94 research outputs found
Unifying phantom inflation with late-time acceleration: scalar phantom-non-phantom transition model and generalized holographic dark energy
The unifying approach to early-time and late-time universe based on phantom
cosmology is proposed. We consider gravity-scalar system which contains usual
potential and scalar coupling function in front of kinetic term. As a result,
the possibility of phantom-non-phantom transition appears in such a way that
universe could have effectively phantom equation of state at early time as well
as at late time. In fact, the oscillating universe may have several phantom and
non-phantom phases. As a second model we suggest generalized holographic dark
energy where infrared cutoff is identified with combination of FRW parameters:
Hubble constant, particle and future horizons, cosmological constant and
universe life-time (if finite). Depending on the specific choice of the model
the number of interesting effects occur: the possibility to solve the
coincidence problem, crossing of phantom divide and unification of early-time
inflationary and late-time accelerating phantom universe. The bound for
holographic entropy which decreases in phantom era is also discussed.Comment: 13 pages, clarifications/refs added, to match with published versio
Universal procedure to cure future singularities of dark energy models
A systematic search for different viable models of the dark energy universe,
all of which give rise to finite-time, future singularities, is undertaken,
with the purpose to try to find a solution to this common problem. After some
work, a universal procedure to cure all future singularities is developed and
carefully tested with the help of explicit examples corresponding to each one
of the four different types of possible singularities, as classified in the
literature. The cases of a fluid with an equation of state which depends on
some parameter, of modified gravity non-minimally coupled to a matter
Lagrangian, of non-local gravity, and of isotropic turbulence in a dark fluid
universe theory are investigated in detail
On Isotropic Turbulence in the Dark Fluid Universe
As first part of this work, experimental information about the decay of
isotropic turbulence in ordinary hydrodynamics, u^2(t) proportional to
t^{-6/5}, is used as input in FRW equations in order to investigate how an
initial fraction f of turbulent kinetic energy in the cosmic fluid influences
the cosmological development in the late, quintessence/phantom, universe. First
order perturbative theory to the first order in f is employed. It turns out
that both in the Hubble factor, and in the energy density, the influence from
the turbulence fades away at late times. The divergences in these quantities
near the Big Rip behave essentially as in a non-turbulent fluid. However, for
the scale factor, the turbulence modification turns out to diverge
logarithmically. As second part of our work, we consider the full FRW equation
in which the turbulent part of the dark energy is accounted for by a separate
term. It is demonstrated that turbulence occurrence may change the future
universe evolution due to dissipation of dark energy. For instance,
phantom-dominated universe becomes asymptotically a de Sitter one in the
future, thus avoiding the Big Rip singularity.Comment: 10 pages, no figures, significant revision. Matches published versio
Unifying inflation with dark energy in modified F(R) Horava-Lifshitz gravity
We study FRW cosmology for a non-linear modified F(R) Horava-Lifshitz gravity
which has a viable convenient counterpart. A unified description of early-time
inflation and late-time acceleration is possible in this theory, but the
cosmological dynamic details are generically different from the ones of the
convenient viable F(R) model. Remarkably, for some specific choice of
parameters they do coincide. The emergence of finite-time future singularities
is investigated in detail. It is shown that these singularities can be cured by
adding an extra, higher-derivative term, which turns out to be qualitatively
different when compared with the corresponding one of the convenient F(R)
theory.Comment: LaTeX 12 pages, typos are correcte
Repetitive corrugation and straightening effect on the microstructure, crystallographic texture and electrochemical behavior for the Al-7075 alloy
Anti-corrosion susceptibility is one of the top criteria for selecting metallic materials for several industrial applications. This work studies the corrosion performance of an Al-7075 alloy obtained by repetitive corrugation and straightening (RCS). This processing method generated a microstructure formed by randomly distributed micro-, submicro-, and nano- metric grain sizes. The samples exhibited a drop in corrosion resistance for a longer duration in the electrolyte and higher deformation. However, the samples processed by RCS showed better electrochemical stability than the non-deformed condition. The improved electrochemical stability could be associated with the particular microstructure generated during the RCS process
