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.

Astronomers are looking to future missions to help address the mystery surrounding an epoch known as reionization. This Hubble Space Telescope image, the result of 841 orbits of telescope viewing time, contains ∼10,000 galaxies, extending back in time not long before before reionization, which took place about 500 million years after the Big Bang. Image courtesy of NASA, European Space Agency, H. Teplitz, and M. Rafelski (Infrared Processing and Analysis Center/California Institute of Technology), A. Koekemoer (Space Telescope Science Institute), R. Windhorst (Arizona State University), and Z. Levay (Space Telescope Science Institute).

In the first fractions of a second after the Big Bang, the cosmos expanded exponentially, then became a blistering soup of fundamental particles and energy, and eventually cooled to a point where protons and electrons could combine to form neutral hydrogen. A few hundred million years later, the universe was filled with billowing clouds of hydrogen gas, entering what cosmologists call the cosmic dark ages.

Then everything changed. About 500 million years after the Big Bang, intense UV radiation suddenly began to burn into the gas, creating massive hollow holes that grew and merged. The energy onslaught continued for nearly a half-a-billion more years, ripping the neutral hydrogen back into its constituent protons and electrons and transforming the entire universe. This was the start of the era of reionization, a strange and important epoch that nonetheless remains poorly understood.

Cosmologists think that during reionization, the first stars and galaxies switched on and enormous black holes consumed anything within their reach. “The Big Bang was the beginning,” says Richard Ellis, an astronomer at the California Institute of Technology in Pasadena, California. “But the moment when the universe was bathed in starlight 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 preliminary answers. But soon, powerful new space- and ground-based telescopes, along with instruments that can probe the distribution of neutral hydrogen gas throughout the cosmos, will allow astronomers to examine 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’s between the quasars and Earth partially absorbs their light, imparting telltale clues about the intervening material. Cosmological models from the time suggested that a significant amount of matter was not bound up inside galaxies, but rather was floating in intergalactic space as neutral hydrogen. But Gunn and Peterson’s search turned up far less hydrogen than expected. Instead of being in a neutral state, most of the hydrogen had been ionized and was thus invisible to the quasar probes.

This finding created a mystery. Ionization requires intense UV light and nobody knew the source of all this energy. Some of the most obvious candidates were quasars, which are actually powered by supermassive black holes at the centers of galaxies. Matter falling into such black holes heats up through friction and shoots out powerful jets of light that can be 100-times brighter than the host galaxy.

But whereas quasars are relatively common later in the universe’s history, starting about 1 billion years after the Big Bang, they are scarce within the reionization period itself. “We’ve so far found only one,” says astronomer Casey Papovich of Texas A&M University in College Station, Texas. “And people are looking really hard” (2). All of the available data suggest there just weren’t enough quasars during the first billion years to split apart the universe’s hydrogen.

That left stars and galaxies as potential culprits. But before 2009, astronomers had only spotted a handful of galaxies within the tail end of reionization. They couldn’t be certain that there had been enough stars and galaxies earlier to plausibly reionize the universe. Then, in 2009, astronauts installed the infrared Wide Field Camera 3 on the Hubble Space Telescope, which was 40-times more efficient than the instrument’s previous infrared camera. This allowed scientists to discover more than 1,000 galaxies from when the universe was between 500 million and a billion years old, and they’re now reasonably certain that stars and galaxies caused much of reionization, says Steven Finkelstein of the University of Texas at Austin.

The most distant galaxy found to date was around when the universe was 600 million years old. Its discovery was announced in August (3). The search for ever-more distant galaxies continues.

Star Power

Meanwhile, this most-distant known galaxy is already giving cosmologists clues to what triggered reionization. The bright galaxy seems to contain a population of unusually hot stars. The finding lines up well with researchers’ expectations that the first generations of stars were probably behemoths unlike anything seen today, and would have generated the kind of UV radiation necessary to break apart hydrogen.

