Hubble Constant
The Hubble constant is used in astrophysics to characterize the expansion of the universe. The constant specifically relates the distance to a galaxy to that galaxyüs velocity of recession. Ironically, because gravitational forces depend on the distance between objects, the Hubble constant is actually a function of the expansion of the universe (i.e., it changes as the universe expands).
In 1929, American astronomers Edwin Hubble and Milton Humason (1891-1972), expanding upon the work of Vesto M. Slipher (1875-1969), discovered that light from distant galaxies is systematically increased in wavelength. This increase in wavelength, called a red shift, is denoted (z). Hubble interpreted observed red shifts as a consequence of the Doppler effect, which implied that the galaxies in his sample were all receding from the Sun with a velocity (v) determined by the equation, v = cz, where (c) is the speed of light. Of profound significance to cosmology, and in contrast to prevailing theories describing a static universe, Hubble and Humason discovered proof of an expanding universe.
Hubble also realized that there was a linear relationship between the spatial velocity away from the Sun and the distance to each galaxy. The farther one looks into space, the faster galaxies are moving uniformly away from the Sun. Incidentally, this does not imply that the Sun is at the center of the universe. In accordance with the cosmological principle, there is no center and every location in the universe should see the same nature of the expansion of the universe.
Hubble quantified the linear relationship between expansion velocity (v) and distance (d) via the equation, v = Hd, known as Hubble's law. The constant term (H), determined from the slope of the linear relation between velocity and distance, has come to be known as the Hubble constant.
In an expanding universe, the Hubble constant provides the expansion rate of the universe in units of inverse time. Thus, the reciprocal of the Hubble constant, called the Hubble time, gives a rough estimate of the age of the universe. This is a rough estimate because it relies on the assumption that the expansion rate of the universe has been constant with time. In all likelihood, the expansion rate has slowed due to the mutual gravitational attraction of the constituent material of the universe.
As of 2000, astronomers remain unable to determine a precise value of the Hubble constant better than to say it lies between 50 and 100 inverse time units of kilometers per second per Megaparsec. Such obscure units are used because it makes the analysis easier as red shifts are often measured in kilometers per second and galaxy distances are measured in Megaparsecs (the prefix Mega denotes one million and one parsec equals 3.26 light-years). Because of the uncertainty placed on the Hubble constant, astronomers have been loosely divided into two camps; those who subscribe, such as Alan Sandage and Gustav Tammann, to a value near 50; and those, like the late Gérard de Vaucouleurs, who argue for a value near 100. Finally, the modern accepted range of values for the Hubble constant place the age of the universe between 10 and 20 billion years.
The inability to pin down the Hubble constant is almost entirely due to the difficulty associated with measuring accurate distances to galaxies. Although obtaining accurate red shifts for galaxies is relatively easy with high-tech spectroscopic instruments, accurate distance determinations, especially to galaxies that are over 100 million light-years away, is highly prone to error. For example, Edwin Hubble's first calculation of the Hubble constant yielded a whopping value of 530 kilometers per second per Megaparsec because of a severe underestimate in the distances to galaxies resulting from a misidentification of the types of stars used to calibrate distance. Hubble's errors in distance determination propagated through his subsequent calculations and led to the extraordinarily high magnitude for the Hubble constant.
The manner in which a recessional velocity for a galaxy associates with a measured red shift is also highly dependent upon the cosmological model used. A direct correlation between Doppler velocity and red shift, as Hubble assumed and as outlined above, is valid only for a flat Euclidean space. The red shift that astronomers measure due to the expansion of the universe is known as a cosmological red shift. Most importantly, galaxies are not racing away from each other through space, but rather with space. As a result the entire universe is expanding. An important consequence of this expansion to astronomers is that the wavelengths of radiation from distant galaxies become stretched as a result their propagation through expanding space.
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