Why the 'big bang' is a red herring


Most cosmologists accept that the universe was once small, dense and hot . . . not that it arrived with a bang

The “big bang” is surely one of the best-known concepts of modern science. It vividly conveys the idea of a universe born in a cataclysmic event, expanding and cooling over billions of years. The idea has long since passed into popular culture, and is widely used as a metaphor in public discourse.

So it is quite a surprise to learn that the term “big bang” is one of science’s great misnomers; physicists do not, in fact, claim to have a clear picture of the origin of our universe.

Certainly, it is true that science can describe much of the history of the universe in astonishing detail, from a tiny, hot and dense state billions of years ago to the gigantic ensemble of galaxies we see today. However, the beginning itself – or indeed, whether there was a beginning – remains uncertain.

The problem is that as one traces the evolution of the universe back towards time zero, our best theories run into infinities (not unlike the way the mathematical function 1/x has no definition for the value x = 0).

Our understanding of the cosmos is based on Einstein’s general theory of relativity and is strongly supported by astronomical observation; however, the theory breaks down when one tries to describe the universe when it was extremely small.

It has long been expected that this problem can be circumvented by formulating a “quantum” theory of gravity (quantum physics is the physics of the world of the very small). However, efforts to formulate such a theory have proved unsuccessful so far. In truth, we have a very successful “after the bang” theory, but know nothing about the “bang”itself.

‘Primeval atom’

So where does the name come from? The concept of an origin for the universe was first mooted by the Belgian priest and physicist Fr Georges Lemaître in 1931. Lemaître first showed that the recession of the galaxies, a phenomenon familiar to astronomers, could be explained in terms of a universe that was expanding, as predicted by Einstein’s relativity. Realising that an expanding universe must have been smaller in the past, Lemaître then suggested that the cosmos was not eternal, but began many billions of years ago as some sort of “primeval atom”.

The idea was not popular at first, but it was investigated in detail a decade later by the Russian nuclear physicist George Gamow. With his colleagues Ralph Alpher and Robert Herman, Gamow developed a model of a universe that started in an extremely hot and compact state and then expanded and cooled over billions of years (known to many physicists as “the evolving universe”).

The British astrophysicist Fred Hoyle was deeply sceptical of the idea of a universe with a finite age, evolved from some tiny initial state. He made his views clear in a BBC broadcast, stating: “I cannot believe that our whole universe began as some sort of big bang.” It was a rather unfair comment, as the Gamow group had said nothing about a bang, or indeed where the primeval fireball came from; however, Hoyle’s comment became well known and the name stuck.

In time, astronomical evidence favoured the evolving model of the Gamow group. It was discovered that galaxies from earlier epochs look very different to those of today, and radiation left over from the hot, infant universe was detected in 1965 .Thus the concept of a universe that was once extremely small, hot and dense was eventually accepted by most cosmologists. Unfortunately, it became known as the “big bang” universe in popular literature, although physicists disliked the term.

In recent times, the expression has become widespread among scientists themselves, perhaps a case of science following the media.

Does it matter? One problem is that the term “big bang” is misleading, as well as inaccurate. It pretends a knowledge that scientists do not have, or claim to have, creating confusion.

For example, public debates concerning the implications of the “big bang” for religion or philosophy are common. Most cosmologists are bemused by such talk, as science does not claim to describe the origin of the universe, or even whether it is meaningful to talk of such things. For example, one model that is quite popular in cosmology is that where the “big bang” was preceded by a “big crunch”, which may in turn have been preceded by earlier cycles. Such ideas are highly speculative, but cannot at present be ruled out.

Dr Cormac O’Raifeartaigh lectures in physics at Waterford Institute of Technology and writes the science blog Antimatter

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