In the past century, Einstein's special relativity and general relativity have stood the test of experiments and can be used to explain many physical phenomena, including the origin of the universe. But in the late 1990s, some physicists challenged a basic hypothesis of special relativity. They think that the speed of light is not constant: in the early universe, the speed of light was faster than it is now.
Since it was put forward, the variable speed of light has been controversial. According to a paper published in Physical Review D, this theory is expected to be verified in the near future. If the experimental results support the variable speed of light, it means that the laws of nature are not as eternal as we understand them today, and we also need to re-examine Einstein's theory of gravity. If the speed of light remains the same, light can only travel about 65.438+037 billion light years in the universe from BIGBANG about 65.438+037 billion years ago. The distance of light propagation is limited by this, which means that the visible range of the universe has a boundary, and the radius of this boundary is about 47 billion light years (although light can only travel through 65.438+037 billion light years, we should also take into account the expansion effect of the universe).
We imagine the visible universe as a big ball with a radius of 47 billion light years, and we sit in the center of the ball. The boundary of the sphere, that is, the horizon of the universe, is where the cosmic microwave background (CMB) is emitted. The cosmic microwave background is the radiation generated about 400,000 years after the Big Bang, and it is the earliest image of the universe that human beings can obtain. No matter where you are in the universe, you are in the center of your cosmic horizon, and the observed cosmic microwave background is 65.438+037 billion light years away.
The problem is that the distance from any point in the universe to the place where the microwave background radiation is emitted is 65.438+037 billion light years, so the distance between the microwave background radiation at both ends of the cosmic horizon is about 27.4 billion light years. This means that the universe is too big for light to travel from one end to the other within the lifetime of the universe. However, this is contrary to the uniformity of microwave background radiation we observed.
BIGBANG's process is similar to an extended bathtub, but the microwave background radiation that can reflect the early temperature of the universe is very uniform, rather than cold and hot fluctuations. What's going on here?
At present, among all hypotheses that try to solve the "horizon problem", the inflation theory has the highest voice. According to the inflation theory, the reason why the cosmic microwave background radiation is so uniform is that the universe reaches a uniform state in a small and dense stage, and then continues to be uniform during the inflation process. This is equivalent to mixing the water in the bathtub, reaching a uniform temperature, and then starting to spread out quickly.
Although inflation theory can be compatible with the invariance of light speed, it needs the existence of an "inflation field", which only existed for a short time in the early universe.
However, supporters of variable speed of light claim that if the speed of light in the early universe is much higher than that in the present universe, then the problem of horizon can be solved without expansion. In this way, in the process of the expansion of the universe, the two far apart ends of the universe can remain "connected", resulting in uniform microwave background radiation throughout the universe.
However, for theoretical physicists who support the expanding universe model, allowing the speed of light to change is like reversing a symbol in special relativity.
Two physicists pointed out that the speed of light in the early universe was at least 32 orders of magnitude faster than the current 300,000 km/s, which is only a lower limit. As time approaches BIGBANG, the speed of light will also tend to infinity.
According to the viewpoint that the speed of light is variable, the speed of light is faster because the temperature in the early universe was extremely high. Afsholdi pointed out that their theory required the early universe to be a melting pot with a temperature of at least 1028 degrees Celsius. In contrast, the highest temperature that humans can get on the earth is only 10 16 degrees Celsius, which is two orders of magnitude different from the early universe.
With the expansion of the universe, its temperature drops below 1028 degrees Celsius. Just as liquid water freezes when the temperature drops to a certain extent, light will change phase here, and the speed of light will become 300,000 km/s now ... Ice will not be harder because of lower temperature, and the speed of light will not be slower because of lower temperature, so light has maintained this value since then.
If Magueijo and Afshordi's variable light speed are correct, then we can predict the process of light speed slowing down, which means that we can measure the attenuation of light speed as long as there are enough accurate measuring devices. This is the content of their latest paper.
Afsholdi said that galaxies and other structures in the universe can exist because the density of the early universe fluctuated. These density fluctuations are reflected in the "spectral index" of cosmic microwave background radiation, which we can imagine as different "colors" of the early universe. The central reference value of the spectral index is 1, which corresponds to a universe with the same gravitational fluctuation on all scales. When the spectral index is higher than 1, the universe is "blue", which represents the fluctuation biased towards short wavelength; If the spectral index is lower than 1, the universe is "red", representing fluctuations biased towards long wavelengths.
If the variable speed of light is confirmed, it will subvert one of the main axioms of Einstein's special theory of relativity and force physicists to reconsider the nature of gravity. Afsholdi said, however, Einstein's gravity theory can't contain the whole picture of physics, and there must be a quantum gravity theory to replace general relativity in the future. At present, physicists have put forward many possible schemes to compete for the throne of quantum gravity theory, but if the variable speed of light is verified to be true, the optional range of quantum gravity theory will be greatly reduced.
"If we want to observe the phenomenon of quantum gravity, we'd better abandon the concept of inflation," Magueijo said. "The inflation model is actually avoiding basic physical problems. It separates the observable universe from physics outside the theory of relativity. Changing the speed of light is the basis of revisiting physics and trying to break through the theory of relativity. This is the best way to open new ideas and new theories. "