60 research outputs found
Uniting cosmological epochs through the twister solution in cosmology with non-minimal coupling
We investigate dynamics of a flat FRW cosmological model with a barotropic
matter and a non-minimally coupled scalar field (both canonical and phantom).
In our approach we do not assume any specific form of a potential function for
the scalar field and we are looking for generic scenarios of evolution. We show
that dynamics of universe can be reduced to a 3-dimensional dynamical system.
We have found the set of fixed points and established their character. These
critical points represent all important epochs in evolution of the universe :
(a) a finite scale factor singularity, (b) an inflation (rapid-roll and
slow-roll), (c) a radiation domination, (d) a matter domination and (e) a
quintessence era. We have shown that the inflation, the radiation and matter
domination epochs are transient ones and last for a finite amount of time. The
existence of the radiation domination epoch is purely the effect of a
non-minimal coupling constant. We show the existence of a twister type solution
wandering between all these critical points.Comment: 22 pages, 5 figs; (v2.) 27 pages, 12 figs, JCAP in pres
Universe from vacuum in loop-string cosmology
In this paper we study the description of the Universe based on the low
energy superstring theory modified by the Loop Quantum Gravity effects.This
approach was proposed by De Risi et al. in the Phys. Rev. D {\bf 76} (2007)
103531. We show that in the contrast with the string motivated pre-Big Bang
scenario, the cosmological realisation of the -duality transformation is not
necessary to avoid an initial singularity. In the model considered the universe
starts its evolution in the vacuum phase at time . In this phase
the scale factor , energy density and coupling of the
interactions . After this stage the universe evolves to the
non-singular hot Big Bang phase . Then the
standard classical universe emerges. During the whole evolution the scale
factor increases monotonically. We solve this model analytically. We also
propose and solve numerically the model with an additional dilaton potential in
which the universe starts the evolution from the asymptotically free vacuum
phase and then evolves non-singularly to the emerging dark energy
dominated phase with the saturated coupling constant .Comment: JHEP3 LaTeX class, 19 pages, 9 figures, v2: added some comments and
references, v3: new numerical result added, new figure
De Sitter ground state of scalar-tensor gravity and its primordial perturbation
Scalar-tensor gravity is one of the most competitive gravity theory to
Einstein's relativity. We reconstruct the exact de Sitter solution in
scalar-tensor gravity, in which the non-minimal coupling scalar is rolling
along the potential. This solution may have some relation to the early
inflation and present acceleration of the universe. We investigated its
primordial quantum perturbation around the adiabatic vacuum. We put forward for
the first time that exact de Sitter generates non-exactly scale invariant
perturbations. In the conformal coupling case, this model predicts that the
tensor mode of the perturbation (gravity wave) is strongly depressed.Comment: 9 page
Variable Modified Chaplygin Gas in Anisotropic Universe with Kaluza-Klein Metric
In this work, we have consider Kaluza-Klein Cosmology for anisotropic
universe where the universe is filled with variable modified chaplygin gas
(VMCG). Here we find normal scalar field and the self interacting
potential to describe the VMCG Cosmology. Also we graphically
analyzed the geometrical parameters named {\it statefinder parameters} in
anisotropic Kaluza-Klein model. Next, we consider a Kaluza-Klein model of
interacting VMCG with dark matter in the Einstein gravity framework. Here we
construct the three dimensional autonomous dynamical system of equations for
this interacting model with the assumption that the dark energy and the dark
matter are interact between them and for that we also choose the interaction
term. We convert that interaction terms to its dimensionless form and perform
stability analysis and solve them numerically. We obtain a stable scaling
solution of the equations in Kaluza-Klein model and graphically represent
solutions.Comment: 11 pages, 13 figure
UV stable, Lorentz-violating dark energy with transient phantom era
Phantom fields with negative kinetic energy are often plagued by the vacuum
quantum instability in the ultraviolet region. We present a Lorentz-violating
dark energy model free from this problem and show that the crossing of the
cosmological constant boundary w=-1 to the phantom equation of state is
realized before reaching a de Sitter attractor. Another interesting feature is
a peculiar time-dependence of the effective Newton's constant; the magnitude of
this effect is naturally small but may be close to experimental limits. We also
derive momentum scales of instabilities at which tachyons or ghosts appear in
the infrared region around the present Hubble scale and clarify the conditions
under which tachyonic instabilities do not spoil homogeneity of the
present/future Universe.Comment: 22 pages, 7 figures; Presentation modified substantially, results and
conclusions unchanged. Journal versio
- …