and Space Administration
The Hubble Constant (Ho) is one of the most important numbers
in cosmology because it is needed to estimate the size and age of the
universe. This long-sought number indicates the rate at which the universe
is expanding, from the primordial "Big Bang."
The Hubble Constant can
be used to determine the intrinsic brightness and masses of stars in nearby
galaxies, examine those same properties in more distant galaxies and galaxy
clusters, deduce the amount of dark matter present in the universe, obtain
the scale size of faraway galaxy clusters, and serve as a test for theoretical
In 1929, American astronomer Edwin Hubble announced
his discovery that galaxies, from all directions, appeared to be moving
away from us. This phenomenon was observed as a displacement of known
spectral lines towards the red-end of a galaxy's spectrum (when compared
to the same spectral lines from a source on Earth). This redshift appeared
to have a larger displacement for faint, presumably further, galaxies.
Hence, the farther a galaxy, the faster it is receding from Earth.
The Hubble Constant can be stated as a simple mathematical expression, Ho
= v/d, where v is the galaxy's radial outward velocity (in other words,
motion along our line-of-sight), d is the galaxy's distance from earth,
and Ho is the current value of the Hubble Constant.
a true value for Ho is very complicated. Astronomers need two measurements.
First, spectroscopic observations reveal the galaxy's redshift, indicating
its radial velocity. The second measurement, the most difficult value
to determine, is the galaxy's precise distance from earth. Reliable "distance
indicators," such as variable stars and supernovae, must be found in galaxies.
The value of Ho itself must be cautiously derived from a sample of galaxies
that are far enough away that motions due to local gravitational influences
are negligibly small.
The units of the Hubble Constant are "kilometers
per second per megaparsec." In other words, for each megaparsec of distance,
the velocity of a distant object appears to increase by some value. (A
megaparsec is 3.26 million light-years.) For example, if the Hubble Constant
was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift
corresponding to a radial velocity of 500 km/s.
The value of the Hubble
Constant initially obtained by Edwin Hubble was around 500 km/s/Mpc, and
has since been radically revised because initial assumptions about stars
yielded underestimated distances.
For the past three decades, there have
been two major lines of investigation into the Hubble Constant. One team,
associated with Allan Sandage of the Carnegie Institutions, has derived
a value for Ho around 50 km/s/Mpc. The other team, associated with Gerard
DeVaucouleurs of the University of Texas, has obtained values that indicate
Ho to be around 100 km/s/Mpc. A long-term, key program for HST is to refine
the value of the Hubble Constant.