Scientists have ventured to the farthest reaches of the solar system to collect the most accurate measurements yet of the faint glow that fills the universe, at least virtually. The combined light from all sources outside our Milky Way galaxy is known as the cosmic optical background.

In a recently published study Astrophysics JournalExperts analyzed data from NASA’s New Horizons spacecraft, which flew past Pluto in 2015 and is now about 8.5 billion miles from Earth.

The faint light of a trillion galaxies

According to study co-author Michael Shull, an astrophysicist at the University of Colorado at Boulder, the study addresses a seemingly simple but profound question: “Is the sky really dark?”

“At the beginning of time, the universe was a sea of ​​light. But as it expanded, it cooled, darkened, and matter came to the fore. About 14 billion years after the Big Bang, space was now cold and dark, the researchers said.

“Although our horizon contains nearly a trillion galaxies that formed during this time, they are extremely faint and we would need our most powerful telescopes to detect their presence directly.”

Although space appears black to the human eye, scientists believe that it is not completely devoid of light. Throughout cosmic history, trillions of star-filled galaxies have appeared and disappeared, leaving behind a faint, almost imperceptible light that resembles a cosmic night light.

Measuring the brightness of the universe

Shull and his team, led by Mark Postman of the Space Telescope Science Institute in Baltimore, calculated the intensity of this faint glow. They found that the cosmic optical background is about 100 billion times fainter than the sunlight reaching the Earth’s surface, too faint for the human eye to detect.

The results could help scientists better understand the history of the universe since the Big Bang.

“We’re kind of like cosmic accountants who collect every light source we can count in the universe,” Shull said, noting that this effort to measure the cosmic optical background has fascinated scientists for nearly half a century.

Light is fading throughout the universe

Shull explained that after decades of research, astrophysicists believe they have a pretty clear picture of how the universe evolved. Galaxies first formed a few hundred million years after the Big Bang, during a period called the Cosmic Dawn. The light from these galaxies peaked about 10 billion years ago and has been waning ever since.

Accurate measurements of the cosmic optical background could help confirm whether this understanding of the universe is correct or whether there are hidden, undiscovered objects providing light to space. But measuring this faint light is not easy, especially from Earth.

A rare opportunity to observe the glow of the universe

The space around the Earth is filled with dust and debris. Sunlight reflects off this material, blocking out all signals from the cosmic optical background.

“The metaphor I use is: If you want to see the stars, you have to get out of Denver,” Shull said. “You have to go very far, to the northeast corner of Colorado, where all you have is South Dakota and Nebraska in front of you.”

New Horizons has given scientists a rare opportunity to achieve something like this in space.

New Horizons mission

The New Horizons mission has deep roots in Colorado. It is led by Alan Stern, a graduate student at the University of Colorado at Boulder under Shull and former senior research associate Jack Brandt. He is currently at the Southwest Research Institute in Boulder, Colorado.

The spacecraft also includes the Student Dust Counter, an instrument designed and built by students at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Boulder. The research team aimed New Horizons’ Long Range Reconnaissance Imager (LORRI) at 25 different areas of the sky in the summer of 2023.

Even at the outer edges of the solar system, the team was still dealing with excess light. The Milky Way galaxy, for example, is in a halo of dust, like the Solar System. “You can’t escape the dust,” Shull said. “It’s everywhere.”

Cosmic optical background

The researchers estimated how much light the Milky Way’s halo might contribute and subtracted this from data collected by LORRI. Once all other light sources were taken into account, they were left with the faint glow of the cosmic optical background.

In scientific terms, this background is about 11 nanowatts per square meter per steradian (a steradian is an area of ​​sky about 130 times the diameter of the Moon).

This figure agrees well with the number of galaxies that scientists believe formed after the Big Bang, Shull explained. That means there don’t seem to be any mysterious objects that emit significant amounts of light, such as exotic particles. But the researchers can’t completely rule out the possibility of such anomalies.

The most accurate estimates of the universe’s brightness

These measurements are likely to remain the most accurate estimates of the brightness of the universe for the foreseeable future. New Horizons is using its remaining fuel for other science missions, and no other spacecraft currently plan to travel to the cold, distant regions of space explored by New Horizons.

“If they use the camera on a future mission and we wait a few decades for it to arrive, we will be able to see more accurate measurements,” Schull concluded.