Far from home, a lone space probe is hurtling into the blackness of space. Located just over 60 AU from the Sun, New Horizons is the most advanced human-made vehicle ever created. This means we now have a spacecraft capable of capturing unprecedented images of the universe, with uncontaminated light from the Sun reflecting off interplanetary dust scattered throughout the Solar System.

Scientists have now measured the true darkness of the universe, making the most accurate measurements yet of the faint background of visible light that permeates the universe. This is the cosmic optical background, and the new measurements made by New Horizons show that, unlike previous measurements, there is absolutely nothing strange about it.

“We now have a good idea of ​​how dark space really is,” explains astronomer Mark Postman of the Space Telescope Science Institute. “The results show that the vast majority of the visible light we receive from the universe is produced in galaxies. Importantly, we also found that there is no evidence that significant levels of light are produced by sources currently unknown to astronomers.”

Observing the universe from the solar system is like trying to see a room through a dirty container. There are elements of the surrounding space that we have to adjust for, especially the light from the Sun and the diffuse dust and gas suspended between the planets.

For most observations of the interstellar and intergalactic medium, there are ways to overcome such obstacles. The diffuse glow of all visible matter in the universe is too faint to be detected by obstacles in the solar system.

“People have repeatedly tried to measure it directly, but our part of the solar system reflects too much sunlight and interplanetary dust scatters the light around, creating a haze that blocks out the faint light from the distant universe,” says astronomer Todd Lauer of the National Science Foundation’s NOIRLab. “All attempts to measure the COB strength from the inner Solar System suffer from large uncertainties.”

NASA’s New Horizons probe was sent to explore the outer Solar System, including Pluto and various objects in the Kuiper Belt, during a flyby in 2015. Its mission provided the best opportunity to capture light outside the aquarium. There is still a lot of dust at 60 AU from the Sun, but the light from the Sun is faint enough to measure the cosmic optical background.

But the first attempt to measure the cosmic optical background using the New Horizons instruments in 2021 produced some pretty strange results. Yes, there was a background, but it was much brighter than expected, and scientists struggled to explain it.

A new attempt was made in the second half of 2023. This time, scientists used far-infrared data on the dust clouds of the Milky Way collected by the European Space Agency’s Planck mission to calibrate the New Horizons data. This allowed astronomers to correct for the amount of dust in the Milky Way galaxy.

The results showed that the team underestimated the amount of dust in the Milky Way in a preliminary analysis, then overestimated the amount of excess light coming from the rest of the universe. The new observations and analysis show that the universe actually emits as much light as we expected, from the sources we expected.

“The simplest interpretation is that the cosmic optical background is all about galaxies,” Lauer says. “When we look beyond the galaxies, we see darkness and nothing else.”

That’s probably not much comfort, considering that darkness can hide many cosmic horrors. But at least we don’t need to invent new physics… at least not yet. The study was published on: Astrophysics Journal.