Ken Tapping, December 02, 2009

Observations of the expansion of the universe suggest that the Beginning was about 14 billion years ago. The age of the universe is a rather special number, and we would certainly like some ways to check it. The expansion of the universe is a pretty exotic test. However, there are methods we can use that employ basic, well-established laboratory physics.

Let's start with radioactive dating. Radioactive elements are unusual in that their atoms decay into other elements at a precisely fixed rate. When a molten rock containing radioactive elements solidifies, they are frozen in place, along with the elements they produce when they decay. All we need to do is look at the amounts of radioactive elements and decay products and we can calculate how long it has been since that rock solidified.

Most rocks on Earth are quite young, because they are being continuously recycled by plate motions and subduction. However, some rocks seem to have escaped this process. The confirmed oldest rock on Earth found so far is the Acasta Gneiss, in Northwest Canada, which solidified 4.031 billion years ago. It is early days yet, but its record may have been broken by the Narryer Gneiss in Western Australia, which might be as much as 4.4 billion years old!

The Apollo expeditions brought back 382 kg of lunar rocks back to Earth. These proved to be between 2.5 billion and 3.2 billion years. In general the lunar rocks are older than most Earth rocks because any subduction on the Moon must have ended long ago.

Our galaxy, the Milky Way, is difficult to age precisely. However, we can assume that the Milky Way has to be at least as old as the oldest star in it, so we simply search for the oldest stars we can find. Stars that are still actively producing energy are either hard to age precisely or are much younger than the universe. However, white dwarf stars are a different issue altogether.

Stars like our Sun end their lives by sneezing off their outer layers, leaving a bare core, about the size of the Earth, devoid of fuel, and very hot. Calculating the rate at which such a core – a white dwarf star – cools off by radiating its heat into cold space is undergraduate physics, and is well understood. The Hubble Space Telescope and other major telescopes have been used for measuring the temperatures of white dwarfs. Some 13.2 billion years old have been found. None of them is cooler than a few thousand degrees. Apparently our galaxy and probably the universe are not yet old enough for any white dwarfs to have completely cooled off.

These days, scientific arguments over the precise ages of the Earth, Solar System, Milky Way and universe are now confined to possible errors of a few percent. Unless someone comes up with something really weird, our errors should not be worse than that. However, scientists being what they are, they are making great efforts to find that weird something. That's what science is all about.