290 research outputs found
Asymmetric cyclic evolution in polymerised cosmology
The dynamical systems methods are used to study evolution of the polymerised
scalar field cosmologies with the cosmological constant. We have found all
evolutional paths admissible for all initial conditions on the two-dimensional
phase space. We have shown that the cyclic solutions are generic. The exact
solution for polymerised cosmology is also obtained. Two basic cases are
investigated, the polymerised scalar field and the polymerised gravitational
and scalar field part. In the former the division on the cyclic and non-cyclic
behaviour is established following the sign of the cosmological constant. The
value of the cosmological constant is upper bounded purely from the dynamical
setting.Comment: 10 pages, 4 figs, JHEP3.cl
Reheating temperature from the CMB
In the recent paper by Mielczarek \emph{et al.} (JCAP {\bf 1007} (2010) 004)
an idea of the method which can be used to put some constraint for the
reheating phase was proposed. Another method of constraining the reheating
temperature has been recently studied by Martin and Ringeval (Phys.\ Rev.\ D
{\bf 82} (2010) 023511). Both methods are based on observations of the cosmic
microwave background (CMB) radiation. In this paper, we develop the idea
introduced in this first article to put constraint on the reheating after the
slow-roll inflation. We restrict our considerations to the case of a massive
inflaton field. The method can be, however, easily extended to the different
inflationary scenarios. As a main result, we derive an expression on the
reheating temperature . Surprisingly, the obtained equation is
independent on the unknown number of relativistic degrees of freedom
produced during the reheating. Based on this equation and the WMAP 7
observations, we find GeV, which is consistent
with the current constraints. The relative uncertainty of the result is,
however, very high and equal to . As we show, this uncertainty will be significantly reduced with future CMB
experiments.Comment: 6 pages, 3 figures. Matches version published in Phys. Rev.
Observational issues in loop quantum cosmology
Quantum gravity is sometimes considered as a kind of metaphysical
speculation. In this review, we show that, although still extremely difficult
to reach, observational signatures can in fact be expected. The early universe
is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop
Quantum Gravity as one of the best candidate for a non-perturbative and
background-independant quantization of gravity, we detail some expected
features.Comment: 75 pages, invited topical review for Classical and Quantum Gravit
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
Effective dynamics of the closed loop quantum cosmology
In this paper we study dynamics of the closed FRW model with holonomy
corrections coming from loop quantum cosmology. We consider models with a
scalar field and cosmological constant. In case of the models with cosmological
constant and free scalar field, dynamics reduce to 2D system and analysis of
solutions simplify. If only free scalar field is included then universe
undergoes non-singular oscillations. For the model with cosmological constant,
different behaviours are obtained depending on the value of . If the
value of is sufficiently small, bouncing solutions with asymptotic de
Sitter stages are obtained. However if the value of exceeds critical
value then solutions become oscillatory. Subsequently we study
models with a massive scalar field. We find that this model possess generic
inflationary attractors. In particular field, initially situated in the bottom
of the potential, is driven up during the phase of quantum bounce. This
subsequently leads to the phase of inflation. Finally we find that, comparing
with the flat case, effects of curvature do not change qualitatively dynamics
close to the phase of bounce. Possible effects of inverse volume corrections
are also briefly discussed.Comment: 18 pages, 11 figure
Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology
Holonomy corrections to scalar perturbations are investigated in the loop
quantum cosmology framework. Due to the effective approach, modifications of
the algebra of constraints generically lead to anomalies. In order to remove
those anomalies, counter-terms are introduced. We find a way to explicitly
fulfill the conditions for anomaly freedom and we give explicit expressions for
the counter-terms. Surprisingly, the "new quantization scheme" naturally arises
in this procedure. The gauge invariant variables are found and equations of
motion for the anomaly-free scalar perturbations are derived. Finally, some
cosmological consequences are discussed qualitatively.Comment: 19 pages, 1 figure, v2, new comments and references added, minor
correction
Observational hints on the Big Bounce
In this paper we study possible observational consequences of the bouncing
cosmology. We consider a model where a phase of inflation is preceded by a
cosmic bounce. While we consider in this paper only that the bounce is due to
loop quantum gravity, most of the results presented here can be applied for
different bouncing cosmologies. We concentrate on the scenario where the scalar
field, as the result of contraction of the universe, is driven from the bottom
of the potential well. The field is amplified, and finally the phase of the
standard slow-roll inflation is realized. Such an evolution modifies the
standard inflationary spectrum of perturbations by the additional oscillations
and damping on the large scales. We extract the parameters of the model from
the observations of the cosmic microwave background radiation. In particular,
the value of inflaton mass is equal to GeV. In
our considerations we base on the seven years of observations made by the WMAP
satellite. We propose the new observational consistency check for the phase of
slow-roll inflation. We investigate the conditions which have to be fulfilled
to make the observations of the Big Bounce effects possible. We translate them
to the requirements on the parameters of the model and then put the
observational constraints on the model. Based on assumption usually made in
loop quantum cosmology, the Barbero-Immirzi parameter was shown to be
constrained by from the cosmological observations. We have
compared the Big Bounce model with the standard Big Bang scenario and showed
that the present observational data is not informative enough to distinguish
these models.Comment: 25 pages, 8 figures, JHEP3.cl
Effects of the quantisation ambiguities on the Big Bounce dynamics
In this paper we investigate dynamics of the modified loop quantum cosmology
models using dynamical systems methods. Modifications considered come from the
choice of the different field strength operator and result in
different forms of the effective Hamiltonian. Such an ambiguity of the choice
of this expression from some class of functions is allowed in the framework of
loop quantisation. Our main goal is to show how such modifications can
influence the bouncing universe scenario in the loop quantum cosmology. In
effective models considered we classify all evolutional paths for all
admissible initial conditions. The dynamics is reduced to the form of a
dynamical system of the Newtonian type on a 2-dimensional phase plane. These
models are equivalent dynamically to the FRW models with the decaying effective
cosmological term parametrised by the canonical variable (or by the scale
factor ). We find that for the positive cosmological constant there is a
class of oscillating models without the initial and final singularities. The
new phenomenon is the appearance of curvature singularities for the finite
values of the scale factor, but we find that for the positive cosmological
constant these singularities can be avoided. For the positive cosmological
constant the evolution begins at the asymptotic state in the past represented
by the deSitter contracting (deS) spacetime or the static Einstein
universe H=0 or state and reaches the deSitter expanding state
(deS), the state H=0 or state. In the case of the negative
cosmological constant we obtain the past and future asymptotic states as the
Einstein static universes.Comment: RevTeX4, 28 pages, 11 figs; rev.2 new section on exact solutions;
(v3) published versio
Density growth in Kantowski-Sachs cosmologies with cosmological constant
In this work the growth of density perturbations in Kantowski-Sachs
cosmologies with a positive cosmological constant is studied, using the 1+3 and
1+1+2 covariant formalisms. For each wave number we obtain a closed system for
scalars formed from quantities that are zero on the background and hence are
gauge-invariant. The solutions to this system are then analyzed both
analytically and numerically. In particular the effects of anisotropy and the
behaviour close to a bounce in the cosmic scale factor are considered. We find
that typically the density gradient in the bouncing directions experiences a
local maximum at or slightly after the bounce.Comment: 33 pages, 17 picture
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