We finally know just how dark it is in deep space. NASA鈥檚 New Horizons spacecraft has made the first precise measurements of the ambient light that suffuses the universe, called the cosmic optical background.
The cosmic optical background is so dim that it is impossible to measure with any precision from Earth 鈥� the glow of objects in the inner solar system far outshines it. 鈥淓very time you try to measure it from Earth, or from near Earth, you鈥檙e going to have a lot of uncertainty, and there鈥檚 nothing you can do about it,鈥� says at the Space Telescope Science Institute in Maryland. 鈥淚t鈥檚 like trying to see the faint stars in the Milky Way from Broadway in NYC.鈥�
But New Horizons has travelled far enough to avoid some of that light pollution. After visiting Pluto in 2015, the spacecraft continued moving away from our planet 鈥� it is now about 8.7 billion kilometres from Earth. That is still within the bounds of our solar system, but far from its bright lights.
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With its camera shadowed by the body of the spacecraft, pointed away from both the sun and the disc of the Milky Way, New Horizons took 24 images of dark areas of space.
Postman and his colleagues calculated how much of the light in those images was due to light from stars and dust within our galaxy and subtracted that to find the total amount of background light. The result was an astonishingly dim glow of roughly 11 nanowatts per square metre per steradian.
It is as if you were in a deserted cabin on a moonless night, more than a kilometre from your nearest neighbour, and that neighbour鈥檚 refrigerator door light was on, says Postman. 鈥淭he change in light on the wall of your cabin when they open their refrigerator one mile [1.6 kilometres] away is the intensity of the light that we鈥檙e measuring,鈥� he says. 鈥淚t鈥檚 100 times darker than even the darkest sky you can see from Earth.鈥�
While it may be faint, this background is composed of all of the light from all of the galaxies that we can see in the cosmos. Earlier, less precise measurements hinted that there could be some unexplained extra light there, but these new observations dispel that idea. 鈥淭his is an important part of our understanding of the universe,鈥� says Postman.
The Astrophysical Journal
