Cosmological model-building has made impressive advances in the past year. However much of this rests on supernovae being standard candles. This is a demanding requirement, given that we lack complete models for supernovae. Consider a Type I supernova, for which one popular model consists of a close pair of white dwarfs. We do not know a priori whether a pair of merging white dwarfs will explode or not, or will self destruct or leave a neutron star relic. Other models involve mass transfer onto a white dwarf by an evolving close companion: again, we do not know the outcome, whether the endpoint is violently explosive or mildly quiescent. No doubt some subset of accreting or merging white dwarfs are SNIa, but we do not know how to select this subset, nor how evolution of the parent system would affect the outcome in the early universe.
One of the largest uncertainties in interpreting the SCUBA
submillimeter sources is the possible role of AGN and quasars in
powering the high infrared luminosities. The absence of a hot dust
component in some high redshift ultraluminous infrared galaxies
(ULIRGs) with CO detections argues for a star formation interpretation
of infrared luminosities as high as 1013 L
. Observations
of far infrared line diagnostics suggest thatup to ~ 20% of ULIRGs
may be AGN-powered,
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but nearby examples such as Arp 220 suggest that
even in these cases there may be comparable amounts of star
formation-induced infrared luminosity. Interpretation of the hard
(~ 30 keV) x-ray background requires the mostly resolved sources
responsible for the background to be self-absorbed AGN surrounded by
dusty gas that reemits the absorbed AGN power at far infrared
wavelenghts and can at most account for ~ 10-20% of the diffuse
far infrared background.
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An independent argument is as follows:
the correlation of central black holes in nearby
galaxies with spheroids (Mbh
0.005 M*)
suggests that with an accretion efficiency that is expected to be
a factor f ~ 10-30 larger than the nuclear burning efficiency for
producing
infrared emission, the resulting contribution from AGN and quasars to the
far infrared background should be ~ 15 (f / 0.03)% of the
contribution from star formation.
There are too many unresolved issues in the context of structure formation to be confident that we have converged on the correct prescription for primordial fluctuations in density, nonlinear growth, and cosmological model. And then we must add in the complexities of star formation, poorly understood in the solar neighbourhood, let alone in ultraluminous galaxies at high redshift. One cannot expect the advent of more powerful computers to simply resolve the outstanding problems. Rather it is a matter of coming to grips with improved physical modelling of star-forming galaxies. Phenomenological model building is likely to provide more fruitful returns than brute force simulations, but the data requirements are demanding even on the new generations of very large telescopes. Fluctuation spectra will be measured with various CMB experiments, although disentangling the various parameters of cosmology and structure formation will take time.
However I am optimistic that the anticipated influx of new data, from optical, infrared, x-ray and radio telescopes will go far towards resolving these uncertainties. It is simply that the journey will be long with many detours, before we have deciphered the ultimate model of cosmology.
Acknowledgments
I thank Ana Mourao and Pedro Ferreira for the gracious hospitality provided in Faro.