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2025-01-19 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >
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Shulou(Shulou.com)11/24 Report--
Thanks CTOnews.com netizen Hua Ke high achiever's clue delivery! It is said that the cosmic microwave background is the "first beam of light" of the whole universe, which is like a set of snapshots recording the original appearance of the universe. Since it is the "first beam of light", it should never be seen again after it arrives on Earth. Why can we keep seeing it?
The short answer is generally: because it is the background radiation that permeates the entire universe. But such an answer lacks an intuitive impression, and it's hard to associate it with the "first beam of light" you just said. "Why does the first beam of light become background radiation? why does the background radiation keep flowing?"
Before we answer this question, we need to know where the photons that make up the background radiation come from. The story starts 13.8 billion years ago.
At that time, the universe had just experienced a stage of inflation, and hadrons made up of protons and neutrons had appeared one after another in a piece of quark-gluon soup. Hadron is ready, and then it's the lepton's turn to take the stage.
Leptons refer to particles such as electrons and neutrinos that do not participate in strong interactions and are relatively light in mass. With the emergence of leptons, the universe has officially entered the "lepton period". During this period, the universe originally had no photons, only some leptons and their antiparticles, such as electrons and positrons. But with the annihilation of positive and antimatter, some matter is converted into energy and released in the form of photons, so the universe has the first photons.
However, the photons at this time are not the photons of today's background radiation. Because the annihilation process of positron and positron is reversible in an environment of more than 6 billion degrees, the newly emerging photons will return to a pair of positron and positron in this extreme environment, and the positron and positron will annihilate into photons again after collision. During this constant pull, the temperature of the universe continues to drop until the reverse reaction no longer occurs, when the electrons and photons in the universe stabilize.
Although there are photons, the universe is still dark at this time. Why? Because these photons are trapped.
The remaining electrons in the annihilation reaction were supposed to combine with the nucleus to form atoms, but at this time the photons were so energetic that they bumped around like headless flies. As soon as the electrons were ready to "hold hands" with the nucleus, they were knocked off by the sudden appearance of the "light bulb". But for Photon, it is also very aggrieved: I was just about to go out to play, but I was full of dates everywhere. I was bumped back without two steps, and I couldn't even walk.
What happens here is Compton scattering, and the existence of free electrons allows photons to travel a short distance, showing that the universe is like a mass of high-density matter, opaque.
As the temperature continues to drop, the photons are tired, and without the interference of the "light bulb", electrons and nuclei can finally get married. At the same time, for photons, fewer and fewer people are rushing to the appointment, and the surroundings are becoming more and more spacious. Get rid of the shackles, the photons like a runaway Mustang began to gallop wantonly, this stage is called "photon decoupling", also known as "decoupling period". Because this stage occurs relatively quickly on a cosmic scale, the time for photons to unbind (that is, the last scattering) is very short, so it is called the "final scattering surface".
Aren't photons all over the universe? why are they called "noodles" here? Because this is about the observer's us as the center.
Since photons also take time to move, this means that only those photons that take long enough to reach the earth can be seen by us. This creates a spherical region centered around us with a radius of 13.8 billion light-years. The surface of this spherical region is the so-called final scattering surface, and it also represents the farthest place we can see, the boundary of the observable universe. Considering that the universe has been expanding, this boundary has now expanded to about 46.5 billion light-years away. This also answers the old question: why the universe is only 13.8 billion years old, but its radius reaches 46.5 billion light-years.
What does all this have to do with background radiation? Well, the background radiation we see today is actually made up of photons that are separated from the final scattering surface. Because these photons fill the whole universe, the radiation they bring is also a kind of radiation that fills the universe, so it is called "background radiation".
After understanding the concept of the final scattering surface, it is easy to understand the question of "why the light from the background radiation continues to flow".
Suppose there is a God's perspective outside the universe, and before the photons are decoupled, the universe in its eyes is a hot and opaque chaotic state.
About 380,000 years after the Big Bang, when the temperature of the universe had dropped to about 3000 degrees, photons began to free themselves. From God's point of view, all these photons come out at once, and the universe as a whole becomes transparent.
