NEWS FEATURE NEWS FEATURE News Feature: Reionizing the universe A slew of current and planned space projects should help scientists better understand the mysterious star- and galaxy-forming epoch that followed the Big Bang. Adam Mann Science Writer grew and merged. The energy onslaught continued for nearly a half-a-billion more years, ripping the neutral hydrogen back In the first fractions of a second after the Big filled with billowing clouds of hydrogen gas, into its constituent protons and electrons Bang, the cosmos expanded exponentially, entering what cosmologists call the cosmic and transforming the entire universe. This then became a blistering soup of fundamental dark ages. wasthestartoftheeraofreionization,a particles and energy, and eventually cooled to Then everything changed. About 500 strange and important epoch that nonetheless a point where protons and electrons could million years after the Big Bang, intense remains poorly understood. combine to form neutral hydrogen. A few UV radiation suddenly began to burn into Cosmologists think that during reioniza- hundred million years later, the universe was the gas, creating massive hollow holes that tion, the first stars and galaxies switched on and enormous black holes consumed any- thing within their reach. “The Big Bang was the beginning,” says Richard Ellis, an astron- omer at the California Institute of Technology in Pasadena, California. “But the moment whentheuniversewasbathedinstarlight is the birth of us, because we’re made of star stuff.” Were these first stars and galaxies and supermassive black holes responsible for the UV light that ripped apart hydrogen? Our current best observatories offer only pre- liminary answers. But soon, powerful new space- and ground-based telescopes, along with instruments that can probe the distri- bution of neutral hydrogen gas throughout the cosmos, will allow astronomers to exam- ine the era of reionization in unprecedented detail and fill in the holes in the story of the universe’s evolution. Matter Mystery Comes to Light Reionization was first recognized in 1965, when astronomers James Gunn and Bruce Peterson were observing bright and distant quasars (1). These luminous objects are used as cosmic probes because anything that’sbe- tween the quasars and Earth partially absorbs their light, imparting telltale clues about the intervening material. Cosmological models from the time suggested that a significant Astronomers are looking to future missions to help address the mystery surrounding an epoch amount of matter was not bound up inside known as reionization. This Hubble Space Telescope image, the result of 841 orbits of tele- galaxies, but rather was floating in intergalactic scope viewing time, contains ∼10,000 galaxies, extending back in time not long before before space as neutral hydrogen. But Gunn and reionization, which took place about 500 million years after the Big Bang. Image courtesy of Peterson’s search turned up far less hydrogen NASA, European Space Agency, H. Teplitz, and M. Rafelski (Infrared Processing and Analysis than expected. Instead of being in a neutral Center/California Institute of Technology), A. Koekemoer (Space Telescope Science Institute), R. Windhorst (Arizona State University), and Z. Levay (Space Telescope Science Institute). See companion article on page 12243. www.pnas.org/cgi/doi/10.1073/pnas.1517740112 PNAS | October 6, 2015 | vol. 112 | no. 40 | 12225–12227 Downloaded by guest on September 28, 2021 state, most of the hydrogen had been ionized discover more than 1,000 galaxies from when a particularly massive amount of gas has to and was thus invisible to the quasar probes. the universe was between 500 million and a come together, so that its gravity can over- This finding created a mystery. Ionization billion years old, and they’re now reasonably come this heat pressure, and when it does, the requires intense UV light and nobody knew certain that stars and galaxies caused much of resulting star can be hundreds of times big- thesourceofallthisenergy.Someofthemost reionization, says Steven Finkelstein of the ger and dozens of times hotter than the sun. obvious candidates were quasars, which are University of Texas at Austin. Because they shone brightly and expelled actually powered by supermassive black holes Themostdistantgalaxyfoundtodatewas energetic UV radiation, giant early stars fit at the centers of galaxies. Matter falling into around when the universe was 600 million well into the story of how reionization hap- such black holes heats up through friction years old. Its discovery was announced in pened. But the details of their lives raise and shoots out powerful jets of light that can August (3). The search for ever-more distant questions about exactly how much reioniza- be 100-times brighter than the host galaxy. galaxies continues. tion can be attributed to these first stars: their lifecycle could have either suppressed or en- But whereas quasars are relatively com- Star Power mon later in the universe’s history, starting couraged further star