Big Bang Theory Proposed

In 1929, the American astronomer Edwin Hubble published evidence that distant galaxies are moving away from us, and that the farther away a galaxy is, the faster it recedes. The finding showed that the universe is expanding, and it became the observational foundation for what is now called the Big Bang theory: the idea that the cosmos grew from an extremely hot, dense early state. Hubble did not propose the theory himself, and the 1929 date marks the discovery that made it plausible rather than the theory's full formulation. The event's listing under Europe reflects the theory's roots in European physics, since the key theoretical ideas came from there.

The background lies in Einstein's general theory of relativity, published in 1915. When physicists applied its equations to the universe as a whole, they found solutions in which space itself could grow or shrink. Einstein, favoring a static cosmos, added a "cosmological constant" to hold things steady. The Russian mathematician Alexander Friedmann, however, explored expanding solutions in the early 1920s. The Belgian priest and physicist Georges Lemaître independently derived an expanding universe in 1927 and suggested that the recession of galaxies should follow from it. Meanwhile, Vesto Slipher had measured the spectra of spiral nebulae and found that most were redshifted, a sign they were moving away.

Hubble's contribution was to measure distances to these galaxies. Working at Mount Wilson Observatory in California, he used Cepheid variable stars, whose brightness reveals their true luminosity, to estimate how far away several galaxies lay. Combining those distances with measured redshifts, he found a roughly linear relationship between distance and recession speed. His assistant Milton Humason soon extended the data to fainter, more distant galaxies. Hubble's original estimate of the expansion rate was far too high because of calibration errors, and later work revised it substantially, though the exact value is still debated today.

Interpreting the result was the next step. If galaxies are separating now, then running time backward implies they were once much closer together. Lemaître made this argument explicit in 1931, describing a "primeval atom" from which the universe evolved. The phrase "Big Bang" came later, reportedly coined by the astronomer Fred Hoyle in a 1949 radio broadcast, where he favored a rival steady-state model.

The theory gained decisive support in 1964–65, when Arno Penzias and Robert Wilson detected the cosmic microwave background, a faint glow widely understood as leftover radiation from the early universe. The observed abundances of light elements such as hydrogen and helium also matched Big Bang predictions. Today, measurements place the universe's age at roughly 13.8 billion years. The theory describes how the universe evolved from a very hot, dense state, but it does not explain what, if anything, came before, and that question remains open.