As I have been careful to stress (far too carefully for a real debate), the basic tenets of Inflation + Cold Dark Matter have not yet been confirmed definitively. However, a flood of high-quality cosmological data is coming, and could make the case soon. Regardless, the flood of information will make cosmology exciting for the next decade and beyond. Here is my version of how ``quite possibly'' becomes ``yes.''
Map of the Universe at 300,000 yrs. COBE mapped the CMB with an angular resolution of around 10°; two new satellite missions, NASA's MAP (launch 2000) and ESA's Planck Surveyor (launch 2007), will map the CMB with 100 times better resolution (0.1°). From these maps of the Universe as it existed at a simpler time, long before the first stars and galaxies, will come a gold mine of information: Among other things, a definitive measurement of 0; a determination of the Hubble constant to a precision of better than 5%; a characterization of the primeval lumpiness; and possible detection of the relic gravity waves from inflation. The precision maps of the CMB that will be made are crucial to establishing Inflation + Cold Dark Matter (see e.g., Bennett et al. 1997).
Map of the Universe today. Our knowledge of the structure of the Universe is based upon maps constructed from the positions of some 30,000 galaxies in our own backyard. The Sloan Digital Sky Survey (SDSS 1998) will produce a map of a representative portion of the Universe, based upon the positions of a million galaxies. The Anglo-Australian Two-degree Field survey will determine the position of several hundred thousand galaxies (2dF 1998). These surveys will define precisely the large-scale structure that exists today, answering questions such as, ``What are the largest structures that exist?'' Together, the CMB map of the young Universe and the SDSS/2dF map of the Universe today will definitively test the Cold Dark Matter theory of structure formation, and much more.
Cold dark matter. A key element of theory is the cold dark matter particles that hold the Universe together; until we actually detect cold dark matter particles, it will be difficult to argue that cosmology is solved. Experiments designed to detect the dark matter that holds are own galaxy together are now operating with sufficient sensitivity to detect both neutralinos and axions (Sadoulet 1999). In addition, experiments at particle accelerators (Fermilab and CERN) will be hunting for the neutralino and its other supersymmetric cousins.
Nature of the dark energy. If the Universe is indeed accelerating, then most of the critical density exists in the form of dark energy. This component is poorly understood. Equally puzzling is why it is just now come to be the dominant component of the mass/energy budget: its energy density is constant (or slowly varying) and the matter density decreases as the volume of the Universe increase, and thus in the past it was unimportant and in the future matter will be unimportant. Independent evidence for the existence of this dark energy, e.g., by CMB anisotropy, the SDSS and 2dF surveys, or gravitational lensing, is crucial. Additional measurements of SNeIa could help shed light on the precise nature of the dark energy: there are interesting possibilities beyond vacuum energy. The dark energy problem is not only of great importance for cosmology, but for fundamental physics as well. Whether it is vacuum energy or quintessence, it is a puzzle for fundamental physics and likely a clue about the unification of the forces and particles.
Present expansion rate H0. Direct measurements of the expansion rate using standard candles, gravitational time delay, SZ imaging and the CMB maps will pin down the elusive Hubble constant once and for all. It is the fundamental parameter that sets the size - in time and space - of the observable Universe. Its value is critical to testing the self consistency of Cold Dark Matter.
Dark matter bookkeeping. Our best knowledge of the amount of matter in the Universe is based upon clusters of galaxies. Two new X-ray observatories - NASA's AXAF and ESA's XMM - will be launched in 1999, and data they take will strengthen and refine our understanding of dark matter based upon clusters of galaxies. Further, a powerful new tomographic technique for studying clusters when combined with x-ray measurements will sharpen measurements of dark matter in clusters. (The technique, Sunyaev - Zel'dovich or SZ imaging, uses the fact that some fraction of CMB photons that pass through a cluster have their energies changed slightly.) Until a decade ago, almost all knowledge of the distribution of matter in the Universe was based upon the distribution of light. Gravitational lensing by dark matter has begun to reveal the distribution of matter; this technique, which requires CCD cameras with 100s of millions of pixels and telescopes with wide fields of view, will undoubtedly help us to better understand the distribution of dark matter and test the Cold Dark Matter hypothesis (Tyson 1993).
Big-bang nucleosynthesis. We should not forgot possible insights that could come from more precisely probing the standard cosmology. The Tytler - Burles deuterium measurement and pegging of the density of ordinary matter makes it possible to very precisely predict the big-bang abundance of 4He, 24.6% ± 0.1%. Current measurements of the primeval 4He abundance are not nearly so precise, 24% ± 1%. Further measurements of the 4He abundance have the potential to test this powerful probe of the hot big-bang model and to strengthen the foundations of cosmology (or to shake them!).