16,800 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
$f(R)$-gravity with non-linear curvature terms $R^2$ and $R^3$ ($R$ being the
Ricci scalar curvature). It is different from $f(R)$-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 $f(R)$-gravitational equations. It has
advantage over $f(R)$- dark energy models in the sense that the present model
satisfies WMAP results and expands as $\sim t^{2/3}$ during matter-dominance.
So, it does not have problems due to which $f(R)$-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 $(3.87 t_0 + 694.4 {\rm kyr})$ ($t_0$ being the present
age of the universe) and after this epoch, it will contract and collapse by the
time $(336.87 t_0 + 694.4 {\rm kyr})$. 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

### Light-Front QCD in Light-Cone Gauge

The light-front (LF) quantization of QCD in light-cone (l.c.) gauge is
discussed. The Dirac method is employed to construct the LF Hamiltonian and
theory quantized canonically. The Dyson-Wick perturbation theory expansion
based on LF-time ordering is constructed. The framework incorporates in it
simultaneously the Lorentz gauge condition as an operator equation as well. The
propagator of the dynamical $\psi_+$ part of the free fermionic propagator is
shown to be causal while the gauge field propagator is found to be transverse.
The interaction Hamiltonian is re-expressed in the form closely resembling the
one in covariant theory, except for additional instantaneous interactions,
which can be treated systematically. Some explicit computations in QCD are
given.Comment: Presented at VII Hadron Physics 2000, Caraguatatuba, Sao Paulo,
Brazil, 10-15 April 200

### Light-Front-Quantized QCD in Covariant Gauge

The light-front (LF) canonical quantization of quantum chromodynamics in
covariant gauge is discussed. The Dirac procedure is used to eliminate the
constraints in the gauge-fixed front form theory quantum action and to
construct the LF Hamiltonian formulation. The physical degrees of freedom
emerge naturally. The propagator of the dynamical $\psi_+$ part of the free
fermionic propagator in the LF quantized field theory is shown to be causal and
not to contain instantaneous terms. Since the relevant propagators in the
covariant gauge formulation are causal, rotational invariance---including the
Coulomb potential in the static limit---can be recovered, avoiding the
difficulties encountered in light-cone gauge. The Wick rotation may also be
performed allowing the conversion of momentum space integrals into Euclidean
space forms. Some explicit computations are done in quantum electrodynamics to
illustrate the equivalence of front form theory with the conventional covariant
formulation. LF quantization thus provides a consistent formulation of gauge
theory, despite the fact that the hyperplanes $x^{\pm}=0$ used to impose
boundary conditions constitute characteristic surfaces of a hyperbolic partial
differential equation.Comment: LaTex, 16 page

### A Hybrid Model for QCD Deconfining Phase Boundary

Intensive search for a proper and realistic equations of state (EOS) is still
continued for studying the phase diagram existing between quark gluon plasma
(QGP) and hadron gas (HG) phases. Lattice calculations provide such EOS for the
strongly interacting matter at finite temperature ($T$) and vanishing baryon
chemical potential ($\mu_{B}$). These calculations are of limited use at finite
$\mu_{B}$ due to the appearance of notorious sign problem. In the recent past,
we had constructed a hybrid model description for the QGP as well as HG phases
where we make use of a new excluded-volume model for HG and a
thermodynamically-consistent quasiparticle model for the QGP phase and used
them further to get QCD phase boundary and a critical point. Since then many
lattice calculations have appeared showing various thermal and transport
properties of QCD matter at finite $T$ and $\mu_{B}=0$. We test our hybrid
model by reproducing the entire data for strongly interacting matter and
predict our results at finite $\mu_{B}$ so that they can be tested in future.
Finally we demonstrate the utility of the model in fixing the precise location,
the order of the phase transition and the nature of CP existing on the QCD
phase diagram. We thus emphasize the suitability of the hybrid model as
formulated here in providing a realistic EOS for the strongly interacting
matter.Comment: 22 pages, 10 figures. corrected version published in Physical Review
D. arXiv admin note: substantial text overlap with arXiv:1201.044

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