As vast and empty as space may seem, even the darkest parts of the universe aren’t completely devoid of light. In fact, astronomers have long sought to measure the faint glow known as the Cosmic Optical Background (COB). This light provides key insights into the universe’s energy balance, one of the holy grails of cosmology.
Thanks to new data from the New Horizons spacecraft, currently 57 times farther from the Sun than Earth, scientists believe they’ve finally quantified the COB. This faint glow comes from the accumulated light of billions of galaxies that have formed since the beginning of time.
Imagine if you could capture this light from every direction in the darkest corners of space. It would glow no brighter than a firefly seen from a meter away. While this aligns with previous theories, it comes as a relief to astronomers who once thought an unknown light source skewed their data. In 2021, a study found an unexplained excess of light—double the predicted amount.
The Challenge of Measuring Cosmic Light
Marc Postman, lead author and astronomer at the Space Telescope Science Institute, explained, “In our earlier study, we couldn’t account for as much light as we could measure.” The team realized they had underestimated the effect of dust in the Milky Way, which had skewed their results.
Measuring space’s darkness may sound simple, but it’s surprisingly tricky. For instance, measurements from within the inner solar system are unreliable due to zodiacal light—a natural glow caused by sunlight scattering off dust particles orbiting the Sun.
To avoid this interference, researchers turned to New Horizons, far out in the Kuiper Belt, where it escapes the zodiacal light. Although New Horizons wasn’t specifically designed to measure the COB, its unique location made it ideal for the task.
Investigating the Universe’s Glow
In their first analysis in 2021, the team used archived data where New Horizons’ cameras pointed away from the Milky Way’s disk toward its poles, areas chosen to minimize dust interference. However, their analysis showed the COB appeared brighter than expected, suggesting there was more to the story.
To dig deeper, the team conducted a broader survey with New Horizons’ Long Range Reconnaissance Imager (LORRI), covering 23 fields of view. They meticulously avoided bright stars and the Milky Way’s inner disk, while comparing their data to a galactic dust map from the European Space Agency’s Planck mission. This allowed them to correct for the dust’s impact on their measurements.
The results were revealing: the extra brightness they detected wasn’t a mystery after all. It was actually light scattered by infrared cirrus clouds, thin, dusty structures within the galaxy. These cirrus clouds act like the cirrus clouds in Earth’s atmosphere, scattering light from distant stars across the sky.
Now that these contributions from galactic cirrus clouds have been accounted for, the COB aligns closely with the expected brightness from galaxies. “It’s pretty clear now that what we’re seeing is the sum of all the light from galaxies,” Postman said.
Cosmic Mysteries: Room for New Discoveries
While the team’s measurement of the COB is still slightly higher than estimates of all known galactic light, this leaves room for potential new discoveries. Postman noted that the simplest explanation remains: the COB is the accumulated light from all star formation across the universe’s history.
Exploring the Entire Electromagnetic Spectrum
Beyond optical light, astronomers are eager to measure other parts of the electromagnetic spectrum—from radio waves to X-rays. Understanding the full spectrum provides a complete picture of the universe’s energy balance.
The first big breakthrough came with the discovery of the cosmic microwave background (CMB) in 1965. This relic of the Big Bang was found purely by accident when two radio astronomers detected persistent noise coming from all directions in space. This signal represents the light from the universe before stars, galaxies, or black holes existed.
For decades, astronomers have tried to detect an optical counterpart to the CMB, but only New Horizons—equipped with its small 8-inch telescope and black-and-white camera—could finally capture the faint light of the COB.
What’s Next in Understanding the Universe’s Energy?
The next big task for astronomers is to measure the infrared portion of the spectrum, though the CMB remains the largest contributor to the universe’s energy budget. Despite being made up of low-energy photons, the sheer volume of these photons makes them a significant source of cosmic energy.
Thanks to Postman’s research, astronomers are one step closer to understanding the cosmic optical background and, by extension, the vast energy that powers the universe.
As Postman puts it, “It was a learning process, but in the end, it resulted in the most significant detection of the cosmic optical background that’s ever been made.”
