The Big Bang Theory

As discussed on the previous page, Hubble’s law and Einstein’s general theory of relativity together imply that the universe had a beginning. The first scientist to propose a theory of that beginning was none other than Georges Lemaître himself, the same physicist (and Catholic priest) who had predicted Hubble’s law two years before its discovery. In 1931, Lemaître suggested that the universe began with an incredibly tiny but unfathomably powerful burst of energy—a single “quantum” with enough energy to produce all of the matter in the entire universe.Georges Lemaître, “The Evolution of the Universe: Discussion,” Nature 128 (1931), 699 - 701.

Lemaître’s hypothesis was greeted with skepticism, at first, in the scientific community. The idea that the universe had a beginning was viewed with particular suspicion by some atheists, who worried that Lemaître’s proposal was really a creationist hypothesis in disguise. During a BBC radio broadcast in 1949, one of the theory’s outspoken opponents—the prominent atheist astronomer Fred Hoyle—mockingly referred to Lemaître’s hypothesis as the “Big Bang” theory, and the name stuck.See this article for more details.

The so-called “bang” would not have been accompanied by any sound, because there was no air or other matter to carry sound waves. According to the contemporary version of the theory, as we’ll see, the initial event was more like an incomprehensibly brilliant flash of light (and other forms of energy), which later “cooled” to form protons, neutrons, and eventually whole atoms.

A few scientists continued to resist the Big Bang theory decades after it had been accepted by the majority of astronomers and cosmologists. In 1989, while serving as editor of the journal in which Lemaître’s theory had been published, Sir John Maddox wrote a brief editorial expressing his opinion that the Big Bang theory was “philosophically unacceptable.” The problem, for Maddox, is that the unexplained burst of energy that brought the universe into being “is an effect whose cause cannot be identified or even discussed.” The cause of the universe can’t be identified or discussed scientifically, that is. As Maddox goes on to admit:

Creationists and those of similar persuasions seeking support for their opinions have ample justification in the doctrine of the Big Bang. That, they might say, is when (and how) the Universe was created. The reality of the event is accepted. The question of its cause, in the absence of time, is a matter for the imagination.John Maddox, “Down with the Big Bang,” Nature 340 (10 August 1989), 425.

A few others maintained similar opinions. Hoyle, who had coined the term “Big Bang” in the first place, remained a stalwart opponent of the theory until his death in 2001.

Nevertheless, the Big Bang theory steadily gained popularity as its predictions were confirmed by new observations, measurements, and calculations. (We’ll talk about some of that evidence on the next page.) Today, an updated version of the Big Bang theory is considered the standard theoretical model of cosmology. Cosmologists call it the ΛCDM (Lambda-CDM) model. The Greek letter “Λ” (lambda) represents a number called the cosmological constant and the letters “CDM” stand for “cold dark matter.” The cosmological constant and dark matter will be discussed near the end of this chapter. I’ll refer to it simply as the Big Bang model, though there are numerous other theories—most of which are variations or extensions of the ΛCDM model—which also claim that the universe began with an incomprehensibly powerful burst of energy.

Why call it the Big Bang model, rather than the Big Bang theory? Scientists typically use the term “model” to describe mathematical representations of a specific physical system (in this case the entire universe), especially when the representation is based on a broader theory or theories about physical systems in general. The Big Bang model is a mathematical representation of that sort, as I will explain in what follows.

Unlike Lemaître’s hypothesis, the contemporary Big Bang model isn’t a theory about the initial burst of energy known as the Big Bang itself. The model doesn’t claim that the universe began with a single quantum of energy, nor does it say exactly what form (or forms) of energy first appeared. Instead, the model focuses on what happened after the Big Bang. Using the mathematical laws of general relativity and quantum physics, the Big Bang model describes how the vast expanse of stars and galaxies that we see today may have formed from the intense light (and other forms of energy) that filled the rapidly-expanding universe shortly after that unexplained initial event. In other words, the Big Bang model isn’t a theory of how the universe came into being. It is a mathematical model representing the history of the universe after the earliest moments that can be described with our present theories of physics.

Another important difference between Lemaître’s hypothesis and the contemporary Big Bang model is that the latter leaves open the possibility that the universe may be infinite in size. No one knows (and there may be no way to find out) how far the universe extends beyond what we can see. Perhaps it goes on forever. In that case, the universe must have been infinite in size even at the time of the Big Bang. Nevertheless, according to the Big Bang model, the part that ended up forming the observable universe began as a tiny (but unfathomably powerful) burst of energy.

Cosmologists know very little (though some are happy to speculate) about what happened at the very beginning of the universe, because our most successful theories of fundamental physics—relativity and quantum field theories—break down at the Big Bang singularity. These theories probably don’t provide an accurate picture of what happened during the earliest moments after the singularity either, for reasons that we’ll discuss in what follows.

A summary of important stages in the history of the universe, along with evidence for the Big Bang model, will be presented on the next page.