THE CENTRAL FACTS FROM THE COURSES YOU ALWAYS MEANT TO TAKE, in 25 lectures

Week 1 Day 2 Big Bang

Empty space looks empty, but in extreme close-up it is writhing with immensely small fluctuations. Let one of those get caught, and pressure builds up. And builds up. And builds up some more. And then our universe starts exploding out.

It doesn't look like much of a universe at first. It is ridiculously small - able to float, if anyone could measure it, inside a thimble without touching the sides. There is no solid matter, no miniature little galaxies or stars or palm trees waiting to be pumped up and sent out. Instead there are only dense storms of energy, occasionally coalescing into solid particles, but quickly being knocked apart as more energy crashes past.

Because our universe was moving so fast when it came into being, it quickly enlarged, and in doing, spreading more thinly, it naturally cooled down. Within a second it grew enough to swell for many miles, and within two minutes - though still searingly hot by our standards - the cores of the first ordinary atoms began to hold together.

Almost as soon as those atom constructions began, though, the continued expansion cooled everything even more; and by four minutes or so, this fast construction period was largely over. There was energy, and there were the cores of hydrogen and helium, all moving very fast - but that was all.

It is tempting to think of this all spreading out from one point, some sort of gushing tap in the sky, which astronomers might one day locate, like the single vanishing point of Renaissance perspective. But the Big Bang is stranger than that. What was created, at the start, wasn't just the bits of matter that tumble forward through space, out of some suddenly created hole, but also the space itself in which they travel.

This really is odd. Distant galaxies speed away from us, but not because we were each shot out from the Big Bang in a different direction. Rather, the very space stretching between us is steadily opening up, in a rush left over from the initial blast. The concept made no sense before Einstein, as previous thinkers didn't see space as an active substance, capable of a greater or lesser tautening.

As space continued loosening, the first stars ignited, about one billion years after creation. The largest ones built up enormous solid iron cores, which ended up ripping the space at their centres into black holes, so sucking the main body of the star rather uselessly in behind. Less corpulent stars avoided that fate, and often conveniently burst apart outwards, sending the heavy elements that they had squeezed into existence drifting off through space, to end up where one minor planetary system was being formed.

Fast forward 4.5 billion years and the result, each morning, is mobile chunks of carbon atoms (that's us, folks) wading through clouds of star- created oxygen atoms, stirring a caffeine-dense liquid of Big Bang hydrogen atoms, as they read about how they came to exist.

It is a good story, and pleasingly well supported. Particles really have been seen to emerge spontaneously out of "empty" vacuum in the lab; the two-minute interval for the original construction of atom cores matches the amounts of helium left in the oldest stars; and star explosions that create further elements are so frequently photographed that you can call them up on the Web.

Even the future can be peered at. In five billion years, the hydrogen at the core of our sun will be used up, its outer layers igniting and roaring towards Earth, finally returning our carbon atoms or any distant descendants back into space, for another turn in the great galactic recycling machine.

Another 30 billion years, and the galaxies will have increasingly used up their fuel, the night sky everywhere becoming darker, and colder. The fabric of space will continue rushing outward, from the long distant Big Bang, carrying the rock fragments and dead stars ever farther apart in the swirling cold.

Pascal wrote about the terror one could feel, trapped in a universe of infinite silences; isolated from God. I don't know how much he was satisfied with the answers he struggled with, for the very question was only posed once science - by uncovering those vast spaces so exactly - had disturbed, perhaps forever, the religion he sought. But I do wonder what he would have made of the most recent view.

If our universe did begin from a tiny fluctuation in an apparent vacuum, our Big Bang didn't have to be the only one. Universes could be whizzing off from ours all the time, inflating at great velocity, albeit in dimensions we can't see. Ours, in turn, would just have spun off from one of the many universes existing before.

It is not what Pascal would have expected, yet if it is true, we are not alone at all, but rather surrounded, encased, by these infinite bubbles of existence.

Tomorrow, Evolution

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