Cosmic Distance Ladder

The cosmic distance ladder is the succession of overlapping methods astronomers use to measure distances across the universe, moving from techniques applicable nearby, such as parallax for the closest stars, to standard candles like Cepheid variable stars and Type Ia supernovae that calibrate progressively larger distances, up to the redshift-based methods used for the most distant galaxies. Each rung is calibrated against the one before it, so a systematic error at any step propagates outward through the whole ladder, which is why refining its early rungs remains an active area of research. The ladder underlies essentially every distance measurement beyond the reach of direct geometric parallax.