Late Time Exponential Trend

Compilations of H(z) measurements — the Hubble parameter as a function of redshift, derived from cosmic chronometers (passively evolving galaxies), BAO, and supernovae — reveal a trend in the late-time expansion history that shows mild but systematic deviation from the flat ΛCDM prediction at redshifts z < 1. The expansion appears to be not quite following the expected pattern of matter domination transitioning smoothly to Λ domination. Several independent analyses find that allowing a dynamical dark energy component with time-varying w improves the fit to H(z) data at 2–3σ over the standard Λ = const model.

ΛCDM's expansion history in the late universe is fully determined by Ω_m and Λ once H₀ is fixed. If H(z) observations systematically prefer a different trajectory — one consistent with evolving dark energy rather than a cosmological constant — then the fundamental assumption of ΛCDM is challenged. The DESI 2024 results amplified this concern significantly, finding a preference for w₀ > −1 and w_a < 0 in a w₀w_a CDM parameterization. While each individual dataset's deviation is moderate, the consistent direction of the deviation across multiple probes (BAO, supernovae, chronometers) suggests a coherent physical signal rather than noise.