94 research outputs found
Equivalence Principle (EP) and Solar System Constraints on model of Gravity
Experiments on the violation of equivalence principle (EP) and solar system
give a number of constraints in which any modified gravity model must satisfy
them. We study these constraints on a kind of gravity as . For this investigation we use of chameleon
mechanism and show that a spherically body has thin-shell in this model. So
that we obtain an effective coupling of the fifth force which is suppressed
through a chameleon mechanism. Also, we obtain which is agreement with experiment results. At last, we show
that for this model is consistent with EP, thin shell
condition and fifth force of chameleon mechanism for .Comment: 6 pages, no figure, Accepted for publication in Astrophysics & Space
Scienc
Quark-Hadron Phase Transition in DGP Brane Gravity with Bulk Scalar Field
A DGP brane-world framework is picked out to study quark-hadron phase
transition problem. The model also includes a bulk scalar field in agreement
with string theory prediction. The work is performed utilizing two formalisms
as: smooth crossover approach and first order approach, and the results are
plotted for both branches of DGP model. General behavior of temperature is the
same in these two approaches and it decrease by passing time and expanding
Universe. Phase transition occurs at about micro-second after the big bang. The
results show that transition time depends on brane tension value in which
larger brane tension comes to earlier transition time.Comment: 14 pages, 13 figure
QCD phase transition with a power law chameleon scalar field in the bulk
In this work, a brane world model with a perfect fluid on brane and a scalar
field on bulk has been used to study quark-hadron phase transition. The bulk
scalar field has an interaction with brane matter. This interaction comes into
non-conservation relation which describe an energy transfer between bulk and
brane. Since quark-hadron transition curly depends on the form of evolution
equations therefore modification of energy conservation equation and Friedmann
equation comes into some interesting results about the time of transition. The
evolution of physical quantities relevant to quantitative of early times namely
energy density temperature and scale factor have been considered
utilizing two formalisms as crossover formalism and first order phase
transition formalism. The results show that the quark-hadron phase transition
in occurred about nanosecond after big bang and the general behavior
temperature is similar in both of two formalism.Comment: 22 pages, 8 figure
Tachyon constant-roll inflation
The constant-roll inflation is studied where the inflaton is taken as a
tachyon field. Since in this approach the second slow-roll parameter is assumed
to be of order one instead of being small, then the perturbation parameters
will be considered again. The results are compared with observational data, and
it is confirmed that the model could stand as a proper candidate for inflation.Comment: 16 pages, 8 figures, Typos corrected, references adde
Intermediate inflation driven by DBI scalar field
Picking out DBI scalar field as inflation, the slow-rolling inflationary
scenario is studied by attributing an exponential time function to scale
factor; known as intermediate inflation. The perturbation parameters of the
model are estimated numerically for two different cases and the final result is
compared with Planck data. The diagram of tensor-to-scalar ratio versus
scalar spectra index is illustrated, and it is found out that they are in
acceptable range, as suggested by Planck. In addition, the acquired values for
amplitude of scalar perturbation reveals the ability of the model for depicting
a good picture of the universe in one of the earliest stage. As a further
argument, the non-Gaussianity is investigated displaying that the model
prediction stands in CL regime; according to latest Planck data.Comment: 8p pages, 6 figures, 3 table
Viscous Warm Inflation: Hamilton-Jacobi formalism
Using Hamilton-Jacobi formalism, The scenario of warm inflation with viscous
pressure is considered. The formalism gives a way of computing the slow-rolling
parameters without extra approximation, and it is well-known as a powerful
method in cold inflation. The model is studied in detail for three different
cases of dissipation and bulk viscous pressure coefficients. In the first case
where both coefficients are taken as a constant, it is shown that the case
could not portray warm inflationary scenario compatible with observational data
even it is possible to restrict the model parameters. For other cases, the
results shows that the model could properly predicts the perturbation
parameters in which they stay in perfect agreement with Planck data. As a
