We investigate observational constraints on the Brans-Dicke cosmological model using observational data coming from distant supernovae type Ia, the Hubble function H(z) measurements, information coming from the Alcock-Paczyński test, and baryon acoustic oscillations. Our analysis is based on the modified Friedmann function resulting form dynamical investigations of Brans-Dicke cosmology in the vicinity of a de Sitter state. The qualitative theory of dynamical systems enables us to obtain three different behaviors in the vicinity of this state. We find for a linear approach to the de Sitter state ωBD = −0.8606 −4.5459 . We obtain the mass of the Brans-Dicke scalar field at the present epoch as m φ ∼ H0. The Bayesian methods of model comparison are used to discriminate between obtained models. We show that observational data point toward vales of the ωBD parameter close to the value suggested by the low-energy limit of the bosonic string theory.PACS numbers: 04.50. Kd, 95.36.+x, 95.35.+d

We study new FRW type cosmological models of modified gravity treated on the background of Palatini approach. These models are generalization of Einstein gravity by the presence of a scalar field non-minimally coupled to the curvature. The models employ Starobinsky's term in the Lagrangian and dust matter. Therefore, as a by-product, an exhausted cosmological analysis of general relativity amended by quadratic term is presented. We investigate dynamics of our models, confront them with the currently available astrophysical data as well as against ΛCDM model. We have used the dynamical system methods in order to investigate dynamics of the models. It reveals the presence of a final sudden singularity. Fitting free parameters we have demonstrated by statistical analysis that this class of models is in a very good agreement with the data (including CMB measurements) as well as with the standard ΛCDM model predictions. One has to use statefinder diagnostic in order to discriminate among them. Therefore Bayesian methods of model selection have been employed in order to indicate preferred model. Only in the light of CMB data the concordance model remains invincible.

Recent astronomical observations have indicated that the Universe is in a phase of accelerated expansion. While there are many cosmological models which try to explain this phenomenon, we focus on the interacting CDM model where an interaction between the dark energy and dark matter sectors takes place. This model is compared to its simpler alternative-the CDM model. To choose between these models the likelihood ratio test was applied as well as the model comparison methods (employing Occam's principle): the Akaike information criterion (AIC), the Bayesian information criterion (BIC) and the Bayesian evidence. Using the current astronomical data: type Ia supernova (Union2.1), h(z), baryon acoustic oscillation, the AlcockPaczynski test, and the cosmic microwave background data, we evaluated both models. The analyses based on the AIC indicated that there is less support for the interacting CDM model when compared to the CDM model, while those based on the BIC indicated that there is strong evidence against it in favor of the CDM model. Given the weak or almost non-existing support for the interacting CDM model and bearing in mind Occam's razor we are inclined to reject this model.

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 m = (2.6 ± 0.6) · 10 13 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 γ < 1100 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.

Abstract. We show that future singularities which have appeared in the Palatini cosmological models investigated in [1] are of finite size at finite time type [2]. Doomsday scenarios for cosmological models are subject of numerous investigations and speculations. For example the standard LCDM model predicts a thermal death caused by the expansion forever. In this short note we solve similar problem for the Palatini cosmological models investigated already in [1].First of all we recall that, similarly to LCDM case, Palatini models are described by dynamical systems of Newtonian type. They are fully determined by e ective potential functions V(a) defined by Friedmann equation (see [1]). The evolution of the Universe described by such system is given by:where a denotes the cosmic (FRWL) scale factor. From the potential plot ( fig . 1) one can see that: for some finite a s < I singular (final) point. a

In this paper we investigate power spectrum of a smoothed scalar field. The smoothing leads to the regularisation of the UV divergences and can be related with the internal structure of the considered field or the space itself. We apply procedure of smoothing to the quantum fluctuations generated during the phase of cosmic inflation. We study whether this effect can be probed observationally and conclude that the modifications of the power spectrum due to the smoothing on the Planck scale are negligible and far beyond the observational abilities. Subsequently we investigate whether smoothing in any other form can be probed observationally. We introduce phenomenological smoothing factor e −k 2 σ 2 to the inflationary spectrum and investigate its effects on the spectrum of CMB anisotropies and polarisation. We show that smoothing can lead to suppression of high multipoles in the spectrum of the CMB. Based on five years observations of WMAP satellite we indicate that the present scale of high multipoles suppression is constrained by σ < 2.86 Mpc (95% CL). This corresponds to the constraint σ < 100 µm at the end of inflation. Despite this value is far above the Planck scale, the other processes of smoothing can be possibly studied with this constraint, e.g. diffusion or decoherence of primordial perturbations.

New constraints on previously investigated Palatini cosmological models [arXiv:1109.3420] have been obtained by adding Gamma Ray Burst data [arXiv:1205.2954].Comment: 4 pages, 5 figure