Red Shift


edwin hubble


"Equipped with his five senses, man explores the universe around him and calls the adventure Science." - Edwin Hubble

Edwin Hubble, the American astronomer, was instrumental in helping our understanding of an expanding universe. His intense observations, which at times bordered on obsession, led to the discovery that the recessional speed of a distant galaxy is directly proportional to the distance to it from the earth: the greater the distance, the greater the speed.

NASA named the space telescope in his name to honour the giant in the astronomical realm.

Red shift provides us with the concrete evidence that our universe is expanding. A source that emits waves, when moving away from a receiver two things happen:

  • The waves stretch out and the wavelength increases
  • Since the product of the wavelength and the frequency - velocity - remains the same, the frequency goes down
High Frequency   Low Frequency
Visible Spectrum of light


Therefore, light that comes from an object moving away from the earth, must have a longer wavelength or low frequency. The spectrum shows, that type of light lies at the red-end.

Since the light, that comes from distant galaxies, shows a tendency towards to be at the red end of the spectrum, it comes from a source that moves away from us. This phenomenon is called Red shift

Therefore, the universe is expanding. Red shift is the evidence.

The following animation shows how it happens.


Edwin Hubble said that the further the galaxy, the faster it moves away from us. This has become known as Hubble's Law.

Please answer the following questions.


  1. There are galaxies that come towards us. If someone analyzes the light that comes from them, where will that light be heading towards in the spectrum?
  2. It has been predicted that the universe cannot go on expanding for ever. What is the limiting factor? What change will this factor bring about?
  3. How does Red shift lead to the idea of Big Bang? Is there any other credible evidence to prove that Big Bang really took place?