But as observers in the universe, what we see is not the same. Because of the speed limit of light, we first see photons that are closer to us (because they can reach us faster), then photons that are farther away, then photons that are farther away, and so on.
In other words, the background radiation photons we see all the time are actually photons farther away than the last moment, and every background radiation image we see is actually older than the last one.
See, the background radiation and the final scattering surface are for a specific observer. Observers in different parts of the universe actually see different background radiation from others. However, because these photons are uniformly distributed in the universe, there is not much difference in the background radiation seen by different observers, which is also a sign of uniform and isotropic background radiation.
Now you know how outrageous it is to "let the background radiation flicker" in the "three-body"? So even the three-body civilization can only be realized by falsifying background radiation.
If you think of the universe as a stage, the stars and galaxies you usually see, including quasars, are just actors on the stage, while the background radiation is the background of the whole stage. What about behind the stage background? In the more distant background, where the photons are still trapped, it is still a dark place of chaos for us.
So, in the end, the scattering surface is actually like a barrier, dividing the universe into two parts. For every observer in the universe, whether human or alien, the existence of this barrier prevents us from peeping into the early secrets of the universe, especially the ultimate mystery of the origin of the universe. The cosmic microwave background is the earliest view of the universe that we can see directly, and theoretically we still have a chance to understand what happened after that, but we can never "see it with our own eyes" about what happened before that. Unless there's some information that can break through this barrier.
Microwave background radiation is also called "3K background radiation" because its current temperature is close to 3K. In fact, there is more than this kind of background radiation in the universe, for example, there is a kind of radiation that is weaker than 3K radiation, about less than 2K, which is neutrino background radiation.
Neutrino background radiation is similar to microwave background radiation, but it formed earlier, about two seconds after the birth of the universe. As I just said, the first photons in the universe appeared at about that time, but due to the influence of baryons and leptons, photons were imprisoned by them for about 380,000 years. Why are the neutrinos that appeared at the same time not trapped?
Yes, because neutrinos do not participate in electromagnetic interactions or strong interactions, and their mass is still very small (perhaps even as massless as photons), the gravitational effect can be completely ignored. therefore, from the moment of birth, neutrinos can travel freely through the universe, and the only thing that can limit it is speed.
The universe is filled with a large number of neutrinos, and these neutrinos, which were born in the early days of the universe, are called "residual neutrinos". As the name implies, they are the neutrinos left over from the beginning of the universe. In theory, there are about 300 residual neutrinos per cubic centimeter, all of which carry information from the other end of the barrier.
But "Cheng Ye Xiao he, failure also Xiao he", although neutrinos with its super penetration, let us hope to get a glimpse of the secrets of the birth of the universe, but it is also because its penetration is so strong that it is already difficult to detect neutrinos, not to mention the background, which is cooler than the microwave background and has less than 2K.
But in addition to neutrinos, in theory, there is another thing that is more penetrating and can break through the barrier, and that is gravity. There may be some violent quantum fluctuations in the early universe, which produce a special gravitational wave, called the primary gravitational wave. Different from the common gravitational waves produced by the merging of black holes or neutron stars, the frequency of the original gravitational wave is very low, which is several orders of magnitude lower than that of the Nahertz gravitational wave mentioned last time, which can be said to be the lowest in the gravitational wave family. So we can imagine the difficulty of detection.
However, if the original gravitational wave does exist, it will theoretically affect the photon distribution on the final scattering surface, which may lead to some special imprints in the microwave background image. So although it is still very difficult to detect the primary gravitational waves directly, maybe we can find some clues in the microwave background to help us solve the long-standing mystery of the origin of the universe.
Reference:
[1] https://en.wikipedia.org/wiki/Cosmic_microwave_background
[2] https://en.wikipedia.org/wiki/Chronology_of_the_universe
[3] https://en.wikipedia.org/wiki/Observable_universe
[4] https://en.wikipedia.org/wiki/Cosmic_neutrino_background
[5] https://www.zhihu.com/question/66071582/answer/2779916560
[6] http://www.ihep.ac.cn/kxcb/kjqy/201604/W020160403557369479073.pdf
This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo
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