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Look at the night sky and if you are away from the city lights, you will see the stars. The space between those bright points of light is, of course, filled with darkness like ink.
Some astronomers have wondered about all that dark space, how dark it really is.
“Is space really black?” says Tod Lauer, an astronomer at the National Optical Astronomy Observatory in Arizona. It says that if you could look at the night sky without stars, galaxies and anything else known to give off visible light, “does the universe itself glow?”
It’s 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 been able to do so, using a spacecraft that travels far beyond the dwarf planet Pluto. The group has posted their work online and will soon appear in The Astrophysical Journal.
New Horizons was originally designed to explore Pluto, but after it sped past the dwarf planet in 2015, the intrepid spaceship continued to function. It is now more than 4 billion miles from home, nearly 50 times farther from the sun than Earth is.
This is important because it means that the spacecraft is away from major sources of light contamination which make it impossible to detect any slightest light signal from the universe itself. Around the Earth and the inner solar system, for example, space is filled with dust particles that are illuminated by the sun, creating a diffuse glow across the sky. But that dust isn’t a problem where New Horizons is. Also, the sunlight out there is much dimmer.
To try to detect the faint glow of the universe, the researchers examined the images taken by the simple telescope and probe camera and searched for the incredibly boring ones.
“The images were all what you just call the empty sky. There’s a splash of faint stars, there’s a splash of faint galaxies, but it looks random,” Lauer says. “What you want is a place that doesn’t have a lot of bright stars in the images or bright stars even out of range that can scatter light into the camera.”
Then they processed these images to remove all known sources of visible light. Once they subtracted the light from the stars, plus scattered light from the Milky Way and any stray light that could be the result of camera quirks, they are left with the light coming in from beyond our galaxy.
So they went a step further, stealing the light they could attribute to all the galaxies that were thought to be out there. And it turns out that, once done, there was still a lot of inexplicable light.
In fact, the amount of light from mysterious sources was nearly equal to all the light from known galaxies, says Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore. So maybe there are unrecognized galaxies out there, he says, “or some other light source that we don’t know what it is yet.”
The new findings are sure to get astronomers talking.
“They say there is so much light outside than it is inside galaxies of galaxies, which is a pretty hard pill to swallow, frankly,” notes Michael Zemcov, an astrophysicist at the Rochester Institute of Technology, who was not part of research team.
A few years ago, Zemcov and some colleagues analyzed the New Horizons data in a similar way. Using fewer images, they made a less accurate measurement, but it was still compatible with the current results.
He says that for 400 years astronomers have studied visible light and the sky seriously and yet somehow apparently “lost half of the light in the universe.”
“It’s very hard to turn around and say to the astronomical community, like, ‘Hey, guys, we’re missing half of the stuff out there,'” Zemcov says. However, buy the results: “I think the work is really solid”.
So where does the light come from? Perhaps, he says, there are much smaller and fainter dwarf galaxies and other faint regions on the periphery of galaxies that instruments such as the Hubble Space Telescope cannot detect and therefore scientists are simply not aware of. Or maybe there’s more dust out there interfering with the measurements than scientists expected.
Or perhaps there is a more exotic explanation: an unknown phenomenon in the universe that creates visible light. It is also possible that it is something associated with dark matter, a mysterious form of matter that exerts a gravitational pull 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 Postman. “We would like to think that the components that emit light are something that we can really get a good sense of and understand why there is so much light.”
But to do that, Postman notes, it’s really essential to first understand how much light there is to take into account, and that’s where a study like this can help.
“It’s a new measurement, with the capabilities that we have because we’re in one place with a camera that can take advantage of that clean place,” says Lauer.
However, he adds, “The space is dark”. Even after all this analysis, “it’s still pretty dark”.
Copyright 2020 NPR. To find out more, visit https://www.npr.org.
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