Redshift isn't specific to hydrogen. Redshift is a phenomenon observed in light from distant objects. It's the stretching of wavelengths of light due to the expansion of the universe. This stretching shifts the light towards the red end of the spectrum, hence the name "redshift."
Hydrogen's spectral lines are used to measure redshift. Astronomers often use the spectral lines of hydrogen to measure redshift. Hydrogen is the most abundant element in the universe, and it has a very distinct set of spectral lines. When these lines are shifted towards the red end of the spectrum, it indicates that the object emitting the light is moving away from us.
To measure redshift, we need to know the object's original spectral lines. The amount of redshift is measured by comparing the observed position of hydrogen's spectral lines to their known laboratory values.
In summary, "redshift of hydrogen" is a shortcut that refers to using hydrogen's spectral lines to measure redshift. It's not a specific value or property of hydrogen itself.
