Light from outside our galaxy brighter than expected

Study led by RIT scientists uses data taken by LORRI on NASA’s New Horizons mission.


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The cosmic optical background (COB) — the sum of light emitted by stars beyond the Milky Way over the universe’s history. If the COB brightness is different 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 the COB was twice as large as initially believed.

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

In the new study, scientists analyzed hundreds of images of background light 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 environment 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 see more light than we should see based on the populations of galaxies that we understand to exist and how much light we estimate they should produce. Determining what produces that light could change our understanding of how the universe formed over time.”

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

Zemcov said“This has gotten to the point where it’s an actual mystery that needs to be solved. I hope that some of the experiments we’re involved in here at RIT, including CIBER-2 and SPHEREx, can help us resolve the discrepancy.”

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