16,764 research outputs found
Acceleration and Deceleration in Curvature Induced Phantom Model of the Late and Future Universe, Cosmic Collapse as Well as its Quantum Escape
Here, cosmology of the late and future universe is obtained from
-gravity with non-linear curvature terms and ( being the
Ricci scalar curvature). It is different from -dark enrgy models, where
non-linear curvature terms are taken as gravitational alternative of dark
energy. In the present model, neither linear nor no-linear curvature terms are
taken as dark energy. Rather, dark energy terms are induced by curvature terms
in the Friedmann equation derived from -gravitational equations. It has
advantage over - dark energy models in the sense that the present model
satisfies WMAP results and expands as during matter-dominance.
So, it does not have problems due to which -dark energy models are
criticized. Curvature-induced dark energy, obtained here, mimics phantom.
Different phases of this model, including acceleration and deceleration during
phantom phase, are investigated here.It is found that expansion of the universe
will stop at the age ( being the present
age of the universe) and after this epoch, it will contract and collapse by the
time . Further,it is shown that universe will
escape predicted collapse (obtained using classical mechanics) on making
quantum gravity corrections relevant near collapse time due to extremely high
energy density and large curvature analogous to the state of very early
universe. Interestingly, cosmological constant is also induced here, which is
very small in classical domain, but very high in quantum domain.Comment: 33 page
Thermal Particle and Photon Production in Pb+Pb Collisions with Transverse Flow
Particle and photon production is analyzed in the presence of transverse flow
using two approximations to describe the properties of the hadronic medium, one
containing only , and mesons (simplified equation of
state) and the other containing hadrons and resonances from the particle data
table. Both are considered with and without initial quark gluon plasma
formation. In each case the initial temperature is fixed by requiring
550 in the final state. It is shown that most observables are
very sensitive to the equation of state. This is particularly evident when
comparing the results of the simplified equation of state in the scenarios with
and without phase transition. The hadronic gas scenario leads to a
substantially higher rate for the -distribution of all particles. In the
complete equation of state with several hundreds of hadronic resonances, the
difference between the scenarios with and without phase transition is rather
modest. Both photon and particle spectra, in a wide range, show very
similar behavior. It is therefore concluded that from the spectra it will
be hard to disentangle quark gluon plasma formation in the initial state. It is
to be stressed however, that there are conceptual difficulties in applying a
pure hadronic gas equation of state at SPS-energies. The phase transition
scenario with a quark gluon plasma present in the initial state seems to be the
more natural one.Comment: 9 pages RevTeX figures in postscript forma
New analytical and numerical models of solar coronal loop: I. Application to forced vertical kink oscillations
Aims. We construct a new analytical model of a solar coronal loop that is
embedded in a gravitationally stratified and magnetically confined atmosphere.
On the basis of this analytical model, we devise a numerical model of solar
coronal loops. We adopt it to perform the numerical simulations of its vertical
kink oscillations excited by an external driver. Methods. Our model of the
solar atmosphere is constructed by adopting a realistic temperature
distribution and specifying the curved magnetic field lines that constitute a
coronal loop. This loop is described by 2D, ideal magnetohydro- dynamic
equations that are numerically solved by the FLASH code. Results. The vertical
kink oscillations are excited by a periodic driver in the vertical component of
velocity, acting at the top of the photosphere. For this forced driver with its
amplitude 3 km/s, the excited oscillations exhibit about 1.2 km/s amplitude in
their velocity and the loop apex oscillates with its amplitude in displacement
of about 100 km. Conclusions. The newly devised analytical model of the coronal
loops is utilized for the numerical simulations of the vertical kink
oscillations, which match well with the recent observations of decay-less kink
oscillations excited in solar loops. The model will have further implications
on the study of waves and plasma dynamics in coronal loops, revealing physics
of energy and mass transport mechanisms in the localized solar atmosphere.Comment: 6 Pages; 5 Figures; A&
On the Asymmetric Longitudinal Oscillations of a Pikelner's Model Prominence
We present analytical and numerical models of a normal-polarity quiescent
prominence that are based on the model of Pikelner (Solar Phys. 1971, 17, 44 ).
We derive the general analytical expressions for the two-dimensional
equilibrium plasma quantities such as the mass density and a gas pressure, and
we specify magnetic-field components for the prominence, which corresponds to a
dense and cold plasma residing in the dip of curved magnetic-field lines. With
the adaptation of these expressions, we solve numerically the 2D, nonlinear,
ideal MHD equations for a Pikelner's model of a prominence that is initially
perturbed by reducing the gas pressure at the dip of magnetic-field lines. Our
findings reveal that as a result of pressure perturbations the prominence
plasma starts evolving in time and this leads to the antisymmetric
magnetoacoustic--gravity oscillations as well as to the mass-density growth at
the magnetic dip, and the magnetic-field lines subside there. This growth
depends on the depth of magnetic dip. For a shallower dip, less plasma is
condensed and vice-versa. We conjecture that the observed long-period
magnetoacoustic-gravity oscillations in various prominence systems are in
general the consequence of the internal pressure perturbations of the plasma
residing in equilibrium at the prominence dip.Comment: 24 Pages; 16 Figures; Solar Physic
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