HERA, The Hydrogen Epoch of Reionization Array, may provide our first look at the oldest stars in the universe.
If you’re looking for the oldest stars in the universe, the best place to start is in a remote region of South Africa’s Karoo desert. This is the home of HERA, a radio telescope whose array of large mesh dishes looks totally alien against the stark backdrop of bitterbos shrubs and Karoo Koppies, the distinctive flat-top hills endemic to the region.
When HERA comes online in the coming months it will consist of 350 dishes that form a hexagonal footprint nearly a kilometer wide. If everything goes as planned, the telescope may provide our first glimpse of the cosmic “dark ages.” This period, and the epoch of reionization, form a billion-year blank spot in our understanding of the history of the universe that spans from the formation of the cosmic microwave background to the formation of the first stars.
Most modern radio telescope arrays, like ALMA in Chile, consist of massive mechanical radio dishes that cost hundreds of millions of dollars to build. Each dish in these arrays pushes the limits of engineering and requires construction that is precise down to the micrometer. In this sense, HERA is unlike any radio telescope in existence. Although it will also do cutting edge cosmological science, it is remarkable in that its dishes are largely made from off-the-shelf components, available at any hardware store.
In contrast to the gleaming white, custom-made dishes of other radio telescopes, much of HERA could be repurposed for plumbing. Each dish is about 45 feet in diameter and stands about ten feet tall. PVC pipes extend like the spokes in a wheel from the wooden perimeter of the dish and naturally sag to form a curve, which is covered with a fine mesh similar to chicken wire. HERA’s receivers are suspended above the dish using three telephone poles placed around the perimeter, a configuration similar to the massive Arecibo radio telescope in Puerto Rico. HERA may not look fancy, but when it comes online it will be one of the most sensitive radio telescopes ever built.
“HERA was designed to be very cost-effective, and so we did try and use material you can get at the hardware store,” David DeBoer, HERA’s project manager tells Supercluster. “The first prototype I built here in the US was sourced by me from my local Home Depot. This is the beauty of low frequency radio astronomy: one doesn’t need custom, expensive hardware. The wavelengths of the signals correspond to meters long, so ‘precision’ for us is a few centimeters.”
HERA is formally known as the Hydrogen Epoch of Reionization Array. It is specially designed to detect the ultra-faint, low-frequency radio waves produced by the neutral hydrogen gas that dominated the universe before the first stars and galaxies were formed. This period in the history of the universe is known as the epoch of reionization and is considered the “last frontier” for observational cosmology.
Cosmologists can see the early universe as it looked shortly after the Big Bang by studying radiation from the Cosmic Microwave Background (CMB), which formed about 380,000 years after the Big Bang. But cosmologists lack the observational data to back up theories that explain how the universe transitioned from the dense plasma of hydrogen gas we see in the CMB into the stars and galaxies we see today. This transition period is divided into two eras—the dark ages and the epoch of reionization—and as far as our understanding of the early universe is concerned, it remains a mystery. With any luck, HERA will be the instrument that starts solving it with observational data.
“Trying to understand the evolution of the neutral hydrogen that pervaded the early universe is really the last unexplored space on our cosmic map,” says Chris Carilli, chief scientist at the National Radio Astronomy Observatory and a contributor to the HERA project. “Here be dragons.”
AN EAR FOR THE EOR
So why is the epoch of reionization so hard to observe? Well, the Big Bang produced a dense, ultra-hot plasma made of fundamental particles, and after about 250,000 years, this plasma had sufficiently cooled to allow the particles to combine and form neutral hydrogen. Cosmologists can study the thermal ghost of this period, known as the Cosmic Microwave Background, which is the most ancient light cosmologists can see. As more and more stars and galaxies formed over the past 12 billion years, they drowned out the faint signals produced by the hydrogen gas that pervaded the universe during the dark ages and epoch of reionization.
