New Horizons discovered the abnormal brightness of the Universe’s background starlight

Astronomers estimated the Universe’s background starlight — the amount of light pervading the cosmos. The New Horizons spacecraft was used for this experiment, it was directed to a part of the sky almost devoid of nearby stars and galaxies.

The Universe’s background starlight far from the Sun turned out to be abnormally bright, as recorded by the New Horizons apparatus. Photo: NASA

The fact is, if you remove all the glowing objects from the night sky, whether they are stars, galaxies or interstellar dust, the background starlight still remains. This background starlight comes from the cosmic sea of distant galaxies, the first burnt stars, distant interstellar gas. In theory, the level of the Universe’s background starlight should correspond to measurements of the total amount of light emanating from galaxies throughout the history of the Universe. However, as the New Horizons experiment found out, this turned out to be fundamentally wrong. The researchers report this in the Astrophysical Journal Letters. There is something else that eludes researchers.

“Turns out, galaxies constitute about half of the true brightness level of the Universe’s background starlight.” — says Tod Lauer, an astronomer at the NOIRLab National Science Foundation in Tucson, Arizona.

Measuring the Universe’s background starlight

For decades, astronomers have been measuring extragalactic background emission at different wavelengths, from radio waves to gamma rays. This gave the researchers clues about the processes that emit these types of starlight. But the background visible starlight, called the cosmic optical background, is difficult to measure from the inner Solar System. There is a lot of interplanetary dust scattered by sunlight. 

However, the New Horizons spacecraft, which flew past Pluto in 2015, is quite far from our luminary. Therefore, the scattered sunlight does not obscure the view of the spacecraft. In September 2021, Lauer and his colleagues pointed the LORRI camera on board the spacecraft at the darkest part of the sky, and took a series of photos. Then they removed all known light sources — individual stars, nearby galaxies, even heat from the spacecraft’s nuclear power source — and measured what was left to assess the brightness level of the Universe’s background starlight.

Sunlight scattered by dust near the Earth creates a great photo, but makes it difficult to observe the faint cosmic background. Photo: ESA

Then the scientists used large archives of observations of galaxies. For example, using the Hubble Space Telescope to calculate the light emitted by all the galaxies in the Universe. The measured background of the Universe turned out to be about twice as bright as theoretical calculations. 

“This is obviously an anomaly. Now we should try to understand and explain this phenomenon.” — says co-author Mark Postman, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

Causes of the anomaly

There are several astronomical reasons that could explain the discrepancy. Perhaps the reason is rogue stars torn out of galaxies and remaining in intergalactic space. Or there is a very weak population of compact galaxies that are just below the limits of detection by existing tools. NASA’s James Webb Space Telescope will probably be able to see these dim galaxies.

Recall that Stephen Hawking’s main paradox about black holes was previously solved.