The following is a brief history of how astronomers have developed ways to measure the Universe's expansion rate.
Harvard astronomer Henrietta Leavitt begins measuring the brightnesses of stars in a class known as Cepheid variables, bright, young stars with masses of perhaps 5 to 20 times that of our own Sun. She measures the distances of stars in the Small Magellanic Cloud, a diffuse-looking nebula (from the Latin word ``fuzzy''), visible in the Southern Hemisphere. Leavitt discovers that these stars reveal their intrinsic brightness by the way their light varies. This makes them reliable milepost markers for measuring astronomical distances.
Albert Einstein develops his General Theory of Relativity in 1917. Applying Einstein's theory to the evolution of the Universe, several theoreticians discover the possibility that the Universe is expanding or contracting. But Einstein dismisses this possibility because there was no evidence that the Universe is in motion. He believed the Universe is static, and proposes the existence of a hypothetical ``repulsive force,'' called the cosmological constant that prevents galaxies from falling together.
Astronomer Edwin Hubble discovers Cepheid variable stars in several nebulae. These nebulae, he concluded, are galaxies far outside our Milky Way Galaxy, and that they were similar in size and structure to our Milky Way.
Astronomer Vesto Slipher makes measurements of the velocities of spiral nebulae, which shows they are all receding from Earth, but he does not realize they are remote galaxies.
In 1929, Hubble made another startling discovery: The more distant the galaxy from Earth, the faster it moves away. Hubble discovered a correlation between the distance of a galaxy and its recession velocity. This relationship is called the Hubble law and the relationship between the distance and velocity is known as the Hubble Constant. Both theories have helped astronomers better understand the evolution of the Universe. Astronomers need an accurate value for the Hubble Constant to estimate the size and age of the Universe.
Hubble's observations lead to the realization that, in a uniformly expanding universe, galaxies would have been closer together in the past. Early in the Universe, the density (and temperature) of matter would have been very high. This leads to a model for the evolution of the Universe, called the Big Bang theory. The theory says that the Universe began in an extremely hot and dense state and has been expanding and cooling ever since then. To test and constrain the Big Bang theory, astronomers work on making solid measurement of the expansion rate (needed to determine the size and age) and check this against an independent estimate based on the ages of the oldest stars in the Universe.
Before calculating an accurate value for the Hubble Constant, astronomers try to fine tune the cosmic distances. In 1952, Carnegie astronomer Walter Baade finds that the distance scale to galaxies is wrong because of an error in the luminosity scales of stars.
Astronomers detect the cosmic microwave radiation left over from the Big Bang, as predicted by theory.
Measurements of the density of light elements (such as hydrogen and helium) in the early universe also provide support of the Big Bang theory.
In the mid-1970s, Carnegie astronomer Allan Sandage discovers that some stars used by Edwin Hubble to estimate distances weren't as bright as once thought.
Though, distances to the nearest galaxies have been measured using Cepheids and other methods, unfortunately, astronomers cannot see Cepheids in distant galaxies. NASA begins construction on Hubble Space Telescope. One of the primary goals is to find Cepheids in more distant galaxies, opening the way to pin down an accurate value for the Hubble Constant.
Carnegie astronomer Wendy Freedman and Caltech astronomer Barry Madore conclude that dust in the spiral galaxies where Cepheids are located, significantly dims and reddens these stars, causing an error in the distance scale.
Astronomers refine ``secondary'' methods for measuring the relative distances among galaxies. Among them are measuring the brightnesses and rotational velocities of entire galaxies and the measurement of another class of younger, more massive supernovae (exploding stars). Relative distances, however, do not alone provide a measure of the Hubble Constant. The situation is like the case of a road map with no scale printed on it. Two cities may be closer to each other than to a third city. Without a scale, no one will know the actual distances between those cities. Similarly, to measure the Hubble Constant, astronomers must know the actual distances to galaxies. Following the road map analogy, if the actual distance between two cities is known, then the actual distances among all other cities are established. Cepheids provide the absolute distance scale for celestial objects.
Using the Hubble Space Telescope, 14 internationally-based astronomers move toward pinning down the Hubble Constant. The astronomers' proposal, called the ``Key Project on the Extragalactic Distance Scale,'' has three goals. The first is to measure Cepheid distances to about 20 galaxies and calibrate five secondary methods for measuring the relative distances to galaxies. The second is to measure Cepheid distances to galaxies in two of the nearest massive clusters of galaxies, Virgo and Fornax. The third is to check for errors in the Cepheid distance scale.