It is frustrating that probably the most significant moment in the history of the universe – its first – is cloaked in almost complete darkness. True, total ignorance envelops only the first one-millionth of a second (approximately) of the universe’s existence – cosmologists have many theories about what happened over the subsequent 13,700,000,000 years. Still, our lack of knowledge of the instant of the universe’s supposed creation – the Big Bang – is profound indeed. Not only is it unknown what happened in that initial tiny fraction of a second; it is not even clear that it happened at all!
Among today’s scientists, the Big Bang cosmology is the most widely accepted view of the origin and evolution of the universe. According to this theory, the universe originated in a catastrophic event in which all matter, infinitely compressed at that instant into a single dimensionless point, began to expand and cool extremely rapidly. It was this explosion that put in train the sequence of events that resulted, approximately 13.7 billion years later, in the vast structure of innumerable stars and galaxies that exists today. The concept of an expanding universe is now acknowledged to be the unifying theme of modern cosmology.
The uncertainty in this account concerns not so much the subsequent evolution – although the details of this are naturally the focus of intense speculation – as the Big Bang itself. The idea of an ‘initial singularity’, as the Big Bang is more formally called, follows, with apparent inevitability, from Einstein’s general theory of relativity. The various cosmological models that have been derived in this way all feature a singularity in which the density and temperature of matter and the curvature of space–time are infinite; from this point expansion begins, with matter becoming less dense and cooling in the process. The difficulty arises because physicists today doubt that the equations of general relativity would remain valid in the extreme physical conditions that would obtain at the moment of singularity. At such high densities, it is now widely believed, the normal laws of physics will cease to apply and general relativity will need to be replaced by a more complete theory of a kind known generically as ‘quantum gravity’. So-called ‘superstring theory’ is a leading candidate for such a role, but it is unknown whether this or any of its rivals will predict a singularity. So, while the majority of cosmologists favour the idea of a single hot Big Bang, a number of alternative, non-singular cosmologies have been proposed.
Standing the test of time There is no doubt, at least, that the current universe looks as if it underwent a Big Bang-type explosion in the past, and there is compelling evidence to suggest that it did. The most important strand of this evidence is the fact of expansion, which is implied in the equations of general relativity, first published by Einstein in 1916. Indeed, Einstein himself recognized this implication, but to salvage his own belief that the universe was static, he introduced a compensatory pressure called the ‘cosmological constant’ – a move which he later described as his ‘biggest blunder’. Observational evidence of expansion, accumulated sporadically during the 1910s and 1920s, culminated in 1929 in the formulation by US astronomer Edwin Hubble of his eponymous law. Hubble, assisted in particular by Milton Humason, observed that the light emanating from nearby galaxies was ‘redshifted’ – it had moved to the red end of the colour spectrum. A phenomenon analogous to the Doppler effect in sound, this indicated that the light waves were stretched and hence that the galaxies involved were travelling away from our galaxy. Repeated measurements showed that the velocities at which the galaxies were receding were proportional to their distance – the essence of Hubble’s law.
‘You don’t have to search far to locate where the Big Bang occurred, for it took place where you are now as well as everywhere else; in the beginning, all locations we now see as separate were the same location.’
Brian Greene, US superstring theorist, 1999
When something expands, we normally think of it as occupying space; expansion, in this sense, is expansion in or through space. But at the instant of the Big Bang, there was no space ‘outside’ to expand into: the subsequent expansion was (and is) the expansion of space and occurred everywhere at once. The galactic recession that Hubble observed is not a matter of galaxies moving away from us through space. They and we remain in broadly the same positions relative to each other and everything else; it is the space between us that expands and carries us apart. This kind of expansion has been compared to the swelling-up of a fruitcake in which all the raisins, corresponding to galaxies, get further and further apart as the cake cooks; except that, in the case of the universe, as there is no outside, there is no edge and no centre. This is one aspect of the so-called ‘cosmological principle’, according to which the universe is essentially the same in all directions and its expansion is the same for all observers: there is no privileged position for us or any other notional observer.
Apart from the recession of galaxies, there are two other especially important strands of evidence in support of the hot Big Bang cosmology. According to the theory, the atomic nuclei of lighter elements, especially hydrogen and helium, would form in the first few instants after the bang, when temperatures had fallen to a few billion degrees. The abundances of these elements detected in the universe today accord closely with the levels predicted by the theory. Even more direct evidence that the universe went through a hot, dense phase is provided by the cosmic microwave background (CMB). This low-energy radiation, a relic of the hot early universe, suffuses all space, bathing the earth in a faint glow that comes from all directions. The existence of the CMB was predicted, as a remnant of the Big Bang, in 1948 and detected somewhat fortuitously in 1965.
The discovery of the CMB not only provided further corroboration of the Big Bang cosmology but largely put paid to its main rival at that time, the steady state theory. This, no more than other rival theories, was unable to provide as satisfactory an account of the various strands of empirical evidence. The Big Bang, having so far rebuffed all major challengers, remains the cornerstone of modern cosmology.
the condensed idea
The beginning of space and time