Observational constraints on late-time Lambda(t) cosmology

The cosmological constant, i.e., the energy density stored in the true vacuum state of all existing fields in the Universe, is the simplest and the most natural possibility to describe the current cosmic acceleration. However, despite its observational successes, such a possibility exacerbates the well known cosmological constant problem, requiring a natural explanation for its small, but nonzero, value. In this paper we study cosmological consequences of a scenario driven by a varying cosmological term, in which the vacuum energy density decays linearly with the Hubble parameter. We test the viability of this scenario and study a possible way to distinguish it from the current standard cosmological model by using recent observations of type Ia supernova (Supernova Legacy Survey Collaboration), measurements of the baryonic acoustic oscillation from the Sloan Digital Sky Survey and the position of the first peak of the cosmic microwave background angular spectrum from the three-year Wilkinson Microwave Anisotropy Probe.Comment: Some important revisions. To appear in Physical Review

Is the H0H_0 tension suggesting a 4th neutrino's generation?

Flavour oscillations experiments are suggesting the existence of a sterile, $4$th neutrino's generation with a mass of an eV order. This would mean an additional relativistic degree of freedom in the cosmic inventory, in contradiction with recent results from the Planck satellite, that have confirmed the standard value $N_{eff} \approx 3$ for the effective number of relativistic species. On the other hand, the Planck best-fit for the Hubble-Lema\^itre parameter is in tension with the local value determined with the Hubble Space Telescope, and adjusting $N_{eff}$ is a possible way to overcome such a tension. In this paper we perform a joint analysis of three complementary cosmological distance rulers, namely the CMB acoustic scale measured by Planck, the BAO scale model-independently determined by Verde {\it et al.}, and luminosity distances measured with JLA and Pantheon SNe Ia surveys. Two Gaussian priors were imposed to the analysis, the local expansion rate measured by Riess {\it et al.}, and the baryon density parameter fixed from primordial nucleosynthesis by Cooke {\it et al.}. For the sake of generality, two different models are used in the tests, the standard $\Lambda$CDM model and a generalised Chaplygin gas. The best-fit gives $N_{eff} \approx 4$ in both models, with a Chaplygin gas parameter slightly negative, $\alpha \approx -0.04$. The standard value $N_{eff} \approx 3$ is ruled out with $\approx 3\sigma$.Comment: includes some improvements in analysis, matches accepted version in PR

Active galactic nuclei, gravitational redshifts, and cosmological tensions

Gravitational redshift is a classical effect of Einstein's General Relativity, already measured in stars, quasars and clusters of galaxies. We here identify the signature of gravitational redshift in the emission lines of active galaxies due to supermassive black holes and discuss their impact on cosmological inference from type Ia supernovae. Firstly, from the full width at half maximum of $H_{\beta}$ lines of 75 Seyfert type I galaxies of the AGN Black Hole Mass Database, we derive a gravitational redshift $z_g = (2.4 \pm 0.9) \times 10^{-4}$. Expanding this analysis to 86755 quasars from DR14 of SDSS we have a mean value $z_g \approx 2.7 \times 10^{-4}$. Then, by comparing the redshifts of 34 lines measured at the central and outer regions of LINER galaxies in the SAMI survey we obtain $z_g = (0.68 \pm 0.09) \times 10^{-4}$. These numbers are compatible with central black holes of $\approx 10^9$ solar masses and broad line regions of $\approx 1$~pc. For non-AGN galaxies the gravitational redshift is compatible with zero and, as they constitute most of SNe Ia host galaxies, the impact on the cosmological parameters is negligible.Comment: 5 pages, 5 figure