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Redshift - Expansion of the Universe

Redshift is a concept often used by astronomers, it refers to the shift of light to the red end of the spectrum, which happens through the stretching of light waves.


Redshift and the Doppler's Effect

Wavelength shifting to the red end of the spectrum.

The redshift concept is very similar to the Doppler’s Effect. The Doppler’s Effect is commonly illustrated using an example of an ambulance. The Doppler’s Effect refers to mainly sound waves, when an object moves towards a reference point the pitch is higher as compared to an object moving away from a reference point where the pitch is lower. Light behaves in the same way, the only difference is that instead of listening for pitch, scientists look for color to determine whether an object is moving towards or away.


Redshift - refers to a celestial object emitting light, moving away, since the waves occur at a lower frequency.

Blueshift - refers to a celestial object emitting light, moving towards since the waves occur at a higher frequency.


Expansion of the Universe


In 1929, Edwin Hubble discovered the universe was expanding through the concept of redshift. He noticed that galaxies appeared to be moving away from us, their light shifted to the red end of the spectrum.


While the doppler’s effect is about relative motion between a source and the observer, redshifts occur due to the expansion of space themselves. The galaxies appear to move away from Earth because the fabric of space itself is expanding.


One common analogy used to express this is the “bread” analogy. Suppose two galaxies are raisins in dough, they are a certain distance apart from each other. When the bread is being baked, the distance between the two raisins increases even though the raisins were stationary to each other. This is exactly what is happening with the space between galaxies in the universe.


Astronomers use spectroscopy to measure the redshift of a celestial object:

  1. They obtain a spectrum of a galaxy

  2. They then analyze it to see which blueprint of bands corresponds to which atom

  3. They then compare it with the wavelengths of those atoms and molecules from a laboratory on earth.

They are then able to measure how much redshift/ blueshift has occurred with that celestial body from Earth.


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