3,195 research outputs found
Form-factors of exponential fields in the sine-Gordon model
An integral representation for form-factors of exponential fields in the
sine-Gordon model is proposed.Comment: 8 pages, harvmac.tex, added the formula (25) for two soliton
form-factors at the reflectionless point
Testing the equation of state for viscous dark energy
Some cosmological scenarios with bulk viscosity for the dark energy fluid are considered. Based on
some considerations related to hydrodynamics, two different equations of state for dark energy are
assumed, leading to power-law and logarithmic effective corrections to the pressure. The models are tested
with the latest astronomical data from type Ia supernovae (Pantheon sample), measurements of the Hubble
parameter HðzÞ, baryon acoustic oscillations and cosmic microwave background radiation. In comparison
with the ΛCDM model, some different results are obtained and their viability is discussed. The power-law
model shows some modest results, achieved under negative values of bulk viscosity, while the logarithmic
scenario provide good fits in comparison to the ΛCDM model.Ministerio de Economía, Industria y Competitividad (project FIS2016-76363-P)Agencia de Gestión de Ayudas Universitarias y de Investigación (project 2017 SGR 247)CANTATA COST action (grant CA15117
Inhomogeneous Equation of State of the Universe: Phantom Era, Future Singularity and Crossing the Phantom Barrier
The dark energy universe equation of state (EOS) with inhomogeneous,Hubble
parameter dependent term is considered. The motivation to introduce such a term
comes from time-dependent viscosity considerations and modifications of general
relativity. For several explicit examples of such EOS it is demonstrated how
the type of future singularity changes, how the phantom epoch emerges and how
crossing of phantom barrier occurs. Similar cosmological regimes are considered
for the universe with two interacting fluids and for universe with implicit
EOS. For instance, the crossing of phantom barrier is realized in easier way,
thanks to the presence of inhomogeneous term. The thermodynamical dark energy
model is presented where the universe entropy may be positive even at phantom
era as a result of crossing of w=-1 barrier.Comment: LaTeX, 13 pages, refs. added, version to appear in PR
Is exponential gravity a viable description for the whole cosmological history?
Here we analysed a particular type of gravity, the so-called
exponential gravity which includes an exponential function of the Ricci scalar
in the action. Such term represents a correction to the usual Hilbert-Einstein
action. By using Supernovae Ia, Barionic Acoustic Oscillations, Cosmic
Microwave Background and data, the free parameters of the model are well
constrained. The results show that such corrections to General Relativity
become important at cosmological scales and at late-times, providing an
alternative to the dark energy problem. In addition, the fits do not determine
any significant difference statistically with respect to the CDM
model. Finally, such model is extended to include the inflationary epoch in the
same gravitational Lagrangian. As shown in the paper, the additional terms can
reproduce the inflationary epoch and satisfy the constraints from Planck data.Comment: 20 pages, 6 figures, analysis extended, version published in EPJ
Discussing the possibility of observation of parity violation in heavy ion collisions
It was recently argued that in heavy ion collision the parity could be
broken. This Note addresses the question of possibility of the experimental
detection of the effect. We discuss how parity violating effects would modify
the final particle distributions and how one could construct variables
sensitive to the effect, and which measurement would be the (most) conclusive.
Discussing different observables we also discuss the question if the
``signals'' can be faked by ``conventional'' effects (such as anisotropic flow,
etc.) and make estimates of the signals.Comment: LaTeX, 5 pages; some corrections in chapter III; main results are
unchange
f(R,T) gravity
We consider f(R,T) modified theories of gravity, where the gravitational
Lagrangian is given by an arbitrary function of the Ricci scalar R and of the
trace of the stress-energy tensor T. We obtain the gravitational field
equations in the metric formalism, as well as the equations of motion for test
particles, which follow from the covariant divergence of the stress-energy
tensor. Generally, the gravitational field equations depend on the nature of
the matter source. The field equations of several particular models,
corresponding to some explicit forms of the function f(R,T), are also
presented. An important case, which is analyzed in detail, is represented by
scalar field models. We write down the action and briefly consider the
cosmological implications of the models, where is
the trace of the stress-energy tensor of a self-interacting scalar field. The
equations of motion of the test particles are also obtained from a variational
principle. The motion of massive test particles is non-geodesic, and takes
place in the presence of an extra force orthogonal to the four-velocity. The
Newtonian limit of the equation of motion is further analyzed. Finally, we
provide a constraint on the magnitude of the extra-acceleration by analyzing
the perihelion precession of the planet Mercury in the framework of the present
model.Comment: 14 pages. V2: minor corrections, to appear in PR
Properties of singularities in (phantom) dark energy universe
The properties of future singularities are investigated in the universe
dominated by dark energy including the phantom-type fluid. We classify the
finite-time singularities into four classes and explicitly present the models
which give rise to these singularities by assuming the form of the equation of
state of dark energy. We show the existence of a stable fixed point with an
equation of state and numerically confirm that this is actually a
late-time attractor in the phantom-dominated universe. We also construct a
phantom dark energy scenario coupled to dark matter that reproduces singular
behaviors of the Big Rip type for the energy density and the curvature of the
universe. The effect of quantum corrections coming from conformal anomaly can
be important when the curvature grows large, which typically moderates the
finite-time singularities.Comment: 17 pages, 6 figures, references are added, version to appear in
Physical Review
Non-Markovian Random Walks and Non-Linear Reactions: Subdiffusion and Propagating Fronts
We propose a reaction-transport model for CTRW with non-linear reactions and
non-exponential waiting time distributions. We derive non-linear evolution
equation for mesoscopic density of particles. We apply this equation to the
problem of fronts propagation into unstable state of reaction-transport systems
with anomalous diffusion. We have found an explicit expression for the speed of
propagating front in the case of subdiffusion transport.Comment: 7 page
Can Quantum-Corrected BTZ Black Hole Anti-Evaporate?
Kaluza-Klein reduction of 3D gravity with minimal scalars leads to 2D
dilaton-Maxwell gravity with dilaton coupled scalars. Evaluating the one-loop
effective action for dilaton coupled scalars in large and s-wave
approximation we apply it to study quantum evolution of BTZ black hole. It is
shown that quantum-corrected BTZ BH may evaporate or else anti-evaporate
similarly to 4D Nariai BH as is observed by Bousso and Hawking. Instable higher
modes in the spectrum indicate also the possibility of proliferation of BTZ BH.Comment: LaTeX file and ps file of figure
Holographic entropy and brane FRW-dynamics from AdS black hole in d5 higher derivative gravity
Higher derivative bulk gravity (without Riemann tensor square term) admits
AdS-Schwarzschild black hole as exact solution. It is shown that induced brane
geometry on such background is open, flat or closed FRW radiation dominated
Universe. Higher derivative terms contributions appear in the Hawking
temperature, entropy and Hubble parameter via the redefinition of 5-dimensional
gravitational constant and AdS scale parameter. These higher derivative terms
do not destroy the AdS-dual description of radiation represented by
strongly-coupled CFT. Cardy-Verlinde formula which expresses cosmological
entropy as square root from other parameters and entropies is derived in
gravity. The corresponding cosmological entropy bounds are briefly discussed.Comment: LaTeX file, 19 page
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