Origins of the Big Bang Theory

l Vesto Slipher (1914) - 
"Nebulae" receding from the earth
l Alexander Friedman (1922) -  Expansion of the universe
l Hubble (1929) - Law of Red-Shifts

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Vesto Slipher (1914): Presented findings at an obscure astronomy meeting which showed that several "nebulae" were receding away from the earth. A graduate student named Edwin Hubble was in attendance and realized the implications of these findings. Hubble proved that these nebulae were actually galaxies, composed of billions of stars.

Alexander Friedman (1922): This Russian mathematician predicted the expansion of the universe.

Hubble (1929): Proposed the law of red shifts. Galaxies which are moving away from the earth demonstrate emission spectra with bands that are shifted toward the red (or longer wavelengths) end of the spectrum.