Scientists have traveled to the sides of the solar system, virtually, at the very least, to capture essentially the most accurate measurements thus far of the faint glow that permeates the universe — a phenomenon referred to as the cosmic optical background.
The brand new study, published Aug. 28 in The Astrophysical Journal, draws on observations from NASA’s Latest Horizons spacecraft, which whizzed past Pluto in 2015 and is now nearly 5.5 billion miles from Earth. The research seeks to reply a deceptively easy query, said co-author Michael Shull, an astrophysicist on the University of Colorado Boulder.
“Is the sky really dark?” said Shull, professor emeritus within the Department of Astrophysical and Planetary Sciences.
Space may look black to human eyes, but scientists imagine that it isn’t completely dark. For the reason that dawn of the cosmos, trillions of galaxies containing countless stars have formed and died, abandoning an imperceptibly faint light. Consider it because the night light in space.
Shull and the team, led by Marc Postman on the Space Telescope Science Institute in Baltimore, calculated just how vivid that glow is. Their findings suggest that the cosmic optical background is roughly 100 billion times fainter than the daylight that reaches Earth’s surface¬ — far too faint for humans to see with the naked eye.
The outcomes could help scientists shine a lightweight on the history of the universe because the Big Bang.
“We’re sort of like cosmic accountants, adding up every source of sunshine we will account for within the universe,” Shull said.
Into the dark
It is a form of number crunching that has captured the imagination of scientists for nearly 50 years, he added.
Shull explained that, after a long time of research, astrophysicists think they’ve a reasonably good idea of how the cosmos evolved. The primary galaxies formed during an epoch referred to as the Cosmic Dawn several hundred million years after the Big Bang. The starlight from galaxies within the distant universe reached its brightest point about 10 billion years ago and has been dimming ever since.
Precise measurements of the cosmic optical background could help scientists confirm whether this picture of the cosmos is sensible — or if there are mysterious, as-of-yet-undiscovered objects casting light into space.
Taking those sorts of measurements, nevertheless, is not easy, especially not from Earth.
Earth’s neighborhood is teeming with tiny grains of dust and other debris. Sunlight glints off this mess, washing out any signals that could be coming from the cosmic optical background.
“A metaphor I take advantage of is that if you need to see the celebrities, you should get out of Denver,” Shull said. “You could have to go way out, right to the northeast corner of Colorado where all you’ve gotten ahead of you might be South Dakota and Nebraska.”
Latest Horizons has given scientists a once-in-a-lifetime opportunity to do something similar in space.
Cosmic accounting
The mission has uniquely Colorado origins. Alan Stern, who studied as a graduate student at CU Boulder under Shull and former Senior Research Associate Jack Brandt, leads the Latest Horizons mission. He’s currently based on 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).
Over the course of several weeks in summer 2023, the researchers pointed Latest Horizons’ Long Range Reconnaissance Imager (LORRI) at 25 patches of sky.
Even at the sting of the solar system, the team still had a number of extra light to contend with. The Milky Way Galaxy, for instance, sits inside a halo that, like our solar system, gathers dust.
“You’ll be able to’t get away from dust,” Shull said. “It’s in all places.”
He and his colleagues estimated how much light that halo could generate, then subtracted it from what they were viewing with LORRI. After eliminating additional sources of sunshine, the team was left with the cosmic optical background.
In scientific terms, that background amounts to about 11 nanowatts per square meter per steradian. (A steradian is a patch of sky with a width about 130 times the diameter of the moon).
Shull said that this value lines up well with what number of galaxies scientists imagine must have formed because the Big Bang. Put in another way, there are not any strange objects, equivalent to exotic sorts of particles, on the market in space producing a number of light. However the researchers cannot rule out such anomalies completely.
The team’s measurements are prone to be the perfect estimates of the universe’s glow for a very long time. Latest Horizons is using its remaining fuel supplies to pursue other scientific priorities, and no other missions are currently heading toward those cold and dark corners of space.
“In the event that they put a camera on a future mission, and all of us wait a few a long time for it to get on the market, we could see a more exact measurement,” Shull said.
Other co-authors of the brand new study include SWRI’s Alan Stern and Tod Lauer on the U.S. National Science Foundation National Optical Infrared Astronomy Research Laboratory. Researchers from the Johns Hopkins University Applied Physics Laboratory, University of Texas at San Antonio and University of Virginia also participated.
