About the origin of the universe

The question of the origin of the universe is a bit like this old question: which came first, the chicken or the egg? In other words, what created the universe and what created the universe? Maybe the universe, or what created it, has existed for infinite time and does not need to be created. Until recently, scientists have been trying to avoid such problems, thinking that they belong to metaphysics or religion rather than science. However, in the past few years, people have found that the laws of science are valid even at the beginning of the universe. In that case, the universe can be self-sufficient and completely determined by scientific laws.

The debate about whether and how the universe began runs through the whole recorded history. There are basically two schools of thought. Many early traditions, as well as Judaism, Christianity and Islam, believe that the universe was created in the fairly recent past. /kloc-in the 7th century, Bishop Wu Schell calculated that the date of the birth of the universe was 4004 BC, which was obtained by adding up the ages of the characters in the Old Testament. It is a fact that there has been an obvious evolution of human culture and technology since modern times to support the above viewpoint. We remember the founder of that performance or the developer of this technology. It can be said that we can't exist that long; Because otherwise, we should be more advanced than now. In fact, the date of creation of the Bible is similar to the end of the last ice age, and it seems to be the earliest time for modern humans to appear.

On the other hand, some people, such as the Greek philosopher Aristotle, don't like the idea that the universe has a beginning. They think this means god's intervention. They prefer to believe that the universe already exists and will continue to exist indefinitely. Immortal things are more perfect than things that must be created. Their answer to the above questions about human progress is that periodic floods or other natural disasters repeatedly bring human beings back to their original state.

Both schools believe that the universe will basically not change with time. It is either created in its present form or continues indefinitely in its present form. This is a natural belief, because human life-the whole recorded history is so short that the universe has never changed significantly during this period. In a stable cosmic framework, whether it exists in infinite time or was born in limited time is really a metaphysical or religious question: any theory explains it. 178 1 year, the philosopher Immanuel Kant wrote a landmark and very vague work, Critique of Pure Reason. In this book, he concludes that there are equally valid arguments to support the belief that the universe has a beginning or that the universe has no beginning. As the title of his book implies, he simply draws a conclusion by reasoning, in other words, he simply ignores the observation of the universe. After all, what is there to observe in an unchanging universe?

However, in the nineteenth century, evidence began to accumulate, which showed that the earth and other parts of the universe actually changed with time. Geologists realize that the formation of rocks and fossils in them takes hundreds of millions or even billions of years. This is much longer than the age of the earth calculated by creationists. The so-called second law of thermodynamics put forward by German physicist ludwig boltzmann also provides further evidence that the total amount of disorder in the universe (measured by a quantity called entropy) always increases with time, just as the argument about human progress implies that it can only run for a limited time, otherwise, it should have degenerated to a completely disordered state by now, in which everything is at the same temperature.

Another difficulty with the idea of stabilizing the universe is that according to Newton's law of gravity, every star in the universe must attract each other. If so, how can they keep a constant distance from each other and stay there?

Newton knew the problem. In a letter to Richard Bentley, a famous philosopher at that time, he agreed with the view that a limited group of stars could not stand still, and they would all fall on a central point. However, he concluded that an infinite number of stars would not fall together because there was no central point for them to fall. This argument is an example of the trap people will encounter when talking about infinite systems. Adding up the forces of infinite stars in the universe acting on each star in different ways will give different answers to whether the stars keep a constant distance. We now know that the correct step is to consider the limited area of stars and then add more stars that are roughly evenly distributed outside this area. Stars in a limited area will collapse together. According to Newton's law, adding more stars outside this area can't stop them from collapsing. In this way, an infinite number of stars cannot be at rest. If they don't move relative to each other at a certain moment, the attraction between them will make them start to fall in each other's direction. In another case, they may be moving away from each other, and gravity slows down this return.

Despite these difficulties in the concept of a constant universe, in the seventeenth, eighteenth and nineteenth centuries, no one even suggested that the universe might evolve with time, and Newton and Einstein lost the opportunity to predict that the universe would either shrink or expand. Because Newton lived 250 years before the expansion of the universe was observed, people really can't blame him. But Einstein should know better. His prediction of 19 15' s general theory of relativity is expanding. But he was so fascinated by the stable universe that he had to add a factor to his theory to reconcile it with Newton's theory and use it to fight gravity.

1929 Edwin Hubble's discovery of cosmic expansion completely changed the discussion about its origin. If we trace the movement of galaxies back to the past time direction, it seems that there should be an overlap between10 billion years and 20 billion years ago. This time is called the singularity of the Big Bang, and the density of the universe and the curvature of space-time should be infinite. In this case, all known scientific laws are invalid. This is a disaster for science. All science can tell us is that the present state of the universe is so because it was in that state in the past. But science can't explain why it was like that at the moment after the big bang.