formation. Meanwhile, this most-distant known galaxy ’ about 1 billion years after the Big Bang, they That s because as these first stars burned is already giving cosmologists clues to what hydrogen fuel, they produced heavy elements are scarce within the reionization period triggered reionization. The bright galaxy itself. “We’ve so far found only one,” says like oxygen, carbon, and iron in their cores. seems to contain a population of unusually When such stars eventually exploded as astronomer Casey Papovich of Texas A&M hot stars. The finding lines up well with re- “ supernovae, they would have spewed out University in College Station, Texas. And searchers’ expectations that the first genera- ” the heavy elements, polluting the surround- people are looking really hard (2). All of tions of stars were probably behemoths unlike the available data suggest there just weren’t ing pristine gas. Heavy elements can radiate anything seen today, and would have gener- away energy more efficiently than hydrogen, enough quasars during the first billion years ated the kind of UV radiation necessary to making it easier to cool the gas and form to split apart the universe’s hydrogen. break apart hydrogen. subsequent stars. But at the same time, these That left stars and galaxies as potential Stars form when pockets of gas cool and extremely bright early stars should have gen- culprits. But before 2009, astronomers had collapse into dense cores, the centers of which erated a lot of heat, raising the temperature of only spotted a handful of galaxies within canheatuptothesearingtemperatures their surroundings and suppressing the pro- the tail end of reionization. They couldn’tbe needed for nuclear fusion. But the pristine duction of new stars. certain that there had been enough stars and — gas of the early universe mostly hydrogen “Do the first stars shut down star forma- — galaxies earlier to plausibly reionize the uni- with traces of helium was a poor medium tion in their vicinity for a while or actually verse. Then, in 2009, astronauts installed the for star formation. These light elements are stimulate star formation nearby?” asks cos- infrared Wide Field Camera 3 on the Hubble limited in their ability to radiate away energy. mologist Miguel Morales of the University of Space Telescope, which was 40-times more The gas cannot cool below a certain tem- Washington in Seattle. “Those are the kinds ’ efficient than the instrument spreviousin- perature, which keeps it swirling and prevents of the questions we’re looking to answer.” frared camera. This allowed scientists to it from collapsing. In order for a star to form, Galactic Impact? The first stars are not the only objects con- founding cosmologists. Galaxies and their formation during the era of reionization pose problems too. Computer simulations suggest that during the cosmic dark ages (so named be- cause there was no starlight), dark matter—a mysterious and invisible substance that is thought to make up roughly 85% of the universe’smass—was clustered unevenly throughout space. Denser spots attracted more dark matter and grew even denser. Hydrogen gas and other ordinary matter got sucked in too and, about 100 million years after the Big Bang, these dense pockets of ordinary matter became the seeds of the first proto galaxies. It remains an open question just how quickly these seeds formed to create the gal- axies we see today. With our telescopes, we see the process in reverse: large modern- day galaxies giving way to smaller and smallergalaxiesthefartherawaywelook. The rate of galactic formation seems to decrease smoothly for most of cosmic his- A color image of the newly found, most-distant galaxy, EGS8p7. Image is from the Hubble tory, until around 650 million years after the Space Telescope and Spitzer telescopes and courtesy of I. Labbé (Leiden University) and NASA/ Big Bang, when “it looks like it just goes off European Space Agency/Jet Propulsion Laboratory-California Institute of Technology. a cliff,” says George Becker, who works at 12226 | www.pnas.org/cgi/doi/10.1073/pnas.1517740112 Mann Downloaded by guest on September 28, 2021 the Space Telescope Science Institute in holes blew gigantic bubbles of ionized hy- the head of the MWA power spectrum NEWS FEATURE Baltimore, Maryland (4). drogen in the neutral gas during reionization. observing team. This precipitous decline could simply be But the Milky Way is obscuring our view If that weren’t enough, researchers are an artifact of the fact that astronomers— of this radiation. “Our galaxy is a very ex- already eyeing the next generation of fa- lacking powerful enough telescopes—haven’t citing place, with huge, buzzing amounts of cilities, such as PAPER’s successor, the found enough galaxies from that time. But energy,” says radio astronomer Aaron Par- 568-antenna Hydrogen Epoch of Reioni- if the drop is real, it could mean that there sons of the University of California, Berkeley, zation Array (HERA), which should be weren’t enough galaxies around in the early who leads the Precision Array for Probing complete in 2019.