The problem of cosmic flatness can be understood from two aspects: critical density and triangular geometry.

Simply put, the universe is a four-dimensional space of time and space, and the matter and energy contained in it will produce a gravitational field, which will deform the four-dimensional space. Ancient light, especially the microwave emitted 376,000 years after BIGBANG, should have been bent by this deformed four-dimensional space on the way to the earth after 65.438+0.37 billion years, instead of the microwave that we have measured and transmitted to our radio ears in a straight line.

Our present universe contains the density of matter plus energy, which is very close to the critical density of 1. Relativity shows that the universe with critical density of 1 has four-dimensional spatial characteristics of measurement and linearity, and microwaves propagate linearly in this universe.

Scientists like to use the critical density of 1 to describe the flat mystery of our universe, which is based on a simple formula under the guidance of relativity:

(ω0- 1- 1)ρa2 =-3κC2/8πG

We don't need to know how this formula reveals the ultimate mystery of the universe came from, but it is indeed the crystallization of the smartest wisdom of mankind in the 20 th century. We can't know how Beethoven wrote the Symphony of Destiny, but we can still be fascinated. Let's enjoy this formula together!

In this formula, ω 0 (pronounced omega zero) is the critical density, and κ (pronounced kappa) is the curvature of the universe. The measured data now show that the critical density ω 0 of our universe is very close to 1, which makes the curvature κ value very close to 0. Conclusion: Our universe is flat!

For outsiders, the critical density is too abstract, and it is more direct and easy to understand to explain the flattening phenomenon of the universe with plane geometry.

According to middle school plane geometry, the three internal angles of a triangle on the plane add up to 180 degrees. Draw a triangle on the sphere, and the three internal angles add up to more than 180 degrees. Draw a triangle on the saddle, and the three internal angles add up to less than 180 degrees. Therefore, if we can draw a super-large triangle with tens of billions of light-years in the universe and measure its three internal angles, we can judge whether our universe is straight or not (Figure 1).

Spherical universe means that the gravitational field generated by the total mass with equal intrinsic functions is large enough and the critical density is greater than 1, and the universe will eventually end in a big collapse. Saddle universe means that the gravitational field generated by the total mass with equal intrinsic functions is not large enough, and the critical density is less than 1, and the universe will eventually end with a big gap. The critical density of the flat universe is equal to 1, which just makes the universe move towards a state of no tearing and no collapse.

However, we live on a small earth, how to find a triangle with the size of billions of light years in the universe and how to accurately measure the sum of its three internal angles? Answer, yes! This is a kind return from the universe.

Microwave after 376,000 years back in BIGBANG (Figure 2). Although this microwave distribution is extremely uniform, there is still a change of 65438+ one in ten thousand. This subtle change is the result of the struggle between the gravitational field of the universe and the ubiquitous plasma at that time before the microwave set off. Humans have completely mastered this acoustic oscillation phenomenon, and theoretically calculated the linear size of microwave inhomogeneity to be 376,000 light years. 376,000 light years is a standard measure for us to stay away from the edge of the universe. After 65.438+0.37 billion years, if the opening angle of this ruler is about 654.38+0 degrees, it is confirmed that the total internal angle of this long and narrow triangle is 654.38+0.80 degrees, and our universe is straight; If the opening angle is greater than 1 degree and the sum of the internal angles of the triangle is greater than 180 degree, our universe is spherical; If the opening angle is less than 1 degree, the sum of the internal angles of the triangle is less than 180 degree, and our universe is saddle-shaped.

Mankind assembled all the forces, invested a lot of money, sent several expensive satellites, as well as high-altitude balloon detectors around the South Pole, and carefully measured the opening angles of this standard scale in all directions. It turns out that the opening angle is about 1 degree. Our universe is flat!

Humans probably feel so lucky and have been asking, why is it so coincidental? Is our universe specially designed for human beings? Of course, human beings have a lofty position in the universe, but it is better to be modest. I think this statement should be reversed: it is because the universe is straight, the critical density is close to 1, the cosmic environment has no ups and downs, and life is particularly friendly and caring that human beings have the opportunity to originate, develop wisdom and civilization and ask such great questions. In the universe far from the critical density 1, the matter in the universe is tossed back and forth, and there is no time to condense at all. Stars and galaxies are stillborn at best, and life can never originate. It is meaningless whether the universe is straight or not and whether there is a problem of horizon.

