How Old Is the Universe?

Before the advent of the Hubble Space Telescope, astronomers couldn'tdecide if the universe was 10 billion or 20 billion years old. The scale sizeof the universe had a range so vast that it didn't allow astronomers to confrontwith any certainty manyof the most basic questions about the origin and the eventual fate of theuniverse.

The Hubble team

The answer to this cosmic puzzle was finally solved earlier this year by NASA'sHubble Space Telescope Key Project team, led by Wendy Freedman of CarnegieObservatories. The team, a groupof 27 astronomers from 13 different U.S. and international institutions, announcedin May that it had determined the age of the universe with precise accuracy.After an eight-year effort to measure the far-flung galaxies of the expandinguniverse, the Key Projectastronomers confidently concluded that the universe is approximately 12 billionyears old.

The magic number

The team's precise measurements are the key to learning about theuniverse's rate of expansion, named the Hubble constant (H_o),after the American astronomer Edwin P. Hubble. He was the first to realizethat the galaxies were running away fromeach other at a proportional distance, i.e., the farther away, the fasterthe recession. It was in 1929 that Hubble discovered this important linearrelationship.

Finding the definitive value of Hubble's constant was one of themajor goals for the Space Telescope when it was launched in 1990. The constantis one of the most important numbers in cosmology because it is needed toestimate the size and age of theuniverse. This long-sought number indicates the rate at which the universehas been expanding since the primordial "Big Bang."

1 Megaparsec = 3.26 mil. light-years

The units of the Hubble constant are "kilometers per second,per Megaparsec." A Megaparsec (Mpc) is 3.26 million light-years. The Key Project team's measurement now gives the precisevalue of Hubble's constant at70 km/s/Mpc, with an uncertainty of ten percent. This means that a given galaxyappears to be moving 160,000 miles per hour faster for every 3.3 million light-yearsaway from Earth.

Cepheid stars: Tools of measurement

The team used the Hubble Telescope to observe 18 galaxies out to65 million light-years. They discovered almost 800 Cepheid variable stars,a special class of pulsating stars used for accurate distant measurement.Although Cepheids are rare, they providea very reliable "standard candle" for estimating intergalacticdistances. The team used the stars to calibrate many different methods formeasuring distance.

Combining Hubble's constant measurement with estimatesfor the density of the universe, the teamdetermined that the universe is approximately 12 billion years old—similarto the oldest stars. This discovery also cleared up a nagging paradox thatarose from previous estimates that the universe appeared to be younger thanits oldest stars.

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