by Dr. Peter V. Mason, California Institute of Technology, ret. Jet Propulsion Laboratory, CSA Fellow, pmason@alumni.caltech.edu
We sit at the center of the universe, with the edge about 13.8 light-years away, receding at the speed of light and back in time. As far as any experimental evidence is concerned, this is the beginning of our universe. About 500 light-years after the creation there is a curtain that we can’t peer though. Beyond that curtain, the universe was ionized and the photons just bounced off the ions. Once the universe deionized, the photons could travel freely, which is why we see all those messy things like stars, galaxies, clusters of galaxies, superclusters of galaxies, etc. We can still look past the mess and see the cosmic microwave background at 13.3 billion light-years, but not beyond.
But wait! There are shifting shadows on that curtain. There are dances going on behind that curtain, and we can make out the vague figures of the dancers. As an electrical engineer (yes, electrical engineers can become cryogenicists), I know these as patterns of electric and magnetic fields, and I can measure them. Because of the red-shift, these patterns are in the infrared part of the spectrum—even though they started out life at 3,000K, so we need to observe at 1 or 2 mm wavelength. Since the atmosphere has many molecules vibrating in the infrared, and since water vapor supplies most of these, we need to be high, dry and where there is as little ozone as possible. Space would be ideal, but space is remote and expensive, and it takes a long time to fund, build, fly and analyze data from space. Almost as good is Antarctica, in particular the South Pole. At 10,000 feet, it’s the driest place on Earth and the ozone hole is our friend. In fact, in many ways it is better than space because we can run for several years, improving the telescopes as we learn their weaknesses.
The first BICEP (Background Imaging of Cosmic Extragalactic Polarization), BICEP1, observed with 49 spider-web bolometers for three years. BICEP2 carried 256 dual polarization transition-edge superconducting detectors of greatly increased sensitivity. It also observed for three years. We report here on the results of these two instruments. These were followed by the Keck array of five BICEP2 instruments whose full results are still being analyzed. In turn this will be followed by BICEP3, which will be of even higher performance. BICEP 3 will become operational in early 2015.
The BICEP1, BICEP 2 and Keck instruments scanned the parts of the sky around the South Pole, where there are the fewest nearby objects (stars, galaxies, etc.) to interfere, for six years. Professor Andrew Lange of Caltech initiated the ideas, which were developed by the BICEP team, led by Professor John Kovac, formerly at Caltech and more recently a professor at Harvard, and Professor Jamie Bock, Caltech, who took over from Professor Lange. The results from BICEP1 and BICEP2 have been published in the last few months in the New York Times, the LA Times, Time magazine (March 30) and many other news and scientific publications.
BICEP1 put a very low upper limit on the B-mode, while BICEP2 actually detected B-mode signals with a certainty of about 5 sigma, i.e. far more than the 3 sigma usually defined as true detection. The B-mode polarization is characterized by a square array of magnetic vectors (Figure 1), and this was seen distinctly. The importance of the B-mode polarization is that it is the key signature of gravitational waves. Scientists all over the world have been looking for gravitational waves and this is the first real detection of their existence.
Instrument
Both instruments are housed in cryostats about 35 cm in diameter and 1.2 m long (Figure 2). Inside is a mirror about 60 cm in diameter. A small aperture refracting telescope provided angular resolution of 0.93° and 0.60° at 100 and 150 GHz, respectively. A signal of 300 microKelvin can be detected in one second. Sensitivity increases as the square root of observing time, so in three years one can detect about 1 nano-Kelvin variation on the sky.
The detector array was enclosed in the 1.2 m cryostat. Inside the cryostat was a closed-cycle three-stage refrigerator, He4, He3 and He4/He3 dilution, which achieved about 270 milliKelvin at the focal plane (Figure 3). BICEP1 operated from the Dark Sector Lab at Amundsen-Scott South Pole Station from January 2006 through December 2008. It was followed by BICEP2, with many more detectors, operating from 2009 through 2011. Results are to be published as arXiv:1403.3985 and arXiv:1403:4302. When the article is accepted, I will inform my readers where to find it.
