Hubble's law

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Hubble's law,

in astronomy, statement that the distances between galaxies (see galaxygalaxy,
large aggregation of stars, gas, dust, and usually dark matter, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P.
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) 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 spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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), 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 effectDoppler effect,
change in the wavelength (or frequency) of energy in the form of waves, e.g., sound or light, as a result of motion of either the source or the receiver of the waves; the effect is named for the Austrian scientist Christian Doppler, who demonstrated the effect
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. 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, based on studies of type 1a supernovas, which have a known brightness, using Cepheid variable stars to determine the supernovas distances. 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, and a third, more recent method that was based on type 1a supernovas but used red giants to determine distances resulted in an estimate of about 43 mi (70 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. Current estimates place the age of the universe at around 13.799 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 cosmologycosmology,
area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories
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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 GalaxyAndromeda Galaxy,
cataloged as M31 and NGC 224, the closest large galaxy to the Milky Way and the only one visible to the naked eye in the Northern Hemisphere. It is also known as the Great Nebula in Andromeda. It is 2.
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) 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).

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Hubble's law

The law, first proposed by the American astronomer Edwin Hubble in 1929, stating that the recession velocity, v, of a distant extragalactic object (one outside the Local Group) is directly proportional to its distance, D. The constant of proportionality is known as the Hubble constant, H 0, thus
v = H 0 D

The law is a direct consequence of a uniformly expanding isotropic Universe. The uniform outward streaming motion of all galaxies due to the cosmological expansion is called the Hubble flow. See also recession of the galaxies.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006