The concordance model is now well established, and there seems little room left for any dramatic revision of this paradigm. A measure of the strength of that statement is how difficult it has proven to formulate convincing alternatives. For example, one corner of parameter space that has been explored is the possibility of abandoning the dark energy, and instead considering a mixed dark matter model with m = 1 and = 0.2. Such a model fits both the 2dF and WMAP data reasonably well, but only for a Hubble constant h < 0.5 [27, 46]. However, this model is inconsistent with the HST key project value of h, the results from SNe Ia, cluster number density evolution, and baryon fraction in clusters.
Should there indeed be no major revision of the current paradigm, we can expect future developments to take one of two directions. Either the existing parameter set will continue to prove sufficient to explain the data, with the parameters subject to ever-tightening constraints, or it will become necessary to deploy new parameters. The latter outcome would be very much the more interesting, offering a route towards understanding new physical processes relevant to the cosmological evolution. There are many possibilities on offer for striking discoveries, for example:
The cosmological effects of a neutrino mass may be unambiguously detected, shedding light on fundamental neutrino properties;
Detection of deviations from scale-invariance in the initial perturbations would indicate dynamical processes during perturbation generation, for instance, by inflation;
Detection of primordial non-Gaussianities would indicate that non-linear processes influence the perturbation generation mechanism;
Detection of variation in the dark energy density (i.e., w - 1) would provide much-needed experimental input into the question of the properties of the dark energy.
These provide more than enough motivation for continued efforts to test the cosmological model and improve its precision.
Over the coming years, there are a wide range of new observations, which will bring further precision to cosmological studies. Indeed, there are far too many for us to be able to mention them all here, and so we will just highlight a few areas.
The cosmic microwave background observations will improve in several directions. The new frontier is the study of polarization, first detected in 2002. Data are imminent from balloon-based experiments including Maxipol and Boomerang, and with WMAP continuing to take data, they should be able to measure a polarization spectrum, as well as improve measures of the temperature-polarization cross-correlation (which is easier to measure as the temperature anisotropies are much larger). Dedicated ground-based polarization experiments, such as CBI and QUEST, promise powerful measures of the polarization spectrum in the next few years, and may be able to separately detect the two modes of polarization. Another area of development is pushing accurate power spectrum measurements to smaller angular scales, typically achieved by interferometry, which should allow measurements of secondary anisotropy effects, such as the Sunyaev-Zel'dovich effect, whose detection has already been tentatively claimed by CBI. Finally, we mention the Planck satellite, due to launch in 2007, which will make high-precision all-sky maps of temperature and polarization, utilizing a very wide frequency range for observations to improve understanding of foreground contaminants, and to compile a large sample of clusters via the Sunyaev-Zel'dovich effect.
Concerning galaxy clustering, the Sloan Digital Sky Survey is well underway, and currently expected to yield around 600,000 galaxy redshifts covering one quarter of the sky. Large samples of galaxy positions at high redshifts (z ~ 1) will begin to be obtained, for instance, by the DEEP2 survey using the Keck telescopes, and the VIRMOS survey on the VLT. The 6dF survey aims to take high-quality redshift and peculiar velocity data for a large sample of nearby galaxies, and has already taken around 40,000 of the planned 170,000 redshifts.
Still awaiting final approval is the SNAP satellite, which seeks to carry out a survey for Type Ia supernovae out to redshifts approaching two, which should in particular be a powerful probe of the dark energy. With large samples, it may be possible to detect evolution of the dark energy density, thus measuring its equation of state. SNAP is also able to carry out a large weak gravitational lensing survey, complementing those becoming possible with large-format CCDs on ground-based telescopes. Before SNAP, the ESSENCE project will significantly increase the size of the SNe Ia dataset.
The development of the first precision cosmological model is a major achievement. However, it is important not to lose sight of the motivation for developing such a model, which is to understand the underlying physical processes at work governing the Universe's evolution. On that side, progress has been much less dramatic. For instance, there are many proposals for the nature of the dark matter, but no consensus as to which is correct. The nature of the dark energy remains a mystery. Even the baryon density, now measured to an accuracy of a few percent, lacks an underlying theory able to predict it even within orders of magnitude. Precision cosmology may have arrived, but at present many key questions remain unanswered.
ARL was supported in part by the Leverhulme Trust. We thank Sarah Bridle and Jochen Weller for useful comments on this article, and OL thanks members of the Cambridge Leverhulme Quantitative Cosmology and 2dFGRS Teams for helpful discussions.