Audio Guide：

Astronomer Vesto Melvin Slipher applied the theory of Doppler Effect to observe the spectral lines of celestial bodies in order to find out which celestial objects are moving closer to Earth and which ones are moving away. He found that most celestial objects are actually redshifting. In 1929, American astronomer Edwin Hubble discovered that planets farther away from the Earth are also receding at a higher velocity. This is known as the famous “Hubble’s Law” and proof that the universe is expanding. Cosmological redshift is a result of increase in wavelength when light travels through expanding space.