Over countless years, the stars and galaxies in the universe have illuminated the cosmos, creating an extremely faint night light referred to as the cosmic optical background. NASA’s New Horizons spacecraft has made a journey to the farthest reaches of our solar system and provided the most precise measurement of this glow to date.
Researchers have ventured to the outer limits of the solar system, at least in a virtual sense, to obtain the most precise assessments yet of the subtle light that fills the cosmos — a phenomenon termed the cosmic optical background.
The recent study, released on August 28 in The Astrophysical Journal, utilizes data gathered from NASA’s New Horizons spacecraft, which flew past Pluto in 2015 and is now approximately 5.5 billion miles away from Earth. This research aims to address a seemingly straightforward inquiry, as explained by co-author Michael Shull, an astrophysicist from the University of Colorado Boulder.
“Is the sky really dark?” Shull, an emeritus professor in the Department of Astrophysical and Planetary Sciences, posed.
While space may appear black to our eyes, scientists suspect it is not entirely devoid of light. Since the universe’s inception, trillions of galaxies, filled with innumerable stars, have emerged and faded away, leaving behind an almost imperceptible light — similar to a night light found in space.
Shull and his team, under the leadership of Marc Postman from the Space Telescope Science Institute in Baltimore, evaluated the intensity of this glow. Their results imply that the cosmic optical background shines at around 100 billion times dimmer than the sunlight that arrives at Earth’s surface — too faint for human vision.
These findings hold potential to illuminate the universe’s history since the Big Bang.
“We’re like cosmic accountants, tallying every source of light we can find in the universe,” Shull remarked.
Into the dark
This figure-crunching pursuit has fascinated researchers for nearly half a century, he noted.
Shull mentioned that after years of exploration, astrophysicists believe they have a solid understanding of the cosmos’s evolution. The first galaxies are thought to have emerged during a period known as the Cosmic Dawn, a few hundred million years after the Big Bang. The brightness from distant galaxies peaked about 10 billion years ago and has gradually dimmed since then.
Accurate measurements of the cosmic optical background might assist scientists in confirming whether this cosmic narrative is correct or if there are unknown objects contributing light in the universe.
However, obtaining such measurements is challenging, especially from our planet.
The vicinity of Earth is filled with minute dust particles and other debris. Sunlight reflects off this clutter, obscuring any potential signals from the cosmic optical background.
“To see the stars, you need to escape Denver,” Shull said metaphorically. “You have to travel far out, to the northeast corner of Colorado where only South Dakota and Nebraska lie ahead.”
New Horizons has provided scientists with an extraordinary chance to do something analogous in space.
Cosmic accounting
This mission has roots in Colorado. Alan Stern, who studied as a graduate student at CU Boulder under Shull and former Senior Research Associate Jack Brandt, heads the New Horizons mission. He is currently affiliated with the Southwest Research Institute in Boulder, Colorado. The spacecraft also includes the Student Dust Counter, an instrument designed and created by students at CU Boulder’s Laboratory for Atmospheric and Space Physics (LASP).
During several weeks in summer 2023, researchers directed New Horizons’ Long Range Reconnaissance Imager (LORRI) toward 25 sections of the sky.
Even at the solar system’s edge, the team had to contend with substantial light interference. For instance, the Milky Way Galaxy is surrounded by a halo that collects dust, just like our solar system does.
“Dust is inescapable,” Shull commented. “It’s omnipresent.”
He and his team estimated the light produced by this halo and then subtracted it from their observations with LORRI. Once they eliminated other light sources, they were able to isolate the cosmic optical background.
In scientific terms, this background generates about 11 nanowatts per square meter per steradian (with a steradian referring to a region of the sky roughly 130 times the moon’s diameter).
Shull stated that this measurement aligns well with estimates of how many galaxies are thought to have formed since the Big Bang. In other words, no unusual objects, like exotic particles, seem to be emitting considerable light in the universe. Nonetheless, they cannot completely dismiss the possibility of such anomalies.
The measurements taken by this team are likely to remain the best approximation of the universe’s glow for an extended period. New Horizons is now using its remaining fuel to pursue additional scientific objectives, and no other missions are currently aimed at exploring those remote and shadowy areas of space.
“If a future mission includes a camera and we wait a couple of decades for it to reach those regions, we might obtain more precise measurements,” Shull noted.
Other co-authors of this recent study include SWRI’s Alan Stern and Tod Lauer from the U.S. National Science Foundation’s National Optical Infrared Astronomy Research Laboratory. Researchers from Johns Hopkins University Applied Physics Laboratory, University of Texas at San Antonio, and University of Virginia also contributed.
