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.
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