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NASA’s New Horizons is billions of kilometers away from the Sun having flown past Pluto and Kuiper Belt Object Arrokoth. This position gives it a unique position to answer an interesting question: how dark is space? And it is probably darker than previously thought.
First of all, let’s deal with Olbers’ paradox. Why is the universe dark in the first place? There are stars and galaxies in every direction you can go! Shouldn’t it be bright? Well, no. That's because the universe is expanding and the speed of light is finite, so as the universe becomes bigger and bigger the light from distant objects is stretched beyond what we can see.
But what we can’t see clearly has an effect and this is where New Horizons’ research comes in. We can see the light of many billion galaxies. The light of the rest should be a sort of general feeble glow hinting at their presence. It turns out that this glow is weaker, suggesting there are fewer galaxies than expected in the visible Universe.
Astronomers using the Hubble Space Telescope have previously estimated that the orbiting observatory was capable to see only 10 percent of all the galaxies in the Universe and placed the total number of galaxies to about 2 trillion. The estimate from New Horizons instead suggests that that number is way too high. The glow from these unresolved galaxies is now bright enough to come from 1.8 trillion galaxies. The team thinks that Hubble can see about half of all the galaxies in the Universe.
"It's an important number to know—how many galaxies are there?" a lead author of the study, Marc Postman of the Space Telescope Science Institute in Baltimore, said in a statement. "We simply don't see the light from two trillion galaxies."
New Horizons is 7.4 billion kilometers (4.6 billion miles) from the Sun. All the way out there, it does not experience the glare of the Zodiacal Light which sunlight reflects on dust particles abundant in the inner solar system (and also subject matter of the Ph.D. thesis of Queen guitarist Dr Brian May). New Horizon’s ambient sky is 10 times darker than the darkest sky accessible to the Hubble Space Telescope.
"These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time," added Tod Lauer of NSF's NOIRLab, a lead author on the study. "New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it."
The precise composition of the glow’s origin is uncertain. It could be very distant galaxies, it could be very faint dwarf galaxies orbiting larger and close-by galaxies, and it could even be numerous stars inhabiting the spherical halo of galaxies. With the launch of the James Webb Space Telescope later this year, we might finally get the answers to this question.
The current work has been accepted for publication in The Astrophysical Journal and was presented at the virtual 237th meeting of the American Astronomical Society.
