Dynamical dark energy in light of the latest observations

A flat Friedmann-Robertson-Walker universe dominated by a cosmologicalconstant (Λ) and cold dark matter (CDM) has been the workingmodel preferred by cosmologists since the discovery of cosmicacceleration^1,2. However, tensions of various degrees ofsignificance are known to be present among existing datasets within theΛCDM framework^3-11. In particular, the Lyman-αforest measurement of the baryon acoustic oscillations (BAO) by theBaryon Oscillation Spectroscopic Survey³ prefers a smallervalue of the matter density fraction Ω_M than thatpreferred by cosmic microwave background (CMB). Also, the recentlymeasured value of the Hubble constant, H₀ = 73.24 ±1.74 km s^-1 Mpc^-1 (ref. ¹²), is3.4σ higher than the 66.93 ± 0.62 km s^-1Mpc^-1 inferred from the Planck CMB data⁷. In thiswork, we investigate whether these tensions can be interpreted asevidence for a non-constant dynamical dark energy. Using theKullback-Leibler divergence¹³ to quantify the tension betweendatasets, we find that the tensions are relieved by an evolving darkenergy, with the dynamical dark energy model preferred at a 3.5σsignificance level based on the improvement in the fit alone. While, atpresent, the Bayesian evidence for the dynamical dark energy isinsufficient to favour it over ΛCDM, we show that, if the currentbest-fit dark energy happened to be the true model, it would bedecisively detected by the upcoming Dark Energy Spectroscopic Instrumentsurvey¹⁴.