Pantheon+ Supernova Tension

Type Ia supernovae are the workhorses of cosmic distance measurement, used to map the expansion history of the universe across billions of light-years. The Pantheon+ compilation, the largest standardized supernova dataset to date (1,701 supernovae spanning z = 0.001 to 2.26), combined with SH0ES Cepheid calibrations, yields a Hubble constant of H₀ = 73.04 ± 1.04 km/s/Mpc. This is in direct conflict with the Planck CMB value of H₀ = 67.4 ± 0.5 km/s/Mpc. Beyond the Hubble constant, Pantheon+ also shows hints of deviations in the dark energy equation of state from w = −1 at low to moderate redshifts.

Within ΛCDM, supernovae brightness–distance relationships should follow a precise, predictable pattern set by the cosmological parameters. Pantheon+ data, when analyzed without the CMB prior, prefers a universe that expands faster locally than ΛCDM predicts from early-universe boundary conditions. Additionally, different subsets of the supernova sample — divided by host galaxy type, stellar mass, or redshift — show systematic differences in the inferred Hubble constant at the 2–3σ level, suggesting either unresolved astrophysical systematics in the supernova standardization or genuine large-scale variation in expansion rate. Neither possibility is comfortable within standard ΛCDM.