Implications of a transition in the dark energy equation of state for the H0 and σ8 tensions

We explore the implications of a rapid appearance of dark energy between the redshifts (z) of one and two on the expansion rate and growth of perturbations. Using both Gaussian process regression and a parameteric model, we show that this is the preferred solution to the current set of low-redshift (z<3) distance measurements if H₀=73~ km\, s^-1\, Mpc^-1 to within 1\% and the high-redshift expansion history is unchanged from the CDM inference by the Planck satellite. Dark energy was effectively non-existent around z=2, but its density is close to the CDM model value today, with an equation of state greater than -1 at z<0. 5. If sources of clustering other than matter are negligible, we show that this expansion history leads to slower growth of perturbations at z<1, compared to CDM, that is measurable by upcoming surveys and can alleviate the ₈ tension between the Planck CMB temperature and low-redshift probes of the large-scale structure.