Cosmic background: from quantum to cosmos

Craig Hogan

Craig Hogan, director of the Center for Particle Astrophysics, wrote this column.

The first great breakthrough of 20th-century physics came just as it dawned, in late 1900, when Max Planck derived from simple quantum principles an exact universal formula for the spectrum, or amount of light at each frequency, emitted by opaque matter.

A related breakthrough in cosmology came many decades later, when it was found that radiation with precisely Planck’s spectrum is found not only in the laboratory, but also coming from all directions in the sky. This simple fact carries a profound message about cosmic history: The entire universe is expanding from a state when matter everywhere was once hot, dense and opaque. The cosmic radiation is left over from the earliest moments of the cosmic expansion—the big bang.

In recent decades, measurements have shown that the cosmic radiation is not at exactly just one temperature, but varies by a tiny amount in different directions—a little colder here, a little hotter there. The early universe was not perfectly uniform, which is a good thing, because those tiny variations eventually led to the formation of galaxies and, of course, us.

The rich complexity of cosmic structure now provides more things to measure than just a single temperature. Cosmological information today is so precise and diverse that it informs us about new physics, not just the other way around. The structure and evolution of the universe is measured in different ways—for example, with surveys of galaxies and maps of the background radiation. By comparing different probes, we learn about the physics that connects them as well the new physics that creates structure. Perhaps the most famous example is dark energy, a behavior of nature measured only by its effects on the largest scales.

Measurements of cosmic background radiation have advanced rapidly in the last year with new high-resolution detectors in Chile and at the South Pole and with the release in March of definitive all-sky data from the Planck satellite. Some of these results offer tantalizing hints of new physics beyond the Standard Model, and future data may provide a reliable independent way to measure particle properties, such as neutrino masses. The new data radically constrain models of inflation, the new physics that powered the start of the cosmic expansion and created its originally quantum fluctuations. Although many earlier flights of theorists’ imaginations have now been permanently grounded, the paradigm of inflationary cosmology has been spectacularly confirmed.

Fermilab scientists invented many techniques of precision cosmology, helped create the Sloan Digital Sky Survey that defines the state of the art in precision measurement of cosmic structure with galaxies, and are about to start operating a still deeper cosmic mapping project, the Dark Energy Survey. Exciting choices lie ahead as we plan our participation in future experiments, perhaps including measurements of cosmic background radiation.