Scientific notes: Universal problem solved with triangles
Tuesday 16 March 1999
I admit that the breakthrough would not have been possible without access to a pretty sophisticated "theodolite" - the Hubble Space Telescope (HST), orbiting above the Earth's atmosphere. But the way we made the measurement is schoolroom science.
We know the Universe is expanding because of the famous redshift in the light from distant galaxies, which tells us they are moving apart. Clearly, in that case there was a time, long ago, when they were all piled on top of each other - the Big Bang, in which the Universe was born. But, in order to find out when that happened, we need to know the distances to galaxies, as well as their speeds. Then, working out how long it has been since the Big Bang is as simple as working out how long it takes to get from one motorway junction to another in a car travelling at 50mph if the two junctions are 50 miles apart. Since redshifts are known, distances are the key to the age of the Universe. But how do you measure distances to galaxies?
The galaxies we are interested in are disc-shaped star systems like our Milky Way, typically about 100,000 light years across (so it takes light 100,000 years to get from one side to the other) and each containing several hundred billion stars like the Sun. Even the nearest of these disc galaxies are millions of light years away, and it takes the power of the HST to pick out even the brighter individual stars within those galaxies.
But, with the aid of the HST, comparing the brightness of individual stars in nearby galaxies with essentially identical stars in our Milky Way, it was possible, by the mid-1990s, to measure accurate distances to those nearby galaxies. This answered a question that had nagged astronomers for decades. Were the other disc galaxies big objects like the Milky Way, and relatively far away, or smaller islands in space, relatively nearby? It turns out that they are big and distant, and that the Milky Way is almost exactly average in size.
But distances to the nearest galaxies are not enough to tell us how the Universe as a whole is expanding. Happily, though, there are thousands of disc galaxies, all with known redshifts, so far away that even the HST could not pick out individual stars in them, but for which telescopes on Earth (and the HST) can measure their angular size (the apparent width of the disc) on the sky. Because (thanks to the HST) we now know the range of sizes of disc galaxies, Simon Goodwin, Martin Hendry and I were able to calculate distances to these more distant objects, representative of the Universe at large, from how small they look on the sky.
I am always reminded of the classic episode of the television comedy Father Ted where Ted tries to explain to Dougal the difference in size between a toy model of a cow in his hand, and a live cow on the far side of a field. Big things look small when far away. It's just a matter of perspective - and if we know how big galaxies really are, we know how far away they are from the angle each one subtends on the sky.
And that - really - is all there is to measuring the age of the Universe. The answer, if you really want to know, comes out as between 13 and 16 billion years (three to four times the age of the Earth), which, happily, is comfortably older than the ages of the oldest stars. And it's all done with triangles.
John Gribbin is the author of `The Birth of Time' (Weidenfeld & Nicolson, pounds 20)
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