What are Redshift and Blueshift?

Redshift and blueshift describe how light shifts toward shorter or longer wavelengths as objects in space (such as stars or galaxies) move closer or farther away from us. The concept is key to charting the universe's expansion.  Visible light is a spectrum of colors, which is clear to anyone who has looked at a rainbow. When an object moves away from us, the light is shifted to the red end of the spectrum, as its wavelengths get longer. If an object moves closer, the light moves to the blue end of the spectrum, as its wavelengths get shorter.

To think of this more clearly, imagine yourself listening to a police siren as the car rushes by you on the road.

Everyone has heard tìhe increased pitch of an approaching police siren and the sharp decrease in pitch as the siren passes by and recedes. The effect arises because the sound waves arrive at the listener's ear closer together as the source approaches, and further apart as it recedes.

This sound effect was first described by Christian Andreas Doppler in the 1800s and is called the Doppler effect. Since light also emanates in wavelengths, this means that the wavelengths can stretch or crunch together depending on the relative position of objects. That said, we don't notice it on daily-life-sized scale because light travels so much faster than the speed of sound — a million times faster, ESA noted.