The cosmic optical background (COB) — the sum of light emitted by stars beyond the Milky Way over the history of the universe. If the COB brightness differs from the known light from galaxies, it suggests that there may be other optical light sources in the universe.

Earlier this year, an independent team of scientists reported that the COB was twice as large as initially believed.

Those results were no fluke, as confirmed using a much broader set of LORRI observations in the new study by Symons, RIT Associate Professor Michael Zemcov and researchers from the Jet Propulsion Laboratory at Caltech, UC Irvine, UC Berkeley and Johns Hopkins University.

In the new study, scientists analyzed hundreds of backlight images taken by the Long-Range Reconnaissance Imager (LORRI) on NASA’s New Horizons mission to calculate the cosmic optical background (COB). They found new measurements showing that the light emitted by stars outside our galaxy is two to three times brighter than the light from known populations of galaxies.

The study challenges assumptions about the number and neighborhood of stars in the universe.

Teresa Symons’ 22 Ph.D. (astrophysical sciences and technology), who led the study for her dissertation and is now a postdoctoral researcher at the University of California Irvine, said: “We are seeing more light than we should be seeing based on the populations of galaxies that we understand exist and how much light we estimate they should be producing. Determining what produces that light would improve our understanding of how the universe formed over time may change.”

Dust between the planets makes it challenging to measure COB clear of Earth. Still, the New Horizons mission is at the frontier of our solar system, where this foreground is low, providing a much clearer picture for this kind of research. The scientists hope that new missions and tools can be created to better investigate the discrepancy.

Zemkov said, “This has gotten to the point where it is a real mystery that needs to be solved. 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.”

The study results, led by scientists from the Rochester Institute of Technology, have been posted to ArXiv and accepted for publication in The Astrophysical Journal.