100 research outputs found
Dark matter and dark energy as a effects of Modified Gravity
We explain the effect of dark matter (flat rotation curve) using modified
gravitational dynamics. We investigate in this context a low energy limit of
generalized general relativity with a nonlinear Lagrangian , where is the (generalized) Ricci scalar and is parameter
estimated from SNIa data. We estimate parameter in modified
gravitational potential .
Then we compare value of obtained from SNIa data with parameter
evaluated from the best fitted rotation curve. We find which
becomes in good agreement with an observation of spiral galaxies rotation
curve. We also find preferred value of from the combined
analysis of supernovae data and baryon oscillation peak. We argue that although
amount of "dark energy" (of non-substantial origin) is consistent with SNIa
data and flat curves of spiral galaxies are reproduces in the framework of
modified Einstein's equation we still need substantial dark matter. For
comparison predictions of the model with predictions of the CDM
concordance model we apply the Akaike and Bayesian information criteria of
model selection.Comment: Lectures given at 42nd Karpacz Winter School of Theoretical Physics:
Ladek, Poland, 6-11 Feb 200
Finite-temperature effective potential for an open universe
We investigate one-loop effects at finite temperatures for an open Robertson–Walker Universe, and v e obtain formulae for a scalar field Coleman-Weinterg potential. The meaning of non-zero temperature and negative curvature for the space-time topology R PS is discussed. Zero-point fluctuation of energy are determined
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
Dynamical meaning of the adiabatic quantum phase
The interaction between the "macrosystem" (corresponding to the so-called slow variables) and the "microsystem" (corresponding to the so-called fast variables) is considered. By using the Born–Oppenheimer adiabatic approximation it is shown that such interaction modifies dynamics of the "macrosystem". We find the form of modified Poisson brackets for the "macrosystem". We also show how symmetries and laws of conservation of the "macrosystem" are influenced by this interaction
Overview remarks on homogeneous, N = 1, d = 11 supergravity cosmologies
The dynamics of the full class of homogeneous N = 1, d = 11 supergravity world models is investigated. By using the classification of Lie algebras of Lie groups which act simply transitively on 6- and 7-dimensional compact spaces some conclusions are drawn concerning the non-existence of the chaotic regime near the singularity. This is illustrated with some new solutions having a richer structure of the microspace. The significance of known solutions is briefly discussed
Dynamical dimensional reduction in multidimensional Bianchi I models
We discuss some mechanisms of isotropization in the class of n-dimensional Bianchi I models. We show that these models can isotropize but this process leads to the breakdown of dimensional reduction. Thus we ought to look for such a mechanism of dimensional reduction that isotropizes the space
Non-integrability of density perturbations in the FRW universe
We investigate the evolution equation of linear density perturbations in the
Friedmann-Robertson-Walker universe with matter, radiation and the cosmological
constant. The concept of solvability by quadratures is defined and used to
prove that there are no "closed form" solutions except for the known Chernin,
Heath, Meszaros and simple degenerate ones. The analysis is performed applying
Kovacic's algorithm. The possibility of the existence of other, more general
solutions involving special functions is also investigated.Comment: 13 pages. The latest version with added references, and a relevant
new paragraph in section I
The Mixmaster Universe in Five Dimensions
We consider a five dimensional vacuum cosmology with Bianchi type-IX spatial
geometry and an extra non-compact coordinate. Finding a new class of solutions,
we examine and rule out the possibility of deterministic chaos. We interpret
this result within the context of induced matter theory.Comment: 13 page
Brane universes tested by supernovae
We discuss observational constrains coming from supernovae Ia
\cite{Perlmutter99} imposed on the behaviour of the Randall-Sundrum models. In
the case of dust matter on the brane, the difference between the best-fit
general relativistic model with a -term \cite{Perlmutter99} and the
best-fit brane models becomes detectable for redshifts . It is
interesting that brane models predict brighter galaxies for such redshifts
which is in agreement with the measurement of the supernova
\cite{Riess01} and with the New Data from the High Z Supernovae Search Team
\cite{schmit02}. We also demonstrate that the fit to supernovae data can also
be obtained, if we admit the "super-negative" dark energy
on the brane, where the dark energy in a way mimics the influence of the
cosmological constant. It also appears that the dark energy enlarges the age of
the universe which is demanded in cosmology. Finally, we propose to check for
dark radiation and brane tension by the application of the angular diameter of
galaxies minimum value test.Comment: 4 pages, 5 figures, REVTEX4, amended versio
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
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