The hot plasma of hydrogen gas that makes up the Cosmic Microwave Background is like a backlight against all the other objects in the universe. It’s relatively easy to see more recently formed objects, like galaxies and stars, against this background because their radio emissions are “loud” compared to faint emissions from the CMB. As the CMB cooled, its emissions became fainter—the cosmic equivalent of turning off the lights. This period, appropriately known as the dark ages, lasted until the universe was about 400 million years old. During this time, dark matter pulled neutral hydrogen together and the densest pockets of hydrogen slowly began to collapse from their own gravity.
The resulting temperature imbalances and concentrations of energy led to the formation of the first stars and black holes. This marked the end of the dark ages and the beginning of the so-called “Cosmic Dawn.” The first stars are estimated to have been about 30 to 300 times the size of our sun, but millions of times brighter. The high-energy radiation from these first stars began to re-ionize the neutral hydrogen throughout the universe, which meant that light could once again travel through the cosmos.
This epoch of reionization finished about 1 billion years after the Big Bang, which marks the limit of the oldest stars and black holes we can observe. The universe becomes increasingly dominated by neutral hydrogen as you work your way backwards from the end of reionization and this makes it difficult to observe the light of the earliest stars. If observational cosmologists want to study this period they have to find light from exceptionally energetic stars and black holes, as if they were mariners lost at sea looking for a lighthouse in a thickening fog.
Indeed, cosmologists trying to probe the epoch of reionization first did so by studying ancient quasars, a type of supermassive black hole that emits an enormous amount of energy as gas surrounding the black hole falls in. These observations and experiments involving the Cosmic Microwave Background helped establish boundaries on the beginning and end of the epoch of reionization. They provided strong evidence that the universe was in fact dominated by neutral hydrogen for hundreds of millions of years, but a “signal” from this period remained elusive.
“We know exactly what we should see, we just need to find it and characterize it,” Carilli says.
“The signal must be there unless we really don’t understand the universe.”
The problem is that studying individual quasars can’t provide a big-picture view of the evolution of the early universe. Eventually, astronomers reach a point far enough back in time where there are no quasars, just an endless sea of neutral hydrogen. Since light can’t pass through neutral hydrogen, astronomers needed to study the radio emissions from the hydrogen itself. This radiation is referred to as the 21 centimeter line (the approximate wavelength produced by neutral hydrogen) and is especially tricky to observe from the epoch of reionization because its weak signal is overwhelmed by radio signals from more recently formed objects. To put this into context, hydrogen signals from the epoch of reionization are roughly 100,000 times fainter than emissions from the Milky Way.
Once HERA is up and running, Jacobs says the team will be able to filter out a lot of this noise by taking an average of the foreground and removing it from the data. This should, in theory, make signals from the epoch of reionization stand out in the observations, provided the signal is bright enough to be detected in the first place. While filtering out this noise is far from trivial, it’s relatively straightforward when compared to the challenge of building and calibrating an instrument that is able to detect ultra-faint emissions from sources 13 billion light-years away.
HOW TO DETECT THE UNDETECTABLE
“There was a sort of big debate at the beginning about how to go about this measurement,” says Danny Jacobs, an astronomer at Arizona State University and HERA’s commissioning scientist. “The argument is that sensitivity was never going to be the issue. It was always going to be systematics—understanding your instruments and controlling them.”
This was the conclusion from PAPER, or the Precision Array for Probing the Epoch of Reionization, and the precursor to HERA. PAPER was a specialized radio telescope array designed to make the first statistical detection of the low-frequency radiation from neutral hydrogen produced more than 13 billion years ago. In 2005, the first four PAPER antennas were deployed at the Green Bank Radio Telescope in West Virginia. These initial antennas were used to test and make improvements to the hardware prior to their larger deployment in South Africa. In 2009, the first 16 PAPER antennas were deployed in the Karoo desert to begin the search for a hydrogen signal from the epoch of reionization. By studying variations in this signal, cosmologists could in principle create a 3D map of the evolution of the structure of the universe.
The number of South African antennas grew to 128 by 2013, but despite this increase in sensitivity and field of view, the hydrogen signal remained elusive.
