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Geology 130F

Lecture One


The age of the universe

The observation that nearly all known galaxies are moving away from us, and that more distant galaxies are moving away more rapidly, suggests that the universe as a whole is expanding. This is the motivation for the big bang theory. If we imagine running the picture backward, all the galaxies will approach each other in a much smaller region of space. Since we know roughly how far away the galaxies are now, and roughly how fast they are moving away from us, we can calculate the time of the big bang; the best current estimates are that it occurred between about 10 and 15 billion years ago.

An independent estimate of the minimum age of the universe can be found by assuming that the universe is older than anything in it. For example, we can estimate the age of the oldest stars that we see. These are found in globular clusters such as M15, which you can see on a clear summer night in the constellation of Hercules. By looking at the temperature and brightness or luminosity of stars in M15, and using theoretical stellar models, we find ages of order 14-15 billion years. This is uncomfortably close to the upper limit of the age of the universe as measured by the expansion time described above.

A third estimate is found by examining the population stellar remnants known as white dwarfs. These are the collapsed cores of stars that have exhausted their nuclear hydrogen fuel. Since they are no longer burning hydrogen into helium, they are cooling off. By measuring the temperatures of white dwarfs, and calculating the rate at which they cool off, astronomers estimate that the oldest white dwarfs in our galaxy are 10 billion years old.

Interesting Ages

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