Lighting Up the Dark Ages

LIGHTING UP THE DARK AGES by Marcus Y. Woo NOT LONG AFTER THE BIG BANG-a mere 370,000 years later-the universe entered what astronomers now call the dark ages. The cosmos had yet to see the light of stars and galaxies; in fact, a cloak of hydrogen haze smothered almost any photon that might have tried to illuminate the infant universe. A - years, however, that hydrogen gas began to coalesce into increasingly dense clouds-coaxed together br the gravitational pull of dark matter, the unseen and unknown stuff that accounts for mact at the unterce e mace Those hydr gen clouds eventually collapsed into dense balls that were hot enough to ignite nuclear fusion, thus giving birth to the first stars. Over the course of another couple of hundred million years, more and more stars flared into existence and galaxies formed, flooding the universe with high-energy photons- ultraviolet light that carried enough energy to strip the single electrons from the hydrogen atoms that pervaded the cosmos. At this time, most of the hydrogen gas in the universe was electrically neutral- a single electron loined with a proton— and neutral hydrogen blocks light by absorbing it. But with the influx of ultraviolet licht. the neutral ionized. Without neutral hydrogen to absorb them, photons could now travel freely. This epoch—the one that saw the first stars and galaxies and the ionization of hydrogen—is known as the cosmic dawn, and its light brought an end to the dark ages. Together, both periods represent the last frontier of observational cosmology-an era so distant and therefore so faint that it's out of reach (though only just so) of today's telescopes. But because astronomers, ever the pioneers, love a good frontier, scientists like Caltech's Richard Ellis are pushing the limits of those present-day telescopes, trying to capture images of the first generation of stars right after they formed. Recently, they got tantalizingly close when Ellis led an international team of astronomers as they discovered seven of the earliest galaxies ever seen—the first-ever census of such primitive galaxies. The observations setting the stage for the next generation of telescopes to peer even deeper into this mysterious era of cosmic history. "It's exciting," he says. "We're very close now " FILLING THE GAP The universe is roughly 13.8 billion years old— a staggering number compared to our human timescales. But, remarkably, scientists have been able to sketch out its general history pretty well. The beginning of the dark ages, for instance, is marked by the omnipresence of the cosmic microwave background (CMB)—a microwave-frequency glow that provides a snapshot of the universe when it was 370,000 years old. Studying the CMB allows astronomers to piece together a blow-by-blow account of the early universe beyond that snapshot, down to the first fractions of a second after the Big Bang. There have been a variety of increasingly advanced experiments, 24 ENGINEERING & SCIENCE WINTER 2013 WINTER 2012 FNGINFEPING & COTENCE 13.7 人（→ JANE- 1000 COSMIC "DARK AGES" such as the balloon-borne BOOMERanG project, which was led by Caltech's late Andrew Lange at the turn of the century, as well as other Caltech-led balloon and Earth-based programs at the South Pole, and experiments on the Planck spacecraft (whose U.S. participation is based at JPL). Thanks to these efforts, astronomers have been able to probe the CMB and the baby universe with startling precision over the last couple of decades. While such experiments are telling, telescopes offer the ability to actually peer back into different stages of cosmic history. The light that reaches astronomers' telescopes is an image from the past: the more distant a galaxy, the longer its light will have to travel; so the farther away the object, the more primitive it is. Bigger and better telescopes have allowed astronomers to chronicle periods of cosmic history deeper into the past— as far back in time as about 12.8 billion years ago. But, until recently, telescopes hadn't been able to reach much farther than that. Between the window the CMB gives us into the earliest days of the universe and the telescopic record going back more than 12 billion years from today, however, there's a gap. And in that gap are the dark ages and the cosmic dawn. As Ellis says, "it's the last uncharted period of cosmic history." Which isn't to say that nobody's tried to chart it, nor that they haven't had some success. In 2009, for instance, a team of astronomers led by Garth Illingworth of UC Santa Cruz and Rychard Bouwens of Leiden University pointed the Hubble Space Telescope at what's known as the Hubble Ultra Deep Field. Hubble had first observed this tiny patch of sky in the constellation Fornax— only about 1/50th the area of the moon as it appears in the sky- -between 2003 and 2004. The astronomers had chosen that region because it lacked bright foreground