. Cyclooxygenase–prostaglandin E2 signaling within the ventrolateral periaqueductal grey (vlPAG) is pro-nociceptive in naïve and acutely inflamed animals but its contributions in more prolonged inflammation and, importantly, secondary hypersensitivity remain unknown. In naïve rats, prostaglandin EP3 receptor (EP3R) antagonism in vlPAG modulated noxious withdrawal reflex (EMG) thresholds to preferential C-, but not A-, nociceptor activation, and raised thermal withdrawal thresholds in awake animals. In rats with inflammatory arthritis, secondary mechanical and thermal hypersensitivity of the hind-paw developed, and this was associated with spinal sensitization to Anociceptor inputs alone. In arthritic rats, blockade of vlPAG EP3R raised EMG thresholds to C-nociceptor activation in the area of secondary hypersensitivity to a degree equivalent to that evoked by the same manipulation in naïve rats
Exact solutions in a scalar-tensor model of dark energy
We consider a model of scalar field with non minimal kinetic and Gauss Bonnet
couplings as a source of dark energy. Based on asymptotic limits of the
generalized Friedmann equation, we impose restrictions on the kinetic an
Gauss-Bonnet couplings. This restrictions considerable simplify the equations,
allowing for exact solutions unifying early time matter dominance with
transitions to late time quintessence and phantom phases. The stability of the
solutions in absence of matter has been studied.Comment: 30 pages, 2 figures, to appear in JCA
Slow-roll Inflation with the Gauss-Bonnet and Chern-Simons Corrections
We study slow-roll inflation with the Gauss-Bonnet and Chern-Simons
corrections. We obtain general formulas for the observables: spectral indices,
tensor-to-scalar ratio and circular polarization of gravitational waves. The
Gauss-Bonnet term violates the consistency relation r = -8n_T. Particularly,
blue spectrum n_T > 0 and scale invariant spectrum |8n_T|/r << 1 of tensor
modes are possible. These cases require the Gauss-Bonnet coupling function of
\xi _{,\phi } \sim 10^8/M_{Pl}. We use examples to show new-inflation-type
potential with 10M_{Pl} symmetry breaking scale and potential with flat region
in \phi \gtrsim 10M_{Pl} lead to observationally consistent blue and scale
invariant spectra, respectively. Hence, these interesting cases can actually be
realized. The Chern-Simons term produce circularly polarized tensor modes. We
show an observation of these signals supports existence of the Chern-Simons
coupling function of \omega _{,\phi } \sim 10^8/M_{Pl}. Thus, with future
observations, we can fix or constrain the value of these coupling functions, at
the CMB scale.Comment: 21 pages, 5 figure
Scalar cosmological perturbations from inflationary black holes
We study the correction to the scale invariant power spectrum of a scalar
field on de Sitter space from small black holes that formed during a
pre-inflationary matter dominated era. The formation probability of such black
holes is estimated from primordial Gaussian density fluctuations. We determine
the correction to the spectrum by first deriving the Keldysh propagator for a
massless scalar field on Schwarzschild-de Sitter space. Our results suggest
that the effect is strong enough to be tested -- and possibly even ruled out --
by observations.Comment: 41 pages, 11 figures, published versio
Scalar field exact solutions for non-flat FLRW cosmology: A technique from non-linear Schr\"odinger-type formulation
We report a method of solving for canonical scalar field exact solution in a
non-flat FLRW universe with barotropic fluid using non-linear Schr\"{o}dinger
(NLS)-type formulation in comparison to the method in the standard Friedmann
framework. We consider phantom and non-phantom scalar field cases with
exponential and power-law accelerating expansion. Analysis on effective
equation of state to both cases of expansion is also performed. We speculate
and comment on some advantage and disadvantage of using the NLS formulation in
solving for the exact solution.Comment: 12 pages, GERG format, Reference added. accepted by Gen. Relativ. and
Gra
Generalized thermodynamics and Fokker-Planck equations. Applications to stellar dynamics, two-dimensional turbulence and Jupiter's great red spot
We introduce a new set of generalized Fokker-Planck equations that conserve
energy and mass and increase a generalized entropy until a maximum entropy
state is reached. The concept of generalized entropies is rigorously justified
for continuous Hamiltonian systems undergoing violent relaxation. Tsallis
entropies are just a special case of this generalized thermodynamics.
