From probing the Dark Ages of the universe to building a telescope on the moon, pinpointing the light of the first star will be no easy task.
Jack Burns is interested in the Dark Ages of the universe. His primary research team—consisting of 22 scientists from CU, UCLA, NASA Goddard, Arizona State and more—wants to know how things began. Specifically, they are working to investigate the time when the universe's very first stars were born, a period that's clouded in mystery and darkness. And they are going to do it by launching a telescope to the moon.
To make such an elusive discovery as the light of the first stars, Burns is leading a number of projects to ultimately allow astronauts to remotely pilot rovers on the lunar surface from a space habitat orbiting near the moon. In time, those systems could lead to a radio telescope constructed on the far side of the moon and maybe even pave the way for manned flights to Mars and beyond. But for Burns, who is interested in the realms of space and time that lie far beyond and far before our Milky Way, the amazing machines are the means, not the end.
"The engineering is fun. The technology is fun. But for us, it's all about the science," Burns tells me at the University of Colorado at Boulder, where he teaches and works.
The Beginning of Time
A lot happened in the first few seconds.
Immediately after the Big Bang, the universe was much smaller but rapidly expanding. After just 10−43 seconds came the Planck epoch, when temperatures and pressures were so high that the four fundamental forces of physics were not distinct from one another and existed as one force. They separated during the Quark epoch (10−12 seconds), the earliest conditions of the universe that we can simulate in the Large Hadron Collider. Then came the Hadron epoch, when the first protons and neutrons formed from quarks, about 1 second after the Big Bang. At roughly 10 seconds, the universe entered the Photon epoch, when atomic nuclei, electrons and photons floated freely in a plasma, though things were still too hot for stable atoms to form.
Things stayed more or less this way for about 380,000 years, until a period known as "recombination." During that time, cooler temperatures allowed electrons to bond to nuclei and form stable atoms of neutral hydrogen and helium. (This happened for the first time—the term recombination is a misnomer from before Big Bang cosmology became the leading theory of astrophysicists.) The universe became transparent. For the very first time, photons of light could travel freely through space, which astronomers observe today as the Cosmic Microwave Background (CMB).
And yet, despite this liberation of light, what came next is what cosmologists call the Dark Ages. For the next hundreds of millions of years, the universe was a relatively calm sea of neutral atoms, mostly hydrogen with some helium mixed in. This persisted until enough gas finally coalesced to spark the fires of nuclear fusion, forming our first stars.
But no one knows precisely when this happened.
Estimates for the first stars range from about 100 million years to 400 million years after the Big Bang. Part of the problem is that during the Dark Ages, the only electromagnetic radiation emitted came from neutral hydrogen and had a wavelength of about 21 cm and a frequency of about 1,400 MHz —known as the hydrogen line or the 21-centimeter line. This incredibly old light has been traveling through the expanding universe for over 13 billion years, stretching its wavelength from 21 cm to between 5 and 10 meters and decreasing its frequency from 1,400 MHz to roughly 40 MHz. Light that was released in the Dark Ages as microwaves has redshifted into radio waves by the time it reaches our telescopes today.
Studying this redshifted line could reveal the illumination of the very first stars. There's just one problem: Our planet produces too much radio interference to get an accurate reading, with too many errant signals flying around from all of our electronics. But one of the most radio-silent places in the entire solar system is relatively nearby, on the far side of the moon.
The Dark Ages Radio Explorer
Jack Burns' Dark Ages Radio Explorer (DARE) is a space radio telescope currently under review by NASA and planned for launch in mid-2023. The spacecraft is spec'd to launch in a SpaceX Falcon 9, though NASA will officially select the launch vehicle at a later date. The craft will be built by Ball Aerospace, and it will settle into an orbit around the moon for two years of science before plunging into the lunar surface.
Because the moon is tidally locked with the Earth, its far side always faces out to space, and the moon's body acts as a shield from Earth's cacophony of radio waves. It's the perfect spot for DARE to search for ancient stars.
The stars DARE seeks are called Population III stars. They are incredibly massive and volatile. They weigh in at 100 times the mass of the sun or more, are made almost entirely of hydrogen and helium, and have short lives, burning through their nuclear fuel in only three million years or so before going supernova. However, this is all based on models and simulations, as no one has ever observed a Population III star (though some astronomers speculate that they could still exist in dwarf galaxies today).
"What we'll be able to do is pin down for the first time when those first stars turn on, which, we don't know," says Burns.
All the elements heavier than hydrogen and helium—including the matter that created all of us—formed in the nuclear fusion furnaces at the cores of stars. The heaviest of elements, like platinum, gold, and uranium, were fused from lighter elements in volatile cosmic events such as supernova.
Population I stars, like our sun, are the youngest generation of stars, relatively rich with metals that already formed in the fiery explosions of older stars. Population II stars are older, metal-poor stars that are generally found in the outer regions of galaxies where stellar formation is less common and fewer heavy elements are produced. Population III stars have almost no metals at all, and they formed at a time when the universe itself had almost no heavy elements. The belief is that Population III stars must have ignited, fused their fuel, and gone supernova to eject heavy elements into the universe before Population II stars could form at all.
