![]() For example, they subtracted the light from the galaxies expected to exist that are too faint to be identifiable. To tease out the feeble background glow, they had to correct for a number of other factors. The team analyzed existing images from the New Horizons archives. “New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it.” A lot of people have tried to do this for a long time,” said Lauer. “These kinds of measurements are exceedingly difficult. At its distance (more than 4 billion miles away when these observations were taken), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble. Fortunately the New Horizons spacecraft, which has delivered the closest ever images of Pluto and the Kuiper Belt object Arrokoth, is far enough to make these measurements. To escape the zodiacal light, the team had to use an observatory that has escaped the inner solar system. Sunlight reflects off those particles, creating a glow called the zodiacal light that can be observed even by skywatchers on the ground. The inner solar system is filled with tiny dust particles from disintegrated asteroids and comets. Although located in space, Hubble orbits Earth and still suffers from light pollution. “It puts a constraint on the total starlight from galaxies that have been created, and where they might be in time.”Īs powerful as Hubble is, the team couldn’t use it to make these observations. “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. ![]() 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. 13th at a meeting of the American Astronomical Society, which is open to registered participants. These results will be presented on Wednesday, Jan. “Take all the light from 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 new findings, which relied on measurements from NASA’s distant New Horizons mission, finds only about half as much light as that earlier Hubble study but still twice as much light as existing catalogs of observed galaxies can account for. That study also estimated the combined light from those two trillion galaxies. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. “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.”Īn estimate of the total number of galaxies has been extrapolated from very deep sky observations by NASA’s Hubble Space Telescope and suggested there are about two trillion galaxies in the cosmos. New measurements of that weak background glow show that the unseen galaxies may be emitting more light than can be accounted for by existing surveys of the sky. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies. And yet even there, space isn’t absolutely black. Above the Earth’s atmosphere outer space dims even further, fading to an inky pitch-black. How dark does space get? If you get away from city lights and look up, the sky between the stars appears very dark indeed. ![]()
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