Age of the Universe

The idea that the universe had a beginning is common to various religions and mythologies. However, astronomical evidence that the universe truly has a finite age did not appear until early in the twentieth century. The first clue that the universe has a finite age came at the end of World War I, when astronomer Vesto Slipher noted that a mysterious class of objects, collectively called spiral nebula, were all receding from Earth. He discovered that their light was stretched or reddened by their apparent motion away from Earth—the same way an ambulance siren's pitch drops when it speeds away from a stationary observer.

Hubble's Contribution

In the early 1920s American astronomer Edwin P. Hubble was able to measure the distances to these receding objects by using a special class of mile-post marker stars called Cepheid variables. Hubble realized that these spiral nebulae were so far away they were actually galaxies—separate cities of stars—far beyond our own Milky Way.

By 1929, Hubble had made the momentous discovery that the farther away a galaxy is, the faster it is receding from Earth. This led him to conclude that galaxies are apparently moving away because space itself is expanding uniformly in all directions. Hubble reasoned that the galaxies must inevitably have been closer to each other in the distant past. Indeed, at some point they all must have occupied the same space. This idea led theoreticians to conceive of the notion of the Big Bang, the theory that the universe ballooned from an initially hot and dense state.

Hubble realized that if he could measure the universe's speed of expansion, he could easily calculate the universe's true age. Assuming the universe's expansion rate has not changed much over time, he calculated an age of about 2 billion years. One problem with this estimate, however, was that it was younger than geologists' best estimate for the age of Earth at the time.

Astronomers since then have sought to refine the expansion rate—and the estimate for the universe's age—by more precisely measuring distances to galaxies. Based on uncertainties over the true distances of galaxies, estimates for the universe's age have varied from 10 billion to 20 billion years old.

More Recent Estimates

A primary task of the Hubble Space Telescope (HST), launched in 1990, was to break this impasse by observing Cepheid variable stars in galaxies much farther away than can be seen from ground-based telescopes. The HST allowed astronomers to measure precisely the universe's expansion rate and calculate an age of approximately 11 to 12 billion years.

Estimating the age is now complicated, however, by recent observations that show the universe expanded at a slower rate in the past. This is due to some mysterious repulsive force, first envisioned by physicist Albert Einstein as part of his so-called fudge factor in keeping the universe balanced. The presence of such a repulsive force pushing galaxies apart means that the universe is more likely to be 13 to 15 billion years old.

Using Stars to Estimate Age.

The universe's age can also be estimated independently by observing the oldest stars. Astronomers know that stars must have started forming quickly after the universe expanded and cooled enough for gas to coagulate into stars. So the oldest star must be close to the true age of the universe itself. The oldest stars, which lie inside globular clusters that orbit our galaxy, are estimated to be at least 12 billion years old. These estimates are difficult because they rely on complex models and calculations about how a star burns its nuclear fuel and ages.

A simpler cosmic clock is a class of star called white dwarfs, which are the burned-out remnants of Sun-like stars. Like dying cinders, it takes a long time for dwarfs to cool to absolute zero—longer than the present age of the universe itself. So the coolest, dimmest dwarfs represent the remnants of the oldest stars. Because they are so dim, these dwarfs are hard to find. Astronomers are using the HST to pinpoint the very oldest white dwarfs in globular clusters.

The HST has uncovered the very faintest and coolest dwarfs in the Milky Way galaxy, with ages of 12.6 billion years, thus giving an age estimate for the universe of 13 to 14 billion years. This is a very successful and entirely independent confirmation of previous age estimates of the universe.

Astronomers now know the age of the universe to within a good degree of accuracy. This is quite an achievement considering that less than a century ago, astronomers did not even realize the universe had a beginning.

SEE ALSO COSMOLOGY (VOLUME 2); HUBBLE, EDWIN P. (VOLUME 2); STARS (VOLUME 2).

Ray Villard

Bibliography

Guth, Alan H., and Alan P. Lightman. The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Reading, MA: Addison-Wesley Publishing, 1998.

Hogan, Craig J., and Martin Rees. The Little Book of the Big Bang: A Cosmic Primer. New York: Copernicus Books, 1998.

Livio, Mario, and Allan Sandage. The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos. New York: John Wiley & Sons, 2000.