Rocky III

Craig Hogan

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

No, this column is not about a movie featuring an American comeback slugger. Instead, I’m providing an update on a science strategy report that came out this summer.

“Rocky III” is the nickname for the latest in a series of reports on dark energy chaired by Edward “Rocky” Kolb of the University of Chicago. Rocky’s team was asked to comment on the status and opportunities for DOE’s program on dark energy research.

The report has already had an important impact on agency policy. One of its recommendations—to do a deep, wide-field spectroscopic survey—is now under serious consideration. It’s been given first-stage approval (CD-0) by the Department of Energy, meaning that it fills a mission need for the agency.

The basic idea is to make a detailed map of the universe so we can better understand the mysterious forces that influence its expansion and structure. The Dark Energy Camera, now being commissioned in Chile, will soon create the best and biggest imaging survey of the universe ever made. This survey, however, still has limited resolution on how far away the galaxies are because it only measures light in a few different colors.

The report points out the scientific value of a spectroscopic survey to measure spectra—the mixture of thousands of colors—of tens of millions of galaxies and quasars over a wide area of the sky. This spectral information gives a cosmic map with far better resolution in depth: a third dimension in space.

The spectrum tells us approximately how far away an object is from Earth—the missing third dimension—because the wavelength of light stretches in proportion to the size of the expanding universe: Light from more distant objects is stretched more and appears redder. A detailed spectrum also provides precise information on the velocities of celestial objects not directly associated with the expansion of space, caused by the gravity of cosmic structures, since motion changes the wavelength of the light. Both kinds of information are valuable for studying the effects of dark energy.

Collecting the spectra takes a big new machine, a factory floor full of precision spectrographs fed by thousands of optical fibers, whose other ends are precisely positioned in the focal plane of a telescope. It’s like the wildly successful Sloan Digital Sky Survey, but an order of magnitude bigger—an ambitious project appropriate for DOE laboratories like Fermilab and Berkeley Lab.

Of course, you also need a big, wide-field telescope where one would position the fibers to collect the light from distant galaxies. Now the fun begins. It is up to all of us—funding agencies, science communities and laboratories—to work together in a time of constrained budgets to create the best science program we can to investigate dark energy.