kmsMpc

kmsMpc stands for kilometers per second per megaparsec, the unit commonly used for the Hubble constant in cosmology. It is written as km s^-1 Mpc^-1 and appears in Hubble's law, v = H0 d, where v is a galaxy’s recessional velocity and d its distance. The constant H0 sets the rate of cosmic expansion for nearby galaxies where the expansion dominates over peculiar motions.

A megaparsec is about 3.0857 × 10^19 kilometers, so an H0 of roughly 70 km s^-1 Mpc^-1 implies that a galaxy 1 Mpc away recedes at about 70 kilometers per second. The inverse of H0 provides a rough cosmic timescale for the expansion, though precise ages depend on the cosmological model and other parameters.

Measuring kmsMpc involves diverse methods. The distance ladder uses standard candles such as Cepheid variables and Type Ia supernovae to calibrate distances locally. Cosmic microwave background measurements, notably by the Planck satellite, infer H0 within a cosmological model (Lambda-CDM) from early-universe data. Other approaches include baryon acoustic oscillations, megamaser distances in nearby galaxies, and gravitational lensing time delays.

Current results show a tension between late-time, local measurements (roughly 73–74 km s^-1 Mpc^-1) and early-universe inferences from the CMB (about 67–68 km s^-1 Mpc^-1). This discrepancy, known as the Hubble tension, is a central topic in cosmology, prompting investigations into potential systematics or new physics beyond the standard model.