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:
Table 2. PARTICULAR DIFFICULTIES FOR COSMOLOGY
AS A SCIENCE
|1. ||Only one Universe.
|2. ||Universe opaque for 56/60 decades since Planck era.
|3. ||Need to extrapolate physics over huge distances.
|4. ||Need to work with what we can currently detect. [But ...]
|5. ||Local background very bright.
|6. ||Distances very hard to determine (standard candles).
|7. ||Observational Selection insidious.
|8. ||Distant galaxies hard to measure and interpret unambiguously.
|9. ||Luminosity Functions unreliable.
|10. ||Geometry, astrophysics and evolution often entangled.
|11. ||Physics of early Universe unknown (and unknowable?)
|12. ||Human time-frame so short compared to cosmic.
|13. ||Origin of inertia.
|14. ||The singularity.
- 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.
- 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).
- 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.
- 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.
- Observational astronomy is all about the contrast between
an object and its background
- 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
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
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
- 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
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.