Model-independent cosmological constraints 251We should not rest lightly until the cosmological model preferred by microwave
background measurements is comfortably consistent with all relevant priors
derived from other data sources of comparable precision.
The picture will come into sharper relief over the next two years. The
MAP satellite (http://map.gsfc.nasa.gov), launched by NASA on 30 June 2001,
will map the full microwave sky in five frequency channels with an angular
resolution of around 15 arc minutes and a temperature sensitivity per pixel of
a part in a million. Space missions offer unequalled sky coverage and control of
systematics and, if it works as advertized, MAP will be a benchmark experiment.
Prior to its launch, expect to see the first interferometric microwave data at
angular scales smaller than a half degree from the CBI interferometer experiment
(http://www.astro.caltech.edu/∼tjp/CBI/). In this same time frame, we also may
have the first detection of polarization. The most interesting power spectrum
feature to focus on will be the existence and amplitude of a third acoustic peak. If
a third peak appears with amplitude significantly higher than the putative second
peak, this almost certainly indicates conventional acoustic oscillations with a
high baryon fraction and possibly new physics to reconcile the result with the
deuterium measurements. If, however, the power spectrum remains flat or falls
further past the second peak region, then all bets are off. In a time frame of the
next 5 to 10 years, we can reasonably expect to have a cosmic-variance limited
temperature power spectrum down to scales of a few arcminutes (say,l=4000),
along with significant polarization information (though probably not cosmic-
variance limited power spectra). In particular, ESA’s Planck satellite mission
(http://astro.estec.esa.nl/SA-general/Projects/Planck/) will map the microwave
sky in nine frequency bands at significantly better resolution and sensitivity
than the MAP mission. For a comprehensive listing of past and planned
microwave background measurements, see Max Tegmark’s experiments web
page, http://www.hep.upenn.edu/∼max/cmb/experiments.html.
7.6 Model-independent cosmological constraints
Most analysis of microwave background data and predictions about its ability
to constrain cosmology have been based on the cosmological parameter space
described in section 7.5.1. This space is motivated by inflationary cosmological
scenarios, which generically predict power-law adiabatic perturbations evolving
only via gravitational instability. Considering that this space of models is broad
and appears to fit all current data far better than any other proposed models, such
an assumed model space is not very restrictive. In particular, proposed extensions
tend to be ratherad hoc, adding extra elements to the model without providing
any compelling underlying motivation for them. Examples which have been
discussed in the literature include multiple types of DM with various properties,
non-standard recombination, small admixtures of topological defects, production
of excess entropy, or arbitrary initial power spectra. None of these possibilities