The Cosmic Distance Ladder

For distant galaxies, the expansion of the universe stretches their light to longer, redder wavelengths in proportion to their distance. Measure the redshift, apply the Hubble–Lemaître law, and you have the distance — the top rung of the ladder.

Objects whose true brightness is known, so that how faint they appear gives their distance. Cepheid variables and Type Ia supernovae are the great standard candles that carry the distance ladder out into the universe.

Cepheid variable stars pulse with a period that depends on their true luminosity — Henrietta Leavitt's discovery. Measure the period, know the luminosity, and the apparent brightness gives the distance. Cepheids calibrated the scale of the galaxy and beyond.

The chain of methods, each calibrating the next, that measures the universe: geometric parallax for nearby stars, standard candles for nearby galaxies, and redshift for the far reaches. The mismatch between its rungs is at the heart of today's Hubble tension.