Determining the Hubble Constant
Obtaining a true value for H is complicated. Two measurements are required.
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.
The value of H itself must be
derived from a sample of galaxies that are far enough away that motions
due to local gravitational influences are negligibly small (these are called
peculiar motion, and they represent deviations from the Hubble Law).
Units for Hubble's Constant
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.
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.
Current Value of the Hubble Constant
The value of the Hubble constant initially obtained by 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 H around 50 km/s/Mpc. The
other team, associated with Gerard DeVaucouleurs of the University of
Texas, has obtained values that indicate H to be around 100 km/s/Mpc.
Reference: Space Science Short
National Aeronautics and
Space Administration
NASA Headquarters
Washington, D.C. (October, 1994)
