Volume 4, Issue 25 February 2007
Evidence for Inflation
Astrophysicist Adrian Lee is developing POLARBeaR, an instrument designed to obtain the first concrete evidence of the Big Bang. He is standing before one of his previous projects, APEX, located on Chile's Atacama Plateau. Image courtesy of Adrian Lee.
In the beginning, there was the Big Bang. Traveling faster than the speed of light, the universe swelled from an unimaginably hot, dense speck into a vast cloud of elementary particles—all in a trillion-trillionth of a second, a period known as "inflation." Then, over the next 14 billion years, that ball of particles continued to spread and grow at a far more steady rate, evolving into the protons, atoms, elements, stars, planets, and galaxies we know today.
At least, that's the creation story that cosmologists currently believe. But the fact is, despite many hints implying that inflation occurred, and abundant theories describing how it might have happened, no one has detected any direct evidence of that singular, mind-boggling expansion.
If UC Berkeley astrophysicist Adrian Lee has his way, this dearth of data about the newborn universe should disappear very soon. Lee and colleagues have devised an experiment called POLARBeaR (Polarization of Background Radiation), designed to detect patterns in space that could only have formed due to inflation.
At present, the most compelling inflation evidence is the cosmic microwave background, or CMB. This faint radiation permeates the entire observable universe—evidence that it was present early on, about 400,000 years after the birth of the universe. The other striking aspect of the CMB is its remarkably consistent temperature. The simplest explanation is that at some point the entire universe was small enough to be at thermal equilibrium. "I like to think of finding pieces of a hot potato scattered around a house," says Lee. "All of those pieces are the same temperature. The logical conclusion would be that they all belonged to the same potato, and that someone heated that potato, cut it up, and spread the pieces from room to room."
Lee has engineered a way to miniaturize and mass produce the photon detectors needed to obtain signals about primordial gravitational waves. He can now fit 1,000 detectors, called bolometers, onto a 9" x 9" silicon wafer. Image courtesy of Adrian Lee.
Minute variations in the temperature of the CMB were found in the early 1990s by George Smoot of UC Berkeley and John Mather of NASA Goddard Space Flight Center. They won the 2006 Nobel Prize in physics partly for this work.
Lee, too, studies the CMB, but seeks an even more subtle pattern within it. If inflation occurred, it would have created ripples in the fabric of space-time called gravitational waves. These waves should have etched a distinctive pattern into the CMB
"Imagine you're looking at the sky with eyes that can see microwaves, but you're also wearing polarized sunglasses. If you looked from place to place, you'd see faint fluctuations in the polarization intensity of the sky. If you could see the lines of the polarization, you would see that they are pointing in different directions in different points of the sky," Lee says.
The patterns would resemble the lines of force around a magnet, or the celestial gyres depicted in Vincent Van Gogh's painting Starry Night. "If we see these swirling patterns of vectors, they would be a smoking gun that inflation occurred," Lee says.
The POLARBeaR instrument will be built at 17,000 feet on Chile's Atacama Plateau, where vicunas, wild relatives of the llama, run wild. Image courtesy of Adrian Lee.
But detecting these patterns will be astoundingly difficult. The signals are very faint—a difference of billionths of a degree atop the 2.7 Kelvin of the CMB.
The best way to read CMB signals is with supersensitive radiation detectors called bolometers. From all the photons in the night sky, a bolometer can pick out the few that that are the right wavelength to have come from the CMB. Only 20 bolometer pixels, each as wide as a finger and a foot across, were needed in the CMB temperature fluctuation experiments. Each was hand built, hand tuned, and chilled to minus 272.5 degrees Celsius to improve their sensitivity.
But the polarization signals Lee seeks are a thousand times fainter. That means he needs at least a thousand bolometers for POLARBeaR. Handcrafting that many bolometer pixels, and chilling each, would be daunting.
Instead, Lee has engineered a way to build bolometer arrays using photolithography, the way computer chips are made. This has enabled him to manufacture all 1,000 pixels at once, and shrink them onto a silicon wafer no larger than a brownie pan.
When photons from the night sky hit the lens of the POLARBeaR telescope, they will be focused on the bolometer detector array. A cryostat will keep the bolometers near -273 degrees Celsius. The multiplexor will both amplify the signals from each bolometer and assemble them into an image in a manner similar to that used in digital cameras. Image courtesy of Adrian Lee.
POLARBeaR itself will consist of a telescope with a 3.5 m diameter telescope sited on the Atacama Plateau of Chile. The telescope will focus an image of the sky onto the bolometer array. At 17,000 feet above sea level, staff must wear oxygen prongs to breathe. But the advantages of altitude are well worth the trouble. At sea level, 50 percent of photons are absorbed by atmospheric water vapor; that drops to 10 percent three miles up.
Lee and colleagues at Lawrence Berkeley National Laboratories, UC San Diego, the University of Colorado, McGill University, College de France, and Imperial College London have submitted a $9 million proposal to fund POLARBeaR to the National Science Foundation.
Signals obtained by POLARBeaR would be a phenomenal advance in our understanding of the early universe. "The earliest thing that man could conceive of seeing right now would be these gravitational waves," Lee says. "I'm hoping that this information will allow us to go back and discover things we haven't even predicted yet about that time."