On the next clear night, go outside and look up. If you’re away from city lights, you may be amazed by the darkness of the sky between the stars. But what looks like inky black isn’t really so. Even the darkest of night skies still contains the light of all the stars that ever shone.
Photons, or particles of light, are born in the nuclear furnaces of stars and then jet outward through the empty depths of space. Still more photons are ejected when stars explode as supernovas and from superheated matter that swirls in its death throes before being sucked into a black hole.
Some of these journeying photons will slam into other particles and disappear; a minuscule fraction will be captured by telescopes on Earth. But the vast majority of stellar photons continue traversing the cosmos, creating a ubiquitous if faint glow that scientists call extragalactic background light, or EBL.
The cumulative radiation isn’t enough to light up the night sky. In fact, this cosmic glow is so dim that it’s extremely difficult to spot even with powerful telescopes. But measuring the extragalactic background light is a challenge researchers are eager to take on. Because the EBL has been rattling around for nearly the entire history of the cosmos, it can help astronomers peel back layers of the universe’s history and probe profound questions that other types of observations cannot. “If we can measure all this radiation, we can get fundamental information about the universe,” says Alberto Domínguez, an astrophysicist at the University of California, Riverside.
After decades of hunting the EBL, astronomers are finally close to snaring their quarry. New discoveries are narrowing precisely how much light exists and at what wavelengths, from infrared to visible to ultraviolet. The findings come thanks to a clutch of telescopes that capture energy blazing from the universe’s distant reaches.
By figuring out the amount of EBL at different stages of the universe’s history, researchers can explore how stars and galaxies formed and evolved over time. The EBL is already offering a glimpse at the first generations of stars. Eventually, the EBL could reveal mysterious objects such as “dark stars” that may have burned fast and furious in the early universe and strange shape-shifting particles flitting through intergalactic space.
“The EBL has so much information,” says Eli Dwek, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “You really can see so many different types of physics in it.”
Two glows for the price of one
Astronomers knew they had their work cut out for them when they started hunting the cosmic glow of the stars in the 1970s. Calculations revealed that the EBL would be very faint simply because so many stars are extremely far away. The challenge is akin to holding up a 100-watt lightbulb and trying to determine how much of its light reflects off a sheet of paper 10 kilometers away.
Complicating matters is the fact that Earth nestles in a bright celestial neighborhood. Dust along the plane of the solar system scatters sunlight, creating a diffuse glow of its own known as zodiacal light. Plus, the solar system is embedded in the bright Milky Way. “When you look locally, you’re swamped with light,” says Frank Krennrich, an astrophysicist at Iowa State University in Ames. This flood of nearby light obscures the glow of radiation from all the other stars.
Astronomers first tried to detect the EBL by launching sounding rockets that scanned the sky for minutes at a time before falling back to Earth. Those searches came up empty. In the 1980s, the U.S.-British-Dutch Infrared Astronomical Satellite mapped the sky in infrared wavelengths. It spotted the bright foreground light from the solar system and the Milky Way but provided only inconclusive evidence of a dim background glow.
Then EBL researchers received a gift in the form of NASA’s Cosmic Background Explorer (COBE) satellite, which was launched in 1989 primarily to measure a different type of glow from the universe’s past: the cosmic microwave background. This radiation emerged soon after the Big Bang 13.8 billion years ago when a hot, dense soup of primordial matter cooled enough for photons to break free and travel unimpeded through the cosmos. Over time, as the universe expanded, this relict glow stretched and cooled into microwave energies.
To measure this ancient radiation across the sky, COBE was designed to filter out objects in the foreground that might be obscuring a fainter glow behind. That’s exactly the approach EBL researchers needed to succeed. And in addition to scanning for microwave photons, COBE had a separate instrument that mapped the entire sky in 10 different infrared wavelengths of light. In 1998, scientists using COBE reported glimpsing the EBL for the first time.
