Unfortunately, these calculations of the modern universe's matter and energy leave about 70 percent of the critical density unspecified. To make up the difference, theorists have posited a mysterious component called dark energy, whose relative influence has grown as the universe has expanded. We are thus led by degrees to an improbable conclusion: most of the universe today is composed of invisible dark matter and dark energy. Worse yet, dark matter and dark energy seem to be coincidentally comparable in energy density today, even though the former vastly outweighed the latter at recombination. Physicists dislike coincidences; they prefer to explain the world in terms of cause and effect rather than dumb luck. What is more, another mysterious component, the inflaton, dominated the very early universe and seeded cosmic structure. Why should we believe a cosmological model that is based on the seemingly fanciful introduction of three enigmatic entities?
One reason is that these three entities explain a wealth of previously known facts. Dark matter was first postulated in the 1930s to explain measurements of the local mass density in galaxy clusters. Albert Einstein introduced the concept of dark energy in 1917 when he included the so-called cosmological constant in his equations to counteract the influence of gravity. He later disavowed the constant, but it was resurrected in the 1990s, when observations of distant supernovae showed that the expansion of the universe is accelerating. The energy densities of dark matter and dark energy, as measured from the CMB, are in striking accord with these astronomical observations.
Second, the standard cosmological model has predictive power. In 1968 Joseph Silk (now at the University of Oxford) predicted that the small-scale acoustic peaks in the CMB should be damped in a specific, calculable way. As a result, the corresponding radiation should gain a small but precisely known polarization. (Polarized radiation is oriented in a particular direction.) One might assume that the CMB would be unpolarized because the scattering of the photons in the primordial plasma would have randomized their direction. But on the small scales where damping occurs, photons can travel with relatively few scatterings, so they retain directional information that is imprinted as a polarization of the CMB. This acoustic polarization was measured by the Degree Angular Scale Interferometer (an instrument operated at the Amundsen-Scott South Pole Station in Antarctica) and later by WMAP; the value was in beautiful agreement with predictions. WMAP also detected polarization on larger scales that was caused by scattering of CMB photons after recombination.
Furthermore, the existence of dark energy predicts additional phenomena in the CMB that are beginning to be observed. Because dark energy accelerates the expansion of the universe, it weakens the gravitational-potential wells associated with the clustering of galaxies. A photon traveling through such a region gets a boost in energy as it falls into the potential well, but because the well is shallower by the time the photon climbs back out, it loses less energy than it previously gained. This phenomenon, called the integrated Sachs-Wolfe effect, causes large-scale temperature variations in the CMB. Observers have recently seen hints of this correlation by comparing large structures in galaxy surveys with the WMAP data. The amount of dark energy needed to produce the large-scale temperature variations is consistent with the amount inferred from the acoustic peaks and the distant supernovae. As the data from the galaxy surveys improve and other tracers of the large-scale structure of the universe become available, the integrated Sachs-Wolfe effect could become an important source of information about dark energy.