New Horizons spacecraft answers the question: How dark is space?





«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.»


«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.

This photo shows a phenomenon known as zodiacal light. At lower left, a glowing patch extends to the upper right in the direction of Jupiter, the bright object left of center. Zodiacal light is caused by sunlight reflecting off tiny dust particles in the inner solar system—the disintegrated remains of comets and asteroids. Attempts to measure how dark space is using telescopes like Hubble have been thwarted by this ambient glow. As a result, astronomers relied on NASA’s distant New Horizons spacecraft to observe the sky free from zodiacal light. The faint background they measured is the equivalent of seeing a neighbor’s refrigerator light from a mile away. This very wide, multi-frame panorama was taken in October 2014 at Canyon de Chelly National Monument in northeast Arizona. The zodiacal light is at left, with the northern Milky Way to the right. The Orion constellation is at top right. Jupiter is the brighter object left of center, while a similarly bright object to the right (below Orion) is Sirius. M44 (the Praesepe Cluster) is just above Jupiter. On the horizon, a yellow glow marks the location of the nearby town of Chinle, Arizona. Credit: Z. Levay
«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.»



«These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time,» said Lauer. «New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it.»





This study is accepted for publication in The Astrophysical Journal.

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