One of the strange mysteries of the universe is how bright is the darkness.

Yes, you heard that right: how bright is the darkness.

When we look at the spaces between the visible stars and galaxies, we don’t find perfect darkness. There is a light coming from every point in the sky, and that light comes from the background of stars, distant galaxies, clouds of gas, and things we can’t even imagine yet. This light is called the Cosmic Optical Background or COB.

Measuring the brightness of the darkness, or COB, is exceedingly difficult from inside our solar system. There is dust scattered throughout the solar system that reflects back sunlight, creating a glow that is brightest where the dust is densest along the disk of our solar system. We can try and correct for how bright we think the dust may be, but that isn’t the same as measuring it. To measure it, we need to get beyond the dust and directly measure the COB, which is what we were trying to do in the first place.

Science can sometimes take you on rather circular paths.

Anyway, the New Horizons spacecraft, which has traveled far out beyond Pluto, has highly sensitive cameras and has been trying to measure the darkness, and researchers have discovered that the darkness is twice as bright as we thought. According to team lead Tod Lauer: It turns out that the galaxies that we know about can account for about half of the level we see.

Or it could be that our attempts to measure the cosmic optical background are being thwarted by the dust in our galaxy reflecting light. This is another thing we’re still figuring out, and co-author on this study, Michael Zemcov, reminds us the Milky Way’s dust is a very subtle beast and our uncertainties likely get dominated by it at some point, just because it’s not very well understood.

So the darkness of space is twice as bright as we thought, and it could be there is just a whole lot of stuff out there — intergalactic stars, compact galaxies, things at the edge of the visible universe. Or our galaxy may just be differently dusty than we thought.

The darkness is confusing, but these results are pretty neat and they appear in a new paper in The Astrophysical Journal Letters. The New Horizons team plans to make more observations, looking at more parts of the sky, and as Zemcov puts it: There is something going on that we weren’t expecting, which is where the fun part of science kicks in.

Let’s face it, when we look at other galaxies we see a lot of dust, and it’s kind of easy to see how dust may be a problem here. Our galaxy and the universe beyond are chock full of weird and wonderful stuff we are just starting to understand. Thankfully, we have magnificent spacecraft like New Horizons, to help us out.

More Information

The universe’s background starlight is twice as bright as expected (Science News)

“Anomalous Flux in the Cosmic Optical Background Detected with New Horizons Observations,” Tod R. Lauer et al., 2022 March 3, The Astrophysical Journal Letters