The universe wasn’t always like this. Today it’s filled with glittering galaxies, scattered across space like city lights seen from above. But there was a time when all was dark. Really dark.
First, a very brief history of time: from the Big Bang, the universe burst onto the scene as a tiny but glowing inferno of energy. Immediately, it expanded and cooled, dimming into darkness as particles condensed out of the hot soup like droplets of morning dew. Electrons and protons coalesced into atoms, which formed stars, galaxies, planets, and eventually us.
But a crucial piece still eludes scientists. It’s a gap of several hundred million years that was filled with darkness—a darkness both literal and metaphorical. Astronomers call this period the dark ages, a time that’s not just bereft of illumination, but also devoid of data.
The Big Bang left a glowing imprint on the entire sky called the cosmic microwave background, representing the universe when it was 380,000 years old. Increasingly precise measurements of this radiation have revealed unprecedented details about the earliest cosmic moments. But from then until the emergence of galaxies big and bright enough for today’s telescopes, scientists don’t have any information. Ever mysterious, these dark ages are the final frontier of cosmology.
And it’s a fundamental frontier. It represents the universe’s most formative years, when it matured from a primordial soup to the cosmos we recognize today.
Even without much direct data about this era, researchers have made great strides with theory and computer models, simulating the universe through the birth of the first stars. Soon, they may be able to put those theories to the test. In a few years, a suite of new telescopes with new capabilities will start peering into the darkness, and for the first time, astronomers will reach into the unknown.
The Final Frontier
Considering that it’s the entire universe they’re trying to understand, cosmologists have done a pretty good job. Increasingly powerful telescopes have allowed them to peer to greater distances, and because the light takes so long to reach the telescopes, astronomers can see farther back in time, capturing snapshots of a universe only a few hundred million years old, just as it emerged from the dark ages. Given that the universe is now 13.7 billion years old, that’s like taking a picture of the cosmos as a toddler.
That makes the cosmic microwave background, or CMB, clike a detailed ultrasound. This radiation contains the first photons that escaped the yoke of the universe’s primordial plasma. When the universe was a sea of radiation and particles, photons couldn’t travel freely because they kept running into electrons. But about 380,000 years after the Big Bang, the universe had cooled enough that protons were able to lasso electrons into an orbit to form hydrogen atoms. Without electrons in their way, the newly liberated photons could now fly through the cosmos and, more than 13 billion years later, enter the detectors of instruments like the Planck satellite, giving cosmologists the earliest picture of the universe.
But from this point on, until the universe was a few hundred million years old—the limit of today’s telescopes—astronomers have nothing. It’s as if they have a photo album documenting a person’s entire life, with pictures of young adulthood, adolescence, childhood, and even before birth, but nothing from when the person learned to talk or walk—years of drastic changes.
That doesn’t mean astronomers have no clue about this period. “People have thought about the first stars since the 1950s,” says Volker Bromm, a professor of astronomy at the University of Texas, Austin. “But they were very speculative because we did not know enough cosmology.” Not until the 1980s did researchers develop more accurate theories that incorporated dark matter, the still-unknown type of particle or particles that comprises about 85% of the matter in the universe. But the first key breakthrough came in 1993, when NASA’s COBE satellite measured the CMB for the first time, collecting basic but crucial data about what the universe was like at the very beginning—the so-called initial conditions of the cosmos. Theorists such as Martin Rees, now the Astronomer Royal of the United Kingdom, and Avi Loeb, a professor of astrophysics at Harvard, realized you could plug these numbers into the equations that govern how the first gas clouds and stars could form. “You could feed them into a computer simulation,” Loeb says. “It’s a well-defined problem.”
Both Rees and Loeb would influence Bromm, then a graduate student at Yale. Rees and his early work in the 1980s, in particular, inspired Tom Abel, who was a visiting scientist during the 1990s at the University of Illinois, Urbana-Champaign. Independently, Abel and Bromm would make some of the first computer models of their kind to simulate the first stars. “That really opened the field,” Loeb says. “When I started, there were maybe one or a few people even willing to discuss this subject.”
