Over seven years ago, the New Horizons mission made history by becoming the first spacecraft to fly by Pluto. Ahead of this encounter, the spacecraft provided updated data and images of many objects in the inner and outer solar system. Once out of the orbit of Pluto and its moons, it embarked on a new mission: the first encounter with a Kuiper Belt Object (KBO). This historic flyby occurred approximately four years ago (December 31, 2015) when New Horizons sped past Arrokoth (aka 2014 MU69).
Now that it’s passing through the Kuiper Belt, away from the light pollution of the inner Solar System, it has another lucrative mission: measuring the brightness of the universe. These measurements will allow astronomers to make more accurate estimates of how many galaxies there are, which is still a matter of debate. Light from stars beyond the Milky Way is two to three times brighter than light from known populations of galaxies, according to new measurements from New Horizons – meaning there’s even more out there than we thought!
The study was led by a team from the Center for Detectors (CfD), an academic research group at the Rochester Institute of Technology (RIT). They were joined by researchers from NASA’s Jet Propulsion Laboratory, the Space Exploration Sector (SES) of Johns Hopkins University’s Applied Physics Laboratory (JHUAPL), the University of California Irvine and the Space Sciences Laboratory (SSL) of UC Berkeley. The paper describing their findings recently appeared online and has been accepted for publication in the Astrophysical Journal.
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The overall brightness of the universe is known as the cosmic optical background (COB), which includes the diffuse light given off by all the stars and galaxies in the universe combined. Like the cosmic microwave background (CMB), the remnant radiation from the Big Bang, this value is important to astronomers because it allows them to take stock of all normal matter (also known as “luminous matter”) in the universe. This is a challenge here on Earth due to interference caused by sunlight and the way it reflects off ice particles throughout the solar system (known as Zodiacal Light).
Space telescopes orbiting near Earth are also disrupted by dust between planets, which creates foreground light. But any distracting foreground light is minimal for a mission like New Horizons, which is now deep in the Kuiper Belt and on its way out of the solar system. To calculate the COB, the team analyzed hundreds of background light images taken by New Horizon’s Long-Range Reconnaissance Imager (LORRI). Teresa Symons, a postdoctoral fellow at the University of California Irvine, led the study as part of her dissertation while studying for her Ph.D. at the Rochester Institute of Technology (RIT). As she explained in a recent RIT press release:
“We’re seeing more light than we should be seeing, based on the populations of galaxies we know exist and how much light we think they should be producing. Determining what creates this light could change our fundamental understanding of how the universe formed over time.”
Previous measurements, made in 2021 by researchers at the Space Telescope Science Institute (STScI), showed the COB was brighter than expected. This was followed up earlier this year by an independent team of scientists who found that the COB was twice as large as originally thought. These latest results corroborate these earlier studies using a much broader set of LORRI observations and suggest that there must be additional light sources in the cosmos that we have not yet considered.
Currently exploring the Kuiper Belt, New Horizons is just one of five spacecraft to reach beyond 50 AU on their way out of the Solar System and eventually into interstellar space. Photo credit: NASA/Johns Hopkins APL/SwRI
The New Horizons mission is currently more than 55.85 astronomical units (AU) from Earth (or 8.35 billion km; 5.19 billion mi) – nearly 56 times the distance between Earth and the Sun. At this distance, where foreground light is minimal, astronomers have a much clearer view of the cosmic background and can make more accurate inferences about its galactic population. Symons and her colleagues hope these observations will pave the way for future missions and instruments that can help further explore this discrepancy.
These include Caltech’s Cosmic Infrared Background ExpeRiment-2 (CIBER-2) and NASA’s Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx), which will perform spectrophotometric variations of the cosmic background to learn more about galaxy formation and cosmic evolution since the big bang. Co-author Michael Zemcov, a researcher at NASA JPL and a research professor at CfD and RIT’s School of Physics and Astronomy, will play an important role in the SPHEREx mission and its data pipeline.
“This has gotten to a point where it’s a real mystery that needs solving,” he said. “I hope that some of the experiments we are involved in here at RIT, including CIBER-2 and SPHEREx, can help us resolve the discrepancy.”
Further reading: Rochester Institute of Technology, arXiv