7.4. Probing the Planck Era and ``Beyond''
The second challenge would be to firm up the physics of the ultra-early universe. Perhaps the most `modest' expectation would be a better understanding of the candidate dark matter particles: if the masses and cross-sections of supersymmetric particles were known, it should be possible to predict how many survive, and their contribution to , with the same confidence as that with which we can compute primordial nucleosynthesis. Associated with such progress, we might expect a better understanding of how the baryon-antibaryon asymmetry arose, and the consequence for b.
A somewhat more ambitious goal would be to pin down the physics of inflation. Knowing parameters like Q, the tilt, and the scalar/tensor ratio will narrow down the range of options. The hope must be to make this physics as well established as the physics that prevails after the first millisecond.
One question that interests me specially is whether there are multiple big bangs, and which features of our actual universe are contingent rather than necessary. Could the others have different values of Q, or different Robertson-Walker curvature? Furthermore, will the ``final theory'' determine uniquely what we call the fundamental constants of physics - particle masses and coupling constants? Are these ``constants'' uniquely specified by some equation that we can eventually write down? Or are they in some sense accidental features of a phase transition as our universe cooled - secondary manifestations of some still deeper laws governing a whole ensemble of universes?
This might seem arcane stuff, disjoint from ``traditional'' cosmology - or even from serious science. But my prejudice is to be openminded about ensembles of universe and suchlike. This makes a real difference to how I weigh the evidence and place my bets on rival models.
Rocky Kolb's highly readable history ``Blind Watchers of the Sky'' reminds us of some fascinating debates that occurred 400 years ago. Kepler was upset to find that planetary orbits were elliptical. Circles were more beautiful - and simpler, with one parameter not two. But Newton later explained all orbits in terms of a universal law with just one parameter, G. Had Kepler still been alive then, he'd surely have been joyfully reconciled to ellipses.
The parallel's obvious. The Einstein-de Sitter model seems to have fewer free parameters than any other. Models with low , non-zero , two kinds of dark matter, and the rest may seem ugly. But maybe this is our limited vision. Just as Earth follows an orbit that is no more special than it needed to be to make it habitable, so we may realise that our universe is just one of the anthropically-allowed members of a grander ensemble. So maybe we should go easy with Occam's razor and be wary of arguments that = 1 and = 0 are a priori more natural and less ad hoc.
There's fortunately no time to sink further into these murky waters, so I'll briefly conclude.
A recent cosmology book (not written by anyone at this conference) was praised, in the publisher's blurb, for ``its thorough coverage of the inflammatory universe''. That was a misprint, of course. But maybe enough sparks will fly here in the next few days to make it seem a not inapt description.
The organisers have chosen a set of fascinating open questions. I suspect they'll still seem open at the end of this meeting, but we'll look forward to learning the balance of current opinion, and what bets people are prepared to place on the various options.