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Q. How old is the Universe? — Ashley B.
A. Many astronomers say the Universe is 13.7 billion years old, plus or minus 10 percent.

How do they know? There are four approaches to calculating the age.

1. One method of judging the age of the Universe involves its expansion. Astronomers see the Universe expanding. Galaxies around our Milky Way galaxy are moving away at a significant speed. That is one of the reasons cosmologists believe the Universe began in a Big Bang and has been expanding ever since. Using powerful instruments like the Hubble Space Telescope, astronomers have been able to look at objects dating back most of the way to the Big Bang. This has allowed them to estimate the rate that the Universe has been expanding in the past. Projecting the data backwards, they calculated the Universe shrank to a single point somewhere between 13.5 billion and 14 billion years ago.

2. A second method of measuring the age of the Universe involves the age of white dwarf stars. A white dwarf is a small dense star, about the size of Earth, that has undergone gravitational collapse and is at the final stage of its evolution. Initially all stars are powered by hydrogen fusion but, after they run out of fuel, white dwarfs keep shining because they are hot. However, hot things cool off. White dwarfs gradually cool at a rate astronomers have calculated. The oldest white dwarfs have ages that range from 13.0-13.5 billion years.

3. A third method of assessing the age of the Universe is the study of star clusters. As with white dwarfs, astronomers have found that the oldest star clusters are 13.0-13.5 billion years old.

4. A fourth method of finding the age of the Universe looks at cosmic microwave background radiation. CMB radiation is a faint microwave electromagnetic signal that comes from all points in the sky. Scientists explain it as fossil radiation left over from an early stage in the development of the Universe. They consider CMB to be strong evidence of the Big Bang. They say that during the first 300,000 years of the Universe, it was filled with a foggy plasma. As the Universe expanded and cooled, hydrogen atoms began to form into protons and electrons. As a result, space became increasingly transparent. CMB seen today is light that was released when most of the hydrogen atoms formed and that has been traveling through space for billions of years. By analyzing variations in the intensity of the CMB radiation, astronomers calculate the age of the Universe at 13.72 billion years.

Looking back in time. The deepest image of the Universe recorded by the Hubble Space Telescope shows the faintest objects. Because they are the most distant objects, the image is the equivalent of using a time machine to view the formation of the oldest galaxies. They may have formed fewer than one billion years after the Universe's birth in what cosmologists call the Big Bang.

Source: How Old Is the Universe? by Vanderbilt University astronomy professor David A. Weintraub, published 2010 by Princeton University Press, ISBN: 9780691147314

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