Did the Universe have a beginning? Will it eventually come to an end? How did the Universe evolve into what we can see today: a ‘cosmic web’ of stars, galaxies, planets and, at least on one pale blue planet, what sometimes passes for intelligent life?
Not so very long ago, these kinds of existential questions were judged to be without scientific answers. Yet scientists have found some answers, through more than a century of astronomical observations and theoretical developments that have been woven together to give us the Big Bang theory of cosmology. This extraordinary theory is supported by a wide range of astronomical evidence, is broadly accepted by the scientific community, and has (a least by name) become embedded in popular culture.
We shouldn’t get too comfortable. Although it tells an altogether remarkable story, the current Big Bang theory leaves us with many unsatisfactorily unanswered questions, and recent astronomical observations threaten to undermine it completely. The Big Bang theory may very soon be in crisis.
To understand why, it helps to appreciate that there is much more to the theory than the Big Bang itself. That the Universe must have had a historical beginning was an inevitable consequence of concluding that the space in it is expanding. In 1929, observations of distant galaxies by the American astronomer Edwin Hubble and his assistant Milton Humason had produced a remarkable result.
The overwhelming majority of the galaxies they had studied are moving away from us, at speeds directly proportional to their distances. To get some sense of these speeds, imagine planet Earth making its annual pilgrimage around the Sun at a sedate orbital speed of about 30 kilometres per second. Hubble and Humason found galaxies moving away at tens of thousands of kilometres per second, representing significant fractions of the speed of light.
Hubble’s speed-distance relation had been anticipated by the Belgian theorist Georges Lemaître a few years before and is today known as the Hubble-Lemaître law. The constant of proportionality between speed and distance is the Hubble constant, a measure of the rate at which the Universe is expanding. In truth, the galaxies are not actually moving away at such high speeds, and Earth occupies no special place at the centre of the Universe.
The galaxies are being carried away by the expansion of the space that lies between us, much as two points drawn on a deflated balloon will move apart as the balloon is inflated. In a universe in which space is expanding, everything is being carried away from everything else.
The Big Bang story is almost as fascinating as the story of the Universe itself
To get a handle on the distances of these galaxies, astronomers made use of so-called Cepheid variable stars as ‘standard candles’, cosmic lighthouses flashing on and off in the darkness that can tell us how far away they are. But in the late-1920s, these touchstone stars were poorly understood and the distances derived from them were greatly underestimated, leading scientists to overestimate the Hubble constant and the rate of expansion. It took astronomers 70 years to sort this out.
But such problems were irrelevant to the principal conclusion. If space in the Universe is expanding, then extrapolation backwards in time using known physical laws and principles suggests there must have been a moment when the Universe was compressed to a point of extraordinarily high density and temperature, representing the fiery origin of everything: space, time, matter, and radiation. As far as we can tell, this occurred nearly 14 billion years ago. In a BBC radio programme broadcast in 1949, the maverick British astronomer Fred Hoyle called this the ‘Big Bang’ theory. The name stuck.
Of course, it’s not enough that the theory simply tells us when things got started. We demand more. We also expect the Big Bang theory to tell the story of our universe, to describe how the Universe evolved from its beginning, and how it came to grow into the cosmic web of stars and galaxies we see today. The theorists reduced this to a simple existential question: Why do stars and galaxies exist? To give a proper account, the Big Bang theory has itself evolved from its not-so-humble beginnings, picking up much-needed additional ingredients along the way, in a story almost as fascinating as the story of the Universe itself.
The Big Bang theory is a theory of physical cosmology, constructed on foundations derived from solutions of Albert Einstein’s equations of general relativity – in essence, Einstein’s theory of gravity – applied to the whole universe.
Einstein himself had set this ball rolling in 1917. At that time, he chose to fudge his own equations to obtain a solution describing an intellectually satisfying static, eternal universe. Ten years later, Lemaître rediscovered an alternative solution describing an expanding universe. Although Einstein rejected this as ‘quite abominable’, when confronted by the evidence presented by Hubble and Humason, he eventually recanted.