In this way, it is not surprising that many scientists are dissatisfied with this conclusion. In order to avoid the conclusion that the singularity of the big bang exists and the time has a beginning, people have made several attempts. One of them is called steady-state theory. The idea is that, because stars will not separate from each other, new galaxies are formed through the space between galaxies that are constantly produced. In this way, the universe has existed more or less today and will continue to exist indefinitely.

In order to make the universe continue to expand and create new matter, the steady-state model needs to modify the general theory of relativity. But the required yield is very low: about one particle per cubic kilometer per year, which will not conflict with the observation. The theory also predicts that the average density of galaxies and similar objects must be constant not only in space but also in time. However, the survey of extrariver radio sources conducted by martin ryle and his Cambridge team shows that there are far more weak sources than strong sources. It can be predicted that the weak source should be far away on average. In this way, there are two possibilities: maybe we are in a region of the universe, where strong sources are not as frequent as average sources; In other words, the density of light sources in the past was higher, and when light leaves these light sources and travels to us, it travels farther. These two possibilities are not consistent with the steady-state theory, because the theory predicts that the density of radio source must be constant not only in space but also in time. 1964, arno penzias and robert wilson discovered that the background of microwave radiation came from a place far away from our galaxy, which was a fatal blow to this theory. It has the characteristic spectrum of radiation emitted by hot objects, although the word heat is not appropriate at all in this case, because its temperature is only 2.7 degrees higher than absolute zero. The universe is a cold and dark place! There is no reasonable mechanism to generate microwave with this spectrum in the steady-state theory, so the steady-state theory can't escape the fate of being abandoned.

1963, two Russian scientists Eugnie Lifeizi and Isaac Halanokov put forward another idea, trying to avoid the singularity of the Big Bang. They say that only when galaxies are directly close to or away from each other will they overlap at some point in the past, leading to infinite density. Unfortunately, galaxies still have some lateral velocities, which may have happened in the universe a long time ago. At this time, although the galaxies are very close, they managed to avoid colliding with each other. Then the universe will continue to expand again, without going through the state of infinite density.

When Li Feizi and Haranikov put forward their ideas, I was a graduate student and needed a question urgently to finish my doctoral thesis. Because the question of whether there is a singularity in the Big Bang is of great significance to understanding the origin of the universe, I am very interested. Roger penrose and I have developed a set of mathematical tools to deal with this and similar problems. We point out that if the general theory of relativity is correct, any reasonable model of the universe must start from the singularity. This shows that science can predict that the universe must have a beginning, but it can't predict how the universe should begin: because of this, people must turn to God.

It is very interesting to examine the changes in people's views on strangeness. When I was a graduate student, few people took it seriously. Now, as a result of the singularity theorem, almost no one does not believe that the universe started from the singularity, where the laws of physics failed. But now I think that although there are singularities, the laws of physics can still determine how the universe began.

General relativity is a theory called classical. In other words, it does not take into account the position and velocity of particles that are not precisely defined. Because the uncertainty principle of quantum mechanics is "smoothed" in a very small position and speed range, the uncertainty principle does not allow us to measure two speeds at the same time. Because in general, the curvature of space-time is very uncertain relative to the position of particles, which has no effect on us. However, the singularity theorem points out that in the early stage of the current expansion of the universe, space-time is highly distorted and the radius of curvature is very small. In this case, the uncertainty principle becomes very important. In this way, the singularity of general relativity led to its own collapse. In order to discuss the origin of the universe, we need a theory that combines general relativity and quantum mechanics.

This theory is the theory of quantum gravity. We don't know the exact form that the correct theory of quantum gravity should take. At present, the best candidate we have is superstring theory, but there are still many difficulties to be solved. However, people can expect that any promising theory should have certain characteristics. One of them is Einstein's thought, and the gravitational effect is reflected in the fact that space-time is bent or even curled by matter and energy. An object moves along the trajectory closest to a straight line in a curved space. However, because space-time is curved. So their paths seem to be curved, just like being curved by the gravitational field.

Another predictable element in this ultimate theory is richard feynman's idea, and quantum theory can be expressed as "the sum of history". This idea can be expressed in the simplest form, that is, every particle has traveled any possible path or history in time. Every path or history has a probability, which depends on its shape. In order to make this idea feasible, people must consider the history that takes place in virtual time, not the history of the city that takes place in the real time when we feel that we live. Virtual time sounds a bit like science fiction, but in fact it is a well-defined mathematical concept. In a sense, it can be regarded as the direction of time at right angles to the real time. People add up the history of all particles with certain properties, for example, the probability of passing through certain points at a certain time. Then we should extend this result to the real time and space of our lives. This is not the most well-known means of quantum mechanics, but it gives the same results as other methods.

