That's why knowing the Hubble constant is so important.

The debate over the Hubble constant has divided astronomers into two main camps.

A similarly large change to the Hubble constant would be needed, if this result should be correct.

His measured value for the Hubble constant came very close to the value range generally accepted today.

Thus, with observed time delays and constrained mass model, the cosmological constant like Hubble constant can be inferred.

Dr. Sandage has long argued that the Hubble constant would be low, around 50, and thus the universe would be closer to 20 billion years old.

Used as a standard candle, these objects have consistently given Dr. Sandage and his collaborators Hubble Constants of about 50.

The Hubble constant is a measure of how fast the Universe is expanding, and the age and the size of the Universe are related to it.

Cepheids are pulsating stars regularly used to determine galactic distances, and 20 such stars were observed last year to obtain the high Hubble constant reported in October.

Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established.

When the results were announced in October, Dr. Freedman and her team cautioned that it was premature to believe that the problem of determining the Hubble constant had been solved.

The Hubble constant is a measure of the rate of expansion of the universe, but astronomers using different ways of estimating this rate have calculated wildly differing values over the years.

The value of the Hubble constant is estimated by measuring the redshift of distant galaxies and then determining the distances to the same galaxies (by some other method than Hubble's law).

Although arguments over high and low Hubble constants and young and old universes have been raging for two decades, they came into sharper focus with the observations by Dr. Freedman's group.

Finding the value of the Hubble constant was the result of decades of work by many astronomers, both in amassing the measurements of galaxy redshifts and in calibrating the steps of the distance ladder.

A way to measure the age of the universe would be discovered by Edwin Hubble in the 1930s, but due to observational constraints, an accurate measurement of the Hubble constant would not be forthcoming until the late 1990s.

Dr. P. J. E. Peebles, an astrophysicist at Princeton University, said that among younger cosmologists examining the issue a value of 80 or 90 for the Hubble constant is more popular than Dr. Sandage's 50.

This new, more precise value of the Hubble constant was used to test and constrain the properties of dark energy, the form of energy that produces a repulsive force in space, which is causing the expansion rate of the universe to accelerate.

In their report, Dr. Freedman's team said the new Cepheid observations, combined with other recent measurements by other techniques, "lead us to conclude that the evidence at this time favors a value" for the Hubble constant of 80 kilometers plus or minus 17.

To outsiders, a difference between 12 billion and 15 billion years might seem cosmically (if not comically) insignificant - indeed the error margins overlapped, if only just barely, somewhere around a Hubble constant of 63 and an age of 13.5 billion years.

The value of the Hubble constant was the topic of a long and rather bitter controversy between Gérard de Vaucouleurs who claimed the value was around 100 and Allan Sandage who claimed the value was near 50.

Dr. George Rhee, an astronomer at New Mexico State University in Las Cruces, reports in an article being published today in the journal Nature that the new measurement produced the relatively low value for the Hubble constant of 50.

Applying this value to measurements made over the years of the apparent peak brightness of six other Type 1A supernovas, Dr. Sandage recalculated their distances from Earth, reckoned in their recessional speeds and concluded that the Hubble constant is 57.

The reasons for this are that converting redshift to distance requires knowledge of the Hubble constant which was not accurately measured until the early 21st century, and that at cosmological distances, the curvature of space-time allows one to come up with multiple definitions for distance.

By combining this number with the galaxy's recession velocity, a much easier number to obtain, Dr. Freedman and her colleagues inferred a value for the Hubble constant of 80 plus or minus 11 miles per second per megaparsec (a megaparsec is 3.26 million light-years).