Walking the Hydrogen Line
One of the fundamental questions DARE will answer is: Was the early universe a minefield of rapidly detonating Population III stars for a long time? Or did the Population III stars quickly generate enough heavy elements for Population II stars to form and take over?
"We really don't know, and it's really important to figure out what did happen that led to these second generation stars and then finally to us," says Burns. How Population II stars formed depends partly on how far heavy elements were ejected when Population III stars went supernova.
"It could be timescales of millions of years [between Population III and II], it could be hundreds of millions of years, depending on how these stars explode," says Jordan Mirocha, a UCLA astronomer who developed models of the early universe for DARE—predictions the spacecraft will try to confirm.
DARE will observe the universe that existed from about 80 million years to 400 million years after the Big Bang, with the first stars likely appearing around 100 or 200 million years. The space telescope will zero in on that hydrogen line, the redshifted 21-cm wavelength of electromagnetic radiation. That signal is created by a change in the energy state of a neutral hydrogen atom, called a spin transition or a spin-flip. The name refers to a shift in the angular momentum of a hydrogen atom's single electron from "spin up" to "spin down," or vice versa.
Population III stars are thought to emit vast amounts of ultraviolet light. When the photons from that light hit the ambient hydrogen gas of the early universe, they disrupt the spin-flip process of the electrons in those hydrogen atoms. The temperature of the gas cools as a result in a process similar to laser cooling. This change in temperature—the disturbance of the hydrogen line—is what DARE is looking for, and what will allow scientists to pinpoint the time when the very first stars flared up into existence.
One more thing: Shortly after those first stars form, and possibly immediately after the first ones supernova, the universe gave birth to the first black holes. It could be "just millions of years between stars and black holes," says Burns, "could be a fair bit shorter than that." When the black holes do form, and when they begin to accrete material—forming a massive disk of spiraling debris that is consumed as soon as it crosses the event horizon—then they will emit X-rays and reheat the universe. DARE will be able to detect that disturbance in the hydrogen line, too.
As for the dawn of the first galaxies, "that's something we don't know," says Mirocha. "We sort of suspect that for a time after the first generation of stars, more of these things wouldn't resemble objects that we would call galaxies necessarily." Stars may have become gravitationally bound in clusters early on, but when those stellar clusters evolved into what we recognize as galaxies is still a matter of debate.
"We're Going Back to the Moon"
Jack Burns is a member of President Trump's NASA transition team, and in his quest to study the cosmology of the Dark Ages, he has found himself working on projects in other parts of the agency—planetary science and robotics, for example. Burns wants to use remotely controlled rovers, operated by astronauts in an orbiting lunar space habitat, to build a radio telescope array on the far side of the moon. The astronaut habitat would orbit at the Lagrange 2 (L2) point of the Earth-moon system—hovering just beyond the far side of the moon. After the radio telescope array is deployed on the lunar surface by a team of RC rovers, it could pick up studying the ancient universe where DARE leaves off.
NASA Ames scientist Terry Fong and Burns already led a study in 2013 that involved astronauts on the International Space Station controlling a rover at the Ames Research Center in Silicon Valley. On the ISS, NASA astronaut Chris Cassidy controlled one of the K10 rovers at the NASA Ames Roverscape, an outdoor testing ground for robotics the size of two football fields. He used the rover to deploy strips of composite material that are similar to what would be used to build a radio telescope on the moon. Burns also has a telerobotics team at CU working on machine-vision systems that will one day be crucial to operating surface rovers from orbiting space stations.
"What we want to do is demonstrate next at the moon, but do something that is scientifically interesting—namely, deploy a low-frequency radio telescope on the far side of the moon," says Burns. Such a mission could have multifold benefits, such as providing a testbed for telerobotics technology in space and an opportunity to develop a reusable lunar lander to collect surface samples and dock with the habitat—something Blue Origin has expressed interest in. Ultimately, the data and research from an orbiting lunar habitat would help us create a similar arrangement at Mars, where robots could start building a surface habitat while astronauts control and monitor from above.
As part of Trump's transition team, Burns encouraged NASA to consider launching astronauts on the Space Launch System's (SLS) maiden flight, partly because it would accelerate the timeframe for placing a habitat in orbit at Earth-moon L2. Such a habitat would dock with the Orion spacecraft, offer a place for astronauts to live for months at a time, and provide a platform for astronauts to control rovers on the lunar surface. "If the first crewed mission of Orion takes place in 2019, we're ready to start this in the early 2020s," says Burns.
His focus on missions with the highest scientific output possible—ones that combine cosmological research with planetary science and the development of space infrastructure—is indicative of the new priorities for NASA moving forward. From placing hydrogen line-monitoring spacecraft into orbit to launching telescope-deploying robots to the moon, NASA is attempting to simultaneously achieve its two primary objectives—learn how we came to be out of the early chaos of the universe, and figure out how we are going to climb back out into the abyss and settle other worlds.
"We're sending a message to the American public," Burns says. "Space is back."
Jay Bennett is the associate editor of PopularMechanics.com. He has also written for Smithsonian, Popular Science and Outside Magazine.
