Could there have been a time before the Big Bang? In other words, could the universe have existed before it even began? May there have even been previous universes? Such ideas, once the preserve of high-concept science fiction and philosophical debates, are gaining a new scientific credibility in the 21st century. Some cosmologists are wondering if the Big Bang was merely an intermediate phase and not the true start of the universe at all. Theories such as the ekpyrotic universe, ‘Big Bounce’ models and cyclic cosmology have been around for a while, but data from sensitive space probes could put some of these on a firmer footing. But what exactly was the Big Bang, and why are some scientists now changing their minds about it?
The widely accepted standard cosmological model states that the universe came into being from a superhot, superdense state that was no bigger than an atom and made of pure energy. Not much about that is contentious, but things get precarious with what happens next. This object, known as the ‘initial singularity’, is thought to have been timeless and dimensionless; there was nothing ‘outside’ of the singularity to speak of. Then, 13.82 billion years ago – afigure obtained from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and European Space Agency’s (ESA) Planck satellite – this microscopic singularity expanded rapidly to the size of a football. This was the ‘Big Bang’. But it wasn’t an explosion. The universe never exploded into being. Rather, this initial expansion from microscopic quantum fluctuations birthed space and time and seeded the largescale structure of the universe. This ‘Big Bang’ model has served cosmology well for over 80 years, but there have always been unanswered questions.
Despite the Big Bang theory being the cornerstone of cosmology, a theory called cosmic inflation was proposed in the 1980s to address
