Scientists have ventured to the far reaches of the solar system, at least virtually, to gather the most precise measurements of the faint glow that fills the universe. The combined light from all sources outside of the Milky Way galaxy is known as the cosmic optical background.
In a recent study, published in The Astrophysical Journal , experts analyzed data from NASA’s New Horizons spacecraft, which flew past Pluto in 2015 and is now nearly 5.5 billion miles from Earth.
Faint light of a trillion galaxies
The research addresses a seemingly simple but profound question, according to co-author Michael Shull, an astrophysicist at the University of Colorado Boulder : “Is the sky really dark?”
“At the dawn of time, the universe was a sea of light. But as it expanded, it cooled, dimmed, and matter came to the fore. Nearly 14 billion years after the Big Bang, space is now cold and dark,” noted the researchers.
“While our horizon encompasses almost a trillion galaxies that have formed over that time, they are terribly faint, and we need our most powerful telescopes to tally their presence directly.”
Although space appears black to human eyes, scientists believe it’s not entirely devoid of light. Over the course of cosmic history, trillions of galaxies filled with stars have come and gone, leaving behind a faint, almost imperceptible light - a sort of cosmic night light.
Measuring the glow of the universe
Shull and his team, led by Marc Postman from the Space Telescope Science Institute in Baltimore, calculated the intensity of this faint glow. The findings suggest that the cosmic optical background is about 100 billion times dimmer than the sunlight that reaches Earth’s surface - far too faint for human eyes to detect.
The results may help scientists better understand the history of the universe since the Big Bang.
“We’re kind of like cosmic accountants, adding up every source of light we can account for in the universe,” said Shull, nothing that this effort to quantify the cosmic optical background has fascinated scientists for nearly half a century.
Fading light throughout the universe
Shull explained that after decades of research, astrophysicists believe they have a reasonably clear picture of how the universe evolved.
Galaxies first formed during a period called the Cosmic Dawn , a few hundred million years after the Big Bang. The light from these galaxies reached its peak brightness about 10 billion years ago and has been gradually fading ever since.
Accurate measurements of the cosmic optical background could help confirm whether this understanding of the universe is correct - or if there might be hidden, undiscovered objects contributing light to space. However, measuring this faint light isn’t easy, especially from Earth.
Rare opportunity to observe the universe’s glow
The area around Earth is filled with dust and debris. Sunlight reflects off this material, obscuring any signals from the cosmic optical background.
“A metaphor I use is if you want to see the stars, you need to get out of Denver,” Shull said. “You have to go way out, right to the northeast corner of Colorado where all you have ahead of you are South Dakota and Nebraska.”
New Horizons has provided scientists with a rare opportunity to achieve something similar in space.
New Horizons mission
The New Horizons mission has deep roots in Colorado. Alan Stern, who was a graduate student at the University of Colorado Boulder under Shull and former Senior Research Associate Jack Brandt, leads the mission. He is currently based at the Southwest Research Institute in Boulder, Colorado.
The spacecraft also carries the Student Dust Counter, an instrument designed and built by students at CU Boulder’s Laboratory for Atmospheric and Space Physics (LASP).
During the summer of 2023, the research team aimed New Horizons’ Long Range Reconnaissance Imager (LORRI) at 25 different patches of sky.
Even at the outer edges of the solar system, the team still had to deal with excess light. For instance, the Milky Way galaxy sits within a halo that, like the solar system, contains dust. “You can’t get away from dust,” Shull noted. “It’s everywhere.”
The cosmic optical background
The researchers estimated how much light the Milky Way ’s halo could contribute and subtracted that from the data collected by LORRI. After accounting for all other sources of light, they were left with the faint glow of the cosmic optical background.
In scientific terms, this background equates to roughly 11 nanowatts per square meter per steradian (a steradian being a section of the sky with a width about 130 times the diameter of the moon).
Shull explained that this value aligns well with the number of galaxies scientists believe have formed since the Big Bang.
In other words, there doesn’t appear to be any mysterious objects, such as exotic particles, emitting significant amounts of light. However, the researchers can’t entirely rule out the possibility of such anomalies.
Most accurate estimates of the universe’s glow
The measurements are likely to remain the most accurate estimates of the universe’s glow for the foreseeable future.
New Horizons is using its remaining fuel for other scientific missions, and no other spacecraft are currently planned to venture into the cold and distant parts of space that New Horizons has explored.
“If they put a camera on a future mission, and we all wait a couple of decades for it to get out there, we could see a more exact measurement,” Shull concluded.
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