In 1908, Henrietta Swan Leavitt, a deaf American astronomer at the Harvard College Observatory and one of the Harvard Computers, was studying variable stars in the Small Magellanic Cloud. She noticed that brighter Cepheid variables had longer pulsation periods, and because all the SMC stars were at roughly the same distance, apparent brightness stood in for absolute luminosity. She had discovered the period-luminosity relation of Cepheid variables: a way to read intrinsic brightness from pulsation period, and so distance from how bright the star appeared from Earth. Leavitt's law, published 1912, turned astronomy from a catalogue science into quantitative cosmology. She received almost no recognition during her lifetime.
The ladder's central feature is that each rung calibrates the next, and errors propagate; get the calibration wrong at the bottom, and the Hubble constant shifts. The lowest rung is parallax — the apparent angular shift of a star against the background as Earth orbits — with one arcsecond defining one parsec. Hipparcos extended it to ~1,000 pc; Gaia is now producing parallaxes for 1.8 billion stars at sub-microarcsecond precision. Cepheid variables are the next rung; Leavitt's period-luminosity relation gives intrinsic luminosity from period, useful with HST and JWST out to tens of Mpc. The Tip of the Red Giant Branch — a remarkably constant standard-candle luminosity where low-mass stars undergo the helium flash — is the standard Cepheid alternative. Type Ia supernovae — thermonuclear explosions of binary white dwarfs near the Chandrasekhar limit — are similar enough across events that, with light-curve corrections (the Phillips relation: brighter ones decline more slowly), they serve as standard candles out to z > 1. The 1998 observations of distant Type Ia supernovae by Perlmutter, Riess, and Schmidt — finding them fainter than expected — led directly to the discovery of dark energy (2011 Nobel). Hubble's law then converts redshift to distance for the most distant surveys. The two ends of the ladder — local Cepheid-anchored measurements and CMB-anchored cosmological inferences — do not currently agree; the Hubble tension between them is the central open question in observational cosmology in the 2020s.
Gaia DR3 (June 2022) provides the most-precise stellar parallaxes ever obtained, tightening the bottom of the ladder. JWST is extending Cepheid measurements to galaxies farther than HST could reach; the 2024 Riess et al. result confirmed the SH0ES Cepheid distances, pushing back against the hypothesis that the tension was a Cepheid systematic. DESI released its first cosmological results in 2024, and the BAO + supernova combination shows modest preference for evolving dark energy (w ≠ −1) — a major theoretical development if confirmed. LIGO standard sirens (GW170817 the first such event) provide an independent cosmological distance method that may eventually settle the discrepancy. Every cosmological number you have ever heard flows through this ladder.