Starlight in the background of the universe is twice as bright as expected

Even when you remove bright stars, glowing dust, and other bright spots near the dark, inky sky, a background glow remains. That glow comes from the cosmic sea of ​​distant galaxies, the first stars that burned, distant coalescing gas — and, it seems, something else in the mix that eludes researchers.

Astronomers estimated the amount of visible light entering the cosmos by dragging the New Horizons spacecraft, which flew past Pluto in 2015, to a spot in the sky devoid of nearby stars and galaxies (SN: 12/15/15). This estimate should match measurements of the total amount of light coming from galaxies throughout the history of the universe. But that’s not the case, researchers report in the March 1 Astrophysical Journal Letters.

“It turns out that the galaxies we know about can be about half the level we see,” says Tod Lauer, an astronomer at the National Science Foundation’s NOIRLab in Tucson, Arizona.

For decades, astronomers have measured extragalactic background light in various wavelengths, from radio waves to gamma rays (SN: 08/23/13; SN: 11/29/18). This provides a census of the universe and gives researchers clues to the processes that emit these types of light.

But background visible light — dubbed the cosmic optical background, or COB — is difficult to measure from the inner solar system. Here, a lot of interplanetary dust scatters sunlight, carrying away the much weaker COB. The New Horizons spacecraft visiting Pluto, however, is far enough from the sun that scattered sunlight does not flood the spacecraft’s images.

So, in September 2021, Lauer and his colleagues pointed the spacecraft’s LORRI camera at a patch of sky and took a bunch of photos. They digitally removed all known sources of light – individual stars, nearby galaxies, even the heat from the spacecraft’s nuclear power source (SN: 02/18/16) — and measured what was left to estimate the COB.

Next, they used large archives of galaxy observations, such as those from the Hubble Space Telescope, to calculate the light emitted by all the galaxies in the universe. The measured COB is about twice as bright as this calculation.

While Lauer’s group had previously noted a discrepancy, this new measurement reveals a larger difference and with less uncertainty. “There is clearly an anomaly. Now we have to try to figure it out and explain it,” says co-author Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

Several astronomical reasons could explain this discrepancy. Perhaps, says Postman, rogue stars stripped from galaxies persist in intergalactic space. Or perhaps, he says, there is “a very small population of very compact galaxies that are just below Hubble’s detection limits.” If it’s the latter, astronomers should find out within the next two years, as NASA’s recently launched James Webb Space Telescope will see these galaxies even fainter (SN: 06/10/21).

Another possibility is that the researchers missed something in their analysis. “I’m glad it was done; it is an absolutely necessary measure,” explains astrophysicist Michael Zemcov of the Rochester Institute of Technology in New York, who was not involved in this study. Maybe they’re missing an extra glow from the New Horizons spacecraft and its LORRI instrument, or they haven’t factored in an extra foreground light. “I think there’s a conversation there about the details.”

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The light reflecting off the dust of the Milky Way, for example, is “a very subtle beast,” says Zemcov, “and our uncertainties will probably be dominated by it at some point, simply because it’s not very well understood”. Several projects over the next few years, such as the CIBER-2 experiment and the SPHEREx space mission, could help astronomers understand this dust-scattered light, Zemcov said.

Additionally, he and his research group member, astrophysicist Teresa Symons, are examining hundreds of old LORRI images of the dark sky and performing their own analyses. Meanwhile, Lauer and his colleagues will take more pictures of other patches of sky with LORRI to build their confidence in measuring background light and to better understand intrusions from the spacecraft itself.

“Something is happening that we didn’t expect,” Zemcov says, “that’s where the fun part of science comes in.”