3. THE SPECIFIC DIFFICULTIES OF COSMOLOGY

Table 2 lists some of the special difficulties which cosmology has to face as a science. They are mostly obvious but it is worth emphasising one or two:

1. There is only one Universe! At a stroke this removes from our armoury all the statistical tools that have proved indispensable for understanding most of astronomy.

2. The Universe has been opaque to electromagnetic radiation for all but 4 of the 60 decades of time which stretch between the Plank era (10^-43 sec) and today (10¹⁷ sec). Since as much interesting physics could have occurred in each logarithmic decade, it seems foolhardy to claim that we will ever know much about the origin of the cosmos, which is lost too far back in the logarithmic mists of Time. Even the Large Hadron Collider will probe the microphysics back only as far as 10^-10 secs). [2].

3. Cosmology requires us to extrapolate what physics we know over huge ranges in space and time, where such extrapolations have rarely, if ever, worked in physics before. Take gravitation for instance.. When we extrapolate the Inverse Square Law. ( - dress it up how you will as G.R.) from the solar system where it was established, out to galaxies and clusters of galaxies, it simply never works. We cover up this scandal by professing to believe in ``Dark Matter'' - for which as much independent evidence exists as for the Emperor's New Clothes.

4. Objects at cosmologically interesting distance are exceedingly faint, small and heavily affected by factors such as redshift-dimming and k-corrections, so it will obviously be very difficult, if not impossible, to extract clear information about geometry, or evolution, or astrophysics - all of which are tangled up together.

5. Observational astronomy is all about the contrast between an object and its background [3] - both the background of the local Universe and the background noise in our instruments, which are never perfect. Almost all the galaxies we know of are just marginally brighter than the terrestrial sky - either extraordinary good fortune, or more likely a signal that far more are hidden beneath it [4, 5, 6]. In other words we are in this, as in all other facets of observational astronomy, hapless victims of ``Observational Selection'' - an area in which George Ellis has done some brilliant work [7]. The sky isn't dark. Even at the darkest site of Earth the unaided eye can pick up 50,000 photons a second coming from an area of ``dark sky'' no larger than the full moon. Bigger telescopes are all very well - but they pick up more unwanted foreground light, as well as background signal. When you think that the galaxies at a redshift z of 2 should be dimmer by (1 + z)⁴ ~ 100, and by another large but uncertain factor for the k-correction [i.e. band-pass shifting], it is more than a wonder to me that we can see anything of them at all. Ordinary galaxies at that redshift should be hundreds of times dimmer per unit area than our sky! It is also sobering to realise that only one per cent of the light in the night sky comes from beyond our Galaxy.

6. The tragedy of astronomy is that most information lies in spectra, and yet you need to collect between 100 and 1000 times more radiation to get a spectrum than to see an image. Thus most of the faint galaxies which may have cosmological stories to tell must remain, in spectroscopic terms, tantalisingly out of earshot. If history is anything to go by little good will come of the thousands of nights of big-telescope time now being lavished on the intriguing objects first seen with the Space Telescope, and made famous through the Hubble Deep Field. We will probably learn more cosmology from studying the surprising and diverse histories of star-formation that Hubble is finding among galaxies in the Local Group [8].

   In summary we have very few observations, most of them were accidently made, and all are subject to observational selection. It is therefore outrageous to claim a comparison with all the carefully controlled experiments made by particle physicists. And even if we do get a perfect map of the Cosmic Background Radiation it will only be a map of a moment in time. Celestial mechanics is very precise - but it doesn't tell us how the solar system was formed.