Imagine the universe billions of years ago as a lump of Swiss cheese. About 400,000 years after the Big Bang, it had cooled enough to produce hydrogen atoms—the “cheese.” As stars began to form, they emitted radiation that ionized these hydrogen atoms, a process which caused the holes to appear. The light that this process produced billions of years ago still exists, and can be used to chart the birth of the first galaxies. But does the technology exist to do so? Not even the famed Hubble telescope can reach back to the very first galaxies. In order to glimpse that far into the history of the night sky, Assistant Professor of Physics and Astronomy James Aguirre is using a very special telescope.
“It’s a sort of super FM radio,” says Aguirre. “It uses a system of 128 antennas that look like baskets. Really it’s just PVC pipe from the plumbing store that’s holding up chicken wire that is our reflecting element.” This is PAPER, also known as the Precision Array to Probe the Epoch of Reionization, a groundbreaking array located in the South African desert, where outside frequencies don’t interfere with testing. PAPER combines the capacity to pick up extremely low frequencies with the supercomputing to make sense of the signal. Unlike normal antennas, PAPER’s are stationary and horizontally oriented. As the Earth rotates, they chart what is passing directly overhead as the night goes on. Data is gathered in a series of 12-hour sessions over the course of six months.
“It allows us to effectively lie on our backs in the field and stare deep into space,” says Aguirre. “With traditional advanced telescopes you point it at a patch of the sky and you see the very faint light of galaxies, but this is light from the hydrogen gas filling the space between galaxies. What we’re doing is asking what effect the first galaxies had on their surroundings during the first billion years of the universe’s life.”
PAPER takes advantage of the wave nature of light by recording the electric field at the positions of each antenna. A voltage appears across the antenna, which Aguirre’s team then amplifies, just like a normal radio receiver would. Next, digital records of the voltages are recorded.
“What a lens would do is interfere with the light from two different positions into a point and make an image there,” says Aguirre. “We’re making this process electronic, and instead of looking at one small patch of sky like Hubble, we’re measuring almost all of the sky visible in the southern hemisphere, corresponding to regions billions of light years across.”
Plans to upgrade PAPER have recently been given seed money by the National Science Foundation (NSF). The new array, called HERA (the Hydrogen Epoch of Reionization Array) is a collaboration between the University of Pennsylvania, University of California Berkeley, MIT, the University of Washington, Arizona State University, UCLA, Cambridge University, and the National Radio Astronomy Observatory. In order to upgrade the collecting area on the ground, the “baskets” will be replaced by much bigger parabolas, which will allow Aguirre and his team to measure much fainter signals and, they hope, reach to earlier times by detecting even lower frequencies. “We have a big challenge ahead of us,” says Aguirre. “We are making great strides with PAPER, but HERA will let us really take off.”
As part of his efforts to bring his research to a wider audience, Aguirre has begun a project to convert an unused satellite dish the on the roof of the Enterprise Center in West Philadelphia. “The idea is to involve students directly in the construction, using a combination of commercially available components and the same state-of-the-art electronics used in HERA, providing true crossover between education, community outreach, and cutting-edge research.”
