Traditionally, cosmology was the quest for a few numbers. The first were H, q, and . Since 1965 we've had another: the baryon/photon ratio. This is believed to result from a small favouritism for matter over antimatter in the early universe - something that was addressed in the context of ``grand unified theories'' in the 1970s. (Indeed, baryon non-conservation seems a prerequisite for any plausible inflationary model. Our entire observable universe, containing at least 1079 baryons, could not have inflated from something microscopic if baryon number were strictly conserved)
In the 1980s non-baryonic matter became almost a natural expectation, and b / CDM is another fundamental number.
Another specially important dimensionless number, Q, tells us how smooth the universe is. It's measured by
-- The Sachs-Wolfe fluctuations in the microwave background
-- the gravitational binding energy of clusters as a fraction of their rest mass
-- or by the square of the typical scale of mass- clustering as a fraction of the Hubble scale.
It's of course oversimplified to represent this by a single number Q, but insofar as one can, its value is pinned down to be 10-5. (Detailed discussions introduce further numbers: the ratio of scalar and tensor amplitudes, and quantities such as the ``tilt'', which measure the deviation from a pure scale-independent Harrison-Zeldovich spectrum.)
What's crucial is that Q is small. Numbers like and H are only well-defined insofar as the universe possesses ``broad brush'' homogeneity - so that our observational horizon encompasses many independent patches each big enough to be a fair sample. This wouldn't be so, and the simple Friedmann models wouldn't be useful approximations, if Q weren't much less than unity. Q's smallness is necessary if the universe is to look homogeneous. But it isn't, strictly speaking, a sufficient condition - a luminous tracer that didn't weigh much could be correlated on much larger scales without perturbing the metric. Simple fractal models for the luminous matter are nonetheless, as Lahav will discuss, strongly constrained by other observations such as the isotropy of the X-ray background, and of the radio sources detected in deep surveys.