Starting with a Bang
- Wednesday, August 04, 2010
Even before Friedmann and Lemaître published their ideas, the foundations were being laid for a spectacular experimental demonstration of cosmic expansion. During 1908-1912 Henrietta Leavitt, one of America's first women astronomers, found that within a given stellar ‘cloud', certain stars (called cepheid variables) fluctuated periodically in brightness in a rather special way — those with greater intrinsic luminosity (light output) also had longer periodicities. The relationship was so precise that the periodicity could be used as a measure of the intrinsic luminosity.
Now, for a given intrinsic luminosity, the apparent luminosity or brightness of a star depends on its distance from us, just as a distant car headlight looks dimmer than one close up. So the distance to a cepheid variable star could be measured by comparing its true luminosity, obtained from its periodicity, with its apparent luminosity as recorded on a photographic plate. This, of course, gave only a relative distance — a calibration was needed to find the actual distance. Such a calibration was provided by Ejnar Hertzsprung who used a method called parallax (a form of triangulation applicable only to nearby stars) to measure the distance of several cepheids in our own galaxy. Using this calibration, the distance to any cepheid could be determined, no matter how far away it is. Astronomers had found a tape-measure to the stars.
The next actor in the drama was Edwin Hubble (who gave his name to the modern Hubble Space Telescope). Hubble — who was once tipped as a prospective boxing champion — began working in astronomy in 1919 at the Mount Wilson Observatory in California, where the world's largest and most advanced optical telescope had recently been set up. Back in chapter two I mentioned that the spectral lines (discrete colours) in light from a distant source get shifted in frequency if the source is moving relative to the observer. If the light source is approaching us, we observe a shift towards the blue end of the spectrum (that is, to shorter wavelengths); if it is receding from us the shift is towards the red (to longer wavelengths; the so-called ‘red shift'). Between 1912 and 1922, Vesto Slipher, working at Arizona's Lowell Observatory, had discovered that the light spectra from many distant galaxies were systematically red-shifted, but it was Hubble who in 1929 first realized that the red shift of some cepheid variable stars was directly related to their distance from us. Not only were the galaxies containing these stars moving away from earth (like a starburst, equally in all directions) but the more distant galaxies were receding faster. In fact, the relationship was linear, the recession speed being proportional to distance. Astronomers found themselves in an expanding universe and modern cosmology was born!
The final piece in the jigsaw puzzle was provided in 1963, when Arno Penzias (cited earlier) and Robert Wilson working on a satellite designed to measure microwave radiation, found that microwaves were beaming in on us from space, equally from all directions — the very thing that Gamow, Herman and Alpher had predicted in their theory of the big bang. (Their results were published in The Astrophysical Journal, vol. 142 (1965), p. 1149. Dubbed ‘the cosmic microwave background' it is believed to represent the cooled-down radiation left over from an earlier ultra-hot universe and it convinced most astronomers that the big bang theory was correct. Penzias and Wilson were awarded the 1978 Nobel Prize in Physics for their discovery. By the way, don't worry. These microwaves are far too weak to zap you.
Evaluating the Big Bang
From Einstein in 1915 to Penzias and Wilson in 1963 is less than fifty years, but this period saw a revolution in man's perception of the cosmos. Scientists were at last convinced that the universe had a beginning, just as the book of Genesis had always said. But ever since this unavoidable conclusion was reached, there have been tireless efforts to avoid it! I examine some of these evasive strategies in chapter eight of Who Made God?
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