Hubble's law, in astronomy, statement that the distances between galaxies (see galaxy) or clusters of galaxies are continuously increasing and that therefore the universe is expanding.
Discovery and Explanation of the Red Shift
Edwin Hubble first proposed this law in 1929 based on a study of the light received from the distant galaxies. He observed that the characteristic colors, or spectral lines (see spectrum), emitted by the stars in the galaxies do not have exactly the same wavelengths observed in the laboratory; rather they are systematically shifted to longer wavelengths, toward the red end of the spectrum.
Such "red shifts" could occur because other galaxies are moving away from our own galaxy, the Milky Way. The change in the wavelength of light that results from the relative motion of the source and the receiver of the light is an example of the Doppler effect. The precise definition of the red shift is the increase in the wavelength divided by the original wavelength; for a given relative velocity, this quantity is the same for all wavelengths or colors. For example, a red shift of 0.05 means that all wavelengths are increased by 5% because of the recessional velocity. Thus the velocity of any given galaxy is measured by its red shift.
Subsequent work has confirmed the general features of Hubble's law, but one specific part—Hubble's constant—has been drastically corrected. This value suggests the relative rate at which the scale of the universe changes with time. The value is currently estimated at about 45 to 46 mi (72 to 74 km) per second per megaparsec. There is still some uncertainty in the value of this constant—a more recent estimate based on data from the Planck space observatory was about 42 mi (67 km) per second per megaparsec—although the difference much less what it was in 1990. Hubble's original value for the expansion rate was between five and ten times too large because he underestimated the distances to the galaxies. The Hubble constant has received much attention because its reciprocal can be thought of as a time that represents the age of the universe. A low Hubble's constant implies that the universe is expanding slowly and therefore must be very old to have reached its current size. Conversely, a high estimate implies a rapid expansion and a relatively young universe. In 1996 two teams at the Carnegie Observatories engaged in measuring Hubble's constant by different methods reported converging findings on the age of the universe. One team, led by Wendy L. Freedman, estimated the age at 9–12 billion years. The other, led by Allan Sandage, estimated the age at 11–15 billion years. Current estimates place the age of the universe at around 13.75 billion years.
Relative Motion of the Galaxies
Hubble's law applies to all galaxies or clusters sufficiently distant from one another that gravitational forces are negligible. According to the law, these galaxies are flying away from each other at tremendous speeds as the fabric of space they occupy stretches, such that the greater the distance between any two galaxies, the greater their relative speed of separation. In other words, the expansion of the universe is roughly uniform. This empirical finding strongly supports the theory that the universe began with an explosive big bang (see cosmology).
Hubble's law was deduced from observations that indicate that the more distant a galaxy, the greater its red shift and hence the greater its velocity relative to the Milky Way. The fact that all other galaxies (with the exception of M31, the Andromeda Galaxy) seem to be receding from the Milky Way does not imply that there is anything special about our position in space. Because the expansion of the universe is approximately uniform, it would appear to an observer in any galaxy that all other galaxies, including the Milky Way, were receding from the observer's galaxy.
See E. Harrison, Cosmology (1981).