But PAPER was just the first step on the road to an even larger telescope array designed to detect the low frequency emissions of neutral hydrogen from the early universe: HERA. With nearly three times as many antennas as PAPER, HERA stands to be one of the most sensitive low-frequency radio telescopes ever built. And this achievement is all the more impressive because of the simplicity of its construction — the average handyman would be familiar with most of HERA’s dish components.
In addition to the thousands of PVC pipes and wooden boards, Jacobs estimates that HERA uses enough telephone poles to run a telephone line for more than 30 miles. Getting all this equipment to the middle of the desert was challenging, Jacobs says, but the HERA team collaborated with locals from the nearby town of Carnarvon to do the construction and source materials.
“Carnarvon is a farming town, mostly sheep,” says Jacobs. “There’s a lot of people there that make things and so it was all local craftsmen that did the cutting, sawing, and drilling. Most of HERA by mass and volume was made locally.”
On any given day, there are about 15 people working on site in the Karoo desert building HERA’s dishes while locals in Carnarvon work on getting the materials ready. Jacobs says the on site team has got dish construction down to a science. A team of five people can finish construction on a dish in less than a week.
“The tricky part is the analog and digital side, but folks on site are learning to install those bits now and are making great progress,” Jacobs said. Indeed, the HERA team moved manufacturing of the telescope’s antenna feeds, one of the most technical and high-precision components of the dish, from the United States to South Africa, and so far the team says its seeing great results.
The first few HERA antennas came online in 2016 and since then the array has been steadily expanded. The work has been slow going, due to the complexity involved in connecting 350 separate instruments. Work on HERA was further delayed when the antennas on the dishes were upgraded to push their sensitivity to even lower frequencies that will hopefully allow the team to detect the formation of the first stars in the universe.
At present, the HERA team has completed work on nearly 300 dishes in the array, and expects to finish construction within the next year. But Jacobs says only a few dozen of those dishes are currently operational as the team works to debug the system before observations begin in earnest.
By drawing on design insights from PAPER combined with data analysis techniques and software developed for the Murchison Widefield Array, another low-frequency radio telescope in Australia, Jacobs and his colleagues hope to succeed where past experiments have come up short. Ultimately, the HERA team hopes to create a “movie” of the large-scale evolution of the universe that occurred directly before and after the first stars were formed. It’s the kind of discovery that Nobel Prizes are made of, and would produce a treasure trove of cosmological data. But Carilli is the first to point out that there are no guarantees that HERA will find what it’s looking for.
“I'll be honest, we're not absolutely sure this is the right way to go,” Carilli says. “We’re making progress, but it’s hard. Whether we have found the magic key remains to be seen, but that’s the nature of experimental physics.”
Both Carilli and Jacobs compared the challenges faced by scientists studying the epoch of reionization to those faced by scientists studying gravitational waves. Like the epoch of reionization, cosmological theory said that gravitational waves must exist. Still, it took nearly 50 years of experimental effort before the first gravitational waves were observed.
But even if HERA ultimately turns up empty handed, it will still make invaluable contributions to experimental cosmology. Not only will it help determine new directions to search for this signal, but it will also be used to inform the design of experiments implemented on the Square Kilometer Array, part of which will be located nearby the HERA site.
With a total collection area of one square kilometer, SKA will be the largest radio telescope in the world once it is built. Construction on SKA is expected to begin in late 2020 with observations beginning as soon as 2023. Although SKA won’t be focused solely on studying the epoch of reionization, its sensitivity means that it will likely be able to detect signals further back into the Dark Ages than any telescope that preceded it. Once SKA is online, it will give cosmologists an unprecedented look at the formation of the earliest stars, black holes, and galaxies in the universe.
Like the Greek goddess of childbirth from which the telescope takes her name, HERA will reveal how the stars and galaxies we see all around us were born. In this sense, the telescope array stands as a monument to the beginning of an exciting new era of observational cosmology.
Sure, it’s a monument made of plumbing parts, but at present it’s our best hope for writing the final chapter in the history of the universe.