INTO THE DARK AGES Radioastronomers take aim at the Universe’s first billion years. BY DAVIDE CASTELVECCHI o get an idea of what the Universe looks time when the Universe was just one-thirtieth like from Earth’s perspective, picture a big of its current age of 13.8 billion years. Beyond T watermelon. Our Galaxy, the Milky Way, is those, at the thin, green outer layer of the water- one of the seeds, at the centre of the fruit. The melon skin, lies something primeval from space around it, the pink flesh, is sprinkled with before the time of stars. This layer represents countless other seeds. Those are also galaxies the Universe when it was a mere 380,000 years that we — living inside that central seed — can old, and still a warm, glowing soup of sub atomic observe through our telescopes. particles. We know about that period because its Because light travels at a finite speed, we see light still ripples through space — although it other galaxies as they were in the past. The seeds has stretched so much over the eons that it now farthest from the centre of the watermelon are exists as a faint glow of microwave radiation. the earliest galaxies seen so far, dating back to a The most mysterious part of the observable 298 | NATURE | VOL 572 | 15 AUGUST 2019 ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. FEATURE NEWS A night view of part Universe is another electromagnetic frequency and so a rather long years ago, this afterglow of the Big Bang would of the Murchison layer of the water- wavelength, of slightly more than 21 cm. have looked uniformly orange to human eyes. Widefield Array in melon, the section It was this hydrogen signature that, in Then the sky would have reddened, before Western Australia. between the green the 1950s, revealed the Milky Way’s spiral slowly dimming into pitch darkness; there was shell and the pink structure. By the late 1960s, Soviet cosmolo- simply nothing else there to produce visible flesh. This represents the first billion years gist Rashid Sunyaev, now at the Max Planck light, as the wavelengths of the background of the Universe’s history (see ‘An Earth’s-eye Institute for Astrophysics in Garching, radiation continued to stretch through the view of the early Universe’). Astronomers have Germany, was among the first researchers infrared spectrum and beyond. Cosmologists seen very little of this period, except for a few, to realize that the line could also be used to call this period the dark ages. exceedingly bright galaxies and other objects. study the primordial cosmos. Stretched, or Over time, theorists reckon that the Yet this was the time when the Universe redshifted, by the Universe’s expansion, those evolving Universe would have left three dis- underwent its most dramatic changes. We 21-cm photons would today have wavelengths tinct imprints on the hydrogen that filled know the end product of that transition — we ranging roughly between 1.5 and 20 metres — space. The first event would have begun some are here, after all — but not how it happened. corresponding to 15–200 megahertz (MHz). 5 million years after the Big Bang, when the JOHN GOLDSMITH/CELESTIAL VISIONS JOHN GOLDSMITH/CELESTIAL How and when did the first stars form, and what Sunyaev and his mentor, the late Yakov hydrogen became cool enough to absorb more did they look like? What part did black holes Zeldovich, thought of using the primordial of the background radiation than it emitted. play in shaping galaxies? And what is the nature hydrogen signal to test some early theories Evidence of this period should be detectable of dark matter, which vastly outweighs ordinary for how galaxies formed2. But, he tells Nature, today in the CMB spectrum as a dip in inten- matter and is thought to have shaped much of “When I went to radioastronomers with this, sity at a certain wavelength, a feature that has the Universe’s evolution? they said, ‘Rashid, you are crazy! We will never been dubbed the dark-ages trough. An army of radioastronomy projects small be able to observe this’.” A second change arose some 200 million and large is now trying to chart this terra The problem was that the hydrogen line, years later, after matter had clumped together incognita. Astronomers have one simple source redshifted deeper into the radio spectrum, enough to create the first stars and galaxies. This of information — a single, isolated wavelength would be so weak that it seemed impossible to ‘cosmic dawn’ released ultraviolet radiation into emitted and absorbed by atomic hydrogen, isolate from the cacophony of radio-frequency intergalactic space, which made the hydrogen the element that made up almost all ordinary signals emanating from the Milky Way and there more receptive to absorbing 