3. Redshift & Blueshift

If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. This is just one piece of the electromagnetic spectrum, but changes in this spectrum are one of the key pieces of evidence to support the Big Bang Theory. 

The spectrum will have specific dark bands where elements in the star absorbed light of certain energies. By examining the arrangement of these dark absorption lines, astronomers can actually determine which elements are in a distant star. Each element has a unique set of absorption lines (just like each person has a unique set of fingerprints). By analyzing these "fingerprints", astronomers can determine the star's chemical composition. In fact, the element helium was first discovered in our Sun — not on Earth — by analyzing the absorption lines in the spectrum of the Sun.

When astronomers started to study the spectrum of light from distant galaxies, they noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, as shown in the picture below. This shift of absorption bands toward the red end of the spectrum is known as redshift.

Two light spectrums. The one on the right is redshifted.
Image courtesy of David Bethel. CC BY-SA.

Redshift is a shift in absorption bands toward the red end of the spectrum. Redshift occurs when the light source is moving away from you or when the space between you and the source is stretched.

Redshift occurs when the source of light is moving away from the observer. So when astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth. The strange part is that almost every galaxy in the universe has a redshift, which means that almost every galaxy is moving away from us. This is one piece of evidence astronomers use to support the Big Bang theory because it proves the universe is expanding.

Blueshift is the opposite of redshift. As an object moves towards an observer, the wavelength decreases and light is shifted towards the blue end of the electromagnetic spectrum.  


Image based on one provided by David Bethel. CC BY-SA.


The following video explains redshift and blueshift:


Source

David Bethel
http://ck12.org/flexr/assemble/?fid=732 (CC BY-SA)