Multiple Transitions in Vacuum Dark Energy and H0 Tension

• Hossein Mos'hafi (School of Astronomy, IPM)

The standard model of cosmology called ΛCDM provides good consistency to a large span of observational data. In recent years with the improvement of the number and accuracy of cosmological observations, some discrepancies between key cosmological parameters of the standard model have emerged. The most statistically significant tension is the ~5σ disagreement between predictions of the Hubble constant, H0, made by the early time observations (CMB) and a number of late-time, model-independent determinations of H0 from local measurements of distances and redshifts. In this talk, I review some aspects of cosmic tensions and describe some possible solutions with a focus on a model with multiple transitions in the vacuum dark energy density. In this work, we considered a phenomenological model in which the vacuum energy density undergoes multiple transitions in the early as well as the late-time universe and compare the model's predictions using the three sets of data from the cosmic microwave background, baryonic acoustic oscillations, and supernovae. The transient dark energy can be either positive (dS-like) or negative (AdS-like). We conclude that a transient late-time AdS-type vacuum energy typically yields a higher value of H0, which can alleviate the H0 tension. In addition, to obtain a value of H0 comparable to the value obtained from the local cosmological measurements the spectral index ns moves toward its Harrison–Zel'dovich scale-invariant value. This talk is based on the following paper: https://iopscience.iop.org/article/10.3847/1538-4357