Scientists want to know how dark space really is. And can that tell us something about the number of galaxies in the visible universe?

NASA is using the New Horizons spacecraft to figure out just how dark space really is.

When you look up at the night sky, away from city lights, space between stars appears very dark. Outside the Earth’s atmosphere, outer space gets even darker. But even there, space is not absolutely black.

The universe is lit by a faint glow of countless distant stars and galaxies. But it’s not easy to measure this glow. Satellites and telescopes inside our solar system – like the Hubble Space Telescope – cannot properly measure the light emitted. 

Dust and particles, illuminated by the sun, fill the region around the Earth and the inner solar system. This creates a diffuse glow in the sky.

So, to measure that faint glow that fills the universe, scientists have to use a spacecraft outside our solar system. A probe that is away from light pollution.

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A team of scientists has used observations by NASA’s New Horizons mission to Pluto and the Kuiper Belt to determine the brightness of this cosmic optical background.

New Horizons is currently very far from all major sources of light pollution, billions of kilometers from Earth. The surrounding sky is therefore about 10 times darker than the darkest sky visible to Hubble. This enables the probe to take much more accurate measurements.

New measurements of the dim glow show that the unseen galaxies are less plentiful than some theories suggested. The results show galaxies only number in the hundreds of billions, not 2 trillion galaxies as scientists previously thought.

This means the universe is actually a lot darker than we thought.

“It’s an important number to know—how many galaxies are there?” said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study. “We simply don’t see the light from two trillion galaxies.”

Scientists got the earlier estimate using very deep sky observations by NASA’s Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. In that study, researchers concluded 90 percent of galaxies in the universe were unseeable by Hubble.

“Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” said Tod Lauer of NSF’s NOIRLab, a lead author on the study.

The cosmic optical background that the team sought to measure is the visible-light equivalent of the more well-known cosmic microwave background – the weak afterglow of the big bang itself, before stars ever existed.

“While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the sum total of all the stars that have ever formed since then,” explained Postman. “It puts a constraint on the total number of galaxies that have been created, and where they might be in time.”

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At the moment, we can’t count all the galaxies, but we know their light pervades space with a mysterious dim glow.

Scientists examined images from New Horizons and filtered out all known sources of visible light. The most challenging correction was removing light from Milky Way stars that was reflected off interstellar dust and into the camera. But after they did so, they were left only with light outside our own galaxy.

Although the remaining signal was extremely weak, scientists were still able to measure it.

Postman compares it to living in a remote area, far from the city lights. If you lie in your bed at night with the curtains open and a neighbor a mile away opens their refrigerator, that light can bounce off your bedroom walls. It would then be as bright as the light captured by New Horizons.

Scientists still aren’t sure who or what causes this unexplained light. It might be possible that dwarf galaxies exist in the relatively near universe and are undetectable for some reason. But it may also be that the diffuse halos of stars surrounding the galaxies are brighter than expected. Or there may be many more faint and distant galaxies than theory suggests. Well, no one knows yet.

But NASA’s upcoming James Webb Space Telescope may be able to help solve the mystery. If weak, individual galaxies are the cause, James Webb’s observations should be able to reveal it.

The researchers presented their results on Wednesday, Jan. 13th at a meeting of the American Astronomical Society.

This study is accepted for publication in The Astrophysical Journal.


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