What is the Relativistic Doppler Effect? Why do Distant Galaxies Appear “Redshifted?”

When gazing into the deep expanse of the universe and finding the distant quasars and galaxies, we notice a particular feature that is commonplace no matter where we look: the further away the galaxy or deep sky object is, the redder, or the longer wavelength, the object appears; it seems that a galaxy like the Andromeda Galaxy is far less red than a comparable galaxy that is 1.6 billion light years away. This feature was once noticed by the astrophysicist Edwin Hubble, a physicist that will be spoken of frequently in this chapter, who found that this reddening paradigm was more profound the further one looked out into the universe (a galaxy 100 million light years away will appear less “redshifted” than a galaxy 2 billion light years away, even if the two galaxies have the same compositions). This redshifting determined for Hubble that the universe was expanding, and he proved his findings through what is known as the Doppler Effect, which is the phenomenon that creates the redshifting Hubble used to determine that the universe was expanding. In this chapter, we will discuss the Doppler Effect, how it occurs, and what it represents. References Classical Doppler Effect - Wikipedia https://en.wikipedia.org/wiki/Doppler_effect Relativistic Doppler Effect - Wikipedia https://en.wikipedia.org/wiki/Relativistic_Doppler_effect Cosmological Redshift - COSMOS https://astronomy.swin.edu.au/cosmos/c/cosmological+redshift Cosmological Expansion - Wikipedia https://en.wikipedia.org/wiki/Expansion_of_the_universe Comoving and Proper Distance in Cosmology - David W. Hogg https://cds.cern.ch/record/387177/files/9905116.pdf Future of Cosmological Expansion - Wikipedia https://en.wikipedia.org/wiki/Future_of_an_expanding_universe Heat Death - Wikipedia https://en.wikipedia.org/wiki/Heat_death_of_the_universe Fate of the Universe - Wikipedia https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe Entropy - Wikipedia https://en.wikipedia.org/wiki/Entropy Thermodynamic Free Energy - Wikipedia https://en.wikipedia.org/wiki/Thermodynamic_free_energy