21-cm pho- matter after the Big Bang. The effort to detect from human activity, including FM radio tons. As a result, astronomers expect to see a this subtle signal — a line in the spectrum of stations and cars’ spark plugs. second dip, or trough, in the CMB spectrum at hydrogen with a wavelength of 21 centimetres The idea of mapping the early Universe a different, shorter wavelength; this is the signa- — is driving astronomers to deploy ever-more- with 21-cm photons received only sporadic ture that EDGES seems to have detected1. sensitive observatories in some of the world’s attention for three decades, but technologi- Half a billion years into the Universe’s most remote places, including an isolated raft cal advancements in the past few years have existence, hydrogen would have gone through on a lake on the Tibetan Plateau and an island made the technique look more tractable. The an even more dramatic change. The ultraviolet in the Canadian Arctic. basics of radio detection remain the same; radiation from stars and galaxies would have Last year, the Experiment to Detect the brightened enough to cause the Universe’s Global Epoch of Reionization Signature hydrogen to fluoresce, turning it into a glowing (EDGES), a disarmingly simple antenna in the source of 21-cm photons. But the hydrogen Australian outback, might have seen the first “THIS IS closest to those early galaxies absorbed so hint of the presence of primordial hydrogen much energy that it lost its electrons and went around the earliest stars1. Other experiments PART OF OUR dark. Those dark, ionized bubbles grew bigger are now on the brink of reaching the sensitivity over roughly half a billion years, as galaxies that’s required to start mapping the primordial grew and merged, leaving less and less lumi- hydrogen — and therefore the early Universe — GENESIS nous hydrogen between them. Even today, in 3D. This is now the “last frontier of cosmol- the vast majority of the Universe’s hydro- ogy”, says theoretical astrophysicist Avi Loeb gen remains ionized. Cosmologists call this at the Harvard-Smithsonian Center for Astro- STORY.” transition the epoch of reionization, or EOR. physics (CfA) in Cambridge, Massachusetts. The EOR is the period that many 21-cm It holds the key to revealing how an undistin- many radio telescopes are constructed from radio astronomy experiments, either ongoing or guished, uniform mass of particles evolved into simple materials, such as plastic pipes and in preparation, are aiming to detect. The hope is stars, galaxies and planets. “This is part of our wire mesh. But the signal-processing capa- to map it in 3D as it evolved over time, by taking genesis story — our roots,” says Loeb. bilities of the telescopes have become much snapshots of the sky at different wavelengths, more advanced. Consumer-electronics com- or redshifts. “We’ll be able to build up a whole A FINE LINE ponents that were originally developed for movie,” says Emma Chapman, an astrophysicist Some 380,000 years after the Big Bang, the gaming and mobile phones now allow obser- at Imperial College London. Details of when Universe had expanded and cooled enough vatories to crunch enormous amounts of data the bubbles formed, their shapes and how fast for its broth of mostly protons and electrons with relatively little investment. Meanwhile, they grew will reveal how galaxies formed and to combine into atoms. Hydrogen dominated theoretical cosmologists have been making what kind of light they produced. If stars did ordinary matter at the time, but it neither emits a more detailed and compelling case for the most of the reionization, the bubbles will have nor absorbs photons across the vast majority promise of 21-cm cosmology. neat, regular shapes, Chapman says. But “if of the electromagnetic spectrum. As a result, there are a lot of black holes, they start to get it is largely invisible. DARKNESS AND DAWN larger and more free-form, or wispy”, she says, But hydrogen’s single electron offers an Right after atomic hydrogen formed in the because radiation in the jets that shoot out from exception. When the electron switches between aftermath of the Big Bang, the only light in the black holes is more energetic and penetrating two orientations, it releases or absorbs a photon. cosmos was that which reaches Earth today as than that from stars. The two states have almost identical energies, so faint, long-wavelength radiation coming from The EOR will also provide an unprecedented the difference that the photon makes up is quite all directions — a signal known as the cosmic test for the current best model of cosmic evolu- small. As a result, the photon has a relatively low microwave background (CMB). Some 14 billion tion.