The scientific harvest

21   COBE's DMR detection of temperature fluctuations of amplitude 30 /µ,K on the 10° angular scale gave us the first evidence for the density inhomogeneities that are believed to have seeded all the structure in the universe. For inflation or topological-defect theories, which specify the shape of the Fourier spectrum of density inhomogenei­ties but not its model-dependent amplitude, the accurate (10%) DMR measurement fixes the spectrum on length scales of gigaparsecs and therefore permits extrapolation to the smaller scales relevant for the formation of galaxies, clusters of galaxies and other structures we see today. That immediately leads to more precise scenarios of the evolution of structure. Overnight, the term "COBE nor­malized" has become a part of the cosmological vernacular.

22 In the five years since the COBE detection, anisotropy on angular scales from 100° down to 0.1° has been detected by about twenty different experiments. Because these experiments have had to deal with the effect of the atmosphere, limited sky coverage and calibration difficul­ties, they have all been less precise than COBE. They have not resolved individual multipoles, yielding only broadband power determinations for bins of Δ ℓ ≈ℓ. None­theless, they have added much to our understanding, and a picture is emerging: The anisotropy grows with increas­ing ℓon degree scales and then falls at smaller angles. (See figure 2.) Together with COBE, these experiments now cover almost three decades in angular scale; they are generally consistent with inflation, and they have elimi­nated all models of structure formation that do not incor­porate nonbaryonic dark matter. They also disfavor topo­logical-defect models.

23 Much will happen before MAP and Planck are launched. A new generation of instruments—flown on long-duration balloons or set on high, dry sites like the South Pole or the high desert of Chile—should begin to define the prominent "acoustic peaks" in the multipole spectrum (see box 2) by measuring power in Δℓ≈ 30 windows from

 ℓ= 200 to 2500. (Because ℓand Өare Fourier conjugate variables, one sharpens the resolution in ℓ by covering more sky.) By resolving the position of the first two or three acoustic peaks, these experiments should be able to pin down the mass-density parameter Ω to an ccuracy of 20% and thus test the inflationary prediction of flat universe (Ω = 1). They should also begin to pin down other cosmological parameters, such as the baryon density and the Hubble constant, to a precision of 20% or so.

24   Using low-frequency receivers (22—90 GHz), MAP will determine the angular power spectrum out to ℓ ~ 1000, to a precision close to the sampling-variance limit. Employing both high- and low-frequency detectors, Planck is expected to reach ℓ ~ 2500 with similar precision. Be­tween them, the two satellites should determine 2500 multipoles and come close to reaping almost all the infor­mation encoded in CMB temperature anisotropy.

25 Some of the cosmological parameters, including Ω, can be determined from the CMB without reference to a specific theory. However the full potential of the data is harvested by detailed modeling within a given theoretical framework. (See figure 4.) For a theory like inflation + cold-dark matter, the theoretical angular power spectrum depends upon about ten parameters, including H₀, Ω, the power-law index n that characterizes the spectrum of density perturbations (n = 1 meaning scale invariance), the dark-matter composition and the amount of gravita­tional radiation produced during inflation. These parame­ters will be very overconstrained by the 2500 measured multipoles. Therefore the theory can be thoroughly tested. Furthermore H₀, n, Ω and the baryon density will all be determined within a few percent.

26   The CMB anisotropy should be polarized at the level of about 5 percent, and both MAP and Planck will have the capability of detecting it. The polarization arises because the radiation field was not isotropic before last-scattering and Thomson scattering produces partial po­larization. This polarization, which has yet to be detected, provides a consistency check on the basic picture of an­isotropy formation, and it can improve the accuracy with which cosmological parameters are determined.

27 There probably will still be much to learn from the CMB polarization after MAP and Planck. In particular, polarization may be very useful in separating out the contribution of inflation-produced gravity waves to the CMB anisotropy, because gravity waves and density per­turbations differ in the polarization they engender. De­termining the level of gravitational radiation fixes the energy scale of inflation. Polarization is also crucial for detecting the re-ionization of the neutral, transparent universe by the first generation of stars. The first stars, which are thought to have appeared at redshifts of 10 or 20, ended the "dark age" that began with the last scat­tering of the CMB photons.

RIGHT ASCENSION

figure 5. comparing microwave tomography (contour lines) of a galaxy cluster (CL 0016 + 16) with an x-ray image of its hot intergalactic gas (false colors), provides a way of measuring the Hubble constant without standard candles. The tomography, which exploits the Sunyaev-Zel'dovich scattering of cosmic microwave background photons by the gas, was done with the BIMA and OVRO interferometric radio arrays. The x-ray image comes from the ROSAT satellite. (Figure courtesy of John Carlstrom and Marshall Joy.)

28 Beyond its immense value as a cosmic Rosetta stone, the CMB is being exploited for other purposes. Perhaps the most exciting is "microwave tomography" of clusters of galaxies by means of the Sunyaev-Zel'dovich (S-Z)

effect. In 1972, Rashid Sunyaev and Yakov Zel'dovich pointed out that some of the CMB photons passing through the hot gas in clusters are scattered to higher energy by inverse Compton scattering. This effect produces a small spectral distortion in the CMB, whose amplitude depends on the temperature and density of the cluster gas, but is independent of redshift. The S—Z effect can be used to study the structure of clusters and also to search for high-redshift clusters whose constituent galaxies are too faint to be seen. Furthermore, by comparing S-Z maps with x-ray maps of clusters (see figure 5), one can measure the Hubble constant without recourse to the usual "stand­ard candles." That's because the S—Z distortion is propor­tional to the line-of-sight integral of the electron density, whereas the x-ray intensity is proportonal to the square of that intergral. Thus, comparing the two yields a de­termination of the cluster's absolute size.

Since its discovery in 1965, the Cosmic Microwave Background has played a central role in cosmology. It is one of the cornerstones of the standard hot-Big-Bang theory. The study of CMB anisotropy with microkelvin precision and subdegree angular resolution is likely to have at least as much impact as the original Penzias-Wil­son discovery. It will put to the test our most promising ideas about the earliest moments and it will determine for us the elusive fundamental parameters of cosmology.