Scientists have taken a virtual trip to the far reaches of our solar system to measure the faint glow that fills the universe, called the cosmic optical background. This glow is the dim light left behind by stars and galaxies that have formed and burned out since the beginning of the cosmos. The study, published in The Astrophysical Journal, used observations from NASA’s New Horizons spacecraft, which flew past Pluto in 2015 and is now nearly 8.9 billion kilometers away from Earth.
The main question the researchers wanted to answer was a simple yet profound one: Is the sky truly dark? According to Michael Shull, an astrophysicist from the University of Colorado Boulder and one of the study’s co-authors, space may look black to our eyes, but it isn’t completely dark. From the early days of the universe, countless galaxies filled with stars have formed, burned brightly, and faded away, leaving behind a very faint light—like a tiny nightlight glowing across the cosmos.
Shull and his team, led by Marc Postman from the Space Telescope Science Institute in Baltimore, calculated just how bright this faint glow is. They found that the cosmic optical background is about 100 billion times dimmer than the sunlight we see on Earth, making it far too faint for the human eye to detect.
These findings are important because they could help scientists learn more about the history of the universe since the Big Bang. We are like cosmic accountants. said Shull. We are adding up all the sources of light we can find in the universe.
Understanding the Universe’s Darkness
For nearly 50 years, scientists have been fascinated by the idea of measuring the universe’s faint glow. After decades of research, they have a good idea of how the universe has evolved. The first galaxies formed during an era known as the Cosmic Dawn, which occurred a few hundred million years after the Big Bang. The light from the stars in these early galaxies reached its peak brightness about 10 billion years ago. Since then, the light in the universe has been slowly dimming.
By measuring the cosmic optical background accurately, scientists hope to confirm this understanding of the universe. However, it’s possible that there could be mysterious objects or unknown sources of light that we have yet to discover. Detecting these faint signals is challenging, especially from Earth. Our planet is surrounded by dust and debris, which reflects sunlight and interferes with signals from the cosmic optical background.
Shull explains this with a simple metaphor: If you want to see the stars clearly, you need to get out of Denver. You need to go to a remote place, like the far northeastern corner of Colorado, where the skies are dark and free from city lights. Similarly, to measure the faint glow of the universe, scientists need to get far away from the light pollution of our solar system. This is where New Horizons comes in—it has traveled far enough into space to provide a clearer view.
Measuring the Universe’s Glow
The New Horizons mission has deep roots in Colorado. Alan Stern, who leads the mission, studied at the University of Colorado Boulder and was mentored by Shull and Jack Brandt, a former principal investigator. The spacecraft also carries a unique instrument designed by students from CU Boulder, called the Student Dust Counter, which measures space dust.
In the summer of 2023, scientists used New Horizons’ Long Range Reconnaissance Imager (LORRI) to observe 25 different patches of the sky. Even at the edge of the solar system, the team had to account for extra light from various sources, such as the Milky Way galaxy, which is surrounded by a halo of dust. You can’t escape the dust, said Shull. It’s everywhere.
The researchers estimated how much light could be caused by this dust and other sources, then subtracted it from their measurements. What was left was the cosmic optical background. In scientific terms, this faint glow equals about 11 nanowatts per square meter per steradian (a steradian is a unit used to measure the size of an area in the sky).
Shull noted that this number matches what scientists expected based on the number of galaxies thought to have formed since the Big Bang. This suggests there are no strange, unknown objects producing a lot of light in the universe. However, the possibility of unknown phenomena cannot be entirely ruled out.
These measurements are likely to be the most accurate estimates of the universe’s faint glow for quite some time. New Horizons is running low on fuel and is now focused on other scientific priorities. Currently, there are no other missions planned to explore the distant, dark regions of space where New Horizons has been.
If a future mission includes a camera and we’re willing to wait a few decades for it to reach the distant parts of space, we might get even more precise measurements, said Shull.
For now, the best glimpse we have of the faint glow of the universe comes from New Horizons, and it helps us understand that the universe is mostly filled with darkness, with its light slowly fading away over billions of years.
SOURCES
University of Colorado at Boulder
Marc Postman, Tod R. Lauer, et al., New Synoptic Observations of the Cosmic Optical Background with New Horizons. ApJ 972 95. DOI 10.3847/1538-4357/ad5ffc
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