# What is the redshift of the CMB surface?

May 2, 2018

Redshift of the CMB surface is due to the expansion of the universe.

#### Explanation:

Remember that space is constantly expanding at all points (like the surface of a balloon being blown up). If you are familiar with the Doppler effect, then you know for a stationary observer and a moving target, the observed target frequency will change if the target is moving towards or away from the observer. In the case it is moving away from the observer, the frequency will be decreased. This is equivalent to saying the wavelength will be increased (since frequency and wavelength are inversely proportional: $f \propto \frac{1}{\lambda}$). In the same manner, we are the stationary observer and the photons on the CMB surface are the targets. As the universe expands, the photons are getting moved away from the us which makes their wavelengths increase. If you are familiar with the visible spectrum of light, you know that blue wavelength are shorter and red wavelengths are longer. Hence, if the photons' wavelengths are being observed as longer due to the expansion of the CMB surface, we call that being redshifted.