7.3. Environmental Cosmology: Long Range Prospects
One continuing challenge will be to explore the emergence of structure. This is a tractable problem until the first star (or other collapsed system) forms. But the huge dynamic range and uncertain feedback thereafter renders the phenomena too complex for any feasible simulation.
To illustrate the uncertainty, consider a basic question such as when the intergalactic gas was first photoionized.
There have been many detailed models, but essentially this requires one photon for each baryon (somewhat more, in fact, to compensate for recombinations). A hot (O or B) star produces, over its lifetime, 104- 105 photons for each of its constituent baryons; if a black hole forms via efficient accretion of baryons, the corresponding number is several times 106. Thus, only a small amount of material need collapse into such objects in order to provide enough to ionize all the remaining baryons. But the key questions, of course, are how efficiently O-B stars or black holes can form. This depends on the so-called ``initial mass function'' (IMF), which determines how much mass goes into high mass stars (or black holes) compared with the amount going concurrently into lower-mass stars? The challenge of calculating the IMF - involving gas-dynamical and radiative transfer calculations over an enormous dynamic range - may not have been met even ten years from now. But even if we assume that it has the same form as now, there is the issue of feedback: do the first stars provide a heat input (via radiation, stellar winds and supernovae) that inhibits later ones from forming? More specifically, we can imagine two options; either (a) all the gas that falls into gravitationally bound clumps of CDM turns into stars; or (b) one percent turns into stars, whose winds and supernovae provide enough momentum and energy to expel the other 99 percent. In the first case, the 3-sigma peaks would suffice; in case (b) more typical (1.5 sigma) peaks would be needed, or else larger and deeper potential wells more able to retain the gas.
Even if the clustering of the CDM under gravity could be exactly modelled, along with the gas dynamics, then as soon as the first stars form we face major uncertainties that will still be a challenge to the petaflop simulations being carried out a decade from now.