The universe is a shade too bright. That might be the last news you expected to hear toward the darkening end of a dark year. But that is what a band of astronomers has discovered, using cameras on the New Horizons spacecraft that once visited Pluto to measure the darkness of interplanetary space.
“There’s something out there unknown,” said Tod Lauer of the National Optical-Infrared Astronomy Research Laboratory in Tucson, Arizona, US. “The universe is not completely dark. We don’t yet completely know what it comprises.”
Four billion miles from the sun, far from bright planets and the light scattered by interplanetary dust, empty space was about twice as bright as would be expected, Lauer and his colleagues found. The most likely explanation was that there were more very faint galaxies or star clusters contributing to the background light of the universe than their models indicated. Or even that black holes in the centres of otherwise undistinguished galaxies were pumping extra energy into the void.
A less exciting possibility, Lauer said in an email, was that “we messed up and missed a light source or camera artifact that we should have figured out. This is what I worry about the most.”
A more intriguing, if speculative, suggestion involves what might be called cold, dim matter. The universe is thought to be filled with “dark matter”, its exact substance unknown but whose gravity shapes the visible cosmos. Some theories suggest that this matter could be clouds of exotic subatomic particles that decay radioactively or collide and annihilate themselves in flashes of energy that add to the universal glow.
Lauer and his colleagues prefer to leave such speculations to particle physicists. “Our work is solely concerned with measuring the flux level itself,” he said in an email. “As observers, we offer this up for those who can figure out what to do with it.”
Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore, US, and an author of the report, which was published online in November, said, “It is important to do this to get an estimate of the total energy content of the universe, which helps inform us about the overall cosmic history of star formation.”
For the record, the amount of extra light they found bouncing around the universe is about 10 nanowatts per square metre per steradian, a measure of solid angle on the sky. It takes 4(PIE) steradians to cover the entire sky.
Lauer compared this measurement to the amount of light supplied by the star Sirius or an open refrigerator a mile away. “To make it a little closer to what we did, you can think of lying in bed with the curtains open on a dark moonless night,” he wrote in an email. “Perhaps you’re awake and are staring at the walls. When Sirius clears the mountains, or your neighbour raids his fridge, we would see the light in the room get a little brighter.” However, he noted, “Your distant neighbour eating leftover turkey at three in the morning is not going to wake you up at night from the glare.”
He said the measurement had a five per cent chance of being a fluke; that margin of error is known as two sigma, and is a far cry from the gold standard for a discovery of “5 sigma”, or one chance in 3.5 million of being wrong.
The team’s measurement included only light in the visible wavelengths and needed to be augmented by radio, X-ray and infrared background measurements, according to Postman.
For centuries, the darkness of the night sky was the source of a paradox named after German astronomer Heinrich Wilhelm Olbers. Presumably, in an infinite static universe, every line of sight ends at a star, so shouldn’t the sky appear as bright as the sun?
But astronomers now know that the universe is only 13.8 billion years old and expanding. As a result, most lines of sight do not end on stars but on the fading glow of the Big Bang, and the wavelengths of the glow are now so extended that they are invisible to the eye, making the sky look dark.
But how dark is dark?
It’s no small feat to add up all the light you cannot see. There are distant galaxies too faint to trip the most sensitive detectors on giant telescopes, but which pump energy into the dust and gas that is strewn about space.
The New Horizons spacecraft was launched on January 19, 2006, and sped by Pluto on July 14, 2015. On January 1, 2019, it zoomed past Arrokoth, formerly called Ultima Thule, one of untold numbers of cosmic icebergs that live in the Kuiper belt on the outskirts of the solar system. It is still going.
Lauer’s measurements were based on seven images from the Long-Range Reconnaissance Imager, a camera on New Horizons, that were taken when the spacecraft was some 4 billion miles from Earth. At that distance the spacecraft was well beyond the distracting glow of planets or of interplanetary dust. Indeed going even 10 times farther out, Postman said, would not have produced a cleaner darkness.
“When you have a telescope on New Horizons, way out at the edge of the solar system, you can ask, how dark does space get anyway,” Lauer wrote. “Use your camera just to measure the glow from the sky.” In this case, the images were of distant Kuiper belt objects. Subtract them, and any stars; what remains is pure sky.
The camera, Postman said, is a “white light imager”, receiving light across a wide spectrum spanning visible and some ultraviolet and infrared wavelengths.
Once the team measured the level of light in the sky background, they then had to resort to mathematical models of how many faint galaxies were lurking under the normal limits of detection. When that amount was subtracted from their measurements, an equal amount of light remained of unknown origin.
“It’s as if you counted all the people on Earth but left out Asia,” Postman said. Lauer said this was the most accurate measurement of the background light yet.
NYTNS
