10 research outputs found
Modified Power law Inflation: solution to the graceful exit problem and improvement of dark energy models
We study power law inflation (PLI) with a monomial potential and find a novel
exact solution. It is well known that conventional PLI with exponential
potential is inconsistent with the Planck data. Unlike the standard PLI,
present model does not suffer from graceful exit problem and it agrees fairly
well with recent observations. We have calculated the spectral index and the
tensor-to-scalar ratio which are in very good agreement with recent
observational data and also comparable with other modified inflationary models.
A technique has been used which shows that the large cosmological constant
reduces with expansion of the Universe in case of the power law inflation. The
coupling of the inflaton with gravitation is the main point in this technique.
The basic assumption here is that the two metric tensors in the gravitational
and the inflaton parts correspond to different conformal frames which is in
contradiction with the conventional power law inflation where the inflaton
directly coupled with the background metric tensor. This fact has direct
application to different dark energy models and assisted quintessence theory
Generalized phenomenological models of dark energy
It was first observed at the end of the last century that the universe is
presently accelerating. Ever since, there have been several attempts to explain
this observation theoretically. There are two possible approaches. The more
conventional one is to modify the matter part of the Einstein Field Equations
and the second one is to modify the geometry part. We shall consider two
phenomenological models based on the former, more conventional approach within
the context of General Relativity. The phenomenological models in this paper
consider a term as a function of and
where and are the scale factor and matter-energy density
respectively. Constraining the free parameters of the models with latest
observational data gives satisfactory values of parameters as considered by us
initially. Without any field-theoretic interpretation, we explain the recent
observations with a dynamical cosmological constant.Comment: 6 pages, 7 figure
Lorentzian wormhole in the framework of loop quantum cosmology
In this paper, we construct a traversable static Lorentzian wormhole in the
effective scenario of Loop Quantum Cosmology (LQC), where the field equations
are modified due to the ultraviolet (UV) corrections introduced at large
space-time curvatures. A stable wormhole can be constructed in the effective
scenario without the violation of Null energy condition (NEC) by physical
matter at the throat. The NEC is effectively violated due to the corrections in
the field equations from LQC, resolving the Weyl curvature singularity at the
throat. However, the physical matter does violate the Strong energy condition
(SEC), suggesting the interesting possibility that dark energy can be harnessed
into a wormhole. A possible explanation for this is the presence of inherent
pressure isotropy in the UV-corrected field equations (discussed and compared
to braneworld wormholes in the discussion). No additional exotic ingredient
(violating NEC) is required, avoiding quantum instabilities. The tidal forces
at the throat do not diverge and also the throat is found to be stable. The
wormhole features an attractive geometry. LQC can resolve both types of
curvature singularities appearing at the black hole center and wormhole throat,
without exotic matter.Comment: 10 Pages, 4 Figure
Traversable Lorentzian wormhole on the Shtanov-Sahni braneworld with matter obeying the energy conditions
In this paper we have explored the possibility of constructing a traversable
wormhole on the Shtanov-Sahni braneworld with a timelike extra dimension. We
find that the Weyl curvature singularity at the throat of the wormhole can be
removed with physical matter satisfying the NEC , even in the
absence of any effective -term or any type of charge source on the
brane. (The NEC is however violated by the effective matter description on the
brane arising due to effects of higher dimensional gravity.) Besides satisfying
NEC the matter constituting the wormhole also satisfies the Strong Energy
Condition (SEC), , leading to the interesting possibility that
normal matter on the brane may be harnessed into a wormhole. Incidentally,
these conditions also need to be satisfied to realize a non-singular bounce and
cyclic cosmology on the brane\cite{Sahni4} where both past and future
singularities can be averted. Thus, such a cyclic universe on the brane,
constituted of normal matter can naturally contain wormholes. The wormhole
shape function on the brane with a time-like extra dimension represents the
tubular structure of the wormhole spreading out at large radial distances much
better than in wormholes constructed in a braneworld with a spacelike extra
dimension and have considerably lower mass resulting in minimization of the
amount of matter required to construct a wormhole. Wormholes in the
Shtanov-Sahni (SS) braneworld also have sufficiently low tidal forces,
facilitating traversability. Additionally they are found to be stable and
exhibit a repulsive geometry. We are left with the intriguing possibilty that
both types of curvature singularity can be resolved with the SS model, which we
discuss at the end of the concluding section.Comment: 31 Pages, 16 Figure
Skyrme Fluid in Anisotropic Universes with a Cosmological Constant
Cosmological solutions are obtained in an anisotropic Kantowski-Sachs and
Bianchi Type-I universes considering a cosmological constant with Skyrme fluid
as a matter source. The solutions in both the KS and Bianchi-I universes
obtained here are found to differ significantly, specially with some striking
difference like Bianchi-I universe admitting only oscillatory solutions for a
particular type of matter configuration. Some new and interesting cosmological
solutions are obtained due to the Skyrme fluid in the universe. The anisotropy
parameter with the evolution of the universe for both the cases are determined
and plotted for comparative study.Comment: The analysis is incomplete and vast change is being done which will
reflect major modifications in our obtained result
Lorentzian wormholes in an emergent universe
A non-singular Emergent Universe (EU) scenario within the realm of standard
Relativistic physics requires a generalization of the Equation of State (EoS)
connecting the pressure and energy density. This generalized EoS is capable of
describing a composition of exotic matter, dark energy and cosmological dust
matter. Since the EU scenario is known to violate the Null Energy Condition, we
investigate the possibility of presence of static, spherically symmetric and
traversable Lorentzian wormholes in an EU. The obtained shape function is found
to satisfy the criteria for wormhole formation, besides the violation of the
NEC at the wormhole throat and ensuring traversability such that tidal forces
are within desirable limits. Also, the wormhole is found to be stable through
linear stability analysis. Most , the numerical value of the
emergent universe parameter as estimated by our wormhole model is in
agreement with and lies within the range of values as constrained by
observational data in a cosmological context. Also, the negative sign of the
second EU parameter as obtained from our wormhole model is in agreement
with the one required for describing an EU, which further indicates on the
existence of such wormholes in an emergent universe accounting for
any additional exotic matter field or any modification to the gravitational
sector.Comment: 13 pages, 3 figure