The Science Harvest: From COBE to MAP and Planck

The COBE DMR detection of temperature fluctuations of amplitude K on the provided the first evidence for the density inhomogeneities from which all structure in the Universe is believed to have originated. The level of inhomogeneity is consistent with that needed to account for the structure seen today through the attractive action of gravity over the past 13 billion years. For theories like inflation and topological defects, which specify the shape of the spectrum of inhomogeneity, the accurate (10%) DMR measurement allows extrapolation from COBE scales, 1000's of Mpc, to the length scales relevant for the formation of galaxies, clusters of galaxies, and other structures seen today, leading to more precise predictions about the development of structure. Overnight, the term ``COBE normalized'' became a part of the cosmological vernacular.

In the five years since the COBE DMR detection, anisotropy on angular scales from to 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 been less precise than COBE. These experiments have not resolved individual multipoles, but rather ``broad-band power'' in multipole intervals . Nevertheless, they have added much to our understanding and a picture is emerging: A rise in the level of anisotropy on degree scales and an apparent drop at smaller angular scales (see Fig.2). This is consistent with the first acoustic peak. Together with COBE, these experiments now cover almost three decades in angular scale; they are generally consistent with inflation, have eliminated all models of structure formation that do not incorporate nonbaryonic dark matter, and disfavor topological-defect models.

Much will happen before MAP and Planck. A new generation of experiments, instruments flown on long-duration balloons or located on high-dry sites like the South Pole or Atacama in Chile, should begin to define the prominent acoustic peaks in the multipole spectrum by measuring power in multipole windows from to . (Resolution in is increased by covering more sky, since and are ``Fourier'' conjugate variables.) By resolving the position of the first two or three peaks, these experiments should be able to pin down the density parameter to an accuracy of 20% and in so doing test the inflationary prediction of a flat Universe. They should also begin to determine other cosmological parameters, e.g., the baryon density and the Hubble constant, to a precision of 200r so.

Using low-frequency receivers (22-90GHz), MAP will determine the angular power spectrum out to to a precision nearly limited by sampling variance. Planck is expected to reach with a similar precision, and should have better discrimination against foreground sources by virtue of both high and low frequency coverage. Together, they should come close to reaping almost all the information encoded in CMB temperature anisotropy (see Fig.4).

FIG.4 NASAs MAP satellite which will fly in the year 2000, should easily be able to discriminate between variants of cold-dark matter cosmology: The favored critical density universe to which baryons contribute 5% (black curve with yellow band) or 10% (red); an open universe with (green); and ``tilted'' CDM with n=0.8 (blue). The expected one sigma-error per multipole for MAP is indicated by the thickness of the yellow band (the band blows up at large because the beam smears out smaller features).

Many of the cosmological parameters, including the density parameter , can be determined from the CMB without reference to a specific theory. However the true power comes from detailed modelling within a given theoretical framework. For a theory like inflation + cold dark matter the theoretical angular power spectrum depends upon ten or so parameters, including , , the power-law index n that characterizes the spectrum of density perturbations (n=1 corresponds to scale invariant perturbations), the composition of the dark matter (fraction of critical density in cold dark matter, light neutrinos, and cosmological constant), the amount of gravitational radiation produced during inflation, and a few others. These parameters are very overconstrained by the 2500 multipoles that will be measured, so that the theory can be thoroughly tested. Furthermore the Hubble constant, baryon density, mass density and spectral index will all be determined to within a few percent.

The CMB anisotropy should be polarized at the level of around 5%, and both MAP and Planck will have the capability of detecting it. The polarization arises because Thomson scattering is partially polarized (depending upon the scattering angle) and the CMB radiation field is not isotropic before last-scattering due to the temperature fluctuations induced by density perturbations. Polarization, which has yet to be detected, provides a consistency check on the basic picture of anisotropy formation and has the potential to improve the accuracy with which cosmological parameters can be determined.

FIG.5 Comparing microwave tomography (contour lines) of a galaxy cluster (CL0016+16) with an X-ray image of its hot intergalactic gas (false colors) provides a way of measuring the Hubble constant without standard candles. Figure courtesy of John Carlstrom and Marshall Joy.

It is likely that there will still be much to learn from CMB polarization after MAP and Planck. In particular, polarization might be very useful in separating the contribution of inflation-produced gravity waves to CMB anisotropy, as gravity waves induce a different pattern of polarization than density perturbations do. 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 or quasars. These objects are thought to have appeared at redshifts of around ten to twenty, and ended the ``dark age'' that began with last scattering.

Beyond its immense value as a cosmic Rosetta stone, the CMB is being used for other purposes. Perhaps the most exciting is ``microwave tomography'' of clusters of galaxies using the S-Z effect. In 1972 Sunyaev and 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 (S-Z effect). This leads to a small spectral distortion of 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 the study the structure of clusters as well as to search for clusters at high redshift where the galaxies may be too faint to be seen, or may not even be present. Further, by comparing S-Z maps with x-ray maps of clusters (see Fig.5), the Hubble constant can be determined without recourse to the usual method of ``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 proportional To the integral of the square. Comparing the two yields a determination of the clusters size.

The Cosmic Microwave Background has played a central role in cosmology since its discovery in 1965. It is one of the cornerstones of the standard hot big-bang theory. The study of CMB anisotropy with K precision and fraction of a degree angular resolution is likely to have as least as much impact as the discovery of the CMB. It will put to the test our most promising ideas about the earliest moments and will determine for us the elusive fundamental parameters of cosmology.

• Box 1: Big-bang Basics
• Box 2: The Physics of CMB Anisotropy