Stars form when pockets of gas cool and collapse into dense cores, the centers of which can heat up to the searing temperatures needed for nuclear fusion. But the pristine gas of the early universe—mostly hydrogen with traces of helium—was a poor medium for star formation. These light elements are limited in their ability to radiate away energy. The gas cannot cool below a certain temperature, which keeps it swirling and prevents it from collapsing. In order for a star to form, a particularly massive amount of gas has to come together, so that its gravity can overcome this heat pressure, and when it does, the resulting star can be hundreds of times bigger and dozens of times hotter than the sun.

Because they shone brightly and expelled energetic UV radiation, giant early stars fit well into the story of how reionization happened. But the details of their lives raise questions about exactly how much reionization can be attributed to these first stars: their lifecycle could have either suppressed or encouraged further star formation.

That’s because as these first stars burned hydrogen fuel, they produced heavy elements like oxygen, carbon, and iron in their cores. When such stars eventually exploded as supernovae, they would have spewed out the heavy elements, polluting the surrounding pristine gas. Heavy elements can radiate away energy more efficiently than hydrogen, making it easier to cool the gas and form subsequent stars. But at the same time, these extremely bright early stars should have generated a lot of heat, raising the temperature of their surroundings and suppressing the production of new stars.

“Do the first stars shut down star formation in their vicinity for a while or actually stimulate star formation nearby?” asks cosmologist Miguel Morales of the University of Washington in Seattle. “Those are the kinds of the questions we’re looking to answer.”

A color image of the newly found, most-distant galaxy, EGS8p7. Image is from the Hubble Space Telescope and Spitzer telescopes and courtesy of I. Labbé (Leiden University) and NASA/European Space Agency/Jet Propulsion Laboratory-California Institute of Technology.

Galactic Impact?

The first stars are not the only objects confounding cosmologists. Galaxies and their formation during the era of reionization pose problems too. Computer simulations suggest that during the cosmic dark ages (so named because there was no starlight), dark matter—a mysterious and invisible substance that is thought to make up roughly 85% of the universe’s mass—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 galaxies we see today. With our telescopes, we see the process in reverse: large modern-day galaxies giving way to smaller and smaller galaxies the farther away we look. The rate of galactic formation seems to decrease smoothly for most of cosmic history, until around 650 million years after the Big Bang, when “it looks like it just goes off a cliff,” says George Becker, who works at the Space Telescope Science Institute in Baltimore, Maryland (4).

This precipitous decline could simply be an artifact of the fact that astronomers—lacking powerful enough telescopes—haven’t found enough galaxies from that time. But if the drop is real, it could mean that there weren’t enough galaxies around in the early universe to drive reionization.

Again, future telescopes will help solve these lingering problems by revealing fainter and redder objects than is currently possible. The expanding cosmos stretches traveling light waves in a process known as “redshifting.” This means that most of the ionizing UV radiation produced by early stars has been tugged toward longer and longer infrared wavelengths, making them look redder and redder.

Hubble has difficulty seeing deep into the infrared spectrum but its successor, the James Webb Space Telescope (JWST), is specifically designed for such observations. Not only will JWST have a collecting area five-times larger than Hubble, but its mirror is coated in gold, which absorbs blue wavelengths and will reflect mainly reddish light into the telescope’s cameras. In addition, the new telescope will orbit far from the Earth and be protected by its own sun shade, allowing it to avoid infrared interference from heat radiation.

“With JWST, it’s going to be a complete revolution,” says Pascal Oesch, an astronomer at Yale University in New Haven, Connecticut, who was part of the team that spotted the most distant confirmed galaxy. The telescope is expected to find hundreds of thousands of galaxies within the reionization era.

The space-based JWST will be joined in its mission by the next generation of ground-based instruments, currently being built by different teams around the world. Observatories such as the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope—all of which will dwarf current telescopes—are expected to come online in the early-to-mid 2020s. These advanced facilities will also help spot the earliest stars and galaxies and help explain how they evolved.

