Other meanings of Hubble's law
Astronomy
Hubble's law is the astronomical observation that galaxies recede from Earth at speeds proportional to their distance, implying the expansion of the universe. It is a cornerstone of modern cosmology, providing evidence for the Big Bang and a measure of the universe's age and scale.
Hubble's law states that the recessional velocity v of a galaxy is directly proportional to its distance d from the observer: v = H₀d, where H₀ is the Hubble constant.1 The law is often expressed in terms of redshift z, with v ≈ cz for nearby galaxies. It is a direct consequence of the expansion of space, not a motion through space.
In 1929, Edwin Hubble combined Vesto Slipher's redshift measurements with his own distance estimates to show a linear relation, though earlier work by Georges Lemaître had predicted it.2 Hubble's original value of H₀ was about 500 km/s/Mpc, far larger than today's value, due to calibration errors in Cepheid distances.
Hubble's law underpins the expanding universe model and the Big Bang theory. The reciprocal of H₀ gives the Hubble time, an estimate of the universe's age. Modern measurements from the Planck satellite yield H₀ ≈ 67.4 km/s/Mpc, implying an age of about 13.8 billion years. The law also provides a cosmic distance ladder, enabling distance measurements to far galaxies.
Precise measurements of H₀ have revealed a discrepancy, known as the Hubble tension, between values from the early universe (cosmic microwave background) and the local universe (Cepheids and supernovae).3 This tension may indicate new physics beyond the standard cosmological model, such as dark energy evolving over time or additional relativistic species.
Hubble's law applies to the smooth expansion on large scales; within galaxy clusters, gravitational interactions cause peculiar velocities that deviate from the law. The law is also used in astronomy to estimate distances to galaxies, but it fails for very distant objects where the expansion rate changes over time. The term 'Hubble flow' refers to the recession of galaxies due to expansion, distinct from local motions. The law was anticipated by theoretical work from Alexander Friedmann and Lemaître, and Slipher's redshift measurements were crucial.
Hubble's law is a fundamental relation in observational cosmology, but its precise value remains an active area of research.
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