155 research outputs found
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 tension suggesting a 4th neutrino's generation?
Flavour oscillations experiments are suggesting the existence of a sterile,
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 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 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
CDM model and a generalised Chaplygin gas. The best-fit gives in both models, with a Chaplygin gas parameter slightly negative,
. The standard value is ruled out
with .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 lines of 75 Seyfert type I galaxies of the AGN
Black Hole Mass Database, we derive a gravitational redshift . Expanding this analysis to 86755 quasars from DR14 of
SDSS we have a mean value . Then, by comparing
the redshifts of 34 lines measured at the central and outer regions of LINER
galaxies in the SAMI survey we obtain .
These numbers are compatible with central black holes of solar
masses and broad line regions of ~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
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