Unfortunately for science history, a google search of the Big Bang Theory yields 5 pages about the TV series by that name before a single reference to the scientific theory. Most people interested in cosmology, however, are familiar with the term and theory, “Big Bang.”
What is the Big Bang theory?
The term "Big Bang" was coined by Fred Hoyle in 1949 in a popular BBC science broadcast. It refers to what has become the current Standard Model: that all matter and energy in the universe originated in an initial explosive event.
In 1927, Fr. Georges Lemaitre published a paper that contained mathematical calculations proving that “galactic nebulae” were moving away from us. The shocking conclusion was that the universe was expanding. This idea was contrary to the Steady State model of the universe preferred by most astronomers and scientists—Albert Einstein and Fred Hoyle among them. In the Steady State model, the universe was stable and infinite in time and space.
Fr. Georges Lemaître / Public Domain
Up until the discovery of the Cosmic Microwave Background Radiation (CMBR) in 1965, many astronomers still held to the Steady State model. (See more on CMBR below.)
The background story
In 1929, a well known American astronomer, Edwin Hubble, published a paper documenting the “redshift” of galaxies (the redshift meant that galaxies are moving away from us). For many years after, Hubble received the credit for the idea that the universe is expanding.
Edwin Hubble / Public Domain
This is perhaps understandable for two reasons. First, Lemaitre’s paper of 1927 had received little attention until 1931. That year, Lemaitre’s professor at Cambridge, Sir Arthur Eddington, recognizing the valuable contribution of his pupil, encouraged the Royal Astronomical Society to publish a translation of Lemaitre’s 1927 paper.
Secondly, because more accurate data had been used in Hubble’s 1929 paper, Lemaitre removed his original, less accurate calculations in the translation which he provided. Many scientists were therefore unaware that Lemaitre had calculated the radial velocity of galaxies indicating an expanding universe before Hubble.
Eighty seven years later (in 2018), the International Astronomical Union renamed the Hubble Law in recognition of Lemaitre’s contribution. It is now officially known as the Hubble-Lemaitre Law.
Although the Big Bang theory is considered the standard model among physicists and cosmologists today, it is still useful to examine the evidence that supports the theory.
“Theories are… only coherent unifications of currently available data and observations.” —Fr. Robert Spitzer
Evidence for the Big Bang: Lemaitre/Hubble expansion
By studying the spectra of 46 galaxies, and the Doppler velocities of those galaxies relative to our own Milky Way galaxy, Hubble concluded that they were moving away from us. This famous “red shift” also provided evidence that the farther away galaxies are, the faster they move away from each other. Based on this observation, it became evident that the universe was expanding uniformly.
Assuming that the universe expanded much like a balloon being blown up, the high velocities of galaxies could be explained with almost perfect mathematical accuracy. This is exactly what Lemaitre concluded in his paper: if space was stretching (growing) like the elastic of a balloon being blown up, then the further a galaxy is from us (the observer), the greater its recessional velocity must be. Why? The short videos below illustrates the concept of an expanding universe.
From the expanding universe, Fr. Lemaitre looked beyond the concept of the expanding universe and concluded that the universe was expanding outward from a specific point in the past and must have had its origin in a “primeval atom.” This initial condition would have been one of extreme heat followed by an expansion.
"Timeline of the universe. A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe." / NASA/WMAP Science Team / Public domain
Evidence for the Big Bang: Cosmic Microwave Background Radiation (CMB)
If the universe did begin with an incredibly powerful explosion, the heat from this explosion should be detectable as radiation, a “very faint remnant of the original fireball,” so to speak. That remnant is known as the Cosmic Microwave Background Radiation.
According to a NASA summary, the existence of the CMB radiation was first predicted in 1948. In the 1960s, researchers Robert Dicke at Princeton and Dave Wilkinson of the Wilkinson Microwave and Isotropy Probe (WMAP) science team were devising an experiment to detect the CMB.
Around the same time, in 1965, Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in Murray Hill, New Jersey, were trying to build a radio receiver. They kept encountering excess noise which seemed to be coming from every direction.
Arno Penzias and Robert Wilson's 15 meter Holmdel horn antenna at Bell Telephone Laboratories / NASA, restored by Bammesk / Public domain
When the researchers at Princeton heard about the Bell Labs result, they immediately realized that the CMB had been found. Two papers were published in the Astrophysical Journal (vol. 142) of 1965: one by Penzias and Wilson detailing the observations, and one by Dicke, Peebles, Roll, and Wilkinson giving the cosmological interpretation.
Practically on his deathbed, Fr. Lemaitre was grateful to be informed of its discovery—a confirmation of his Big Bang theory.
Penzias and Wilson shared the 1978 Nobel prize in physics for their discovery. The NASA summary includes this remark:
“This uniformity is one compelling reason to interpret the radiation as remnant heat from the Big Bang; it would be very difficult to imagine a local source of radiation that was this uniform. In fact, many scientists have tried to devise alternative explanations for the source of this radiation, but none have succeeded.”
The Big Bang was subsequently confirmed by data from the cosmic background explorer satellites (COBE) #1 and #2, the Wilkinson Microwave and Isotropy Probe (WMAP), and most recently by the Planck satellite (and here). The new data is additional proof of Fr. Lemaitre’s general concept of the Big Bang. It encompasses other concepts such as quantum gravity, inflationary theory, dark matter, and dark energy.
"The evolution of satellites designed to measure ancient light leftover from the big bang that created our universe 13.8 billion years ago. Called the cosmic microwave background, this light reveals secrets of the universe's origins, fate, ingredients and more." / NASA/JPL-Caltech/ESA / Public domain
Evidence for the Big Bang: Even distribution of matter
CMB radiation also provides evidence for an even distribution of matter in the universe. Star clusters and galaxies might at first glance contradict this idea, but on a large scale, it appears to be true. In 2013, after completing a 4-year scan of the CMB radiation, data from the Planck Space Observatory confirmed this fact. More recent data indicates that matter in the universe might be 10% more uniformly distributed than previous models predicted. Again, this even distribution points to a single origin in time of all matter in the universe.
In spite of this strong line of evidence, other theories have been proposed in order to depose the idea that there was a beginning to our universe. Stay tuned for a look at those alternatives to the Big Bang in the next post.