The COBE data confirmed scientists’ estimates that most of the EBL would appear as infrared light, because light from stars in distant galaxies gets shifted to longer wavelengths as the universe expands. Plot the EBL on a chart, and two peaks appear that resemble a double-humped camel. The first peak, at shorter infrared wavelengths, mostly represents light emitted from ordinary stars, extremely bright supernovas and other explosions. The second peak, at slightly longer wavelengths, represents light that interacted with cosmic dust. Stars are usually born in dusty environments, and photons bouncing around warm up some of that dust, which then re-emits the light at longer wavelengths.
COBE proved that astronomers could detect the EBL; the next challenge was measuring it precisely enough to make conclusions about the distant universe. At first astronomers tried to do that by counting the number of galaxies photographed by orbiting telescopes such as Hubble and Spitzer and estimating how many photons those galaxies emit.
But conducting a photon census this way is sure to miss some important contributors and underestimate the EBL intensity. Even the best telescopes can’t capture the faintest, most distant galaxies. Plus, such a census would miss any stranger, undiscovered sources of light that may also feed into the EBL.
Scanning the fog
That’s why astronomers developed another approach to measuring the EBL. The method relies on studying very high-energy gamma rays that begin their lives in a cloud of gas swirling around a monstrous black hole. As it circles the cosmic drain, the gas heats up furiously and produces gamma rays. A galaxy that happens to emit this powerful radiation directly toward Earth is called a blazar.
Not every gamma ray enjoys a clear path to our planet. Every so often, one of them smashes into a photon from the EBL and breaks apart into a pair of particles — an electron and its antimatter counterpart, a positron. The gamma ray is no more. This annihilation happens to quite a few gamma rays as they travel the billions of light-years from the blazar through the EBL.
These collisions mean that by the time the gamma rays reach Earth, the signal is fainter than scientists might expect. It’s as if a dense fog dimmed the beacon of a lighthouse in the distance. If you knew how bright the beacon was on clear nights, you could look at it on a foggy night and calculate just how much fog there was between it and you. “If we could somehow infer the light that comes out from the blazar, we can infer what was lost on the way to us because of the EBL,” says Domínguez.
It turns out that lower-energy photons aren’t absorbed by the EBL, a fact that allows astronomers to estimate an object’s intrinsic brightness. By seeing how many of those low-energy photons arrive at Earth from a particular blazar, scientists can calculate how many high-energy gamma rays also set sail from the same blazar. In 1992, astrophysicist Floyd Stecker of NASA Goddard and his colleagues suggested using this drop-off between the expected and observed gamma rays to measure how much extragalactic background light exists between the blazar and Earth.
Astronomers accordingly turned a suite of powerful ground-based telescopes to the sky. These Cherenkov telescopes search for showers of particles created when a high-energy gamma ray slams into a particle in the Earth’s atmosphere, triggering a flash of bluish light. Then scientists can trace the evidence back to the general region of the sky that sent the gamma rays this way.
By the 2000s Cherenkov telescopes in Arizona, the Canary Islands and Africa had detected very energetic gamma rays from powerful blazars. But the telescopes also revealed one big problem: They couldn’t probe very distant gamma-ray sources, because the extragalactic background light absorbs so many of their gamma rays en route to Earth.
The Fermi era begins
To extend their reach in the universe, astronomers turned to the Fermi Gamma-ray Space Telescope. NASA launched the spacecraft in 2008 to study blazars and other violent objects. Because Fermi orbits Earth, it sits well above the atmospheric interference that foils gamma-ray measurements.
Last November in Science, an international team of astronomers reported Fermi observations of 150 blazars dating back to about 4 billion years after the Big Bang (SN: 12/15/12, p. 8). The team measured the drop-off in gamma rays — presumably due to the EBL absorbing that radiation — at different distance ranges from Earth. In a cosmic sense, distance represents a sort of time travel into the past: The farther an object is from Earth, the longer its light has been traveling to reach us, and so the object appears to observers as it did in an earlier era.
By calculating the intensity of the EBL at various distances from Earth, the researchers came up with snapshots of how many stars were giving off light during each era. “It’s like having many experiments back in time,” says team member Marco Ajello, an astrophysicist at the University of California, Berkeley.
