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The Universe's Age

Ken Tapping, August 27, 2008

In the sky this week...

> Look for Mercury and Venus in the evening twilight.

> Jupiter dominates the southern sky overnight.

> The Moon will be New on August 30.

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Ever since we first looked up at the stars we have wondered how our universe began, how old it is, and what chain of events eventually led to the birth of stars and planets, the appearance of life, and the evolution of creatures like us. Thanks to the development of powerful new observing tools over the last few decades, together with a lot of hard work by many people, we have been making dramatic progress in answering these questions. In all cases working towards the answers involves observations, analysis and theory, and as many different approaches as possible. For example, let's look at some methods we have used to estimate the age of the universe. A starting point is to look at the ages of the oldest objects we find in it. The universe cannot be younger than its contents.

Let's begin here on the Earth. Some elements are stable and last indefinitely. Others, which we term radioactive, decay into other elements. A good start is to look for uranium in zircons - semiprecious stones we find in some volcanic rocks. These crystallize as some types of lava solidify. Uranium dissolves nicely in zirconium silicate, which is the chemical making up zircons. Lead, however, does not. So in a zircon that has just solidified we expect to find uranium but not lead. However, over time, uranium slowly decays into lead, and since the zircon is now a solid, that lead is trapped along with the uranium that produced it. We know the rate at which uranium turns into lead, so comparing the relative amounts of uranium and lead we can determine when that zircon solidified. The oldest zircons found so far are about 4 billion years old. The Earth has to be at least that old, so the universe has to be that old too. Actually we need to add a few billion years more to allow time for stars to manufacture the materials making up the Earth from the primordial hydrogen formed at the beginning of the universe.

White dwarfs are the cores of old stars that have run out of fuel. They form when an ageing star sneezes off its outer layers. From more than a century of research into how stars work we know what temperature a white dwarf star is when it forms. If we measure its temperature now, we can calculate how long ago the white dwarf formed by determining how much it has cooled. The oldest white dwarfs are about 12.8 billion years old.

Star clusters are made up of a number of stars born at the same time. However, because some of the siblings have higher masses than others, they shine more brightly and age more quickly. If we measure the brightness of the brightest star in the cluster, we can estimate the age of the cluster. The oldest clusters are between 11 and 13 billion years old.

In the 1920's, Monsignor Georges Lemaître, a senior Jesuit, suggested the newly-discovered expansion of the universe could be explained by the universe having begun as something very hot and dense some billions of years ago, which then started rapidly to expand. Using decades of improving observations, we estimate that the universe began its expansion 13.8 billion years ago, which agrees quite nicely with the results obtained using other approaches. Our universe is very old.


Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca