On a cloudless and moonless night, we can see the dark night sky, so how dark is it? This sounds simple question, but even astronomers don’t know it. Fortunately, NASA’s Pluto probe is already quite far away from the sun, and can effectively detect the background color of the universe, only to find that the universe is not as dark as imagined.
Universe Today reported, “Why is the universe dark?” This is the famous “Olbers’ Paradox”, written by the German astronomer Heinrich Olbers. In 1823, he put forward a big question, “If there are countless stars in the night sky, then theoretically the night sky should be filled by distant stars.” Therefore, it is called the “dark night paradox.”
The answer to this question will not be answered until Hubble discovers that the universe is expanding at the beginning of the 20th century. It turns out that the universe continues to expand, so the most distant starlight will never reach the earth, so the universe is dark.
Scientists have designed the darkness of the universe as the term “cosmic optical background”, which is the sum of the comprehensive luminosity emitted by all stars and galaxies in the universe. Like its cousin, the cosmic microwave background, it is an important cosmological value, because it can tell us about the universe.
Marc Postman, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, USA, said: “The darkness of the universe will answer how many galaxies are there in our current universe? And what is left? How much interstellar matter?”
Unfortunately, measuring the optical background of the universe is not as simple as taking photos from space. Because there is light pollution on the earth (light pollution is getting more and more serious), it is unrealistic for telescopes on earth to photograph how dark the space is. But we can’t entrust the Hubble Space Telescope, because that telescope is too close to the earth and will also be affected by the scattered light from the earth. In fact, there are still a large number of tiny ice particles in the entire solar system that reflect the light of the sun. We have seen a phenomenon called “zodiacal light” on the earth, which is caused by small ice particles in the solar system reflecting the sun’s light.
However, after completing the mission to Pluto a few years ago, and flew over the Kuiper Belt celestial body “Arrokoth” (Ultima Thule), which was formerly known as “Ultima Thule” in 2019, it still works well and is far enough away from the sun, so there is no Considering the influence of sunlight and zodiacal light, it is most suitable to measure the background light of the universe.
It turned out that the universe was brighter than expected, twice the expected brightness.
In order to obtain this number, the research team first subtracted some known background light, such as the light in our Milky Way, and the reflected light of some interstellar matter. But even if you subtract these, you still find that the universe has some remaining light.
Astronomers are not yet sure what caused the excess background light. The most likely reason is that the vicinity of the Milky Way is more abundant than expected. There are some dwarf galaxies that have not yet been discovered, and their stars are scattered in the distant suburbs of the Milky Way.
This seems to show that our universe is still full of vitality, there are many luminous stars and galaxies, but we have not seen it on the earth. The upcoming James Webb Space Telescope (James Webb Space Telescope,) will be able to provide us with clues to answer this question after its work.
(Zhongshi News Network)
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