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Session 31 - The Structure and Evolution of The Universe - I.
Topical, Oral session, Tuesday, June 09
Presidio,
There is a standard model now for the growth of structure in the universe which is clearly correct in general form, although the details remain uncertain. Quantum fluctuations in the early universe, amplified by inflation, are probably the original source of perturbations in an otherwise uniform FRW universe. The amplitude of these perturbations is determined by observed CBR fluctuations and the shape is predicted by standard theory, so long as some form of cold dark matter constitutes most of the mass density.
Putting these initial conditions, with standard atomic physics and the equations of hydrodynamics, into a sufficiently detailed computational model, allows one to predict where and when galaxies will form. Very large scale simulations completed to date of models such as the concordance (LCDM, Ostriker and Steinhardt) model indicate quite good agreement with a suite of observational constraints from Lyman-alpha clouds at redshift three to clusters of galaxies at redshift zero. Open models with Omega, matter less than unity, seem best at reproducing all of these properties, as well as the large-scale distribution of galaxies and intergalactic gases.
A particularly interesting and robust conclusion is reached concerning the distribution of baryons. For a wide range of viable cosmological models, the fraction of baryons that condenses to stars and cold gas (in galaxies) is 20 percent plus or minus 10 percent. Of the remaining baryons in the intergalactic medium, the fractions in the hot (T > 10^7K), intermediate (10^7K > T > 10^5K) and warm (10^5 > T) components is (20 \pm 7, 55 \pm 10, 15 \pm 5) percent. The majority of cosmic baryons, in intermediate temperature regions, will be discovered by soft X-Ray techniques.
