The CMB may also provide crucial new evidence that could explain what happened during the very first moments after the big bang. Few aspects of cosmology are more bizarre than the period of inflation. Did the universe really inflate, and, if so, what was the nature of the inflaton, the theoretical field that caused the rapid expansion? Current measurements of the CMB have dramatically strengthened the case for the simplest models of inflation, which assume that the amplitudes of the initial density fluctuations were nearly the same at all scales. But if more detailed observations of the CMB reveal that the amplitudes varied at different scales, the simple inflation models would be in trouble. More baroque alternatives would need to be invoked or altogether different paradigms adopted.
Another exciting possibility is that we could learn about the physics of inflation by determining the energy scale at which it took place. For example, physicists believe that the weak nuclear force and the electromagnetic force were different aspects of a single electroweak force when the universe was hotter than 1015 kelvins. If researchers determine that inflation occurred at this energy scale, it would strongly imply that the inflaton had something to do with electroweak unification. Alternatively, inflation could have occurred at the much higher temperatures at which the electroweak force merges with the strong nuclear force. In this case, inflation would most likely be associated with the grand unification of the fundamental forces.
A distinctive signature in the CMB could allow researchers to settle this issue. In addition to spawning density perturbations, inflation created fluctuations in the fabric of spacetime itself. These fluctuations are gravitational waves whose wavelengths can stretch across the observable universe. The amplitude of these gravitational waves is proportional to the square of the energy scale at which inflation took place. If inflation occurred at the high energies associated with grand unification, the effects might be visible in the polarization of the CMB.
Last, further observations of the CMB could shed some light on the physical nature of dark energy. This entity might be a form of vacuum energy, as Einstein had hypothesized, but its value would have to be at least 60 and perhaps as much as 120 orders of magnitude as small as that predicted from particle physics. And why is dark energy comparable to dark matter in density now and apparently only now? To answer these questions, researchers can take advantage of the fact that CMB photons illuminate structures across the entire observable universe. By showing the amplitude of density fluctuations at different points in cosmic history, the CMB can reveal the tug-of-war between matter and dark energy.
Measurements of two CMB phenomena could be particularly useful. The first, called the Sunyaev-Zel'dovich effect, occurs when CMB photons are scattered by the hot ionized gas in galaxy clusters. This effect allows galaxy clusters to be identified during the crucial period, about five billion years ago, when dark energy began to accelerate the expansion of the universe. The number of galaxy clusters, in turn, indicates the amplitude of density fluctuations during this time. The second phenomenon, gravitational lensing, happens when CMB photons pass by a particularly massive structure that bends their trajectories and hence distorts the pattern of temperature and polarization variations. The degree of lensing reveals the amplitude of the mass density fluctuations associated with these structures.
To conduct these investigations of inflation and dark energy, however, researchers will need a new generation of CMB telescopes that can observe the radiation with even greater sensitivity and resolution. In 2007 the European Space Agency plans to launch the Planck spacecraft, a microwave observatory that will be placed in the same orbit as WMAP. Planck will be able to measure CMB temperature differences as small as five millionths of a kelvin and detect hot and cold spots that subtend less than a tenth of a degree across the sky. Such measurements will enable scientists to glimpse the full range of acoustic oscillations in the CMB and thus sharpen their picture of the inflationary spectrum. A multitude of ground-based experiments are also under way to study CMB effects associated with structure in the current epoch of accelerated expansion.
Although the standard cosmological model appears to work remarkably well as a phenomenological description of the universe, a deeper understanding of its mysteries awaits the findings of these experiments. It seems clear that the cosmic symphony will continue to enchant its listeners for some time to come.