We have discussed the Big Bang theory for countless times and why it became the dominant theory of the origin of the universe. It is not that the Big Bang theory is absolutely correct, but that it is the only world view that we can explain the following three observed phenomena:
We observe distant galaxies and find that the farther away the galaxy is, the faster it is from us! In the 20th century, people have realized that the root cause of this phenomenon is the expansion of space itself!
This also means that according to the current retrogression rate of distant galaxies, these galaxies will disappear from our sight in the near future. It is no exaggeration to say that in the future, humans on earth will only see one galaxy in the universe, that is, the huge elliptical galaxy after the merger of our own galaxy groups! I really can't imagine this situation at that time. At that time, humans did not have any observable data to understand the universe.
So logically speaking, even if we are not physicists and let us reason for ourselves, we can know that the universe is expanding again now, so the universe must have shrunk in the past!
So what are the consequences of a smaller universe? If the universe used to be smaller in space, then it was denser and of course hotter, then the following question is: What will happen when the universe becomes hot enough?
If ordinary matter is heated, atoms will be ionized as long as the temperature is high enough, so the universe was a plasma of nuclei and electrons at some time in the past! When atoms change from ionized plasma to neutral gas, the high-temperature radiation of the universe will spread freely in space through neutral atoms, which is called the "afterglow" of the big heat explosion. After a long journey, this energy should have been transferred to infrared, or farther microwave band. All of the above are very logical reasoning, don't you think?
Cosmic microwave background radiation was first discovered by arno penzias and R Wilson in 1964. At that time, due to the limitation of technology, people did not observe the temperature fluctuation in microwave radiation, and the temperature was completely consistent throughout the day, which brought great trouble to people. The distribution of matter is so absolutely uniform, so how are these structures, such as stars, galaxies and clusters of galaxies, formed?
So the picture above is the accurate measurement of microwave background by COBE satellite in 1990s. The measurement results not only confirm the existence of afterglow of the Big Bang, but also confirm that the temperature and spectral type of microwave radiation (blackbody radiation) are exactly the same as predicted by the Big Bang! Then WMAP (Anisotropy Detection) and Planck satellite accurately measured the tiny fluctuation of microwave radiation!
In fact, if we look back again, what will we see? The temperature of the universe is high enough to destroy a single nucleus, so that only protons, neutrons, electrons and radiation particles remain in the universe!
When the universe is cooled to the point where protons and neutrons can stably combine to form nuclei without being blasted by radiation particles (photons), we can make a very accurate prediction on the number and types of nuclear fusion. So this is the third one!
The Big Bang theory can accurately predict the density of ordinary matter relative to photons and the abundance of other nuclear synthetic elements relative to hydrogen. Through the measurement of microwave radiation spectrum, we find that the abundance of light elements is surprisingly consistent with the theoretical prediction! This is impossible for other models of the universe, so the Big Bang is the dominant theory of the evolution of our universe at present.
BIGBANG is not so perfect, and there are still many things that have not been explained clearly. take for example ...
Why is the universe so smooth? Even at the beginning of the big bang, the universe was flat, and the universe was smooth and uniform on the whole, but how did the tiny fluctuations that were consistent throughout the day appear? Why are there structures such as stars, galaxies and clusters of galaxies?
This is a good question, but the Big Bang itself didn't answer it. So what other questions can't be answered by the Big Bang?
In principle, the universe can be any shape or curvature, because in general relativity, matter and energy determine the curvature of space. When the universe is very young, the density of matter and energy can make the space take on any shape. We found from experiments that the space is very flat. In fact, even if we measure the temperature fluctuation of microwave background today.
We found that the curvature of the universe is less than 2% within the scope of Hubble's volume of 93 billion light years, which is really flat! In fact, many friends will ask such a question, saying that the matter in the universe was extremely high in energy density at first, why didn't it collapse together to form a black hole? This is because the density of matter and energy has been at the critical value since the Big Bang, which is puzzling. What is the curvature of fine-tuning space? What caused the space to be so flat?
The temperature of the cosmic microwave background is roughly the same in all directions of space. Why is this happening? The matter at one end of the universe is completely separated from the matter at the other end, which has been the case since its birth. There has never been communication and information transmission between the two. But their temperature is the same! According to our common sense, there must be a consistent temperature between two objects. After all, after heat transfer, there must be at least contact. The temperature of microwave radiation is the same in the range of 930 light years, as if they have been in contact. What's going on here? The big bang didn't say.
If our high-energy theory is correct, the universe should not only have a microwave background and rich light elements, but also have a bunch of very strange relics, such as magnetic monopoles, cosmic strings and other topological defects. However, no one has found it yet.
None of the above questions can be answered in The Big Bang Theory. Any theoretical physicist like Allen Goose needs to find a new theory to solve all these problems, and a new theory needs to do what a good scientific theory will do: make a new prediction!
This is the theory of cosmic expansion. It created the high temperature, density and expansion state of the Big Bang. At first, the universe had only vacuum energy. The quantum fluctuation of vacuum energy makes the universe begin to skyrocket (exponentially) in tiny areas everywhere. After the skyrocketing, the vacuum energy decayed into matter, creating the initial stage of the Big Bang, and part of the vacuum energy was locked in space. This is what we now call the cosmological constant: dark energy! In this process, skyrocketing has done the following things:
The last point is particularly important, because these fluctuations can be measured not only in the microwave background of the universe (but also in the large-scale structure of our universe when 1979 Gus put forward the inflation theory)!
So when we measure microwave background radiation and large-scale structure, we find that these tiny fluctuations are consistent with the quantum fluctuation prediction of inflation theory!
If we have to say it correctly or not, at present, inflation theory is considered to be 98% correct in explaining the origin of the universe!
This also means that there are some defects in some timeline images of the origin of the universe that we see at present, or at least we should not put the thermal explosion in front of the skyrocketing universe? This is a question that many people are asking. What was before the Big Bang? The current answer is: the universe is skyrocketing under vacuum energy!
Unfortunately, we don't know what happened before the inflation of the universe. As far as the nature of the inflation of the universe is concerned, we will never know what happened before, because inflation also has a name called hairless theorem. Also known as the cosmic horizon, it is no different from the black hole horizon. The exponential expansion of space erased everything before! At least for now, this is the oldest thing that physics allows us to understand the universe.
The inflation was the cause of the Big Bang. According to what we have seen, the skyrocketing leveled the space of the universe, made the temperature of the universe the same in all directions, wiped out some exotic particles that existed before, created appropriate fluctuations on all scales (regardless of size), and grew into the stars, galaxies and galaxy clusters we have now. This is the oldest story we can know about the universe before the Big Bang!