Eyeing Dark Energy

Frigid and bone-dry, with six straight months of night each year, the South Pole is a forbidding place to live or work. But for largely the same reasons, it’s one of the best spots on the planet for surveying the faint cosmic microwave background (CMB) radiation left over from the Big Bang. The 10-meter microwave South Pole Telescope (SPT), which began operating in February 2007, is studying the CMB to gather clues about the birth, evolution and eventual fate of the universe.

The SPT project, led by researchers at the Kavli Institute for Cosmological Physics, University of Chicago, aims to help solve one cosmological mystery in particular – that of dark energy. Little is known about this force, other than that it works against gravity and appears to have sped up the expansion of the universe. Unlike energy as we know it (and measure it), dark energy does not seem to act through any of the fundamental forces of nature other than gravity. It can’t be detected directly, for instance, through light or other manifestations of the electromagnetic force. The evidence for dark energy is indirect. Its existence was first posited in 1998 by scientists seeking to explain unexpected data from distant supernovae. Since then, research using the Hubble Space Telescope and other instruments has traced the impact of dark energy to about nine billion years ago, when the universe was five billion years old and galaxies started flying away from one another at a faster pace.

Understand the Past; Predicting the Future

From studying the CMB and what it tells them about the geometry of the universe, scientists estimate that dark energy makes up 70% to 75% of the universe’s entire mass and energy combined. This is about three times as much as dark matter, which cannot be detected by light or other electromagnetic radiation but exerts a powerful gravitational pull on galaxies. Only about 4% of the cosmos is ordinary matter, the stuff we are made of and the stuff we can see.

So whatever dark energy is, its effect is stronger than anything else on large scales. It also may determine the future of the universe. It might gain strength and end the universe by pulling all matter apart – even atomic nuclei (cosmologists call this the “big rip”). Or it might weaken and allow gravity to re-pack the universe, in a so-called “big crunch,” resulting in something like the infinitely dense condition at the point of the Big Bang. Or perhaps it will simply let the current expansion continue until most stars and galaxies are too distant to be seen.

What can the SPT tell us about the past and future of dark energy? John E. Carlstrom, director of KICP and the S. Chandrasekhar Distinguished Professor in Astronomy and Astrophysics at the University of Chicago, says the telescope is examining clusters of galaxies to learn what role dark energy played in their evolution. “One of the important things we need to learn about dark energy is what influence it has had on structure,” Carlstrom says. If scientists can learn how the density of clusters changed over time, he says they can determine “constraints on the equation of state of dark energy.” That is, they can get a more precise idea of whether dark energy is taking us toward a big rip, a big crunch or something in between. The telescope is looking specifically for the Sunyaev-Zel’dovich (SZ) effect, a distortion of the CMB radiation caused by the highly energized gas of galaxy clusters. When photons originating from the CMB traverse the clusters, they interact with electrons and tend to scatter, creating slight variations in temperature -- shadows against the microwave background – that the SPT detects with a battery of 1,000 sensors chilled to near absolute zero. The SPT will survey about a fifth of the entire southern sky and is expected to detect thousands of clusters. Analyzing follow-up data from optical telescopes, the scientists will determine the mass, distance and age of the clusters. They will then map the clusters in space and time to see how their density and structure evolved over billions of years under the competing pulls of gravity and dark energy. They hope to learn how much power dark energy exerted in the early universe, how it evolved to dominate the universe now, and by extension, how much power it may wield in the future.

Back to the Big Bang

The SPT’s activity will not end with this survey of galaxy clusters. Another project in the works will use the telescope to scan the CMB for tiny fluctuations in its polarization. Like visible light, the microwave radiation from the Big Bang has waves moving in electromagnetic fields at different angles, some up-and-down and other side-to-side. Observations with another South Pole instrument, the degree angular scale interferometer (DASI), have confirmed that the CMB is polarized as expected from prevailing theories about the physics of the Big Bang. Researchers now want to use the more sensitive SPT to look for minute variations in the CMB polarization that mark the presence of huge gravity waves.

Stephan Meyer, associate director of KICP and Professor in Astronomy and Astrophysics at the University of Chicago, says these waves are “a reasonable fraction of the size of the universe” in length and would have been generated in the “inflationary epoch” of the Big Bang. This was the time when the universe was just 10-50 seconds old and matter had not yet coalesced into neutrons and protons. “We don’t really understand the physics of that era,” Meyer says. A new set of sensors, able to detect polarization as well as heat, is being built by the University of Chicago and should be ready for installation on the SPT by the austral summer (the northern winter) of 2009-10.

Carlstrom and Meyer have both made multiple trips to the South Pole since the mid-1990s. Meyer calls it “somewhat monotonic … there are no bugs, no kids” but he says there is “a brutal and stark beauty about it all.” Carlstrom points out that, as remote as the Pole is, it has “very well-developed infrastructure” thanks to the National Science Foundation and its Office of Polar Programs. Still, installing a 75-foot-tall, 280-ton telescope at the South Pole is a major logistical feat. Carlstrom notes with some pride that he and his team (he is principal investigator on the SPT project) took just three months in the austral summer of 2006-07 to assemble the SPT, insulate it and get it up and running.

The SPT was funded with $18.7 million from the NSF, along with additional support from the Kavli Foundation the Gordon and Betty Moore Foundation of San Francisco. The project manager is Steve Padin, Senior Scientist in Astronomy and Astrophysics at the University of Chicago. Senior members of the SPT team include William Hozapfel, Adrian Lee and Helmuth Spieler of UC Berkeley; Joe Mohr of the University of Illinois at Urbana-Champaign; John Ruhl of Case Western Reserve University; Antony Stark of the Harvard-Smithsonian Astrophysical Observatory; Matt Dobbs of McGill University, and Erik Leitch of Jet Propulsion Laboratory.