Space has long fascinated astronomers, not just for its vastness but for its eerie, almost profound darkness. Now, thanks to NASA's New Horizons probe, scientists have gained the most accurate measurement yet of how dark — or rather, bright — deep space truly is. The cosmic optical background (COB), which measures the ambient glow from the births and deaths of countless galaxies and stars, is approximately 100 billion times dimmer than the sunlight we experience on Earth.

The COB can be compared to the cosmic microwave background (CMB) radiation, the faint afterglow from the universe's inception. This incredibly dim light permeates the universe and provides clues about the formation of galaxies and stars over the 13.8-billion-year history of our cosmos.

New Horizons: A Mission Beyond the Solar System

Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore and lead author of the study, explained, "If you hold up your hand in deep space, how much light does the universe shine on it? We now have a good idea of just how dark space really is."

The COB is so faint that even sophisticated telescopes struggle to differentiate it from other sources of light in the inner solar system, such as sunlight scattered by interplanetary dust and debris around Earth. "All attempts to measure the strength of the COB from the inner solar system suffer from large uncertainties," noted Tod Lauer, a study co-author and astronomer at the National Science Foundation's NOIRLab in Arizona.

New Horizons, which passed Pluto in July 2015 and is now 5.5 billion miles (8.7 billion kilometers) from Earth, provided the most accurate measurements of this faint background glow. The spacecraft, which has ventured far beyond the solar system, was ideally positioned to capture the darkest possible skies and gather precise data.

Future Prospects and Ongoing Research

In the summer of 2023, New Horizons used its onboard camera to capture images of two dozen regions of space, focusing on areas away from bright stars and the Milky Way’s core. The probe’s main body shielded the camera from even the dimmest sunlight, allowing for a clearer view of the cosmic background.

By analyzing these images, the research team calculated the COB's brightness to be about 11 nanowatts per square meter across a sky area approximately 130 times the moon's diameter. This estimate aligns with the expected number of galaxies formed since the Big Bang. Importantly, the researchers found no evidence of significant light from previously unknown sources, providing some reassurance following earlier estimates that suggested a higher COB brightness and potential exotic light sources.

"In our previous paper, we found there was as much light we couldn't account for as light we could measure," Postman said. "The real 'gotcha' was that we simply weren't as familiar with the distribution of dust in the Milky Way as we should have been."

The team was able to correct for overestimated dust-scattered light by referencing recent maps from the European Space Agency’s Planck spacecraft. "Looking outside the galaxies, we find darkness there and nothing more," Lauer added.

New Horizons, which is currently exploring the Kuiper Belt in an extended mission mode, faced uncertainty about its future last August when NASA considered disbanding its original science team. However, the mission has been extended until at least 2028, with the possibility of continued operation through 2040.

This groundbreaking research, detailed in a paper published on August 28 in The Astrophysical Journal, offers a profound glimpse into the faint illumination of our universe and its enigmatic darkness.