stars and nearby galaxies, which would have overwhelmed the faint galaxies they wanted to study. During that original set of observations, which were made almost 10 years ago now, the astronomers had kept the telescope trained on that same region for 400 orbits amounting to 11.3 days of total exposure time. Only with such R ENCINERRING & SCIENOR WINTED 2012 HUBBLE 2012 HUBBLE 2009 13.5 13.4 13.0 FIRST GALAXIES K-IONI/AION a long exposure time could they hope to see the cosmic dawn. What they found was an impressive initial haul of galaxies, containing a total of about 10,000 of the most distant galaxies ever seen which meant they were some of the earliest ever detected. But astronomers weren't quite able to see back to the cosmic dawn. By the fall of 2009, however, astronauts had upgraded Hubble with the Wide-Field Camera 3 (WFC3); Ellis says its larger field of view, smaller pixels, and overall improvements made it 40 times more efficient at taking astronomical surveys than its predecessor. Using this new instrument, Illingworth and Bouwens's team was able to observe galaxies that were even more distant-including about three dozen from a time when the universe was just under 800 million years old. The astronomers were also able to pinpoint one galaxy that appeared to be from 480 million years after the Big Bang, making it the single most distant galaxy ever seen. The astronomers were able to date these newly discovered, very distant galaxies by measuring their colors. Astronomers can determine the colors of galaxies using filters that block light at certain wavelengths. For instance, massive stars that quickly burn through their nuclear fuel are hotter, and therefore bluer, than others. Since they use up their fuel so fast, they die faster as well- -after only a few hundred million years, compared to the lifespans of other stars that last billions of years. If you see a blue star, then, it must necessarily have been young i.e., caught early in its lifespan- otherwise, it would already have died. Redder stars, which burn slower, will have lived longer and therefore must be older, caught later in their lifespans. Because the galaxies in the 2009 observations appeared blue, they were likely newborns. That the galaxies were also extremely distant led the scientists to ask whether they might be the long-sought first-generation galaxies of the cosmic dawn. PUSHING THE FRONTIER To see if this was truly the case- -and to see if they could reach even further back in time-Ellis and his colleagues pointed Hubble at the same area once again in 2012, taking more exposures KEushirl (<) over the course of two months and thus gathering an additional 128 orbits of data. They used an additional filter to better narrow down the colors— and therefore the ages of the galaxies. The team also used a "drop-out" filter technique to confirm that the distant galaxies were indeed extremely far away. The neutral hydrogen gas that permeated the early universe absorbed any light that was bluer than a wavelength of 121.6 nanometers. Because the expansion of the universe stretches the wavelength of light, the more distant a galaxy is, the more its light's wavelength gets shifted-a phenomenon known as the cosmological redshift. To measure this shift, the astronomers use different filters to observe each galaxy at shorter and shorter wavelengths until the galaxy is no longer visible because the hydrogen around it has completely absorbed its light. At that point, the galaxy is said to have "dropped out." The wavelength at which a galaxy drops out tells the astronomers how much its light has redshifted, which tells them how far away it is. Caltech's Lee A. DuBridge Professor of Astronomy Chuck Steidel was the first to demonstrate this technique on a large sample of galaxies at the Palomar and Keck Observatories. The results of the 2012 observations, Ellis says, were a breakthrough. The team doubled the 2009 number of known galaxies that existed less than 800 million years after the Big Bang there are now a total of 74 such galaxies. More importantly, he says, the team found seven new galaxies that date from just 500 million years after the Big Bang. With the better data, the 480-million-year-old galaxy that had been previously identified as the most distant galaxy ever spotted now looks as if it came into being when the universe was a mere 380 million years old. (At least for now, says Ellis. Because this galaxy is especially faint, the astronomers aren't as confident about its age; in the future, more Above: A timeline of the history of the universe, with the numbers denoting billions of vears. The location of Hubble 2012 and Hubble 2009 represents the era targeted by the Hubble Ultra Deep Field in 2012 and 2009. WINTER 2012 ENCINFRPINC & SCIENCE galaxy-hnding abilties that appeared at the Hubble Ultra Deep Field soon to dim galaxies out there that Hubble WINTER 2012 ENCINFRPINC & SCIPACE