How to solve the problem of cosmic horizon? How did the linearity of the universe come from?

Inflation theory

Born in 1947, Gusi was born in Depth Charge after World War II. After graduating from MIT, the job market was crowded, and he went his own way, only attacking major physical problems, and the number of published papers was scarce. But the employer knew that he was brilliant, so he was transferred from one postdoctoral fellow to another. Eight years later, he was transferred to Stanford Research Institute, where he remained a promising postdoctoral fellow.

In the late postdoctoral period, Gus founded the inflation theory, and the initial motivation was to solve the historical unsolved problem of magnetic monopole. The theory of particle physics thinks that magnetic monopole should be everywhere, but human beings have been trying to find it. He explained with the expansion theory that in the initial stage of the Big Bang, the universe expanded and its volume increased, diluting the density of magnetic monopoles. In the universe we can see, there are only a few magnetic monopoles that can be assigned values, and the probability of meeting one is too low to help each other. It is very likely that mankind will never have a chance to meet a magnetic monopole.

After the inflation theory satisfactorily explained the reason why humans could not measure magnetic monopoles, the thinking of the ancients flashed. Can't the same theory be used to explain the horizon and straightness of the universe? He quickly wrote "spectacular realization" on the top of the red notebook and framed the sentence with a pen. Solving the problem of magnetic monopole is of course a major event in physics, but it is still a drop in the bucket compared with the two cosmic problems of horizon and straightness.

198 1 After the inflation theory was published, universities competed for Gusi, and finally their alma mater took him back. Now he is a professor at MIT and has won many international awards, and the Nobel Prize is expected.

His "Great Discovery" notebook was later collected and exhibited by the Chicago Museum.

According to the inflation theory, from 10-35 seconds after the big bang to 10-32 seconds, a miracle happened in the universe, and it expanded 1050 times at the speed of light within 10-33 seconds. The magical energy comes from the highest dark energy produced by the "false vacuum" that Gusi didn't know. He suggested that this dark energy was provided by his hypothetical expansion.

Driven by ultra-high pseudo-vacuum energy, the speed is not limited by the speed of light. The cosmic critical density of the new explosion may be only 62 digits after the decimal point (| ω 0- 1 | ~ 10-62), which is different from 1 and close to the idealized 1. After inflation, the pseudo-vacuum energy is exhausted and decays into the mild and low vacuum energy that we are familiar with in the universe. In this universe with low vacuum energy, the critical density may slightly deviate from 1, and we must act according to the rules of relativity. The speed of light is constant and cannot be exceeded.

Before the Big Bang 10-35 seconds, the whole universe contained very little mass and was in a very small period. Light travels easily, and matter and energy are mixed evenly. In the process of superluminal expansion, the uniformity is too late to become uneven. After inflation, all parts of the universe are beyond each other's horizons, forming a strange phenomenon in the future microwave horizon of the universe. The problem of cosmic horizon is so easy to solve. However, there is a quantum fluctuation prediction in the inflation theory, and the microwave inhomogeneity of 65438+ 1 accurately measured at the beginning of the 20th century provides strong evidence for the inflation theory.

As for the flattening of the universe, it is easier to explain it with inflation theory. Pseudo-vacuum can expand the universe by 1050 times in/kloc-0-33 seconds, and the volume of the whole expanded universe is at least 1023 times larger than the universe we can see. Because of its huge size, our small universe looks unusually straight. Just like people on the surface of the earth, they are all flat, but in the distant macro satellite orbit, the curvature of the earth is clearly shown.

In fact, the inflation theory inflates every observable finite universe into a flat universe with a critical density close to 1. If you are a superman, you can travel to other universes outside our universe, not to mention the invisible universe, but as long as you see a universe, it must be a straight universe and you will never fool. This is because every observable finite universe has gone through the processes of condensation, nuclear transformation, luminescence, neutron star, black hole and so on. In the evolutionary history of the universe, it has experienced at least10 billion years of vicissitudes. Only a universe with a critical density close to 1 can provide such a level and stable development environment.

