992. Measuring Time

By C. Claiborne Ray, The New York Times,  January 21, 2013

Modern measurement of time builds on the second, based on atomic oscillation, rather than on the variable time it takes the Earth to orbit the Sun.

As the National Institute of Standards and Technology explains, a second has been defined since 1967 as the time it takes for a cesium atom to oscillate 9,192,631,770 times between two radiation states.

An alternative unit called the cosmic year has been suggested, based on the Sun’s orbit around the center of the Milky Way galaxy about once every 225 million years.

But this “year” is not routinely used by cosmologists, said Andrew J. S. Hamilton, a professor of astrophysics at the University of Colorado.

“The age of the universe is not something that is directly measured by astronomers,” Dr. Hamilton said. “What they measure is the rate of expansion of the universe, as measured by the so-called Hubble parameter” — named for the 20th-century astronomer Edwin Hubble, who was one of the earliest to calculate the expansion rate by noticing that the more distant galaxies are receding at faster speeds.

A value called the reciprocal of the Hubble parameter is approximately equal to the age of the universe, or 13.7 billion years, he added.

“It is not exactly equal,” he added, “because the universe has been decelerating and accelerating under the influence of gravity.