GRB Hubble diagram and constraints on a Λ(t) CDM model.

In previous papers, a cosmological model with constant rate particle creation and vacuum term decaying linearly with the Hubble parameter was shown to lead to a good concordance when tested against precise observations: the position of the first peak in the spectrum of anisotropies of the cosmic microwave background (CMB), the Hubble diagram for Type Ia supernovae (SNe Ia), the distribution of large-scale structures and the distance to the baryonic acoustic oscillations (BAO). That model has the same number of parameters as the spatially flat standard model and seems to alleviate some observational/theoretical tensions appearing in the latter. In this paper, we complement those tests with 109 gamma-ray bursts (GRBs), 59 of them with redshifts above z = 1.4, which permits to extend the Hubble diagram to redshifts up to z ≈ 8. For the calibration of the 50 GRBs with z < 1.4, we use the 288 supernovae of the Sloan Digital Sky Survey project, calibrated with the MLCS2k2 fitter, less model-dependent than other samples like Union2. Our results show a good concordance with the previous tests and, again, less tensions between SNe Ia and GRB best fits as compared to the standard model. [ABSTRACT FROM AUTHOR]