Big eyes

Craig Hogan

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

To create small things you need particles with lots of energy, and to learn about them you need to capture and study lots of particles. So it is not surprising that the worldwide physics community is in the business of building giant accelerators and detectors.

We also find out about new physics without using accelerators by studying the biggest system of all — the cosmos. Such experiments also need big detectors, in particular, giant cameras to make deep, wide-field maps of cosmic structure. For example, Fermilab’s Dark Energy Camera (DECam) is now collecting data for the Dark Energy Survey, using light from distant galaxies gathered by the 4-meter Blanco telescope on Cerro Tololo in Chile. Designed for depth, speed, sensitivity and scientific precision, it’s a behemoth compared to the camera in your phone. By the time you add up all the parts — the detectors, the lenses, the cooling systems, the electronics and the structure to hold them precisely in place 50 feet up in the telescope beam — you have a machine that weighs about 10 tons. That may not seem very big compared to the Tevatron or the thousand-ton telescope the camera is mounted on, but it’s a lot for a digital camera — the biggest ever built.

The giant telescope simulator used to test DECam has recently been removed from the Fermilab building where the camera was put together. In the same space, another giant camera will soon start to take shape. This one will study the cosmic microwave background — the primordial light from the big bang. That light has been cooled by the cosmic expansion to microwave wavelengths, so the camera detectors and even its lenses must be cold to match. About 15,000 advanced superconducting detectors from Argonne National Laboratory will be integrated into a camera system about as big as DECam and then shipped for an experiment to take place under the thin, cold, crystalline skies at the South Pole.

This machine — the SPT-3G camera — will also be the largest of its kind ever built. When it is finished, it will be installed on the South Pole Telescope, where it will map the faint ripples of polarization imprinted on the light since it was created almost 14 billion years ago.

The SPT-3G experiment will advance cosmic mapping by an order of magnitude, but it is also a stepping stone along a path to an even larger Stage 4 CMB project in the following decade. That project, endorsed by the P5 report and supported by a nationwide collaboration of labs and university groups now forming, will carry out a comprehensive survey of the primordial radiation over much of the sky and teach us about new physics ranging from neutrino masses to dark energy.