Ajello’s team lumped the blazars, depending on their distance from Earth, into three separate periods in the past. After measuring how many gamma rays survived their journey from each blazar, the astronomers confirmed earlier work suggesting that star formation has steadily declined since its peak about 3 billion years after the Big Bang.
Although Ajello could not peer back any farther in time directly, his team was able to extrapolate the intensity of the EBL during the era of the very first stars that lit up the universe. These monstrous stars, some 100 times as massive as the sun, probably formed within the first few hundred million years after the Big Bang when vast pockets of hydrogen atoms coalesced and ignited in nuclear fusion.
Despite burning through that nuclear fuel within a few million years, these stars probably played a major role in the universe’s history by sending out photons that collided with hydrogen atoms and imparted an electric charge. This crucial process, called reionization, allowed stars to continue lighting up the universe, sparing the cosmos a cold, dark and featureless fate.
Knowing how many photons were around in this early era could help astronomers better understand how reionization happened. Ajello and his colleagues found that the EBL at these great distances was fainter than suspected, which hints that the first stars formed much more slowly than astronomers had thought. “We’re doing our best to clear out the fog and nail down the numbers about star formation in the very early universe,” Ajello says.
Faint glow, powerful probe
Clearing out the fog may solve other long-standing mysteries of the universe. Dwek hopes that scientists can study the portion of the EBL that has bounced off cosmic dust to better understand the role dust plays in absorbing and re-radiating light.
Studying the EBL may also lead scientists into the realm of exotic physics. Some theories, for instance, suggest that dark stars powered by dark matter — a mysterious, invisible form of matter — might have lurked in the early universe. These stars may have accreted dark matter particles and burned quickly, leaving a signal that should be visible today in the EBL. Recent studies have ruled out the existence of dark stars up to about 100 times the mass of the sun, but heavier ones are still possible.
And then there’s the possibility of axion-like particles, or ALPs, which are lightweight particles no one has ever seen. If they exist, ALPs would have the weird property of being able to shape-shift into a photon and back again. Some physicists have proposed that gamma rays flooding from a blazar could turn into ALPs and travel through the universe unimpeded by the extragalactic background light. The ALPs could then reconvert to an ordinary photon before reaching Earth, and so astronomers observing them would never know the change had happened.
In theory, ALPs disguised as photons could make up a significant percentage of the EBL. Astronomers are searching for fingerprints of ALPs in the spectra of light coming from blazars and other gamma-ray sources.
The EBL is likely to lead to more discoveries soon, even if they are not of the exotic variety. Ajello’s team is digging through Fermi data for more blazars; the satellite has detected at least 1,000 so far, many at great distances that will help reveal star formation in the distant past. The ground-based Cherenkov telescopes are also working busily; HESS, in Africa, was recently expanded and the VERITAS telescope in Arizona has a new upgrade as well.
Observations from these telescopes, cross-checked with photon counts from Hubble and Spitzer, should allow astronomers to pin down precise measurements of the EBL intensity throughout history. “We’re not quite converging yet, but we’re getting close,” says Krennrich.
In a study in the June Astrophysical Journal, Domínguez and colleague Francisco Prada found that EBL estimates obtained from blazars match up closely with those from sky surveys. The study suggests that there aren’t any weird or unusual sources of EBL, like odd stars or faint galaxies, that got missed during the cosmic census. “That means our galaxy surveys are actually detecting most of the light in the EBL,” Domínguez says.
Farther off, new generations of instruments will make unprecedented measurements of the EBL. A planned Cherenkov Telescope Array would consist of dozens of telescopes, far more than the handful used in current Cherenkov systems, to detect more gamma rays disintegrating in the atmosphere. And in space, the James Webb Space Telescope — planned for a 2018 launch — will study very faint, very distant stars and galaxies to help pin down their contributions.
Soon, astronomers say, all the light from all the stars will be measured. “It is curiosity that drives us all,” says Dwek. “By measuring that light we can actually find out what stars were doing over time.”
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