Theorists like Bromm and Abel, now a professor at Stanford, have since pieced together a blow-by-blow account of the dark ages. Here’s how they think it all went down.
Then There Was Light
In the earliest days, during the time that we see in the CMB, the entire universe was bright and as hot as the surface of the sun. But the universe kept expanding and cooling, and after nearly 15 million years, it was as cool as room temperature. “In principle, if there were planets back then, you could’ve had life on them if they had liquid water on their surface,” Loeb says. The temperature continued to fall, and the infrared radiation that suffused the universe lengthened, shifting to radio waves. “Once you cool even further, the universe became a very dark place,” Loeb says. The dark ages had officially begun.
Meanwhile, the simulations show, things began to stir. The universe was bumpy, with regions of slightly higher and lower densities, which grew from the random quantum fluctuations that emerged in the Big Bang. These denser regions coaxed dark matter to start clumping together, forming a network of sheets and filaments that crisscrossed the universe. At the intersections, denser globs of dark matter formed. Once these roundish halos grew to about 10,000 times the mass of the Sun, Abel says—a few tens of millions of years after the Big Bang—they had enough gravity to corral hydrogen atoms into the first gas clouds.
Those clouds could then accumulate more gas, heating up to hundreds of degrees. The heat generated enough pressure to prevent further contraction. Soon, the clouds settled into enormous, but rather dull, balls of gas about 100 light years in diameter, Abel says.
But if the dark matter halos reached masses 100,000 times that of the sun, they could accrue enough gas that the clouds could heat up to about 1000 degrees—and that’s when things got interesting. The surplus energy allowed hydrogen atoms to merge two at a time and form hydrogen molecules—picture two balls attached with a spring. When two hydrogen molecules collide, they vibrate and emit photons that carry away energy.
When that happens, the molecules are converting the vibrating energy that is heat into radiation that’s lost into space. These interactions cooled the gas, slowing down the molecules and allowing the clouds to collapse. As the clouds grew denser, their temperatures and pressures soared, igniting nuclear fusion. That’s how the first stars were born.
These first stars, which formed by the time the universe was a couple hundred million years old, were much bigger than those in today’s universe. By the early 2000s, Abel’s simulations, which he says are the most realistic and advanced yet, showed that the first stars weighed about 30 to 300 times the mass of the sun. Using different techniques and algorithms, Bromm says he arrived at a similar answer. For the first time, researchers had a good idea as to what the first objects in the universe were like.
Massive stars consume fuel like gas-guzzling SUVs. They live fast and die young, collapsing into supernovae after only a few million years. In cosmic timescales, that’s the blink of an eye. “You really want to think of fireworks at these early times,” Abel says. “Just flashing everywhere.”
In general, the first stars were sparse, separated by thousands of light years. Over the next couple hundred million years, though, guided by the clustering of dark matter, the stars started grouping together to form baby galaxies. During this cosmic dawn, as astronomers call it, galaxies merged with one another and became bigger galaxies. Only after billions and billions of years would they grow into those like our own Milky Way, with hundreds of billions of stars.
Lifting the Fog
But there’s more to the story. The first stars shone in many wavelengths, and especially strongly in ultraviolet. The universe’s expansion would’ve stretched this light to visible and infrared wavelengths, which many of our best telescopes are designed to detect. Problem is, during the time of the first stars, a thick fog of neutral hydrogen gas blanketed the whole universe. This gas absorbed shorter-wavelength ultraviolet light, obscuring the view from telescopes. Fortunately, though, this fog would soon lift.
“This state of affairs can’t last for very long,” says Richard Ellis, an astronomer at the European Southern Observatory in Germany. “These ultraviolet photons have sufficient energy to break apart the hydrogen atom back into an electron and a proton.” The hydrogen was ionized, turning into a lone proton that could no longer absorb ultraviolet. The gas was now transparent.
