If our universe has indeed expanded, Friedmann-style, from an exceedingly high density, then after the first 10-12 seconds energies are within the range of accelerators. After the first millisecond - after the quark-hadron transition - conditions are so firmly within the realm of laboratory tests that there are no crucial uncertainties in the microphysics (though we should maybe leave our minds at least ajar to the possibility that the constants may still be time-dependent). And everything's still fairly uniform - perturbations are still in the linear regime.
It's easy to make quantitative predictions that pertain to this intermediate era, stretching from a millisecond to a million years. And we've now got high-quality data to confront them with. The marvellous COBE ``black body'' pins down the microwave background spectrum to a part in 10,000. The ``hot big bang'' has lived dangerously for thirty years: it could have been shot down by (for instance) the discovery of a nebula with zero helium, or of a stable neutrino with keV mass; but nothing like this has happened. The debate (concurrence or crisis?) now focuses on 1 per cent effects in helium spectroscopy, and on traces of deuterium at very high redshifts. The case for extrapolating back to a millisecond is now compelling and battle-tested. Insofar as there's a ``standard model'' in cosmology, this is now surely part of it.
When the primordial plasma recombined, after half a million years, the black body radiation shifted into the infrared, and the universe entered, literally, a dark age. This lasted until the first stars lit it up again. The basic microphysics remains, of course, straightforward. But once non-linearities develop and bound systems form, gravity, gas dynamics, and the physics in every volume of Landau and Lifshitz, combine to unfold the complexities we see around us and are part of.
Gravity is crucial in two ways. It first amplifies ``linear'' density contrasts in an expanding universe; it then provides a negative specific heat so that dissipative bound systems heat up further as they radiate. There's no thermodynamic paradox in evolving from an almost structureless fireball to the present cosmos, with huge temperature differences between the 3 degrees of the night sky, and the blazing surfaces of stars.
It is feasible to calculate all the key cosmic processes that occurred between (say) a millisecond and a few million years: the basic physics is ``standard'' and (according at least to the favoured models) everything is linear. The later universe, after the dark age is over, is difficult for the same reason that all environmental sciences are difficult.
The whole evolving tapestry is, however, the outcome of initial conditions (and fundamental numbers) imprinted in the first microsecond - during the era of inflation and baryogenesis, and perhaps even on the Planck scale . This is the intellectual habitat of specialists in quantum gravity, superstrings, unified theories, and the rest.
So cosmology is a sort of hybrid science. It's a ``fundamental'' science, just as particle physics is. But it's also the grandest of the environmental sciences. This distinction is useful, because it signals to us what levels of explanation we can reasonably expect. The first million years is described by a few parameters: these numbers (plus of course the basic physical laws) determine all that comes later. It's a realistic goal to pin down their values. But the cosmic environment of galaxies and clusters is now messy and complex - the observational data are very rich, but we can aspire only to an approximate, statistical, or even qualitative ``scenario'', rather like in geology and paleontology.
The relativist Werner Israel likened this dichotomy to the contrast between chess and mudwrestling. The participants in this meeting would seem to him, perhaps. an ill-assorted mix of extreme refinement and extreme brutishness (just in intellectual style, of course!).