The fate of (phantom) dark energy universe with string curvature corrections
We study the evolution of (phantom) dark energy universe by taking into
account the higher-order string corrections to Einstein-Hilbert action with a
fixed dilaton. While the presence of a cosmological constant gives stable
de-Sitter fixed points in the cases of heterotic and bosonic strings, no stable
de-Sitter solutions exist when a phantom fluid is present. We find that the
universe can exhibit a Big Crunch singularity with a finite time for type II
string, whereas it reaches a Big Rip singularity for heterotic and bosonic
strings. Thus the fate of dark energy universe crucially depends upon the type
of string theory under consideration.Comment: 6 pages and 3 figures, discussion on dynamically evolving dilaton and
modulus included, references added, version to appear in Physics Letters
Entropy and universality of Cardy-Verlinde formula in dark energy universe
We study the entropy of a FRW universe filled with dark energy (cosmological
constant, quintessence or phantom). For general or time-dependent equation of
state the entropy is expressed in terms of energy, Casimir energy,
and . The correspondent expression reminds one about 2d CFT entropy only for
conformal matter. At the same time, the cosmological Cardy-Verlinde formula
relating three typical FRW universe entropies remains to be universal for any
type of matter. The same conclusions hold in modified gravity which represents
gravitational alternative for dark energy and which contains terms growing at
low curvature. It is interesting that BHs in modified gravity are more entropic
than in Einstein gravity. Finally, some hydrodynamical examples testing new
shear viscosity bound, which is expected to be the consequence of the
holographic entropy bound, are presented for the early universe in the plasma
era and for the Kasner metric. It seems that the Kasner metric provides a
counterexample to the new shear viscosity bound.Comment: LaTeX file, 39 pages, references are adde
Stability analysis of agegraphic dark energy in Brans-Dicke cosmology
Stability analysis of agegraphic dark energy in Brans-Dicke theory is
presented in this paper. We constrain the model parameters with the
observational data and thus the results become broadly consistent with those
expected from experiment. Stability analysis of the model without best fitting
shows that universe may begin from an unstable state passing a saddle point and
finally become stable in future. However, with the best fitted model, There is
no saddle intermediate state. The agegraphic dark energy in the model by itself
exhibits a phantom behavior. However, contribution of cold dark matter on the
effective energy density modifies the state of teh universe from phantom phase
to quintessence one. The statefinder diagnosis also indicates that the universe
leaves an unstable state in the past, passes the LCDM state and finally
approaches the sable state in future.Comment: 15 pages, 12 figure
Dissipative Future Universe without Big Rip
The present study deals with dissipative future universe without big rip in
context of Eckart formalism. The generalized chaplygin gas, characterized by
equation of state , has been considered as
a model for dark energy due to its dark-energy-like evolution at late time. It
is demonstrated that, if the cosmic dark energy behaves like a fluid with
equation of state ; , as well as chaplygin gas
simultaneously then the big rip problem does not arises and the scale factor is
found to be regular for all time.Comment: 6 pages, 2 figures, To appear in Int. J. Theor. Phy
Effect of the microstructure generated by Repetitive Corrugation and Straightening (RCS) process on the mechanical properties and stress corrosion cracking of Al-7075 alloy
This study discussed the effect of the heterogeneous microstructure generated through the Repetitive Corrugation and Straightening (RCS) process on the mechanical and stress corrosion cracking behavior of the AA7075. As a result of the RCS process, significant grain refinement was obtained. The average grain size ranged from 126 to 59 μm, for the initial condition and 4 RCS passes, respectively. The yield strength and hardness increased 170% and 15% from the initial pass, remaining almost constant afterward. The evaluation of stress corrosion cracking showed a decrement in the number of cracks of 21.6% and 23.5% between the initial condition and fourth RCS passes. The cracking and pitting corrosion were the dominant mechanisms in the tested samples. The mechanical and corrosion results were also discussed in terms of the microstructural features
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