About 14 years ago, people thought there was a perfect answer: by observing the microwave radiation in the background of the universe, astronomers finally verified Edwin Hubble's conjecture in 1929, and the universe was born in the Big Bang about 137 billion years ago. Later, with the evolution of the universe, the Milky Way, the solar system, the earth and even ourselves appeared one after another.
From June 5, 2006 to 2006 10, it was with this important achievement that American scientists George F. smoot and John C. Mather shared the Nobel Prize in Physics that year.
But our understanding of the universe is obviously just beginning. Just one month later, the latest research results released by NASA show that a mysterious force called "dark energy" existed at least 9 billion years ago.
In other words, less than 5 billion years after the birth of the whole universe, it began to be affected by dark energy. Previously, scientists generally believed that in the early days of the universe, perhaps this force did not exist, because at that time, the familiar gravity dominated everything.
Although this result still can't tell us what the future of the universe will be, it obviously brings us a new dawn for a thorough understanding of the operation law of the universe. Related papers will also be published in February 2007 in the Journal of Astrophysics.
In an interview with Caijing, Adam Riess, a professor at Johns Hoops University who led the research team, said, "We are still far from truly understanding dark energy. But obviously, this is a very important step because it gives more clues. "
Why does the universe accelerate its expansion?
The discovery of dark energy is very dramatic.
According to the big bang theory, after the big bang, with the passage of time, due to the gravitational effect between substances, the expansion speed of the universe will gradually slow down, just like a car slowly stepping on the brakes. That is to say, galaxies that are relatively far away from the earth should expand relatively slowly.
However, in 1998, two teams led by saul perlmutter, a physics professor at the University of California, Berkeley, a senior scientist at Lawrence Berkeley National Laboratory (LBNL) and Brian Schmidt of the Australian National University observed that those distant galaxies were moving away from us at an increasing speed.
In other words, the universe is accelerating its expansion, just like a car that keeps stepping on the accelerator, instead of slowing down and expanding as predicted by scientists before.
Such a completely unexpected observation has fundamentally shaken the traditional understanding of the universe. So what kind of forces are driving all galaxies or other substances to accelerate away?
Scientists call this source of repulsion opposite to gravity "dark energy". But what does "dark energy" mean? What we can give so far is only a very rough "pyramid diagram" of the universe structure:
The world we are familiar with, that is, the grass, trees, mountains, rivers, stars and moons composed of ordinary atoms, accounts for only 4% of the whole universe, which is equivalent to the piece at the top of the golden pagoda.
The bottom 22% is dark matter. This substance is composed of unknown particles. They do not participate in electromagnetic action and are invisible to the naked eye. But like ordinary matter, it participates in gravity, so it is still possible to detect it.
As 74% of the tower foundation, it is composed of the most mysterious dark energy. It is everywhere, all the time. As we know little about its nature, scientists don't know how to verify its existence in the laboratory. The only means is still to understand its mystery through indirect means of astronomical observation.
At present, observing the explosion of type Ia supernovae is the most important observation method. This supernova is formed by the explosion of a white dwarf star in a binary system, and its brightness is almost constant. In this way, by measuring its brightness, we can know its distance from the earth and then understand its speed.
With the help of sensitive astronomical instruments like Hubble, we can observe at least 9 billion light-years away, that is, understand the information of the universe 9 billion years ago.
Adem Reiss, a professor at Hopkins University, showed us the latest "dark energy" scene as follows:
In the early days after the Big Bang, the universe experienced a rapid expansion stage. Since then, due to the close distance between dark matter and matter, the expansion speed of the universe began to slow down under the action of gravity.
However, at least 9 billion years ago, another force in the universe-dark energy manifested as repulsive force-appeared and began to gradually offset the gravitational effect.
With the expansion of the universe, more and more dark energy finally surpassed gravity about 5 billion to 6 billion years ago. Since then, the universe has changed from slow expansion to accelerated expansion, which has continued to this day.
Einstein's legacy
Li Miao, a professor of physics at the University of Science and Technology of China, once said half jokingly, "There are many dark energy models as there are dark energy experts." Perhaps this statement is not without exaggeration, but the chaotic state of dark energy in theory can also be seen from it.
Among them, the most dramatic theory is the resurrection of Einstein's "cosmological constant". 19 17 years, Albert Einstein, considered as the greatest scientist in the whole 20th century, put forward this concept for the first time in order to establish a steady-state universe model. However, even he himself later admitted that "cosmological constant" was just a wrong concept.
But the existence of dark energy provides a new possibility for the cosmological constant. If dark energy is a cosmological constant, its intensity will only be related to the size of the universe. With the expansion of the universe, its volume gradually increases, so the dark energy will gradually increase. Eventually, it will reach a critical point, which will change the universe from a deceleration state to an acceleration state, and will continue to accelerate.
Zhang Xinmin, a researcher at the Institute of High Energy Physics, Chinese Academy of Sciences, told Caijing that the observation results so far, including the latest results of Reiss, are "very consistent" with Einstein's cosmological constant theory.
