Why is outer space not as dark as we think?

When we look at the night sky, it seems that darkness envelops everything around us, especially if the sky is covered with clouds and no stars are visible. Captured by space telescopes and generously shared with the public, planets, galaxies and nebulae can be seen against the black, cold space. But is space really black? The universe may not be as dark as astronomers thought, according to a new study. Using cameras from the robotic space station New Horizons, which once visited Pluto to measure the darkness of interplanetary space, scientists have concluded that we still have a poor idea of ​​what the universe is. The results of the study showed that six billion kilometers from the Sun, away from bright planets and light scattered by interplanetary dust, empty space was about twice as bright as expected.

The interplanetary space station New Horizons explores outer space.

How dark is it in space?

For centuries, the darkness of the night sky has been 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 look as bright as the sun? Today, astronomers know that the universe is 13.8 billion years old and expanding at an accelerating rate. As a result, most lines of sight end not in the stars, but in the fading glare of the Big Bang, and the light waves are now so extended that they are invisible to the eye. It is what darkens the sky. But how dark is the darkness?

Scientists at the National Optical Astronomical Observatory in Arizona studied light in space with NASA’s New Horizons mission. The New Horizons spacecraft was launched on January 19, 2006 and flew by Pluto on July 14, 2015. On January 1, 2019, New Horizons flew by Arrokoth, formerly Ultima Thule, one of the countless cosmic icebergs found in the Kuiper Belt at the edge of the Solar System. Today, the station continues its successful space journey.

Overview of the Solar System and Kuiper Belt Objects. The yellow line shows the trajectory of the New Horizons mission.

The team’s measurements, published in a new study, are based on seven images from New Horizons’ Long-Range Reconnaissance Thermal Camera, taken at a time when the station was about 2.5 billion kilometers from Earth. At this distance, the spacecraft was well beyond planetary glow or interplanetary dust, which could potentially affect image quality.

“Having a telescope at the edge of the solar system allows us to ask questions about how dark it really is in space,” write the authors of the paper, published on the Arxiv preprint server. “In our work, we used images of distant Kuiper Belt objects. Subtract them and any stars and you have a clear sky.”

Photos from NASA’s New Horizons mission

According to The New York Times, the New Horizons camera is a “white light shaper” that receives light in a wide spectrum, including visible and some ultraviolet and infrared waves. The resulting images were then processed – in all images, all light from all sources known to astronomers was removed, including all relatively nearby stars.

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By processing the obtained images, the researchers also removed the light from galaxies, which, as the authors of the scientific work believe, exist, but have not yet been discovered. The result was images of deep space without any light pollution. Interestingly, despite the removal of all light sources (both known and unknown), there is still a lot of light in the resulting images. Exactly where the remaining light comes from is unknown.

Today, scientists estimate the number of galaxies in the observable universe at two billion.

Scientists believe that the light may come from as yet undiscovered stars or galaxies. However, it cannot be ruled out that the light in the resulting images may be something completely new. No doubt more research will be done as scientists continue to look for sources of light pollution, but so far the source of the extra light photons is a mystery.

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According to Dan Hooper, a physicist at the Fermi National Accelerator Laboratory in Batavia, the mysterious dark matter is the culprit of the extra illumination. In an email to The New York Times, he said that while pondering the possible light source, he and his colleagues have not come up with any new physics that could explain its presence in the images, “except for some really unattractive alternatives.”

It is believed that the universe is full “dark matter”, whose exact contents are unknown, but whose gravity shapes the cosmos we see. According to some theories, this matter may be clouds of exotic subatomic particles that decay radioactively or collide and annihilate in bursts of energy that add light to the universal radiance. Another possible clue could be a common mistake. According to the authors of the study, the possibility that astronomers made a mistake and missed the light source exists, even if it is only 5%. Well, we hope that future research will be able to shed light on this dark region of near space.

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