Application of these results to stellar dynamics, vortex dynamics and Jupiter's
great red spot are proposed. Our prime result is a novel relaxation equation
that should offer an easily implementable parametrization of geophysical
turbulence. This relaxation equation depends on a single key parameter related
to the skewness of the fine-grained vorticity distribution. Usual
parametrizations (including a single turbulent viscosity) correspond to the
infinite temperature limit of our model. They forget a fundamental systematic
drift that acts against diffusion as in Brownian theory. Our generalized
Fokker-Planck equations may have applications in other fields of physics such
as chemotaxis for bacterial populations. We propose the idea of a
classification of generalized entropies in classes of equivalence and provide
an aesthetic connexion between topics (vortices, stars, bacteries,...) which
were previously disconnected.Comment: Submitted to Phys. Rev.
Cosmological background solutions and cosmological backreactions
The cosmological backreaction proposal, which attempts to account for
observations without a primary dark energy source in the stress-energy tensor,
has been developed and discussed by means of different approaches. Here, we
focus on the concept of cosmological background solutions in order to develop a
framework to study different backreaction proposals.Comment: 14 pages, 5 figures; major changes, replaced to match the version
published in General Relativity and Gravitatio
Averaging Robertson-Walker Cosmologies
The cosmological backreaction arises when one directly averages the Einstein
equations to recover an effective Robertson-Walker cosmology, rather than
assuming a background a priori. While usually discussed in the context of dark
energy, strictly speaking any cosmological model should be recovered from such
a procedure. We apply the Buchert averaging formalism to linear
Robertson-Walker universes containing matter, radiation and dark energy and
evaluate numerically the discrepancies between the assumed and the averaged
behaviour, finding the largest deviations for an Einstein-de Sitter universe,
increasing rapidly with Hubble rate to a 0.01% effect for h=0.701. For the LCDM
concordance model, the backreaction is of the order of Omega_eff~4x10^-6, with
those for dark energy models being within a factor of two or three. The impacts
at recombination are of the order of 10^-8 and those in deep radiation
domination asymptote to a constant value. While the effective equations of
state of the backreactions in Einstein-de Sitter, concordance and quintessence
models are generally dust-like, a backreaction with an equation of state
w_eff<-1/3 can be found for strongly phantom models.Comment: 18 pages, 11 figures, ReVTeX. Updated to version accepted by JCA
Energy Conditions in Modified Gravity with Non-minimal Coupling to Matter
In this paper we study a model of modified gravity with non-minimal coupling
between a general function of the Gauss-Bonnet invariant, , and matter
Lagrangian from the point of view of the energy conditions. Such model has been
introduced in Ref. [21] for description of early inflation and late-time cosmic
acceleration. We present the suitable energy conditions for the above mentioned
model and then, we use the estimated values of the Hubble, deceleration and
jerk parameters to apply the obtained energy conditions to the specific class
of modified Gauss-Bonnet models.Comment: 12 pages, no figur, Accepted for publication in Astrophysics and
Space Scienc
Volume averaging in the quasispherical Szekeres model
This paper considers the volume averaging in the quasispherical Szekeres
model. The volume averaging became of considerable interest after it was shown
that the volume acceleration calculated within the averaging framework can be
positive even though the local expansion rate is always decelerating. This
issue was intensively studied within spherically symmetric models. However,
since our Universe is not spherically symmetric similar analysis is needed in
non symmetrical models. This papers presents the averaging analysis within the
quasispherical Szekeres model which is a non-symmetrical generalisation of the
spherically symmetric Lema\^itre--Tolman family of models. Density distribution
in the quasispherical Szekeres has a structure of a time-dependent mass dipole
superposed on a monopole. This paper shows that when calculating the volume
acceleration, , within the Szekeres model, the dipole does not
contribute to the final result, hence only depends on a monopole
configuration. Thus, the volume averaging within the Szekeres model leads to
literally the same solutions as obtained within the Lema\^itre--Tolman model.Comment: 8 pages; calculation of the spatial Ricci scalar added; accepted for
publication in Gen. Rel. Gra