Edwin Hubble’s 1929 discovery, now known as the Hubble Law, tells us that all distant galaxies are retreating from us with a velocity that is directly proportional to their distances from us. In other words, if one galaxy is twice as far from our Milky Way as another, we will find that the galaxy that is twice as far is moving away from us twice as fast. Robert Jastrow (founder of NASA’s Goddard Institute and now head of the Mount Wilson observatory, where Hubble made all his early discoveries) writes: "The Hubble Law is one of the great discoveries in science: it is one of the main supports of the scientific story of Genesis." And Jastrow, we should bear in mind, is a self-proclaimed agnostic (according to his writings as well as his recent interviews with me).

Cosmologists express the precise velocity/distance relationship with a number called the Hubble constant (usually written H 0 ). The value of H 0 is critical because, if we could ascertain this number, we could determine the size and age of the universe. Efforts to establish the Hubble constant have not resulted in universal agreement, and thus cosmologists have been assigning the universe a broad age range between 8 and 20 billion years (meaning that the visible universe spans between 16 and 40 billion light-years across).

Astronomers express the Hubble constant in terms of kilometers per second (velocity) per megaparsec (distance). A parsec is the distance of an object from earth when the object varies one second of an arc when viewed on opposite sides of the earth’s orbit (that is, viewing the object at times six months apart). The parsec (meaning parallax of one second) equals about 3.26 light-years; a megaparsec is a million times this amount. Recent calculations for the Hubble constant range between about 50 and 90 kilometers per second per megaparsec. A Hubble constant at the low end of this range results in an older and larger universe than a Hubble constant at the high end, because slower moving galaxies would take more time to reach their present distances.

How do astronomers calculate the Hubble constant? All distance calculations start with the Cepheid variable stars, the same reference Edwin Hubble used and still the most reliable one. Cepheids are observable up to about 30 million light-years away. Today, other "standard candles" are also employed, each with its own method for determining a light source’s absolute magnitude (that is, its actual brightness, no matter how faint it may appear because of distance or intervening dust).

These standard candles include RR Lyrae stars (old yellow variable stars, observable to about 10 million light-years), planetary nebulae (rings of gas thrown off of dying stars, observable to about 75 million light-years), and spiral galaxies (which can use the Tully-Fisher method up to about 100 million light-years). To apply the Tully-Fisher method to a spiral galaxy, astronomers first make radio observations to determine the galaxy’s rate of rotation. Knowing that a faster rotation rate means that the galaxy has more mass (according to Newton’s laws), astronomers can calculate the galaxy’s absolute brightness. As in all methods, the absolute brightness is then compared to its apparent brightness in order to calculate its distance.

The close agreement between each of these methods have helped to confirm the others. Each yield a Hubble constant that is at the high end of the range, indicating a universe that is relatively young and small. However, astronomers have another standard candle that is not in agreement with the others: the type Ia supernovas. These supernovas briefly shine as brightly as their host galaxy and are observable up to 100 million light-years away. Astronomers like Allan Sandage (considered Hubble’s heir) feel they understand this particular type of supernova very well, having amassed considerable observational data that make the type easily identifiable by its spectrum, light curve, and peak absolute magnitude.

Cosmologists are thus put in the position of having to choose sides. Those who trust the Ia supernovas for their calculations believe the universe is 15 to 20 billion years old. Those who trust the other standard candles believe the universe is closer to 8 to 12 billion years old. Very recently, however, discoveries of the variations in Type Ia supernovas have tended to bring this standard candle almost in line with the others, meaning that the universe may be on the younger side of our range. Today the Hubble Space Telescope is trained on Cepheid variable stars in the Virgo Cluster galaxies in order to resolve the dispute. New observations of Cepheids, which are needed to better calibrate all the other methods, may help settle the issue in the very near future.