NASA / John Hopkins University Laboratory of Applied Physics / Southwestern Research Institute
Look at the night sky and if you are far from the city lights, you will see stars. The space between these bright points of light is, of course, filled with ink darkness.
Some astronomers have wondered about all that dark space – how dark it really is.
“Is space really black?” Says Todd Lauer, an astronomer at the National Observatory for Optical Astronomy in Arizona. He says that if you can look at the night sky without stars, galaxies and everything else that is known to emit visible light, “does the universe radiate brightness?”
This is a difficult question that astronomers have been trying to answer for decades. Now Lauer and other researchers from NASA’s New Horizons space mission say they have finally managed to do so using a spaceship that travels far beyond the dwarf planet Pluto. The group has published its work online and will soon appear in Astrophysical Journal.
New Horizons was originally designed to explore Pluto, but after flying past the dwarf planet in 2015, the fearless spacecraft simply continued. It is now more than four billion miles from home – nearly 50 times farther from the Sun than the Earth.
This is important because it means that the spacecraft is far from the main sources of light pollution, which make it impossible to detect any small light signal from the universe itself. For example, around the Earth and the inner solar system, space is filled with dust particles that are illuminated by the Sun, creating a diffuse glow over the entire sky. But this dust is not a problem where New Horizons are. Plus, the sunlight there is much weaker.
To try to detect the faint glow of the universe, researchers went through images taken by a simple telescope and a spaceship’s camera, and looked for ones that were incredibly boring.
“The images were everything you just call an empty sky. There’s a scattering of faint stars, there’s a scattering of faint galaxies, but it looks random,” Lauer said. “What you want is a place that doesn’t have very bright image stars or bright stars even outside the field that can scatter light back into the camera.”
They then processed these images to remove all known sources of visible light. After removing the light from the stars, plus the scattered light from the Milky Way and any homeless light that could be the result of camera oddities, they were left with light coming from beyond our own galaxy.
Then they went one step further, extracting the light they could attribute to all the galaxies believed to be there. And it turns out that after that was done, there was still a lot of inexplicable light.
In fact, the amount of light coming from mysterious sources was roughly equal to all the light coming from known galaxies, said Mark Postman, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. So maybe there are unrecognized galaxies out there, he says, “or some other light source that we still don’t know what it is.”
The new discoveries will certainly make astronomers talk.
“They say there’s as much light outside the galaxies as there is inside the galaxies, which is a pretty difficult pill to swallow, to be honest,” said Michael Zemkov, an astrophysicist at the Rochester Institute of Technology who was not part of a research team.
A few years ago, Zemcov and some colleagues analyzed data from New Horizons in a similar way. Using fewer images, they made less accurate measurements, but were still compatible with current results.
He says that for 400 years, astronomers have been visibly studying light and the sky in a serious way, and yet somehow “have missed half the light in the universe.”
“It’s very difficult to turn around and tell the astronomical community, ‘Hey, guys, we’re missing half the stuff there,'” Zemkov said, “and yet he buys the results. ‘I think the work is really solid.’
So where does the light come from? Perhaps, he says, there are much smaller, weaker dwarf galaxies and other weak regions on the outskirts of galaxies that instruments like the Hubble Space Telescope can’t detect, so scientists just aren’t aware of them. Or maybe there is more dust that interferes with the measurements than scientists expected.
Or maybe there is a more exotic explanation – some unknown phenomenon in the universe that creates visible light. It may even be associated with dark matter, a mysterious form of matter that exerts gravitational attraction on visible matter but has never been seen directly.
“As a person who studies the universe, I really want to know what the universe is made of and what all the components of the universe are,” says the Postman. “We’d like to think that the components that emit light are something we can really understand and understand why there’s so much light.”
But to do that, the Postman notes, it’s really important to first understand how much light there is that needs to be counted, and a study like this can help here.
“It’s a new measurement of the capabilities we have because we’re in a unique place with a camera that can use that clean space,” Lauer said.
And yet, he adds, “the space is dark.” Even after all this analysis, “it’s still pretty dark.”