The Hubble tension is the persistent 5-sigma discrepancy between the locally measured present-day expansion rate of the universe, H0 ≈ 73 km/s/Mpc from the distance ladder (Cepheids and Type Ia supernovae, especially the SH0ES program), and the lower value H0 ≈ 67 km/s/Mpc inferred indirectly by fitting the early-universe cosmic microwave background anisotropies with the standard six-parameter ΛCDM model (Planck Collaboration 2020; Riess et al. 2022). This disagreement has survived extensive checks for observational systematics on both sides and suggests that the ΛCDM assumption of a single, globally valid FLRW expansion history from a hot dense Big Bang, with fixed early-time physics and a simple dark-energy component, may be incomplete or incorrect (Verde, Treu & Riess 2019; Efstathiou 2024).