Hydrogen Sleuths

Cosmologists are also trying to tackle questions of reionization by looking directly at the neutral hydrogen from the early universe. An atom of neutral hydrogen occasionally emits a photon with a wavelength of 21 centimeters. Because only neutral hydrogen produces this so-called “21-cm line” (ionized hydrogen does not), astronomers can look for it to create maps of the distribution of neutral hydrogen and how it changed over time. This will allow astronomers to watch how the first stars, galaxies, and maybe even some black holes blew gigantic bubbles of ionized hydrogen in the neutral gas during reionization.

But the Milky Way is obscuring our view of this radiation. “Our galaxy is a very exciting place, with huge, buzzing amounts of energy,” says radio astronomer Aaron Parsons of the University of California, Berkeley, who leads the Precision Array for Probing the Epoch of Reionization (PAPER), a 128-antenna array in South Africa searching for the 21-cm line. A great deal of radiation from the Milky Way comes in the form of radio waves ∼100,000 times more intense than the 21-cm signal from the early universe.

Several teams are trying to figure out how to screen out the Milky Way’s interference and see the faint signals from the distant cosmos, including PAPER, the Murchison Widefield Array (MWA) in Australia, and the Low-Frequency Array (LOFAR) in The Netherlands.

“Our galaxy is a very exciting place, with huge, buzzing amounts of energy.” —Aaron Parsons

Although none have yet managed to see the 21-cm hydrogen line from the era of reionization, Parsons says that even this nondetection tells them something interesting about the early universe. According to their models, if the hydrogen gas just before reionization had been cold, the signal should already be visible to current radio telescopes. The fact that it isn’t means that the neutral hydrogen was heated up slightly before reionization, probably by the first generation of stars (5).

Ultimately, PAPER, MWA, and LOFAR are looking to build what’s known as the power spectrum of the 21-cm line, which will tell them the average size of the bubbles of ionized hydrogen and how quickly these bubbles grew. This will help us form a cosmic picture of how ionization progressed, as well as telling us whether reionization was dominated by large or small galaxies, and how pristine the neutral hydrogen gas was at different points in cosmic history, says Morales, the head of the MWA power spectrum observing team.

If that weren’t enough, researchers are already eyeing the next generation of facilities, such as PAPER’s successor, the 568-antenna Hydrogen Epoch of Reionization Array (HERA), which should be complete in 2019. And radio astronomers are eagerly awaiting a massive international project called the Square Kilometer Array (SKA), expected to begin taking data early next decade. As the name suggests, SKA consists of antennas whose combined collecting area will total a square kilometer, allowing it to map the 21-cm radiation across the sky and see to nearly the very beginning of reionization (6). “The cosmic dawn will be the playground for SKA,” says cosmologist Leon Koopmans of the Kapteyn Astronomical Institute in The Netherlands, who is part of both the LOFAR and SKA teams.

To the Dark Side of the Moon

To see even further back, the ultimate instrument for detecting cosmic neutral hydrogen will have to fly into space, far from the noisy radio interference of human technology and Earth’s ionosphere. That’s why some researchers have proposed a project known as the Dark Ages Radio Explorer (DARE), which would travel to the quietest place accessible to us: the far side of the moon. As proposed, DARE will orbit the moon, and each time the moon shields it from Earth during the craft’s lunar orbit, DARE will peer deep into the cosmos and look for hydrogen’s telltale 21-cm signal.

The proposal is currently being submitted to NASA for consideration in its 2017 round of funding. If selected, the satellite could launch early next decade, says astrophysicist Jack Burns of the University of Colorado, Boulder, who leads the project. Over the course of two years, DARE would collect enough data to see back beyond reionization into the universe’s dark ages and watch the cosmos shift from a place of darkness to a place of light.

“It would tell us, how did the first stars and galaxies form, and how did that lead to all the complexity in the universe,” says Burns. “It speaks to that fundamental question of who we are and where we came from.”