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One of the hot research topics in cosmology is the concept of multiverse. If we live in a multiverse, it means that there are multiple universes that are completely independent of what we are familiar with. Why do scientists think that the multiverse may exist? To do this, we need to consider the most puzzling mystery-the question of why we exist.

Our existence requires some very strange coincidences that require the laws of physics to be just right: it needs gravity to work its way to form stars and make planets move around them; it needs nuclear physics that causes stars to burn; more fundamentally, it needs electromagnetic laws and quantum mechanics to keep them as they are, because that's what it takes to form atoms and molecules.

If we change the laws of physics slightly, what will happen to the universe? We can give countless examples to illustrate the consequences of small changes, but here we choose only one example, which involves the mass of protons and neutrons. Neutrons are slightly heavier than protons, only 0.1%, to be exact. Suppose we make a small change and the proton becomes a heavier particle. You might think that this is only a small difference, but the result is huge.

First of all, when the universe was very young, it was made up entirely of hydrogen and helium nuclei. Hydrogen has one proton, while helium contains two protons and two neutrons, which were created in a short time three minutes after the Big Bang. Second, although the nucleus is formed in three minutes, it takes longer to actually form the atom, about 380,000 years. So naked protons exist for a long time before they eventually become hydrogen atoms. Third, we need to know that isolated neutrons decay into protons and other particles in about 15 minutes, and the lifetime of protons seems to be infinite. Neutrons decay into protons instead of protons into neutrons because neutrons are heavier than protons, so if we exchange the mass of protons and neutrons, protons decay into neutrons and other particles.

So in the 380,000 years it takes for hydrogen atoms to form in our universe, all protons decay into neutrons in about 15 minutes. Hydrogen atoms would not be formed without protons, and gravity would not burn as seen in our universe when neutrons are pulled together to form stars. Small changes in the mass of protons and neutrons will cause our entire universe to have completely different characteristics, and human beings will never exist.

Here, I give only one example to show that small changes can make a big difference. But as mentioned earlier, there are a large number of other small changes that can have a huge impact on our universe. Scientists call this the fine-tuning problem, which means that if many physical parameters were not exactly what they are now, we would not exist. This makes the fact that the universe exists seems to be some kind of cosmic coincidence.

Some people think that the problem of fine-tuning is strong evidence of some kind of creator, and according to their ideas, we need a conscious person to create the conditions so that we can exist. Although some people think this explanation is satisfactory, it is not without problems in itself, and many scientists are still looking for different answers. As a result, the multiverse appeared.

Maybe there is not only one universe, but countless universes. In this way, in all countless universes, all countless parameter choices actually take place. In some universes, neutrons are heavier than protons; in others, protons are heavier than neutrons. In some universes, gravity may not exist or it may be a repulsive force. Everything that can happen will happen in at least one universe, and we happen to live in a universe suitable for human beings.

Now you may be skeptical about the whole idea of the multiverse, which is, after all, an untested hypothesis. This means that we must think more carefully about our theory to see if some testable results are possible. For example, the collision of multiple universes in some higher dimensional space may leave a mark on the universe. Some cosmologists believe that the huge cold spots observed when studying the universe on the largest scale may be a sign of such collisions.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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