In response to an enquiry, Davis Martin
posted:
There must be many members of the list who are as, or more, capable than I at providing an answer to your question since I'm not a cosmologist (although the subject fascinates me). Much of this information comes from a colleague at UOW who is, (although, of course, any errors will be his:-)
Almost the only information we have about the universe comes from light, remembering that physicists use this term for electromagnetic radiation across the whole spectrum, from radio to gamma, and not just visible light.
Light behaves somewhat like sound in that it exhibits the Doppler effect. Light from an approaching (receding) source is shifted to higher (lower) frequency. In the case of light these effects are called the blue (red) shift respectively. These are somewhat misleading terms since the detected light is only rarely either blue or red but may, for example, be gamma rays red shifted to x-ray energies, or radio waves blue shifted into the microwave spectrum (or, indeed, infrared *blue shifted* into the red :-)
Anyway, spectral analysis of the sharp lines found throughout the spectra of many luminous objects can be used to determine the objects approach or recession speed (police radar works like this). Further, if the luminous object is rotating, its rotational speed can be found from the *width* of the spectral line since some parts are approaching, some not moving and some receding so that a narrow line is broadened out. In the case of rotating galaxies there is a relation between rotational speed and mass, hence intrinsic brightness (there are more stars in a more massive galaxy), which is called the Tully-Fisher relation.
Finally, if you know an objects intrinsic brightness and you measure its apparent brightness, then you can determine how far away it is. Such objects are so-called "standard candles" or "standard yardsticks".
So much for the physics, now what can we do with it?
The standard candle for "nearby" galaxies is a rare type of star called a cepheid variable. The brightness of these stars varies periodically in a way which is related to the intrinsic brightness, so measuring the period of variation gives the intrinsic brightness, and hence the distance. These measurements have been carried out on galaxies up to distances of about 50 million light years (the most recent measurement, of which I'm aware, was on the galaxy M100 which was determined by the Hubble telescope, appropriately, to be 56 million light years away). Even with the HST, detection of individual stars becomes difficult and the standard candle used is the galaxy itself, whose intrinsic brightness can be determined from the line width (see Tully-Fisher relation above). This has extended, or is in the process of extending, distance measurements out to the most distant galaxies known.
Spectral analysis of the line shifts shows that, with very few exceptions, nearly all the galaxies are receding from us, and hence from each other since there's nothing special about our galaxy in the scheme of things. The further away, the faster they're going. This relation was first discovered by Edwin Hubble (in around 1930?) for more nearby galaxies and the linear relation between distance and recessional speed (Hubble's law) has been confirmed by subsequent measurements.
If galaxies are receding now, then at any time in the past they were closer together and it's relatively (!) easy to calculate how long ago they were all in the same spot (although not in the form of galaxies of course, the temperature would have been so high as to give a soup of fundamental particles). This event, assuming it happened, is called the "big bang" and it is calculated to have occurred about 15 billion years ago.
I say "assuming" because there may have been other factors operating in the earlier universe. The universe may just be gently oscillating and we are in one of its expansion phases, for example.
However, there are other, independent, pieces of evidence for the big bang which make its occurrence as certain as any theory in science. The strongest of these are the relative abundances of hydrogen and helium, which correspond to the cooling and expansion process from a hot particle soup, and the microwave background which is the (still) cooling remnant of the primordial flash.
I hope (after almost writing a textbook) that this is what was required.
Regards,
David.
A. David Martin