further argument, and are drown that show the required
result could stand in acceptable area expressing a compatibility with
observational data.Comment: 20 pages, 11 figure
Hamilton-Jacobi formalism to warm inflationary scenario
Hamilton-Jacobi formalism as a powerful method is being utilized to
reconsider warm inflationary scenario, where the scalar field as the main
component deriving inflation interacts with other field. Separating the context
to strong and weak dissipative regimes, the goal is followed for two popular
function of dissipation coefficient. Applying slow-rolling approximations, the
required perturbation parameters are extracted and by comparison to the latest
Planck data, the free parameters are restricted. Possibility of producing an
acceptable inflation is studied where the result shows that for all cases the
model could successfully suggests amplitude of scalar perturbation, scalar
spectral index , its running, and the tensor-to-scalar ratio.Comment: 12 pages, 18 figure
Quark-hadron phase transition in DGP including BD brane
A DGP brane-world model with a perfect fluid brane matter including a
Brans-Dicke (BD) scalar field on brane has been utilized to investigate the
problem of the quark-hadron phase (QHP) transition in early times of the
Universe evolution. The presence of the BD scalar field comes up with some
modification terms in the Friedmann equation. Since the behavior of phase
transition strongly depends on the basic evolution equations, even a small
change in these relations might come to interesting results about the time of
transition. The phase transition is investigated using two scenarios of the
first-order phase transition and smooth crossover phase transition. For
first-order scenario, which is used for intermediate temperature regime, the
evolution of the physical quantities, such as temperature and scale factor, are
investigated before, during and after the phase transition. The results show
that the transition occurs in about micro-second. In the next part, the
phenomenon is studied by assuming a smooth crossover transition where the
lattice QCD data is utilized to obtain a realistic equation of state for the
matter. The investigation for this part is performed in two regimes of high and
low-temperature. Using trace anomaly in the high-temperature regime specifies a
simple equation of state which states that the quark-gluon behaves like
radiation. However, in the low-temperature regime, the trace anomaly is
affected by discretization effects, and the hadron resonance gas model is
utilized instead. Using this model, a more realistic equation of state could be
found in the low-temperature regime. The crossover phase transition in both
regimes is considered. The results determine that the transition occurs at the
time around a few micro-second. Also, it is realized that the transition in the
low-temperature regime occurs after the transition in the high-temperature
regime.Comment: 11 pages, 11 figure
Effect of an external interaction mechanism in solving agegraphic dark energy problems
Agegraphic dark energy(ADE) and New-ADE models have been introduced as two
candidates for dark energy to explain the accelerated expansion phase of the
Universe. In spite of a few suitable features of these models some studies have
shown that there are several drawbacks in them. Therefore in this investigation
a new version of ADE and New-ADE are studied which can improve such drawbacks
which appear in the ordinary ADE and New-ADE scenario. In fact we consider an
interacting model of scalar field with matter and after re-deriving some
cosmological parameters of the model, we find out the best fit for the model.
Actually by finding the best fitting for free parameters of the model, we show
that our theoretical results are in a good agreement with observational data.Comment: 9 pages, 7 figures. Astrophys Space Sci (2013
Inflationary universe in the presence of a minimal measurable length
In this paper, we will study the effect of having a minimum measurable length
on inflationary cosmology. We will analyze the inflationary cosmology in the
Jacobson approach. In this approach, gravity is viewed as an emergent
thermodynamical phenomenon. We will demonstrate that the existence of a minimum
measurable length will modify the Friedmann equations in the Jacobson approach.
We will use this modified Friedmann equation to analyze the effect of minimum
measurable length scale on inflationary cosmology. This analysis will be
performed using the Hamiltonian-Jacobi approach. We compare our results to
recent data and find that our model may agree with the recent data.Comment: 19 pages, 4 figures, Published version in Annals of Physic
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