BICEP adds muscle to South Pole research
Telescope to measure the polarization of the cosmic microwave background to help scientists understand the moment after the big bang
Posted January 21, 2007
Imagine a universe that exploded into existence and expanded exponentially, faster than the speed of light. Cosmologists operating a new telescope at Amundsen-Scott South Pole Station hope to put that crucial moment and the millennia that followed after the big bang into focus – and, in the process, nudge the study of physics into a new era.
They believe BICEP has the muscle to give them that push. BICEP (Background Imaging of Cosmic Extragalactic Polarization) is an experiment to measure the polarization of the Cosmic Microwave Background (CMB). The telescope was installed last austral summer in the Dark Sector Laboratory. It operated throughout the winter with hardly a hiccup, according to Jamie Bock, the co-principal investigator for the project from NASA’s Jet Propulsion Laboratory.
“Our first season of observations went incredibly smoothly,” said Bock, following a visit to the Pole in December to assist with upgrades and calibration of the instrument. “I think everyone is still a little surprised that we didn’t have some unforeseen problem, which is the usual situation. A lot of the credit for this goes to our first-season winter-over Denis Barkats. We’re still analyzing all of the polarization data from the first season.”
The study of the CMB and events even further back to the very beginning of time involves some rather abstract theories involving such things as “inflation” and “curls.” You don’t have to have a degree in Star Trek physics to appreciate this quest to understand the alpha and omega of the universe, but a little Cosmology 101 may be helpful.
It all started with a bang
Recall that the most widely accepted theory about the existence of the universe is that it all began with a big bang, which created all the mass and energy in the universe. Scientists generally believe that a fraction of a fraction of a second later, the universe burst forth at incredible speed and slowed its expansion almost as quickly.
About 400,000 years after this cataclysmic event, 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 14 billion years as the universe continued to expand. That brilliant light 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.
That faint hiss of microwaves, with a temperature of 2.7 degrees above absolute zero (Kelvin), is the cosmic microwave background or CMB. This radiation comes from every direction in the sky.
“Images of the CMB are basically a picture of the universe at that time,” Bock explained. “It was a much different place than our universe of today. There were no stars or galaxies. Instead, we see an almost perfectly uniform plasma.”
Plasma is an ionized gas of free electrons and protons. It is also opaque. As Bock explained: “You can see this for yourself if you look at a candle and notice that you can’t easily see through the yellow part of the flame. In a similar way, you can’t see past the plasma at the time of the CMB, so maps of the CMB are basically maps of the surface of this opaque plasma.”
The ancient radiation of the CMB possesses much valuable information about the early universe. It contains “hot” and “cold” spots that are slight irregularities on an otherwise uniform plasma. 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.
This opaque plasma causes a problem if you’re interested in learning what happened in the first 400,000 years, when the newborn universe was still rattling with freely roaming atomic particles. In effect, the CMB surrounds us like the walls of a cage, and it takes some ingenuity to see between the bars.
Seeing between the lines
In recent years, scientists have learned something about the polarization of light from the CMB. In this case, polarization is the alignment of light waves as they travel. For instance, polarized sunglasses are able to block road glare because when light hits the road it polarizes in a horizontal direction and the glasses are designed to block photons oriented that way.
Similarly, the alignment of photons emanating from the CMB polarized parallel to the last scattering surface, that cloud of plasma. The polarization contains information about the universe at recombination, when electrons combined with their nuclei.
The rapid expansion of the universe is referred to as inflation. Inflation stretched the universe out to give it a flat appearance sometime in that first fraction of a second after the big bang. This inflation produced gravitational radiation, ripples in space-time that occurred right after the birth of the universe.
Gravitational waves produced a signature after inflation, which squeezes space in one direction and expands it in the other. When these waves interact with the plasma, it gives a polarization with a spiral quality called curl, which would look like hurricanes in a hypothetical map of the polarization of the CMB.
“These gravitational waves make subtle changes in CMB images, because they slightly stretch space when the universe was 400,000 years old,” Bock said. “In particular, there should be a distinctive effect in the polarization of the CMB, and that is what BICEP is looking for. If we see it, then we can say something about what happened in the first moments after the big bang.”
BICEP is specially designed to search to very deep levels for this polarization signal, according to Bock. It might look somewhat “puny” matched up to the 10-meter South Pole Telescope (SPT) currently under construction next to the Dark Sector Laboratory. But the signal scientists are after, while faint, does not require a large aperture. The telescope has an aperture of 30 centimeters but has a wide field of view.
“And our focal plane is packed with detectors, so just because we’re small, it doesn’t mean we aren’t powerful,” he said, adding that being compact has its advantages. For example, the whole instrument can fit into one LC-130 airplane.
Its relatively small size also allowed the team to outfit it with a series of baffles, or plates, to block out “terrestrial contamination” such as the horizon. It is also possible to fill the entire telescope beam with a polarized calibration source, which would be practically impossible to do on a large telescope, according to Bock.
“All of these things make BICEP the ideal instrument for measuring CMB polarization on large angular scales,” he said. “Large telescopes like the SPT are naturally complementary because they access the small angular scales we can’t see.”
A new frontier
Three universities are collaborating with NASA’s Jet Propulsion Laboratory on the experiment – California Institute of Technology, the University of California Berkeley and the University of California San Diego.
The telescope made observations throughout the winter. It easily picked out the temperature signals of the cold and hot spots in the CMB, though the polarization signals were not nearly as strong.
“A few years ago, these temperature signals would have made a big stir, but now we use them to calibrate,” Bock noted. “It just shows we have moved into a new era.”
Cosmologists believe there is an indirect upper limit for how bright the polarization signal from inflation might be. The BICEP team’s goal is to get below that upper limit and determine how faint that signal could be. It is possible BICEP could make a discovery, and there are plans to make a more powerful successor instrument that could go even deeper, according to Bock.
“Detection would be quite a stunning achievement – imagine deducing how the universe behaved just an instant after the big bang,” he said. “This is even more exciting when you consider that the physics driving inflation are out of reach by other means, because inflation probably happens at energy scales beyond even the most capable particle accelerators in high-energy physics.
“So not only would we learn about the universe in its first moments, we would learn new physics.”
NSF-funded research in this story: Jamie Bock, NASA Jet Propulsion Laboratory, and Andrew Lange, California Institute of Technology.