Inflation at South Pole
New telescopes search universe for signs of rapid expansion after the Big Bang
Posted April 1, 2011
The theory of inflation holds that just a fraction of a second after the universe exploded into existence, it expanded exponentially, a burst of energy at a scale almost unfathomable to the human imagination.
The search for the evidence that will finally prove or disprove one of the key concepts of the Big Bang model of cosmology has similarly expanded in the last decade or so. John Kovac is proud to say that astronomers at the South Pole Station have been at the cutting edge of exploration into the microwave spectrum of the universe since almost the beginning.
“We’ve been very successful with cosmic microwave background observations here from the South Pole, particularly in the last 10 years,” said Kovac, an assistant professor of astronomy and physics at Harvard University, as his team readied the first of three receivers for a new telescope that will search for the telltale signature of inflation beginning this winter in Antarctica.
The project is called SPUD, an acronym that incorporates an acronym, for Small Polarimeter Upgrade for DASI. It will join forces in the Dark Sector at South Pole with a second-generation telescope called BICEP (Background Imaging of Cosmic Extragalactic Polarization) that sports a similar receiver.
And with newer receivers and more telescopes planned in future years, experimental physicists believe they will soon be able to learn whether what we think we know about the universe from theorists is in fact reality. So far, so good, according to Kovac.
“It’s been an incredible success story in terms of the interplay between theory and observation, pushing out the frontiers of knowledge,” he said.
From the beginning
The theory about the formation of the universe is elegant. Designing the instruments to figure out if the theorists are on target — or have geeked out on one too many episodes of “Star Trek” — has been a slow but steady progression.
After all, there’s about 14 billion years to cover.
Remember that about 380,000 years after the spark that ignited the Big Bang, the universe cooled enough to allow electrons to combine with nuclei (what’s known as recombination). The universe glowed with light before recombination, but after recombination, it became transparent.
Fast forward about 14 billion years to today. The universe is still expanding — and thanks to another mystery called dark energy, that expansion is accelerating — though at a far slower pace than in its infancy.
The brilliant light from the Big Bang that started as ultra-high energy gamma rays stretched into X-rays, was visible light at recombination, and has now stretched all the way to microwaves.
In the 1960s, Arno Penzias and Robert Wilson of Bell Laboratories found microwave static interfering with a radio experiment. That interference — responsible for some television static — turned out to be remnant radiation from the birth of the universe: the so-called cosmic microwave background (CMB).
CMB radiation, often referred to as a faint hiss of microwaves, comes from every direction in the sky. It is almost a perfectly uniform plasma, with a temperature of 2.7 degrees above absolute zero on the Kelvin scale.
But it contains “hot” and “cold” spots that are slight irregularities in its near-perfect uniformity, which is known as anisotropy. These spots can tell cosmologists something about the geometry of the universe, the amounts and types of dark matter and energy that make up the cosmos, and even something about the universe’s ultimate fate.
The theorists had predicted anisotropy, but it was wasn’t until the 1990s that the experimentalists provided the first data to support the idea from NASA’s Cosmic Background Explorer (COBE) satellite. Many in the field of astrophysics mark those measurements in 1992 as the inception of cosmology as a precise science.
Getting into the act
The South Pole Station sits on the high, dry and cold polar plateau — characteristics appealing for CMB experiments that require a very stable atmosphere for the measurements physicists need to make.
The first effort to measure the anisotropy of the CMB from the South Pole came in the mid-1980s. By 1991, the National Science Foundation, which manages the U.S. Antarctic Program, had established CARA, the Center for Astrophysical Research in Antarctica, as a way to organize the various observing facilities around the station. CARA established the Dark Sector, an area reserved for experiments that search the cosmos.
CARA’s first CMB telescope, Python, debuted at Pole in late 1992, and it detected CMB anisotropy only a few months after the COBE results. In 1994, it became the first CMB telescope to operate during a South Pole winter.
Kovac was fresh out of undergraduate school when he wintered over to work on the Python telescope. The experiment proved it was possible to operate during a South Pole winter, though there were many lessons learned from that first season, according to Kovac.
“It was very exciting time. I was glad to get in as a student, to ride a wave in a field that was really in the process of exploding,” he said.
The next generation of telescopes marched through: VIPER and ACBAR, DASI and QUAD.
The Degree Angular Scale Interferometer, DASI, enjoyed particular success. It was the first telescope to detect the polarization of the CMB. Polarization encodes information about the early universe at recombination when universe cooled down from 3,000 degrees Celsius to allow electrons to combine with nuclei.
Now Kovac and his colleagues are searching the CMB once again for inflation, a discovery that would be on par with the COBE results, which earned the 2006 Nobel Prize in Physics.
“It’s an amazing theory, and it’s so far removed from the energy scales and realms that we can normally test that it is quite impressive to me that we can work it in enough detail to design a machine that says whether or not it’s true,” Kovac said.
