193 research outputs found
The problem of effectiveness of classical dimensional reduction mechanism in homogeneous arbitrary-dimensional cosmology
Thermodynamical functions are determined for the bosonic gas distribution in the external gravitational field described by multidimensional cosmological models having the structure FRW B, where B is any D-dimensional compact space with the scalar curvature R. Universal asymptotics of this function are found for the following situations: 1) at high temperatures, with R 1 and 1, 2) at low temperatures, with R 1 and 1, where R is the scale factor of the physical space, and is the reciprocal of temperature. It is shown that if R / R 1 (where R and R are curvature scalars of macro- and microspace, correspondingly), the Casimir energy is always negative. These results are applied to discuss the dimensional reduction generated by the classical Einstein equations with quantum corrections. The idea of the dynamical dimensional reduction is expressed, in terms of the dynamical system theory, as the problem of the existence of a single stable critical point representing a configuration with the static internal space. It is demonstrated that, in the low-temperature approximation, there is no effective mechanism of the dimensional reduction to a static internal space, if B is a group manifold (with the same scale factor in all internal directions). On the other hand, the effective mechanism of the dimensional reduction to the zero size does exist. The existence of such mechanism for the full class of multidimensional homogeneous cosmologies with the hydrodynamic energy-momentum tensor is also discussed
Emerging singularities in the bouncing loop cosmology
In this paper we calculate corrections from holonomies
in the Loop Quantum Gravity, usually not taken into account. Allowance of the
corrections of this kind is equivalent with the choice of the new quatization
scheme. Quantization ambiguities in the Loop Quantum Cosmology allow for this
additional freedom and presented corrections are consistent with the standard
approach. We apply these corrections to the flat FRW cosmological model and
calculate the modified Friedmann equation. We show that the bounce appears in
the models with the standard quantization scheme is
shifted to the higher energies . Also
a pole in the Hubble parameter appears for corresponding to \emph{hyper-inflation/deflation} phases. This
pole represents a curvature singularity at which the scale factor is finite. In
this scenario the singularity and bounce co-exist. Moreover we find that an
ordinary bouncing solution appears only when quantum corrections in the lowest
order are considered. Higher order corrections can lead to the nonperturbative
effects.Comment: RevTeX4, 8 pages, 4 figures; v2 change of title, more discussion on
co-existence of singularity and bounc
Generalized Chaplygin Gas Models tested with SNIa
The so called Generalized Chaplygin Gas (GCG) with the equation of state was recently proposed as a candidate for dark
energy in the Universe. In this paper we confront the GCG with SNIa data.
Specifically we have tested the GCG cosmology in three different classes of
models with (1) , ; (2) ,
and (3) , , as well as the
model withouth any assumption on . The best fitted models are
obtained by minimalizing the function and levels in the
plane. We supplemented our analysis with confidence intervals
in the plane, as well as one-dimensional probability
distribution functions for models parameter. The general conclusion is that
SNIa data strongly support the Chaplygin gas (with ). Extending our
analysisby relaxing the flat prior lead to the result that even though the best
fitted values of are formally non-zero, still they are close to flat
case. It should be viewed as an advantage of the GCG model since in similar
analysisof CDM model high negative value of were found to
be bestfitted to the data and independent inspiration from CMBR and
extragalactic astronomy has been invoked to fix the curvature problem. Our
results show clearly that in Generalized Chaplygin Gas cosmology distant
supernovae should be brighter than in CDM model.This prediction seems
to be confirmed with new Riess high redshift SNIa sample. Moreover, we argue
that with the future SNAP data it would be possible to differentiate between
models with various value of parameter and/or discriminated between
GCG, Cardassian and CDM modelsComment: 54 pages 29 figures improved version analysis flat prior relaxed high
redshift Riess SNIa sample include
Lyapunov function for cosmological dynamical system
