Is space really black? This question has been asked by a large number of scientists, with an answer that until now seemed obvious to the eyes of any beholder, but actually covers a wide range of possibilities. Now, with new data from space missions, it seems that not everything is as thought. Tod Lauer, an astronomer at the National Optical Astronomy Observatory in Arizona, along with other researchers from NASA’s New Horizons space mission, have come close to an answer to that question, in a study to be published in The Astrophysical Journal. (arxiv.org/abs/2011.03052)

Although it was designed to study Pluto, the New Horizons probe is now 7.4 billion kilometers from Earth. That’s important because it means the spacecraft is far from major sources of light pollution that make it impossible to detect a tiny light signal from the universe itself. Around Earth and the inner solar system, for example, space is littered with dust particles that are illuminated by the Sun, creating a diffuse glow across the entire sky. But that dust isn’t a problem where New Horizons is. Also, out there, the sunlight is much weaker. To try to detect the faint glow of the universe, the researchers examined the images taken by the simple telescope and the spacecraft camera and searched for the ones that were incredibly dull.

“The images were all of what’s just called a blank sky. There’s a pinch of faint stars, there’s a pinch of faint galaxies, but it seems random,” Lauer says. “What you want is somewhere that doesn’t have a lot of bright stars in the images or bright stars even out of field that can scatter light back toward the camera.” They then processed these images to remove all known sources of visible light. Once they subtracted out the light from the stars, plus the scattered light from the Milky Way and any stray light that might be the result of camera quirks, they were left with light coming from beyond our own galaxy.

Then they went a step further, subtracting light that they could attribute to all the galaxies thought to be there. And it turns out, after that was done, there was still a lot of unexplained light left. In fact, the amount of light coming from mysterious sources was roughly equal to all the light coming from known galaxies, Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, tells NPR. So maybe there are unrecognized galaxies out there, he says, “or some other light source that we don’t know what it is yet.” “They’re saying there’s just as much light outside galaxies as there is inside galaxies, which is a pretty hard pill to swallow, frankly,” says Michael Zemcov, an astrophysicist at the Rochester Institute of Technology, who was not part of the research team. .