Firstly, I understand what the Doppler effect is when it comes to sound or light waves.
From everything that I've read, we are told that the universe is constantly expanding since the all the radiation we observed is red-shifted. Assuming we are observing a distant galaxy/start that is moving away from us, the EMR waves incident on us from that galaxy is red shifted. My question is:
how do we know that the light is red shifted? When you measure its wavelength you are given just ONE value Λ, right? How does one know what its original wavelength was to begin with? Its only after we know both the values (the actual wavelength when it was emitted and the wavelength that we measure hear on earth) that we can claim that the galaxy/star is moving away.
I always assumed that one would observe both the wavelength of the photons and the energy and there is some sort of disparity there that tells us the light is red-shifted. But that does not make sense since one usually determines the wavelength based on the energy of the photon......I am confused here. Any answer will be appreciated. Thanks.