There's an unwritten part of the history of the universe, somewhere between the Big Bang and the oldest galaxies Hubble has ever spotted. It's the time when the first stars died, giving way to the first galaxies and matter as we know it today.
Scientists call this period the Epoch of Reionization, and various teams are competing to find evidence of these first galaxies that formed during the missing time period. James Aguirre, a professor at the University of Pennsylvania in physics & astronomy, and Penn grad student Saul Kohn are part of a group looking for evidence in a unique way: an array of radio telescopes made from cheap parts and a few expensive sensors, all rigged up in the middle of nowhere in South Africa.
Very early in the history of the universe, the hydrogen hanging around was ionized by the heat of the big bang itself. Then, by about 400,000 years after the bang, the universe had cooled enough to allow neutral hydrogen to form, which led to the formation of the Cosmic Microwave Background—scientists can still see an echo of that today.
"But then the universe goes dark, and nothing else is putting out any light," Aguirre says. "And then there's a gap in our empirical knowledge, because the next thing we see is galaxies 400 to 600 million years later. By the time you get to 1 billion years after the big bang, you begin to see some things that are actually familiar."
In other words, there's a huge missing piece in our understanding of the history of the universe. These scientists hope to find it, and by doing so, find evidence of the first galaxies—ones the Hubble deep field images have only hinted at. These galaxies don't contain the first stars, mind you—those are thought to be huge, hulking titans 100 times the mass of the sun that burned out quickly and exploded into furious supernovae. These first supernovae, coupled with the primordial hydrogen, provided the seeds from which that the rest of the stars and galaxies grew.
A field an antennas
To hunt for the Epoch of Reionization, Aguirre, Kohn, and colleagues have set up an array of dishes in South Africa called PAPER, the Precision Array for Probing the Epoch of Reionization. Recently, work has begun on an major new project that will grow out of PAPER, called HERA, the Hydrogen Epoch of Reionization Array. Led by UC Berkeley, the team has expanded to include not just the members of PAPER, but now includes collaborators at universities across the U.S. and around the world.
PAPER is a series of small, evenly spaced antennas. They are about the size of a picnic table, and are built out of parts you can find at a hardware store: PVC pipe, chicken wire, coaxial cable, and copper tubing. The only expensive part is an amplifier inside that multiplies faint signals by factors in the millions.
"What you want is a big antenna," Aguirre says. "You put that out where there's not a lot of human-made transmitters and you look at the sky. You need a lot of these things, because the signal is faint."
Unlike a lot of observatories, PAPER is not focused on getting a great resolution picture. Rather than working like Hubble, which points in one area of the sky to get those deep views, it's observing the whole sky. But what PAPER may lack in visuals it's making up for in sensitivity, scanning for the faint whispers of where galaxies aren't.
"The whole idea of PAPER is that it's a very singular telescope, in the sense that it's the only telescope I've ever heard of that doesn't want to take pictures," Kohn says. "Anything we see is just foreground noise compared to what we want to detect."
But they need to scale up, and that's where HERA comes in. Think of it as a cannibalization of PAPER into something more powerful. Each PAPER antenna will be replaced with a 14-meter (46 ft) diameter parabolic dish, with all of the remaining electronics staying the same.
A murky universe
These two projects will hunt for the signal of not-yet-reionized hydrogen throughout the history of the early universe, hinting at what was happening during that crucial missing epoch. Here's what we think happened:
Essentially, that first generation of stars formed black holes when they died. Black holes burst forth with x-rays, which heat up hydrogen, causing it to emit radio waves. The subsequent ionization of hydrogen gas by UV light from galaxies produces a contrast between the radio emissions from the warm hydrogen and the non-emitting ionized regions. Where before the universe had been more or less uniformly murky, reionizing the hydrogen allowed light to peek out more into the interstellar medium.
With HERA, the researchers will be able to observe higher redshifts (greater distances) with more precision and trace interactions of x-rays and hydrogen. The entire array will be devoted just to detecting hydrogen reionization.
"We're an experiment, not an observatory. We're only going to do one thing and we're going to do it really well," Kohn says.
The array will reveal what may have happened in the early universe, and what heat sources created the second generation of stars—and the first galaxies.
"The least interesting thing you'll learn about going to higher redshift in heating the hydrogen is how black holes form," Aguirre says. "The most interesting thing you might find out is that there are other things heating the universe."
Already, there are hints about what happened during this epoch, including the recent announcement of the most distant galaxy ever found. "It appears that the galaxy exists at a time when there are still significant amounts of neutral hydrogen in between galaxies, which for us means that the reionization process is still underway," Aguirre says.
The timeline of the early universe
By September, HERA will be open for observation, starting with 19 antennas and eventually working its way up to 37 in 2016, all tuned into the early universe. By 2017, there will be 127, and within five years, 331.
When HERA turns on in September, it will be searching for a mysterious time in the history of the universe, after a sort of post-big bang dark ages and just before the kinds of galaxies and stars visible today were possible. It may hint at other mysterious forces in the process, including the composition of dark matter. In the process, it will create a timeline of how the early universe began to shine and allow bigger, more stable stars and galaxies to flourish.
And it will do so (mostly) with hardware store parts, a couple of high-end electronics, and a supercomputer in remote South Africa.