When we are seeing an astonomical object, we are actually receiving the electromagnetic radiation that is emitted by the object. The wavelength that we observe on the Earth is not the wavelength that is emitted by the object, but instead, the wavelength is stretching as follows λ –> λ’ = λ + z Δλ , where Δλ = λ’ - λ. λ is the wavelength emmited by the object, λ’ is the wavelength measured by the observer, and *z* is the *redshift*. Underline notes: * Since...