The universe is a shadow too light.
This could be the last news you expected towards the end of a dark year. But that’s exactly what a group of astronomers discovered using cameras from the New Horizons spacecraft, which once visited Pluto, to measure the darkness of interplanetary space.
“There’s something unknown,” said Tod Lauer of the National Research Laboratory for Infrared Optical Astronomy in Tucson, Arizona. “The universe is not completely dark, and we do not yet fully know what it is made of.”
Four billion miles from the Sun, far from bright planets and the light scattered by interplanetary dust, the empty space was about twice as bright as expected, as Dr. Lauer and his colleagues expected. The most likely explanation, he said, was that there were more very faint galaxies or star clusters that contributed to the universe’s backlight than their models indicated. Or even that black holes in the centers of otherwise indistinguishable galaxies pumped additional energy into the void.
A less exciting option, said Dr. Lauer in an email was: “We messed up and missed a light source or camera artifact that we should have found out. This is what I am most concerned about. “
A more intriguing, if speculative, proposal involves what might be called cold dark matter. It is believed that the universe is filled with “dark matter”, the exact substance of which is unknown, but whose gravity shapes the visible cosmos. Some theories suggest that this matter could be clouds of exotic subatomic particles that radioactively decay or collide in flashes of energy and annihilate themselves that contribute to the universal glow.
Dr. Lauer and his colleagues prefer to leave such speculations to particle physicists. “Our work is solely concerned with measuring the flow level itself,” he said in an email. “As observers, we offer this to those who can find out what to do with it.”
Marc Postman, astronomer at the Space Telescope Science Institute in Baltimore and author of the report, which was published online in November, said: “It is important to do this in order to get an estimate of the total energy content of the universe, which helps us through the whole cosmic To inform history of star formation. “
For recording purposes, the amount of additional light that is reflected in the universe is around 10 nanowatts per square meter per steradian, a measure of the solid angle in the sky. (It takes 4𝞹 steradian to cover the entire sky).
Dr. Lauer compared this measurement to the amount of light provided by the star Sirius or an open refrigerator a mile away. “To bring it a little closer to what we did, imagine lying in bed with the curtains open on a dark moonless night,” he wrote in an email. “Maybe you’re awake and staring at the walls. When Sirius clears the mountains or your neighbor raids his refrigerator, the light in the room gets a little brighter. “
However, he noted, “Your distant neighbor who eats turkey scraps at three in the morning won’t wake you up at night from the glare.”
He said the measurement had a 5 percent chance of being a coincidence; This margin of error is known as 2-sigma and is far from the gold standard for detecting “5-sigma” or 1 chance in 3.5 million of being wrong.
The team’s measurement only included light in the visible wavelengths and had to be amplified with radio, x-ray and infrared background measurements, said Dr. Postman.
For centuries, the darkness of the night sky was the source of a paradox named after the 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 is expanding. As a result, most lines of sight stop at the fading glow of the Big Bang rather than stars, and the wavelengths of the glow are now so extended that they are invisible to the eye and make the sky appear dark.
But how dark is dark?
It’s no small thing to add up all of the light you can’t see. There are distant galaxies that are too faint to trigger the most sensitive detectors on giant telescopes, but which pump energy into the dust and gas that are scattered across space.
The New Horizons spacecraft was launched on January 19, 2006 and accelerated by Pluto on July 14, 2015. On January 1, 2019, it sped past Arrokoth, formerly called Ultima Thule and one of the myriad cosmic icebergs that live in the Kuiper Belt on the edge of the solar system. It still works.
Dr. Lauer’s measurements were based on seven images from the Long-Range Reconnaissance Imager, a camera on New Horizons, and were taken when the spaceship was about 4 billion miles from Earth. At this distance the spaceship was far beyond the annoying glow of planets or interplanetary dust. Indeed, said Dr. Postman, exiting ten times would not have created cleaner darkness.
“If you have a telescope on New Horizons at the edge of the solar system, you may be wondering, how dark is space getting anyway?” Wrote Dr. Lurking. “Only use your camera to measure the glow from the sky.” In this case, the images were distant objects from the Kuiper Belt. Subtract it and all the stars, and what remains is pure heaven.
The camera, said Dr. Postman, is a “white light imager” that receives light over a wide range of visible and some ultraviolet and infrared wavelengths.
After measuring the light intensity on the background of the sky, the team had to resort to mathematical models of how many faint galaxies lurked below normal detection limits. When this amount was subtracted from their measurements, an equal amount of light of unknown origin remained.
“It’s like you’ve counted all the people on earth but left out Asia,” said Dr. Postman. Dr. Lauer said this was the most accurate backlight measurement yet.
The study builds on earlier work by Michael Zemcov of the Rochester Institute of Technology, who had to analyze a smaller set of images – four 10-second exposures instead of 195 30-second exposures.
He and his colleagues have derived an upper limit of about 19 nanowatts per square meter and steradian – in the same range as the results of Dr. Lurking.
“This type of measurement aids our understanding of both the instrument and the brightness of light from all things between us and the distant universe,” said Dr. Zemcov in an email. “People have put up a variety of sources, but the jury isn’t sure what it could be yet.”
What we can’t see may change our understanding of the universe, but Dan Hooper, a physicist at Fermi National Accelerator Laboratory in Batavia, Illinois, splashed cold water on the idea that the culprit was dark matter. In an email, he said that while brainstorming he and his colleagues had not come up with new physics that would explain this extra light, “except for some really baroque and otherwise unattractive options.”