In between the thousands of bright galaxies that populate many Hubble Space Telescope photos of the distant cosmos are empty dark spots—tantalizing patches that could be chock-full of more galaxies if only we could see them. Now, astronomers have taken another look at those empty patches and spotted faint light streaming from stars formed only 500 million years after the Big Bang. The new results (pdf) suggest this light came from some of the first galaxies ever formed, which could be 10 times more numerous than previously thought.

This so-called “extragalactic background light” likely dates from roughly 250 million years after the Big Bang. Shortly after the birth of the universe, space was filled with a hot, dense fog of ionized gas. But over hundreds of thousands of years, the gas expanded and cooled, allowing giant clouds of hydrogen and helium to collapse and form the first stars. Ever since these stars first ignited, their light—and all the light from successive generations of stars—has been filling the universe, creating a pervasive glow throughout the darkest depths of space.

Although the extragalactic background radiation has proved arduous to conclusively detect, the light seen in the Hubble photos looks to be the most distant background light yet. Using data from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) and the Great Observatories Origins Deep Survey (GOODS), the team was able to separate out the light from later stars and galaxies, isolating the contribution from the first stars.

Graduate Student Ketron Mitchell-Wynne from the University of California, Irvine, and his colleagues looked for fluctuations in the intensity of the seemingly dark and empty pixels in Hubble photos taken from 2002 to 2012 to measure the elusive first light. The fluctuations helped them statistically determine that they were seeing a faint signal associated with the first stars and not simply noise. They then subtracted any light added by the stars within our galaxy, light added by the nearby galaxies, and even light added by the rogue stars that have been torn from their host galaxies and now occupy intergalactic space, until they were left with light solely from the early universe.  

The three panels show the different components of extragalactic background light detected with the Hubble Space Telescope.
Image Credit: Ketron Mitchell-Wynne, UC Irvine

“It's a really heroic effort to tease out this signal,” says Pascal Oesch from Yale University who was not involved with the study.

The team has calculated there is just a 0.8 percent chance that their measurement is contaminated by non-background light. To boot, the ancient stars contributing the background light look to be radically different from the stars we observe today. Created from only hydrogen and helium, they were hundreds of times more massive than the Sun and so burned brighter and died faster than stars in the nearby universe. “There may have been a flash, when these first stars and galaxies formed and then burned out really fast,” says co-author Matthew Ashby from the Harvard-Smithsonian Center for Astrophysics. “If we can break up that extragalactic background light, then we should see the echo of that flash.”

The team essentially spotted this echo, but can only tell that it occurred within the first 500 million years—something astronomers already knew. They want to pinpoint the time more precisely because these massive early stars dramatically changed the fate of the universe. Their ultraviolet rays heated the surrounding space, carving out bubbles in the gas where the energy of the rays had stripped all hydrogen atoms of their electrons, converting them from neutral atoms to ions. Eventually, the bubbles grew and joined together until the entire universe was ionized again, matching its state after the Big Bang. Ever since then, the universe has remained ionized.

Although astronomers are fairly convinced stars from the first galaxies produced enough light to reionize the universe, Dan Coe, an astronomer from the Space Telescope Science Institute who was not involved with the study, examines the epoch of reionization on the off-chance that galaxies won’t be the leading factor.

“If we found that the galaxies weren't enough to do the reionization then we would have to have some other explanation and it might be another really interesting explanation,” says Coe. After all, 20 years ago, astronomers thought active galactic nuclei—brilliant beacons created by supermassive black holes rapidly swallowing material—produced enough light to reionize the universe. That theory didn’t pan out, and astronomers moved on to their next best educated guess: galaxies. The same scenario could happen again, Coe says, and this time maybe it will actually be something really exotic, like dark matter particles. Perhaps when these poorly understood particles collide, they release energy and that energy is enough to reionize the universe. Coe is careful to say, however, that all lines of evidence still point toward galaxies.

Further clues will come when the James Webb Space Telescope launches in 2018. Indeed the strength of the newly detected extragalactic background light demonstrates that the future orbiting observatory should be able to spot some of the first galaxies currently lurking in those tantalizing dark patches of space.