The reverse is retrograde. For example, some celestial bodies Of Spectral line It is called retrogression to go away from the earth in order to proceed. Retrogressive Movement speed It is called retrograde speed. Hubble Studying Einstein shift The red shift of the existing rules in the spectral lines of distant galaxies is found, which is explained by sight Caused by retrogressive motion in the direction. Because the universe is moving away from us expand The spectral lines from distant galaxies have redshifted towards us. If the red shift is converted to retrograde speed, the line of sight magnitude is converted to distance 。 The retrogression speed is approximately equal to the distance Proportional 。 As the distance between galaxies and us increases, so does the retrogression speed. Formula: V=HD. Where V is Apparent velocity D is the distance, which is proportional to the regression speed and distance constant H， call Hubble constant 。 That is, the retrogression speed of galaxies increases with distance rate 。 The estimated value is between 20-40 km/(s · million light years). The retrograde motion and velocity of distant galaxies have been confirmed by observational facts and become one of the important bases of the universe expansion theory. but a quasar The huge redshift of is still unexplained. Therefore, astronomers have put forward various hypotheses that need to be studied. [1] The "retrogression speed" of galaxies can exceed the speed of light
according to Hubble's law The farther away the galaxy is from us, the faster it "escapes" from us, then its "retrogression speed" can exceed light speed Is it? Special relativity Think the speed of light is Limiting speed 。 whatever Static mass The motion speed of non-zero objects can only approach vacuum The speed of light in cannot reach or exceed the speed of light. Only those with zero static mass particle (e.g photon 、 neutrino Wait The speed of motion is just the speed of light, but it cannot exceed the speed of light in a vacuum. If there are objects or signals Superluminal motion , will invert the causality. Therefore, special relativity strictly prohibits the movement at the speed of light. However, the special theory of relativity does not allow the speed of light to exceed the speed of motion of objects and the speed of signal propagation. Not belonging to the real movement of objects Apparent velocity (e.g“ Visual movement ”）, and the speed at which signals cannot be transmitted (e.g wave Of Phase velocity And so on) are not among the prohibitions. The "retrogression speed" of distant galaxies described by Hubble's law is not among the prohibitions of special relativity. This is because the "retrogression speed" of these galaxies is not their movement speed in space, but space expand Resulting“ Apparent velocity ”Therefore, the "retrogression speed" of galaxies can be "faster than the speed of light". [2] 
