The American astronomer Edwin Hubble uncovered important evidence that the Universe is expanding.
In 1929 he announced his discovery that the further away a galaxy is from another point in space, the faster it appears to recede as the Universe expands - Hubble's Law.
But there was more to be discovered about the expanding Universe - dark energy.
Image: Hubble measured the distances to galaxies such as Andromeda (NOAO/AURA/NSF/T.Rector & B.A.Wolpa)
Hubble measures the Universe's expansion.
Prof Brian Cox studies the colour of stars to understand how the Universe began.
Prof Brian Cox explains how we can understand the origins of the Universe through differing wavelengths of light emitted by stars.
The great astronomer worked at Mount Wilson Observatory.
This clip was filmed at a time when the Hubble Space Telescope had been discovered to have a flawed mirror, hence the reference to "failure". The Hubble Space Telescope went on to become one of the most successful NASA missions. This clip looks at some of the American astronomer Edwin Hubble's most important contributions to astronomy and the equipment he used at the Mount Wilson Observatory. The age of the Universe is now thought to be 13.7 billion years.
Edwin Hubble discovers galaxies outside the Milky Way.
Edwin Hubble discovers galaxies outside the Milky Way and measures how far away they are.
Hubble's law is the name for the observation in physical cosmology that: (1) objects observed in deep space (extragalactic space, ~10 megaparsecs or more) are found to have a Doppler shift interpretable as relative velocity away from the Earth; and (2) that this Doppler-shift-measured velocity, of various galaxies receding from the Earth, is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away. This is normally interpreted as a direct, physical observation of the expansion of the spatial volume of the observable universe.
The motion of astronomical objects due solely to this expansion is known as the Hubble flow. Hubble's law is considered the first observational basis for the expanding space paradigm and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.
Although widely attributed to Edwin Hubble, the law was first derived from the General Relativity equations by Georges Lemaître in a 1927 article where he proposed that the Universe is expanding and suggested an estimated value of the rate of expansion, now called the Hubble constant. Two years later Edwin Hubble confirmed the existence of that law and determined a more accurate value for the constant that now bears his name. The recession velocity of the objects was inferred from their redshifts, many measured earlier by Vesto Slipher (1917) and related to velocity by him.
The law is often expressed by the equation v = H0D, with H0 the constant of proportionality (the Hubble constant) between the "proper distance" D to a galaxy (which can change over time, unlike the comoving distance) and its velocity v (i.e. the derivative of proper distance with respect to cosmological time coordinate; see Uses of the proper distance for some discussion of the subtleties of this definition of 'velocity'). The SI unit of H0 is s−1 but it is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away. The reciprocal of H0 is the Hubble time.