Birth of the Big Bang theory
How we came to understand the universe had a finite beginning in a massive explosion.
The Big Bang theory had a long period of gestation. Although Einstein’s equations permitted an expanding universe, for many years the great man himself clung to the notion that the cosmos is static. Thus when, as early as 1921, the Russian physicist Alexandr Friedman sent Einstein some solutions to his equations describing an expanding universe that starts out explosively from a state of infinite density, Einstein shrugged them aside. In 1929 American Edwin Hubble’s astronomical observations that the galaxies are moving away from each other, persuaded him otherwise. “My greatest blunder” is how Einstein famously referred to his mistake.
By the early 1930s, Friedman’s solutions had become the basis of modern cosmological theory. However, astronomers were wary of extrapolating Friedman’s solutions all the way back to a starting state of infinite density, today termed a “singularity”. Taken at face value, the singular initial state suggested that some time in the past, billions of years ago, the entire universe erupted into being from nothing. Nevertheless, Abbé Georges Lemaître, a Belgian priest and astronomer, championed the idea that the universe had a finite beginning in a massive explosion. But it was only after World War II that the modern conception of the Big Bang theory really took hold.
The term itself was coined in 1949 by the contrarian British cosmologist Fred Hoyle as a term of derision. Hoyle accepted that the universe was expanding, but rejected the notion of a sudden origin. Instead, he proposed the “steady state theory” according to which the universe is eternal and as the galaxies fly apart so new matter is continually created to make new galaxies in the gaps. For many years the steady state theory enjoyed widespread support.
Meanwhile, the well-known physicist George Gamow became a major cheerleader for Big Bang and reasoned in 1948 that if the early universe were very compressed it would also have been very hot, causing nuclear reactions. Intense heat radiation must have filled space. Gamow’s student Ralph Alpher and a colleague Robert Herman pointed out that, as the universe expanded, this radiation would have cooled, but not completely. It should be around today in the form of a universal background radiation.
The primordial radiation has been studied for clues about the early universe.
Alpher and Herman’s idea was elaborated by three Princeton University astronomers, Robert Dicke, James Peebles and David Wilkinson, who began laying plans to look for the radiation using a microwave antenna. Unbeknown to the authors, two radio engineers, Arno Penzias and Robert Wilson, working at the nearby Bell Telephone Company Laboratory had already discovered the radiation by accident. They were puzzling over the source of a weird and persistent hiss in their satellite communication equipment. The Princeton work was published in 1965 alongside the Bell Labs discovery paper.
Now known as the Cosmic Microwave Background (CMB), the primordial radiation has been studied for clues about the early universe. Because the peak of the radiation falls in the microwave region of the electromagnetic spectrum, which is largely blocked by Earth’s atmosphere, it is best observed by satellite. Its discovery all but put paid to the steady state theory and enabled theorists to model nuclear and particle physics processes that occurred in the searing heat just after the Big Bang. These calculations find support in many astronomical observations and further bolster the Big Bang theory.
In spite of the discovery of the CMB, cosmologists still had deep misgivings about taking seriously the initial singularity in Friedman’s universe model – the idea that the universe burst from nothing. Many thought that the solutions would break down well before a singularity was reached. Others pointed out that if there were a singularity at the start of the universe, it would represent an event without a cause and thus set a fundamental limit on how far science could be used to explain the origin of the universe. There was also the vexatious problem that if the Big Bang were not the ultimate beginning, then something must have preceded it. But what?
These scientific and philosophical posers remain with us today, but a new twist was added in the 1980s. James Hartle and Stephen Hawking, drawing upon work in the 1960s by John Wheeler and his former student Bryce DeWitt, recognised that if the universe were sufficiently compressed at the outset, quantum processes, normally discussed in connection with atoms and molecules, would have become critical on a scale involving the whole universe. Thus was the subject of quantum cosmology born. Tentative calculations suggest that quantum effects remove the problematic singularity, although the explosive early expansion may be retained. Paradoxically, quantum cosmology does not compel the universe to have existed before the Big Bang. It may still have a finite existence in the past.
If the polarisation results hold up they are certain to reignite the debate about what, if anything, happened before the Big Bang, or whether our universe is part of a multiverse. Our understanding of the origin of the universe is based on the laws of physics, such as quantum mechanics and the general theory of relativity. Left out of account is the origin of those laws or of deeper laws that may replace them. Whether science can provide answers to those profound questions of existence only time will tell.