However, the cosmological constant is far from a deterministic theory of dark energy. Some scientists say half jokingly that according to this model, the universe will continue to accelerate its expansion, which is quite boring.
Of course, the most fatal thing is that the cosmological constant calculated according to quantum field theory is at least 120 times higher than the upper limit obtained by astronomical observation.
One of the strangest explanations with no lack of scientific basis is "multiple cosmology". Observation and theory may not be wrong. In fact, there are countless other universes besides the one we live in. The number of universes that scientists can imagine is not calculated by tens of thousands or hundreds of millions, but probably as much as 10 1000 power.
Every universe has a different cosmological constant, and we just live in a universe with a very small cosmological constant. There seems to be a "hand of God" in the darkness, which presents us with a universe suitable for intelligent life.
However, there is a great controversy between astronomers and physicists about this "anthropomorphic principle" that hopes for the existence of multiple universes. Zhang Xinmin, a researcher at the Institute of High Energy Physics, Chinese Academy of Sciences, told Caijing that many people think this is just a guess, which is far from the "principle".
More acute critics believe that this explanation is more like a religious belief than a scientific theory.
In order to avoid this conflict, scientists have proposed various dark energy theories to replace the cosmological constant model. There are two representative models: essence model and phantom model. Zhang Xinmin and Li Miao, a professor of physics at the Chinese University of Science and Technology, also put forward quintom and holographic model respectively.
The future of the universe
If these alternative dark energy theories can be established, they will point to a completely different future of the universe:
According to the scalar field model, the future of the universe will be much more complicated. May continue to accelerate the expansion, may slow down the speed of expansion, and even move towards contraction, eventually leading to the "great contraction" contrary to BIGBANG.
According to the ghost model, dark energy will continue to increase, causing the universe to expand at an accelerating rate. Eventually, the universe will move towards a "big tear."
The elf model gives an "oscillating future". Zhang Xinmin told Caijing that according to his theory, the whole universe will be deduced repeatedly between accelerating expansion and decelerating expansion, and neither of the two extreme cases "big collapse" nor "big tear" will appear.
The biggest difficulty is that so far, the means by which we can study dark energy are still very limited. At present, the most mainstream is the observation of supernovae. But some people worry that, especially in the early universe, the brightness of supernovae may not be constant, and it also has its own evolution process.
Even if this concern can be ruled out, given that these supernovae are very, very far away from the earth, it is difficult to observe. In Reese's view, this is like observing a 60-watt light bulb from the distance between two moons. Even though Hubble telescope has very high sensitivity, there are systematic errors that are hard to eliminate.
The study of large-scale cosmic structures (such as galaxy clusters) may provide new clues for dark energy. Once dark energy exists, the formation of galaxy clusters may be slower, because gravity needs to overcome this repulsion first.
At present, Sloan Digital Sky Survey (SDSS), a space exploration project, has completed the first phase of its five-year work. Once completed, this fine optical imaging device, which can cover a quarter of the sky, will undoubtedly reveal more details.
It is reported that Chinese scientists are also trying to observe supernovae by using LAMOST (Large Sky Multi-target Optical Fiber Spectroscopy Telescope) recently launched near Beijing, in order to explore the possibility of conducting dark energy experiments in China for the first time. The use of gamma-ray bursts (cosmic high-energy radiation produced by the explosion of supermassive stars) may provide an indirect means for further study of early dark energy.
Zhu Zonghong, a professor of physics at Beijing Normal University, pointed out in an interview with Caijing that the exploration of gamma-ray burst astronomy is still in its infancy, which is somewhat similar to supernova astronomy when dark energy was just discovered in 1998, but some of its properties may still be used to study dark energy in the long run.
So, is it possible to study dark energy directly in the laboratory? Some people claim that nanotechnology can be used to achieve this goal. In an interview with Caijing, Rice said that some scientists also hope to use short-range gravity experiments to find clues about dark energy.
Sean Carroll, a physicist at the California Institute of Technology (CIT), also stressed to Caijing that to find a more definite model, not only astronomical data but also evidence from particle physics is needed. Especially the Large Hadron Collider (LHC), which will be put into operation in Europe in 2007, maybe "we can look forward to it".
However, because the nature of dark energy, including the reaction mechanism with other substances, is still unclear, many scientists believe that we cannot place too much hope on the work of the laboratory in the short term; A more realistic channel may still come from astronomical observation.
If there are no accidents, Planck probe will be officially launched in the first quarter of 2007, which will detect the sky more accurately. In an interview with Caijing, Piermut also said that the supernova acceleration detector (SNAP) designed by his laboratory will be launched on 20 13 or 20 14 as planned.
"In the next five to ten years, we may have a clearer understanding of the nature of dark energy." Christopher Celiset, a professor at the School of Physics and Astronomy at the University of Nottingham, told Caijing.
Few people deny that dark energy is a revolution for the whole cosmology and even physics. 1979 Steven Weinberg, the winner of the nobel prize in physics, once made it clear that "if we don't solve the obstacle of dark energy, we can't fully understand basic physics." China famous physicist, 1957 Nobel physics.
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