# How Old Is the Universe?

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

The Hubble team

The answer to this cosmic puzzle was finally solved earlier this year by NASA's Hubble Space Telescope Key Project team, led by Wendy Freedman of Carnegie Observatories. The team, a group of 27 astronomers from 13 different U.S. and international institutions, announced in 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 expanding universe, the Key Project astronomers confidently concluded that the universe is approximately 12 billion years old.

The magic number

The team's precise measurements are the key to learning about the universe's rate of expansion, named the Hubble constant (Ho), after the American astronomer Edwin P. Hubble. He was the first to realize that the galaxies were running away from each other at a proportional distance, i.e., the farther away, the faster the recession. It was in 1929 that Hubble discovered this important linear relationship.

Finding the definitive value of Hubble's constant was one of the major goals for the Space Telescope when it was launched in 1990. The constant is one of the most important numbers in cosmology because it is needed to estimate the size and age of the universe. This long-sought number indicates the rate at which the universe has 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 precise value of Hubble's constant at 70 km/s/Mpc, with an uncertainty of ten percent. This means that a given galaxy appears to be moving 160,000 miles per hour faster for every 3.3 million light-years away from Earth.

Cepheid stars: Tools of measurement

The team used the Hubble Telescope to observe 18 galaxies out to 65 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 provide a very reliable "standard candle" for estimating intergalactic distances. The team used the stars to calibrate many different methods for measuring distance.

Combining Hubble's constant measurement with estimates for the density of the universe, the team determined that the universe is approximately 12 billion years old— similar to the oldest stars. This discovery also cleared up a nagging paradox that arose from previous estimates that the universe appeared to be younger than its oldest stars.

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