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Instead, the doppler effect can be used to obtain the rotation.  Around all galaxies there exists is  a hydrogen cloud. As hydrogen atoms change their energy state, they emit photons with a wavelength of approximately 21cm. Due to the doppler effect, these waves are either stretched or squeezed as they travel back to Earth. On the whole, all objects in space are \textit{redshifted}, because they are moving away from us because of the expansion of the universe. However, if the galaxy is spinning, then one side will be moving away more than the other. This difference in redshift can be converted into the speed that the galaxy is spinning at.