412 research outputs found
Covariant Formulation of the Invariant Measure for the Mixmaster Dynamics
We provide a Hamiltonian analysis of the Mixmaster Universe dynamics showing
the covariant nature of its chaotic behavior with respect to any choice of time
variable. We construct the appropriate invariant measure for the system (which
relies on the existence of an ``energy-like'' constant of motion) without
fixing the time gauge, i.e. the corresponding lapse function. The key point in
our analysis consists of introducing generic Misner-Chitr\'e-like variables
containing an arbitrary function, whose specification allows one to set up the
same dynamical scheme in any time gauge.Comment: 11 pages, 1 figur
Cosmological Acceleration from Virtual Gravitons
Intrinsic properties of the space itself and quantum fluctuations of its
geometry are sufficient to provide a mechanism for the acceleration of
cosmological expansion (dark energy effect). Applying
Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy approach to self-consistent
equations of one-loop quantum gravity, we found exact solutions that yield
acceleration. The permanent creation and annihilation of virtual gravitons is
not in exact balance because of the expansion of the Universe. The excess
energy comes from the spontaneous process of graviton creation and is trapped
by the background. It provides the macroscopic quantum effect of cosmic
acceleration.Comment: 6 pages, REVTeX
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
Geometry of dynamics, Lyapunov exponents and phase transitions
The Hamiltonian dynamics of classical planar Heisenberg model is numerically
investigated in two and three dimensions. By considering the dynamics as a
geodesic flow on a suitable Riemannian manifold, it is possible to analytically
estimate the largest Lyapunov exponent in terms of some curvature fluctuations.
The agreement between numerical and analytical values for Lyapunov exponents is
very good in a wide range of temperatures. Moreover, in the three dimensional
case, in correspondence with the second order phase transition, the curvature
fluctuations exibit a singular behaviour which is reproduced in an abstract
geometric model suggesting that the phase transition might correspond to a
change in the topology of the manifold whose geodesics are the motions of the
system.Comment: REVTeX, 10 pages, 5 PostScript figures, published versio
Overlapping functions of the starch synthases SSII and SSIII in amylopectin biosynthesis in Arabidopsis
Background: The biochemical mechanisms that determine the molecular architecture of
amylopectin are central in plant biology because they allow long-term storage of reduced carbon.
Amylopectin structure imparts the ability to form semi-crystalline starch granules, which in turn
provides its glucose storage function. The enzymatic steps of amylopectin biosynthesis resemble
those of the soluble polymer glycogen, however, the reasons for amylopectin's architectural
distinctions are not clearly understood. The multiplicity of starch biosynthetic enzymes conserved
in plants likely is involved. For example, amylopectin chain elongation in plants involves five
conserved classes of starch synthase (SS), whereas glycogen biosynthesis typically requires only one
class of glycogen synthase.
Results: Null mutations were characterized in AtSS2, which codes for SSII, and mutant lines were
compared to lines lacking SSIII and to an Atss2, Atss3 double mutant. Loss of SSII did not affect
growth rate or starch quantity, but caused increased amylose/amylopectin ratio, increased total
amylose, and deficiency in amylopectin chains with degree of polymerization (DP) 12 to DP28. In
contrast, loss of both SSII and SSIII caused slower plant growth and dramatically reduced starch
content. Extreme deficiency in DP12 to DP28 chains occurred in the double mutant, far more
severe than the summed changes in SSII- or SSIII-deficient plants lacking only one of the two
enzymes.
Conclusion: SSII and SSIII have partially redundant functions in determination of amylopectin
structure, and these roles cannot be substituted by any other conserved SS, specifically SSI, GBSSI,
or SSIV. Even though SSIII is not required for the normal abundance of glucan chains of DP12 to
DP18, the enzyme clearly is capable of functioning in production such chains. The role of SSIII in
producing these chains cannot be detected simply by analysis of an individual mutation.
Competition between different SSs for binding to substrate could in part explain the specific
distribution of glucan chains within amylopectin
Observational Constraints on the Generalized Chaplygin Gas
In this paper we study a quintessence cosmological model in which the dark
energy component is considered to be the Generalized Chaplygin Gas and the
curvature of the three-geometry is taken into account. Two parameters
characterize this sort of fluid, the and the parameters. We use
different astronomical data for restricting these parameters. It is shown that
the constraint agrees enough well with the astronomical
observations.Comment: Accepted by IJMPD; 18 pages; 10 Figure
Constraints on alternative models to dark energy
The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. This may be
the evidence of the breakdown of the standard Friemann equation. We consider a
general modified Friedmann equation. Three different models are analyzed in
detail. The current supernovae data and the Wilkinson microwave anisotropy
probe data are used to constrain these models. A detailed analysis of the
transition from the deceleration phase to the acceleration phase is also
performed.Comment: 10 pages, 1 figure, revtex
Stabilization of the Yang-Mills chaos in non-Abelian Born-Infeld theory
We investigate dynamics of the homogeneous time-dependent SU(2) Yang-Mills
fields governed by the non-Abelian Born-Infeld lagrangian which arises in
superstring theory as a result of summation of all orders in the string slope
parameter . It is shown that generically the Born-Infeld dynamics is
less chaotic than that in the ordinary Yang-Mills theory, and at high enough
field strength the Yang-Mills chaos is stabilized. More generally, a smothering
effect of the string non-locality on behavior of classical fields is
conjectured.Comment: 7 pages, 5 figure
(Non)Invariance of dynamical quantities for orbit equivalent flows
We study how dynamical quantities such as Lyapunov exponents, metric entropy,
topological pressure, recurrence rates, and dimension-like characteristics
change under a time reparameterization of a dynamical system. These quantities
are shown to either remain invariant, transform according to a multiplicative
factor or transform through a convoluted dependence that may take the form of
an integral over the initial local values. We discuss the significance of these
results for the apparent non-invariance of chaos in general relativity and
explore applications to the synchronization of equilibrium states and the
elimination of expansions
Fomin's conception of quantum cosmogenesis
The main aim of this paper is to extend the early approach to quantum
cosmogenesis provided by Fomin. His approach was developed independently to the
well-known Tryon description of the creation of the closed universe as a
process of quantum fluctuation of vacuum. We apply the Fomin concept to derive
the cosmological observables. We argue that Fomin's idea from his 1973 work, in
contrast to Tryon's one has impact on the current Universe models and the
proposed extension of his theory now can be tested by distant supernovae SNIa.
Fomin's idea of the creation of the Universe is based on the intersection of
two fundamental theories: general relativity and quantum field theory with the
contemporary cosmological models with dark energy. As a result of comparison
with contemporary approaches concerning dark energy, we found out that Fomin's
idea appears in the context of the present acceleration of the Universe
explanation: cosmological models with decaying vacuum. Contemporary it appears
in the form of Ricci scalar dark energy connected with the holographic
principle. We show also that the Fomin model admits the bounce instead of the
initial singularity. We demonstrate that the Fomin model of cosmogenesis can be
falsified and using SNIa data the values of model parameters is in agreement
with observations.Comment: 12 pages, 4 figures; (v2) 22 pages, references added, figures
improved; (v3) rewritten using revtex4; (v4) minor changes; (v5) improved
formulas and extended statistical analysi
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