Justus Brevik is sipping on perhaps the strongest, darkest cup of coffee at the South Pole. The 24-hour horizontal rotation of the sun makes day and night meaningless during the summer, and the PhD student from the California Institute of Technology has worked tirelessly on troubleshooting some software glitches on the BICEP telescope.
The dark blue pedestal of the telescope dominates this half of the Dark Sector Lab, which also houses the much larger South Pole Telescope (SPT) down the hallway. The 10-meter dish of the SPT makes for dramatic photos. BICEP sports a 25-centimeter aperture.
Brevik explained that the width of the beam from the telescope is inverse to the size of the telescope’s aperture. “We’re very small on the ground, but we actually have very big beams on the sky,” he said.
Those beams are trained on the CMB, through a spot in the Southern Hemisphere called the Southern Hole, a “clean” patch in the night sky that has low emissions of polarized dust that could hamper measurements.
CMB telescopes like BICEP can’t directly revisit the moment of inflation. The cosmic microwave background is opaque, meaning the youngest snapshot physicist have of the early universe is at 380,000 years old.
“In order to see beyond that screen, we need to probe other things. We have to probe the pattern on the cosmic microwave background,” Brevik said. “If you rapidly expand the fabric of space-time, you’re going to generate gravity waves.”
Those gravity waves, referred to as B-mode, leave a distinctive signal on the surface of the CMB when they interact with the plasma. B-mode polarization has a spiral quality called curl, which would look like hurricanes in a hypothetical map of the polarization of the CMB.
“It’s a geometric pattern that can be distinguished from maps of extreme sensitivity,” explained Kovac, who is also a principal investigator on the BICEP experiment.
Spreading out at Pole
Make that BICEP2.
The first BICEP experiment ran from 2006-08. It didn’t detect B-mode polarization but it significantly narrowed the search. Cosmologists believe there is an indirect upper limit for how bright the polarization signal from inflation might be. The goal is to get below that upper limit and determine how faint that signal could be.
“With BICEP2 we’re hoping to make the next best direct constraint — or detect B mode polarization,” Brevik said.
The second-generation BICEP telescope made some technological leaps in hardware that allow it to map the sky about 10 times faster than its predecessor did. “In four days, we roughly achieved the same sensitivity map that BICEP1 would make in 40 days,” Brevik noted.
The SPUD telescope and its three receivers, housed in the nearby Martin A. Pomerantz Observatory on the old DASI telescope mount, are similar in ability to BICEP2. Next year the plan is to add two more receivers to the array.
Even that’s not enough “eyes” on the sky.
Chao-Lin Kuo at Stanford University envisions an even bigger array. He’ll start with Polar-1, a larger telescope than SPUD or BICEP that will be deployed in two years at the South Pole. Its “beam” will be smaller but with the higher resolution that comes with a bigger telescope.
“We want to have clearer maps with less noise, essentially,” Kuo explained over dinner in the South Pole dining room. “We’re going to improve sensitivity and resolution.”
Polar-1 will also join the search for CMB-generated B-mode polarization. But it will move beyond the quest for evidence of inflation to study some of the large-scale structures of the universe between the CMB and Earth using a technique called gravitational lensing.
Astronomers can measure how gravity bends the light of distant objects and calculate the gravitational pull of invisible matter. Gravitational lensing is another way to gain insight into persisting mysteries like dark energy and even neutrinos, subatomic particles with almost no mass produced by high-energy events in the universe.
“You’re using CMB as a tool to study matter distribution,” said Kuo, who is also a collaborator on the SPUD and BICEP programs. “We want to look at mass more directly, so gravitational lensing is one way to do it.”
Kuo said he hopes that Polar-1 lives up to its name, as the first of an array of CMB telescopes spread across the Dark Sector. The first generation will replace BICEP2 at the Dark Sector Lab.
“We eventually need multiple telescopes to get the sensitivity that we want or the survey speed that we want,” Kuo said. “We want Polar-1 to be a pathfinder for that program.”
The South Pole Telescope, currently focused on searching out galaxy clusters in the universe, will also shift to B-mode detection with the addition of a polarization-sensitive receiver in 2012.
Kovac said no one really knew where CMB research at the South Pole was going more than 15 years ago when he wintered with the Python telescope. “We only knew it was an exciting ride, and it hasn’t slowed down since then.”
NSF-funded research in this story: John Kovac, Harvard University, Award No. 1044978; Clement L. Pryke, University of Minnesota-Twin Cities, Award No. 1110087; Chao-Lin Kuo, Sarah Church, James Bock and John Kovac, Stanford University, Award No. 0960243. Background resources for this article included: “CMB from the South Pole: Past, Present and Future” by J.M. Kovac and D. Barkats, California Institute of Technology, Department of Physics; and “Alpha & Omega: The Search for the Beginning and the End of the Universe) By Charles Seife (Penguin Books 2004).
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