The Big Bang & the Origin of the Universe

The Big Bang is the leading scientific account of how the universe began: roughly 13.8 billion years ago, everything we can see was packed into an unimaginably hot, dense state, and it has been expanding and cooling ever since. Stephen Hawking's earliest important work helped put that beginning on a rigorous footing, and much of his later career was spent asking what such a beginning could really mean.

Running the film backwards

The starting point is a simple observation made by Edwin Hubble in the 1920s: distant galaxies are moving away from us, and the farther away they are, the faster they recede. The universe is expanding. That is the single most important fact in cosmology, and it has an obvious implication. If everything is flying apart now, then in the past everything must have been closer together. Run the expansion backwards far enough and the entire universe converges towards a single, incredibly hot and dense point.

That convergence is the Big Bang. It is not the picture most people carry in their heads, so it is worth being clear about what it is not.

What the Big Bang is not

It was not an explosion in space. There was no empty room into which matter was flung. The Big Bang was an expansion of space itself, happening everywhere at once. Galaxies are not rushing outwards through a pre-existing void; the space between them is stretching. There is no centre to the expansion and no edge, just as there is no centre to the surface of an inflating balloon.

And the question "where did it happen?" has no location as an answer, because space and, as Hawking would stress, time itself are part of what came into being.

The evidence

The Big Bang is not a guess. Several independent lines of evidence support it.

The expansion itself is directly observed. The relative amounts of the lightest elements, hydrogen, helium and a trace of lithium, match precisely what the theory predicts should have been forged in the first few minutes. And, most powerfully, the whole sky glows faintly with the cosmic microwave background, the cooled-down afterglow of the hot early universe, discovered in 1965 and mapped in extraordinary detail since. It is, in effect, a photograph of the universe when it was young.

Hawking's contribution

Here is where Hawking enters. In the late 1960s, working with Roger Penrose, he applied the mathematics of general relativity to the expanding universe. The singularity theorems they proved showed that, if general relativity is correct and the universe is expanding as we observe, then it must have begun from a singularity, a moment where density and the curvature of spacetime become infinite and the laws of physics break down.

This was a genuine result, not a story. It established that, taken at face value, our best theory of gravity demands a beginning. The Big Bang was no longer just a plausible extrapolation; it was, under general relativity, unavoidable.

The problem with the beginning

But a singularity is also an admission of defeat. It is precisely the point where the equations stop making sense. Hawking was never content to leave the universe's first moment as a place where physics simply gave up.

So for much of the rest of his career he tried to describe the beginning without a singularity. The most famous result of that effort is the no-boundary proposal, developed with James Hartle, which suggests that the universe may have no sharp starting edge at all, and that asking what came "before" the Big Bang may be as meaningless as asking what lies south of the South Pole.

He also did important early work, sometimes overlooked, on how tiny quantum fluctuations in the infant universe could have been stretched by its rapid early expansion into the seeds of galaxies, the large-scale structure we see today.

The Big Bang, then, is both one of Hawking's earliest triumphs and one of his longest preoccupations: he proved the universe had a beginning, and then spent decades trying to understand what kind of beginning it could possibly have been.

The long-term fate of that expansion now appears to be governed by dark energy.