In the case of quantum gravity, Feynman's thought of historical summation involves the history of different possibilities of the universe, that is, the summation of different curved time and space. These represent the history of the universe and everything in the universe. People must point out what kind of bending space should be included in the summary of history. This kind of space contains a space with singularities, so the theory cannot determine the probability of this kind of space. Instead, they must be given probabilities in some arbitrary way. This means that science cannot predict the probability of this strange history of time and space. In this way, it can't predict how the universe should run. However, the universe may be in a state defined only by the sum of nonsingular curved spaces. In this case, the laws of science completely determine the universe, so people don't have to resort to things outside the universe to determine how the universe began. Determining the state of the universe only by summing up the nonsingular history is a bit like a drunk looking for his key under a lamppost: this may not be the place where he lost his key, but it is the only place he can find it. Similarly, the universe may not be in the state defined by the sum of nonsingular history, but it is the only state that science can predict what it should be like.

1983 Zhan Mu hartle and I proposed that the state of the universe should be given by summarizing some kinds of history. This history consists of a curved space with no singularity and limited scale but no boundary or edge. They are like the surface of the earth, only with two more dimensions. The surface area of the earth is limited, but there is no strangeness, no boundary and no edge. I verified this with experiments. I traveled around the world without falling outside.

The idea that hartle and I put forward can be rephrased as follows: The boundary condition of the universe is that it has no boundary. Only when the universe is in this borderless state can scientific laws themselves determine the probability of every possible history. Therefore, only in this case, the known laws will determine how the universe should operate. If the universe is in any other state, the types of curved spaces in historical summation should include spaces with singularities. People must turn to some principles other than the known laws of science to determine the probability of this strange history. This principle will be something outside our universe. We can't deduce it from our universe. On the other hand, if the universe is borderless, in principle, we can completely determine how the universe should operate in the finite fairy tolerated by the uncertainty principle.

If the universe is borderless, that's great for science, but how do we know if this is the case? The answer is that the borderless hypothesis makes a clear prediction of how the universe should operate. If these predictions are inconsistent with observations, then we can draw the conclusion that the universe is not borderless. Therefore, in the sense defined by philosopher karl popper, the borderless hypothesis is a good scientific theory: it can be falsified by observation.

If the observation is inconsistent with the prediction, we know that there must be strangeness in the possible historical category. However, this is roughly all we know. We can't calculate the probability of this strange history, so we can't predict how the universe should run. Some people may think that if unpredictability only happened in the Big Bang, it wouldn't hinder much. After all, it was10 billion or 20 billion years ago. However, if predictability fails in the very strong gravitational field of the Big Bang, it will also fail as long as the star collapses. This kind of event only happens a few times a week in our galaxy. Even by the standard of weather forecast, our forecasting ability is poor.

Of course, people will also say that we don't have to care about foreseeable failures on distant stars at all. However, anything that is not actually forbidden in quantum theory can and will happen. In this way, if the strange space is included in the possible historical category, these strangeness can happen anywhere, not just the big bang and the collapsing stars. This means we can't predict anything. On the other hand, the fact that we can predict the event is the experimental evidence against singularity and supporting the borderless hypothesis.

So, what is the prediction of borderless vision to the universe? The first prediction is that any quantity people use to measure time must have a maximum and a minimum, because the possible history of the universe is limited in extension. So the universe begins and ends. The real-time start is the big bang singularity. However, in virtual time, this beginning is no longer a singularity. It's a bit like the North Pole of the Earth. If people regard the latitude of the earth's surface as an analogy of time, it can be said that the earth's surface started from the North Pole. However, the North Pole is a completely ordinary point on the earth. Nothing special. The same law applies in the Arctic, just like in other parts of the earth. Similarly, the event we mark as "Chong" is an ordinary point in time and space, just like other points. The laws of science are established from the beginning, just like in other places.

From the analogy with the earth's surface, people may expect that the end of the universe will be similar to the beginning, just as the North Pole is similar to the South Pole. The north and south poles correspond to the extension from virtual time to real time, and we will find that the beginning and end of the universe in real time can be very different.

Jonathan Halliwell and I made an approximate calculation of the meaning of unbounded conditions. We regard the universe as a completely smooth and uniform background, on which there is a small disturbance of density. ? %E