[en] In this article, we elaborate further on the ΛCDM "tension", suggested recently by the authors Lusso et al. (Astron Astrophys 628:L4, 2019) and Risaliti and Lusso (Nat Astron 3(3):272, 2019). We combine Supernovae type Ia (SNIa) with quasars (QSO) and Gamma Ray Bursts (GRB) data in order to reconstruct in a model independent way the Hubble relation to as high redshifts as possible. Specifically, in the case of either SNIa or SNIa/QSO data we find that the current values of the cosmokinetic parameters extracted from the Gaussian process are consistent with those of ΛCDM. Including GRBs in the analysis we find a tension, which lies between 2σ and 3σ levels respectively. Finally, we find that at high redshifts (z>1) the corresponding cosmokinetic parameters significantly deviate from those of ΛCDM, hence the possibility of new Physics is not precluded by the present analysis.

[en] We study the main properties of the warm tachyon inflation model in the framework of the RSII braneworld based on Barrow's solution for the scale factor of the universe. Within this framework we calculate analytically the basic slow-roll parameters for different versions of warm inflation. We test the performance of this inflationary scenario against the latest observational data and we verify that the predicted spectral index and the tensor-to-scalar fluctuation ratio are in excellent agreement with those of Planck 2015. Finally, we find that the current predictions are consistent with those of viable inflationary models. (orig.)

[en] The Gaussian linear model provides a unique way to obtain the posterior probability distribution as well as the Bayesian evidence analytically. Considering the expansion rate data, the Gaussian linear model can be applied for ΛCDM, wCDM and a non-flat ΛCDM. In this paper, we simulate the expansion data with various precision and obtain the Bayesian evidence, then it has been used to discriminate the models. The data uncertainty is in range σ $\in <!--\; ???\; -->$ (0.5,10)% and two different sampling rates have been considered. Our results indicate that considering σ = 0.5% uncertainty, it is possible to discriminate 2% deviation in equation of state from w=-1. On the other hand, we investigate how precision of the expansion rate data affects discriminating the ΛCDM from a non-flat ΛCDM model. Finally, we perform a parameters inference in both the MCMC and Gaussian linear model, using current available expansion rate data and compare the results.

[en] We study the dynamical properties of dark energy based on a large family of Pade parameterizations for which the dark energy density evolves as the ratio between two polynomials in the scale factor of the universe. Using the latest cosmological data we perform a standard likelihood analysis in order to place constraints on the main cosmological parameters of different Pade models. We find that the basic cosmological parameters, namely (Ω_m0, h, σ₈) are practically the same for all Pade parametrizations explored here. Concerning the free parameters which are related to dark energy we show that the best-fit values indicate that the equation of state parameter at the present time is in the phantom regime (w < -1); however, we cannot exclude the possibility of w > -1 at 1σ level. Finally, for the current family of Pade parametrizations we test their ability, via AIC, BIC and Jeffreys' scale, to deviate from ΛCDM cosmology. Among the current Pade parametrizations, the model which contains two dark energy parameters is the one for which a small but non-zero deviation from ΛCDM cosmology is slightly allowed by the AIC test. Moreover, based on Jeffreys' scale we show that a deviation from ΛCDM cosmology is also allowed and thus the possibility of having a dynamical dark energy in the form of Pade parametrization cannot be excluded. (orig.)

[en] We present an investigation of the magnetic activity and flare characteristics of the subgiant stars mostly from F and G spectral types and compare the results with the main-sequence (MS) stars. The light curve of 352 stars on the subgiant branch (SGB) from the Kepler mission is analyzed in order to infer stability, relative coverage, and contrast of the magnetic structures and also flare properties using three flare indexes. The results show the following. (i) Relative coverage and contrast of the magnetic features along with rate, power, and magnitude of flares increase on the SGB due to the deepening of the convective zone and more vigorous magnetic field production. (ii) Magnetic activity of the F- and G-type stars on the SGB does not show dependency on the rotation rate and does not obey the saturation regime. This is the opposite of what we saw for the MS, in which the G-, K-, and M-type stars show clear dependency on the Rossby number. (iii) The positive relationship between the magnetic features’ stability and their relative coverage and contrast remains true on the SGB; though, it has a lower dependency coefficient in comparison with the MS. (iv) Magnetic proxies and flare indexes of the SGB stars increase with increasing relative mass of the convective zone.

[en] The variation of a stellar light curve owing to rotational modulation by magnetic features (starspots and faculae) on the star’s surface can be used to investigate the magnetic properties of the host star. In this paper, we use the periodicity and magnitude of the light-curve variation as two proxies to study the stellar magnetic properties for a large sample of G-type main sequence Kepler targets, for which the rotation periods were recently determined. By analyzing the correlation between the two magnetic proxies, it is found that: (1) the two proxies are positively correlated for most of the stars in our sample, and the percentages of negative, zero, and positive correlations are 4.27%, 6.81%, and 88.91%, respectively; (2) negative correlation stars cannot have a large magnitude of light-curve variation; and (3) with the increase of rotation period, the relative number of positive correlation stars decreases and the negative correlation one increases. These results indicate that stars with shorter rotation period tend to have positive correlation between the two proxies, and a good portion of the positive correlation stars have a larger magnitude of light-curve variation (and hence more intense magnetic activities) than negative correlation stars.