2.4. The Cosmic Microwave Background

Today, CMB photons, while very numerous (there are about 2 billion photons for every hydrogen atom) account for a negligible fraction of the mass-energy budget (about 0.01%). Still, they play a central role in cosmology. First, at early times, the CMB was the dominant part of the mass-energy budget, from which we ascertain that the infant Universe was a hot thermal bath of elementary particles. Second, photons from the CMB interacted closely with matter until the temperature of the Universe had cooled enough for the ionized plasma to combine and form neutral atoms, allowing the photons to stream past. At this "last-scattering surface" of the CMB, the Universe was about 400,000years old, and about 1100 times smaller than it is today. The CMB is a "snapshot" of the Universe at a much simpler time.

The CMB measurements are a striking example of a new level of precision now being made in cosmology. NASA's COBE satellite, a four-year mission launched in 1989, measured the temperature of the background radiation to better than one part in a thousand, T₀ = 2.725 ± 0.001 K (Mather et al., 1999), and discovered tiny (tens of microKelvin) variations in the temperature of the CMB across the sky. These tiny fluctuations arise from primeval lumpiness in the distribution of matter. In the early Universe, outward pressure from the CMB photons, acting counter to the inward force of gravity due to matter, set up oscillations whose frequencies are now seen imprinted in the CMB fluctuations. Evidence of these "acoustic oscillations" can be seen when the fluctuations are described by their spherical-harmonic power spectrum (see Figures 7-9). In late 2002, the DASI Colloboration detected the last feature predicted for the CMB: polarization (Kovac et al., 2002). Because the CMB radiation is not isotropic (as evidenced by the anisotropy seen across the microwave sky) and Thomson scattering off electrons is not isotropic, CMB anisotropy should develop about a 5% polarization.

Figure 7. Anisotropy of the Cosmic Microwave Background: All-sky maps, made by COBE (upper) and by WMAP (lower); range of color scale is ± 200 µKelvin. The consistency of the 30 times higher resolution and higher sensitivity WMAP results with COBE is apparent (courtesy of NASA/WMAP Science Team).

Figure 8. Anisotropy of the Cosmic Microwave Background: Angular power spectrum of the CMB, incorporating all the pre-WMAP data (COBE, BOOMERanG, MAXIMA, DASI, CBI, ACBAR, FIRS, VSA, and other experiments). Variance of the multipole amplitude is plotted against multiple number; as indicated by the top scale, multipole measures the fluctuations on angular scale ~ 200° / . Evidence of the baryon - photon oscillations can be seen as the distinct "acoustic peaks." The theoretical curve is the consensus cosmological model (image courtesy of C. Lineweaver).

Figure 9. Anisotropy of the Cosmic Microwave Background: The WMAP angular power spectrum (also includes data from CBI and ACBAR). The curve is the consensus cosmology model; the grey band includes cosmic variance. The WMAP measurements up to ~ 350 are cosmic variance limited. The lower panel shows the anisotropy cross polarization power spectrum; the high point marked re-ionization is the evidence for re-ionization of the Universe at z ~ 20 (courtesy of NASA/WMAP Science Team).

The precise shape of the angular power spectrum of anistropy and polarization depends in varying degrees upon all the cosmological parameters in Table I, and so CMB anisotropy encodes a wealth of information about the Universe. With a host of ground-based and balloon-borne CMB experiments following COBE, a NASA space mission (the Microwave Anisotropy Probe, MAP) now taking new data, and with an European Space Agency (ESA) mission planned for launch in 2007, we are in the midst of realizing the potential of the CMB as a probe of cosmological parameters. A summary of the progress includes determination of the curvature, ₀ = 1.03 ± 0.03, the power law index of density perturbations, n = 1.05 ± 0.09, the baryon density _B = 4.0 ± 0.6 × 10^-31 g cm^-3, and the matter density _M = 2.7 ± 0.4 × 10^-30 g cm^-3. The uncertainties in all of these quantities are expected to diminish by at least a factor of ten.

As mentioned above, the CMB value for the baryon density is consistent with that determined from BBN. This not only provides confidence that ordinary matter accounts for a small fraction of the total amount of matter, but also is a remarkable consistency test of the entire framework. The CMB provides independent, corroborating evidence for a significant component of dark energy through the discrepancy between the total amount of matter and energy (critical density) and that in matter (1/3 of the critical density). Finally, the measurements of the CMB multipole spectrum are consistent with the emerging new cosmology: a flat Universe with dark matter and dark energy.

Establishing a reliable accounting of the matter and energy in the Universe (see Figure 10) is a major achievement; but, we still have much more to learn about each component and almost everything to understand about the "strange recipe." Moreover, because the energy density of matter, photons and dark energy each change in distinctive ways as the universe expands, the mix we see today must have been different in the past and will be different in the future.

Figure 10. The composition of the Universe today. Because the different components of the mass/energy budget evolve differently, the composition changes with time. For example, at very early times, photons and other relativitic particles were the dominant component; from 10,000 years until a few billion years ago, matter was the dominant component, and in the future dark energy will be the dominant component.