According to the interpretation of the Big Bang theory, the expansion speed of the universe is indeed super-light on a large scale.
However, the expansion speed of space is not limited by the speed of light of special relativity , because space does not transmit any information during the expansion process. Space is just the background of material movement , and of course it is also the background of light flight.
Theoretically, since light travels in a space that expands at a faster than light speed, it means that light can never reach the earth, but why can we still see the light emitted by distant celestial bodies?
On a large scale (usually the distance between galaxies), the expansion speed of space is indeed faster than the speed of light, but in a small local area, the expansion of space is not obvious at all. It would be terrible if even we living on the earth could feel the expansion of space!
For the earth, the solar system, and even the Milky Way , it is difficult to visually observe the expansion of space. Because as mentioned above, space expansion is actually the expansion of space between different galaxies, which shows that the galaxies are accelerating away from each other.
From a macro point of view, it is like an expanding balloon. The distance between each point on the balloon is gradually increasing. However, the distance between two points that are close to each other increases very slowly, while the distance between two points that are farther away from each other is not obvious. The distance between points increases more rapidly. In other words, the further apart the two points are, the greater the expansion speed will be.
This is also the famous " Hubble's law ", which was discovered by the American astronomer Hubble in the 1920s.
Hubble discovered that the speed at which galaxies move away from each other is directly proportional to the distance between them. The formula is υ=H0D. The discovery of Hubble's law has also become one of the powerful evidences for the Big Bang theory.
According to Hubble's law, it can be calculated that every time the distance of space increases by 3.26 million light-years, the expansion speed will increase by about 75 kilometers per second. Only if the expansion speed exceeds the speed of light, the light emitted by the celestial body will not reach the earth.
How can the light from so many distant celestial bodies never reach the earth?
By calculation, this distance is approximately 13.3 billion light years. The implication is that the expansion rate of space 13.3 billion light-years away exceeds the speed of light, and the light emitted by the celestial bodies there can never reach the earth and be seen by us. For us, those celestial bodies do not exist and are meaningless, because nothing that happens there will have any impact on us (the speed of information transmission is also the speed of light).
The light emitted by a celestial body 13.3 billion light years away from the earth does not take 13.3 billion years to reach the earth, because space has been expanding, which is equivalent to the distance that the light travels. After calculation, it takes about 46.5 billion years. to reach the earth.
46.5 billion years is also the radius of the observable universe , centered on the Earth! And due to the isotropy of the universe, no matter whether the radius of the observable universe is measured with any celestial body as the center, the final result is 46.5 billion light-years!
