Hubble Constant

In the standard Big Bang model, the universe expands according to the Hubble law, a simple relation expressed as v=H_od, where v is the velocity of a galaxy at a distance d, and H_o is the Hubble constant. The Hubble constant characterizes both the scale and age of the universe. A measurement of the Hubble constant, together with the ages of the oldest objects in the universe, and the average density of the universe, are all separately required to describe the universe's evolution. Measuring an accurate value of H_o was one of the motivating reasons for building the Hubble Space Telescope (HST).

The measurement of most distances in astronomy cannot be done directly because the size scales are simply too big. In general, the basis for estimating distances in astronomy is the inverse square radiation law, which states that the brightness of an object falls off in proportion to the square of its distance from us. (We all experience this effect in our own lives. A street light in the distance appears fainter than the one beside us.) Astronomers identify objects that exhibit a constant brightness (so-called "standard candles"), or those where the brightness is perhaps related to a quantitythat is independent of distance (for example, period of oscillation, rotation rate, or color). The standard candles must then be independently calibrated (to absolute physical units) so that true distances (in meters or megaparsecs, where 1 megaparsec = 3.08 × 10²² meters) can be determined using the inverse square law.

The most precise method for measuring distances is based on the observations of Cepheid variables, stars whose atmospheres pulsate regularly for periods ranging from 2 to about 100 days. Experimentally it has been established that the period of pulsation is correlated with the brightness of the star. High resolution is the key to discovering Cepheids in other galaxies—in other words, the telescope must have enough resolving power to distinguish Cepheids from other stars that contribute to the overall light of the galaxy. The resolution of the Hubble Space Telescope is about ten times better than can be generally obtained through Earth's turbulent atmosphere.

The reach of Cepheid variables as distance indicators is limited, however, even with the HST. For distances beyond 20 megaparsecs or so, brighter objects than ordinary stars are required; for example, bright supernovae or the brightnesses of entire galaxies. The absolute calibration for all of these methods is presently established using the Cepheid distance scale. A Key Project of the HST has provided Cepheid distances for a sample of galaxies useful for setting the absolute distance scale using these and other methods.

Until recently, a controversy has existed about the value of the Hubble constant, with published distances disagreeing by a factor of two. However the new Cepheid distances from the HST have provided a means of calibrating several distance methods. For the first time, to within an uncertainty of 10 percent, all of these methods are consistent with a value of the Hubble constant in the range of about 60 to 70 kilometers (37.28 to 43.5 miles) per second per megaparsec. This implies an age of the universe of between 13,000 and 15,000 million years.

Hubble, Edwin P. (Volume 2);; Hubble Space Telescope (Volume 2);; Age of the Universe (Volume 2).

Barrow, John. D. The Origin of the Universe. New York: Basic Books, 1994.

Ferguson, Kitty. Measuring the Universe. New York: Walker & Co., 1999.

Freedman, Wendy L. "The Expansion Rate of the Universe." Scientific American 1(1988):92-97.

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