We prove the asymptotic global stability of the de Sitter solution in the Friedmann-Robertson-Walker conservative and dissipative cosmology. In the proof we construct a Lyapunov function in an exact form and establish its relationship with the first integral of dynamical system determining evolution of the flat Universe. Our result is that de-Sitter solution is asymptotically stable solution for general form of equation of state p = (ρ, H), where dependence on the Hubble function H means that the effect of dissipation are included
BANKI PUBLICZNE JAKO INSTYTUCJE MOBILIZUJĄCE FINANSOWANIE GOSPODARKI NISKOEMISYJNEJ
Banki z dominującym udziałem kapitału państwowego, stwarzają możliwości realizacji założeń w zakresie zmian związanych z rozwojem zrównoważonym i ochroną klimatu. W Europie istnieje silna grupa tego typu instytucji bankowych, które realizują różnego rodzaju przedsięwzięcia o znaczeniu społecznym i środowiskowym. Instytucje te stają się coraz bardziej efektywnew przezwyciężaniu głównych barier inwestycyjnych oraz zakłóceń rynku, powstrzymujących inwestycje w niskoemisyjną gospodarkę.Banki państwowe są coraz lepiej przystosowane do transferów kapitału do niskoemisyjnej gospodarki. Mogą elastycznie dostosowywać się do pojawiających się wyzwań i dzięki temuwypracowywać wiele korzyści. Ich cztery kluczowe przewagi są następujące: umiejętność projektowania i implementacji finansowych mechanizmów dostosowanych do określonych celów, wykorzystanie bogatego doświadczenia i wiedzy z zakresu finansów i aspektów technicznych realizowanych przedsięwzięć, występowanie w roli pośredników i mediatorów w procesie realizacji niskoemisyjnych inwestycji oraz silna reputacja długoterminowych inwestorów w przedsięwzięcia o charakterze społecznym i środowiskowym.Celem artykułu jest przedstawienie działań podejmowanych przez wybrane instytucje bankowe, w kierunku rozwoju mechanizmu finansowania gospodarki niskoemisyjnej. Uzyskanewnioski zostały sformułowane na podstawie analizy działań siedmiu wybranych europejskich instytucji bankowych
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
Towards testing interacting cosmology by distant type Ia supernovae
We investigate the possibility of testing cosmological models with
interaction between matter and energy sector. We assume the standard FRW model
while the so called energy conservation condition is interpreted locally in
terms of energy transfer. We analyze two forms of dark energy sectors: the
cosmological constant and phantom field. We find a simple exact solution of the
models in which energy transfer is described by a Cardassian like term in the
relation of , where is Hubble's function and is redshift. The
considered models have two additional parameters
(apart the parameters of the CDM model) which can be tested using SNIa
data. In the estimation of the model parameters Riess et al.'s sample is used.
We also confront the quality of statistical fits for both the CDM
model and the interacting models with the help of the Akaike and Bayesian
informative criteria. Our conclusion from standard best fit method is that the
interacting models explains the acceleration of the Universe better but they
give rise to a universe with high matter density. However, using the tools of
information criteria we find that the two new parameters play an insufficient
role in improving the fit to SNIa data and the standard CDM model is
still preferred. We conclude that high precision detection of high redshift
supernovae could supply data capable of justifying adoption of new parameters.Comment: RevTeX4, 14 pages, 7 figure
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
Constraints on oscillating dark energy models
The oscillating scenario of route to Lambda was recently proposed by us
arXiv:0704.1651 as an alternative to a cosmological constant in a explanation
of the current accelerating universe. In this scenario phantom scalar field
conformally coupled to gravity drives the accelerating phase of the universe.
In our model CDM appears as a global attractor in the phase space. In
this paper we investigate observational constraints on this scenario from
recent measurements of distant supernovae type Ia, CMB R shift, BAO and
observational data. The Bayesian methods of model selection are used in
comparison the model with concordance CDM one as well as with model
with dynamical dark energy parametrised by linear form. We conclude that
CDM is favoured over FRW model with dynamical oscillating dark energy.
Our analysis also demonstrate that FRW model with oscillating dark energy is
favoured over FRW model with decaying dark energy parametrised in linear way.Comment: revtex4, 7 pages, 3 figures; (v2) 12 pages, 11 figures; Phys. Lett. B
(in press
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