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