Hubble constant

(astrophysics) The rate at which the velocity of recession of the galaxies increases with distance; the value is about 70 kilometers per second per megaparsec (or 2.3 × 10^-18 s^-1) with a relative uncertainty of about ± 10%.

A number that characterizes the expansion rate of the universe and is required to determine its age. In the standard big bang model, the universe expands uniformly according to the Hubble law, v = H₀d, where v is the velocity of a galaxy at a distance d, and H₀ is the Hubble constant. The wavelength of radiation is stretched due to the expansion of space so that the spectra of objects become progressively redder at greater distances. (For nearby objects, the observed redshift can be described as a Doppler effect.) The constant is named after Edwin P. Hubble, who discovered that the velocity of recession of a galaxy is proportional to its distance. A reliable and accurate measurement of the Hubble constant, an independent estimate of the ages of the oldest objects in the universe, and a further measurement of the average density in the universe are all separately required in order to test and ultimately provide strong constraints on cosmological models. See also Doppler effect; Redshift.

Measurement of the Hubble constant is extraordinarily difficult in practice. First, measuring distances has turned out to be immensely challenging. Second, while the velocities can be measured very simply and accurately (from measurements of the positions of spectral lines in galaxies), they are perturbed by the gravitational interactions of galaxies with the neighbors (inducing what are referred to as peculiar motions superimposed on the general expansion). Hence, an extragalactic distance scale must be established at distances great enough that peculiar motions of galaxies are small compared to the overall cosmic expansion velocity, the Hubble flow. See also Astronomical spectroscopy.

In general, the basis for estimating distances in astronomy is the inverse-square radiation law. If objects can be identified whose luminosities are either constant (standard candles), or perhaps related to a quantity that is independent of distance (for example, period of oscillation, rotation rate, velocity dispersion, or color), then, given an absolute calibration, their distances can be gauged.

Primary among the distance indicators are the Cepheid variables, stars whose outer atmospheres pulsate regularly with periods ranging from 2 to about 100 days. Empirically it has been established that the period of pulsation (a quantity independent of distance) is very well correlated with the intrinsic luminosity of the star. High resolution is the key to discovering Cepheids in other galaxies. The resolution of the Hubble Space Telescope is about 10 times better than can be generally obtained through the Earth's atmosphere, and, moreover, it is stable. The reach of Cepheid variables as distance indicators is limited, however, even with the Hubble Space Telescope. For distances beyond 20 megaparsecs or so (1 Mpc = 3.08 × 10²² m = 3.26 × 10⁶ light-years), brighter objects than ordinary stars are required, for example, luminous supernovae or the luminosities of entire galaxies. The Cepheid distances from the Hubble Space Telescope Key Project have provided a means of calibrating and comparing a number of relative distance methods, and, to within an uncertainty of ±10%, all of these methods are consistent with a value of the Hubble constant of about 72 kilometers per second per megaparsec.

The Hubble constant sets the expansion time scale for the universe. In order to measure the time since the big bang, it is necessary to determine the expansion rate, and the average matter-plus-energy density of the universe. Increasing evidence suggests that the total matter density of the universe is about 20–30% of the critical density. (Below a critical density the universe will expand forever, whereas above the critical density the universe will recollapse.) The theory of inflation provides a strong theoretical motivation for a critical-density universe; however, if the mass density is only 20–30% of critical, as observations suggest, then inflation would require a missing energy component to bring the total energy density up to the critical density. Measurements of type Ia supernovae provide evidence for such a missing energy component, called the cosmological constant. See also Big bang theory; Cosmology; Inflationary universe cosmology; Universe.

astronomy. Symbol H. The ratio of receding speed to distance (usually as kilometres per second per megaparsec). Deriving from the phenomenon that extragalactic objects in general are receding from an observer on Earth at a speed linearly proportional to their distance away, the value is currently estimated, after various allowances for observational factors, at 71 ± 7 km·s^-1·Mpc^-1 (though originally valued much larger, and currently estimated by some at only 60 km·s^-1·Mpc^-1). See Hubble length.