72 research outputs found
Bulk viscous Zel'dovich fluid model and it's asymptotic behavior
In this paper we have considered a flat FLRW universe with bulk viscous
Zel'dovich as the cosmic component. Being considered the bulk viscosity as per
the Eckart formalism, we have analyzed the evolution of the Hubble parameter
and constrained the model with the Type Ia Supernovae data thus extracting the
constant bulk viscous parameter and present Hubble parameter. Further we have
analyzed the scale factor, equation of state and deceleration parameter. The
model predicts the late time acceleration and is also compatible with the age
of the universe as given by the oldest globular clusters. We have also studied
the phase-space behavior of the model and found that a universe dominated by
bulk viscous Zel'dovich fluid is stable. But on the inclusion of radiation
component in addition to the Zel'dovich fluid, makes the model unstable. Hence,
even though the bulk viscous Zel'dovich fluid dominated universe is a feasible
one, the model as such failed to predict a prior radiation dominated phase.Comment: 14 pages, eight figure
Cosmological horizon entropy and generalised second law for flat Friedmann Universe
We discuss the generalized second law (GSL) and the constraints imposed by it
for two types of Friedmann universes. The first one is the Friedmann universe
with radiation and a positive cosmological constant, and the second one
consists of non-relativistic matter and a positive cosmological constant. The
time evolution of the event horizon entropy and the entropy of the contents
within the horizon are analyses in an analytical way by obtaining the Hubble
parameter. It is shown that the GSL constraint the temperature of both the
radiation and matter of the Friedmann universe. It is also shown that, even
though the net entropy of the radiation (or matter) is decreasing at
sufficiently large times as the universe expand, it exhibit an increase during
the early times when universe is decelerating. That is the entropy of the
radiation within the comoving volume is decreasing only when the universe has
got an event horizon.Comment: 15 pages, 9 figure
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