The Big Bang Theory (BBT) is a cosmological model about the origins of the Universe. Starting out from a high-density and high temperature state, the theory suggests that the Universe has been inflating over the past 13.8 billion years before developing into the present-day Universe. The idea that the Universe can be traced back to a cosmic starting point was introduced in 1931 by Georges Lemaître. The BBT rests on three pillars: the expansion of the Universe, the cosmic microwave radiation and the distribution of elements (Rhee 2013, p. 37):
First, the expansion of the Universe: around 100 years ago, Edwin Hubble discovered that galaxies are moving away from the Milky Way which led to the insight that every galaxy is moving away from every other galaxy. More recent findings even suggest that the distance between cosmic objects across the Universe will double in 9.8 billion years (Riess 2016). Furthermore, based on measurements made by Vesto Slipher and Milton Humason, Hubble concluded in a 1929 paper that there is a linear relationship between the distances of galaxies and the redshift in their light spectra. This relationship which came to be known as ‘Hubble’s law’, indicates that the greater the distance between two galaxies the greater the speed of their separation. In other words, the Universe is expanding with increasing speed (Rhee 2013, pp. 38-44). If this holds true, then by logical implication, the Universe must have been smaller and denser in the the past.
Second, the Cosmic Microwave Background (CMB): in 1948, Ralph Alpher first predicted the existence of ‘relict radiation’ or a radiation remaining from the Big Bang (Alpher 2014). In 1965, Robert Wilson and Arno Penzias captured a low level electrical signal like the one producing black and white static on a television coming from everywhere. They realized that the microwave radiation matched the curve of a (hypothetical) blackbody radiator with a temperature of 2.7K. Due to this finding, it is now believed that the CMB is a distant echo from the moment when the expanding Universe cooled down to a temperature below 3,000K, marking the ‘transparency moment’ when radiation suddenly could escape from opaqueness (May et al. 2015, pp. 28-30). Since the observation matched Alpher’s predictions about the existence of an afterglow they cemented the status of the BBT as the leading theory (Francis 2016).
Third, the distribution of elements: George Gamov and his team first worked out the details of the nuclear processes which supposedly occurred in the first few minutes of the Big Bang when the Universe was hot, dense and small. One of the predictions was that after completion of the so-called ‘nucleosynthesis’, the atomic matter in the universe in terms of mass would consist of 75% hydrogen and 25% helium with only traces of other elements. Again, astronomical observations have confirmed the theory (Rhee 2013, p. 52).
Eventually, the BBT gained universal acceptance as it is better equipped to explain empirical findings than rival models such as the Steady-State Theory according to which the universe continues forever without a beginning.
Alpher, V., Ralph A Alpher, George Antonovich Gamow, and the Prediction of the Cosmic Microwave Background Radiation, in: Asian Journal of Physics, Vol. 23, Nos 1 & 2 (2014), p. 17-26, URL=<https://arxiv.org/pdf/1411.0172.pdf>
Dreifus, C., What Astronomers Are Still Discovering About the Big Bang Theory, in: Smithsonian Magazine, March 2014, URL=<https://www.smithsonianmag.com/smithsonian-institution/what-astronomers-are-still-discovering-about-big-bang-theory-180949794/>
Francis, M., Five facts about the Big Bang, in: Symmetry Magazine, 23 August 2016, URL=<https://www.symmetrymagazine.org/article/five-facts-about-the-big-bang>
May, B., Moore, P., Lintott, C., Bang!: The Complete History of the Universe, 5th edition, Andre Deutsch, London, 2015
Rhee, G., The Cosmic Dawn: The Search for the First Stars and Galaxies, Springer, New York, 2013
Riess, A., NASA’s Hubble Finds Universe Is Expanding Faster Than Expected, Space Telescope Science Institute, Baltimore, Maryland, news release number: STScI-2016-17, URL=<http://hubblesite.org/news_release/news/2016-17>