However, the critical density of the superuniverse in inflation theory may be different from 1. In the whole universe, day and night, anytime, anywhere, inflation, one after another, endless interaction, reincarnation of life and death, beginning and ending but endless. In this huge universe, the critical density is changing rapidly, quantum mechanics is in command, and God is nowhere to be seen.

Spider's thread and horse's trail-clues

The universe outside our universe is invisible and intangible, which is a topic of philosophy. With rigorous scientific quantum field theory, inflation theory has beautifully solved the two cosmic problems of horizon and flatness, and at the same time, on the same solid scientific basis, it has created a super-large universe at least 1023 times larger than our universe.

Is there any hope that we can catch a glimpse of this scientific universe and detect its existence?

The whale jumped from the sea level and plunged back into the water, stirring up thousands of waves. The dislocation of crustal plates will cause seismic waves and even tsunamis. The supersonic plane broke through the sound barrier and the sound shock spread far away. The nuclear bomb exploded and the shock wave spread all over the world.

As long as there is energy release, there will be shock wave generation and propagation, and the cause and effect are clear.

The expansion of the universe is a big action of ultra-high energy release, which will inevitably leave waves in space. This wave is dominated by gravity waves and propagates into the huge universe at the speed of light (Figure 3). The inflation theory predicts that just as there is microwave background radiation in our universe, this kind of gravity wave will also spread to the whole universe, forming cosmic gravity wave background radiation (CGWBR). It treats all isolated small universes fairly, regardless of the critical density value, advantages and disadvantages, and broadcasts to households equally.

In fact, Einstein's theory of relativity has long predicted the existence of gravity waves. Supernova explosion, black hole birth and intergalactic collision can all trigger gravity waves. However, the inflation theory, by going up a flight of stairs, excites a different band of gravity waves with ultra-high pseudo-vacuum energy, which propagates and fluctuates in a larger cosmic model.

Therefore, the detection of this gravity wave with a different wave band from Einstein can at least show that the inflation theory may be correct, and at the same time, it cannot deny the existence of that huge universe.

It is not easy to measure gravity waves. Once again, mankind raised funds and planned to build a space laser interferometer antenna (LISA) flying with the earth in the sun orbit with1500 million dollars. This antenna has a triangular structure, with 5 million kilometers on each side and an astronomical unit from the sun, but its orbit follows the earth at 20 degrees and the inclination angle is 60 degrees. This is the most sensitive gravity wave measuring instrument for human beings. It is expected to be launched between 20 18-2020.

LISA's sensitivity may not fully meet the needs of inflation theory, and more sensitive gravity wave interferometer is needed in the future. However, even if the gravity wave signal is weak and difficult to measure, the inflation theory has another prediction: the gravity wave generated after inflation can act on the electromagnetic background microwave of the universe and polarize the microwave. General light can be polarized by grating, and some sunglasses are designed according to this principle. Therefore, it is as important as gravity wave detection to accurately measure the polarization value of the microwave in Figure 2. In the first quarter of 2009, mankind will launch Planck satellite, and one of its important tasks is to accurately measure the polarization value of cosmic microwave background radiation.

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Gus successfully explained the two difficult problems of horizon and straightness in our universe with inflation theory, but he also pulled out a huge universe with outer space and outer space. The inflation theory predicts that our small and limited universe with a diameter of 93 billion light-years shares the same background radiation of gravity waves as the huge universe with a diameter of 1023 times, and this gravity wave can affect the polarization value of microwaves. Therefore, if there is outer space, humans must measure this gravity wave and polarization. Obtaining these two physical data can't completely prove the existence of another world, but it certainly can't deny its existence.

At present, there are more than 50 versions of inflation theory. The universe outside the universe is invisible and has a huge imagination space. Gravity wave, polarization, every family has its own saying, a hundred flowers blossom, a hundred birds contend, and the popularity is rolling, creating a scientific scene.