During this so-called period of reionization, galaxies continued to grow, producing more ultraviolet light that ionized the hydrogen surrounding them, clearing out holes in the fog. “You can imagine the hydrogen like Swiss cheese,” Loeb says. Those bubbles grew, and by the time the universe was around 800 million years old, the ultraviolet radiation ionized the hydrogen between the galaxies, leaving the entire cosmos clear and open to the gaze of telescopes. The dark ages were over, revealing a universe that looked more or less like it does today.
Seeing into the Dark
Of course, many details have to be worked out. Astronomers like Ellis are focusing on the latter stages of the dark ages, using the most powerful telescopes to extract clues about this reionization epoch.
One big question has been whether the ultraviolet light from early galaxies was enough to ionize the whole universe. If it wasn’t, astronomers would have to find another exotic source—like black holes that blast powerful, ionizing jets of radiation—that would have finished the job.
To find the answer, Ellis and a team of astronomers stretched the Hubble Space Telescope to its limits, extracting as much light as possible from one small patch of sky. These observations reached some of the most distant corners of the universe, discovering some of the earliest galaxies ever seen, during the heart of this reionization era. Their observations suggested that galaxies—large populations of small galaxies, in particular—did seem to have enough ultraviolet light to ionize the universe. Maybe nothing exotic is needed.
But to know exactly how it happened, astronomers need new telescopes, like the James Webb Space Telescope set for launch in 2018. “With the current facilities, it’s just an imponderable,” Ellis says. “We don’t have the power to study these galaxies in any detail.”
Other astronomers are focusing not on the galaxies, but the hydrogen fog itself. It turns out that the spins of a hydrogen atom’s proton and electron can flip-flop in direction. When the spins go from being aligned to unaligned, the atom releases radiation at a wavelength of 21 centimeters, or 8.27 inches, a telltale signal of neutral hydrogen that astronomers call the 21-cm line. The expanding universe would have stretched this signal to the point where it became a collection of radio waves. The more distant the source of light, the more the radiation gets stretched. By using arrays of radio telescopes to measure the extent of this stretching, astronomers can map the distribution of hydrogen at different points in time. They could then track how those holes in the gas grew and grew until the gas was all ionized.
“It’s surveying the volume of the universe on a scale that you can’t imagine doing in any way other than through this method—it’s really quite incredible,” says Aaron Parsons, an astronomer at the University of California, Berkeley, who’s leading a project called HERA, which will consist of 352 radio antennae in South Africa. Once online, the telescope could give an unprecedented view of reionization. “You can almost imagine making a movie of how the first stars galaxies formed, how they interacted, heated up, ionized, and turned into the galaxies we recognize today.”
Other telescopes like LOFAR in the Netherlands and the Murchison Widefield Array in Australia will make similar measurements. But HERA will be more sensitive, Parsons says. And already with 19 working antennae in place, it might be closest to success, adds Loeb, who isn’t part of the HERA team. “Within a couple years, we should have the first detection of the 21-cm line from this epoch of reionization, which would be fantastic because it would allow us to see the environmental effect of ultraviolet radiation from the first stars and first galaxies on the rest of the universe.”
This kind of data is crucial for informing computer models like the kind that Abel and Bromm have developed. But despite their successes, theorists are at the point where they need data to test whether their models are accurate.
Unfortunately, that data won’t be pictures of the first stars. Even the most powerful telescopes won’t be able to see the brightest of them. The first galaxies contain only a few hundred stars and are just too small and faint. “We’ll come ever closer,” Abel says. “It’s very difficult to imagine we’ll actually see those in the near future, but we’ll see their brighter cousins.”
In fact, the darkest of times, during the couple hundred million years between the CMB and the appearance of the first stars, may always remain beyond astronomers’ grasp. “We currently don’t have any idea of how you could get any direct information about that period,” he says.
Still, new telescopes over the next few decades promise to reveal much of the dark ages and whether the story theorists are telling is true or even more fantastic than they had thought. “Even though I’m a theorist, I’m modest enough to acknowledge the fact that nature is sometimes more imaginative than we are,” Loeb says. “I’m open to surprises.”