How old is the universe?

Posted by David Eicher
on Wednesday, April 6, 2011

Welcome to “Dave’s universe,” the new blog by Dave Eicher, editor of Astronomy magazine, and astronomy and science popularizer. I’ll be bringing you new thoughts about astronomy, cosmology, nature, the hobby of astronomy, the sometimes disturbingly pseudoscientific culture we live in, and other miscellany. I hope you’ll enjoy it!

The universe originated about 13.7 billion years ago, making such art forms as the beautiful spiral galaxy NGC 5584 possible. Credit: NASA/STSci
Over the past century, astronomers have deduced several ways to estimate the age of the universe. By using each of these methods, they can draw a bead on a region of time that probably represents the universe’s age. The answer is becoming more accurate than it was 20 or 30 years ago, but it may never be certain. The age of the universe depends on the weight you assign to the method used to determine it.

The most modern approach relies on using an almost magical number in astronomy called the “Hubble constant.” This number describes how fast the cosmos is expanding — how fast galaxies are spreading apart from each other. To find the Hubble constant, astronomers observe very distant galaxies and measure how far away they are and how fast they recede from Earth. Scientists get the distances from measuring the brightness of known objects like variable stars of a particular type. They then determine the Hubble constant by dividing the galaxy’s speed of recession by its distance. Once they decide on a value for the Hubble constant, they can determine the age of the universe by using a simple mathematical equation. The most precise figure often given with this method is 13.7 billion years, the result of highly precise work with the WMAP satellite.

Although the WMAP result is considered quite accurate, other, older approaches attempted to measure the ages of the oldest objects in the universe. Astronomers can measure the decay of radioactive elements, so they can precisely measure the ages of the oldest rocks on Earth at 3.8 billion years and the oldest meteorites at 4.6 billion years, dating the solar system nicely. Applying this technique to old stars points to an age of the universe of 12 to 15 billion years, plus or minus 3 to 4 billion years. One study that measured uranium decay in ancient stars found the universe to be 12.5 billion years old.

Measuring the ages of ancient star clusters offers another avenue. By looking at the brightest stars in a globular cluster, astronomers can measure an upper limit for the age of the cluster. Recent studies of many globulars suggest an age of about 12 billion years for many of their oldest stars.

Astronomers can also measure the ages of white dwarf stars, the shrunken stellar remnants that are as heavy as the Sun but only as large as Earth. By finding the faintest, oldest white dwarfs, astronomers can pinpoint how long the objects have been cooling off. This yields about 10 billion years for the age of the Milky Way’s disk. The authors of that study suggest our galaxy’s disk formed about 2 billion years after the Big Bang, yielding an age of the universe of about 12 billion years.

Astronomers will continue to refine these techniques, and the window of uncertainty will continue to close, providing a pretty accurate estimate of the universe’s age. However, given the precision of the WMAP data, most astronomers currently cite 13.7 billion years as the most accurate estimate.

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