A fundamental parameter in cosmology that describes the rate at which the universe is expanding, typically expressed in kilometers per second per megaparsec. It relates the recession velocity of distant galaxies to their distance from us.
Named after Edwin Hubble, who in 1929 discovered the relationship between galactic distance and recession velocity, providing the first evidence for an expanding universe. The term 'constant' reflects its uniform value throughout space, though it changes over cosmic time.
The Hubble constant isn't actually constant over time - it was much larger in the early universe and is slowly decreasing! This 'constant' is so important that its precise value determines the age of the universe, and astronomers are still debating whether it's 67 or 73 km/s/Mpc.
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