This is how the very early Universe (less than 1 thousand million years old) might have looked as it went through a voracious onset of star formation, converting primordial hydrogen into myriad stars at an unprecedented rate. The sky is ablaze with primeval starburst galaxies. The most massive stars self-detonate as supernovas, exploding across the sky like firecrackers. There is very little dust in these galaxies, since heavier elements have not yet been made through nucleosynthesis in stars. Image credit: A. Schaller (STScI)

In recent years, the search for the fundamental laws of Nature has forced us to think about the Big Bang much more deeply. According to our best theories – string theory and M theory – all of the details of the laws of physics are actually determined by the structure of the Universe; specifically, by the arrangement of tiny, curled-up extra dimensions of space.

This is a very beautiful picture: particle physics itself is now just another aspect of cosmology. But if you want to understand why the extra dimensions are arranged as they are, you have to understand the Big Bang, because that’s where everything came from.

Somehow, until quite recently, fundamental physics had gotten along without really tackling that problem. Even back in the 1920s, Einstein, Friedmann and Lemaitre – the founders of modern cosmology – realised there was a singularity at the Big Bang. That somehow, when you trace the Universe back, everything went wrong about 14 billion years ago. By go wrong, I mean all the laws of physics break down: they give infinities and meaningless results.

Einstein himself didn’t interpret this as the beginning of time; he just said, well, my theory fails. Most theories fail in some regime, and then you need a better theory. Isaac Newton’s theory fails when particles go very fast; it fails to describe that. You need relativity. Likewise, Einstein said, we need a better theory of gravity than mine.

But in the 1960s, when the observational evidence for the Big Bang became very strong, physicists somehow leapt to the conclusion that it must have been the beginning of time. I am not sure why they did so, but perhaps it was due to Fred Hoyle – the main proponent of the rival steady-state theory – who seems to have successfully ridiculed the Big Bang theory by saying it did not make sense because it implied a beginning of time, and that sounded nonsensical.

Then the Big Bang was confirmed by observation. And I think everyone just bought Hoyle’s argument and said, oh well, the Big Bang is true, okay, so time must have begun. So we slipped into this way of thinking: that somehow time began and that the process, or event, whereby it began is not describable by physics. That’s very sad. Everything we see around us rests completely on that event, and yet that is the event we can’t describe. That’s basically where things stood in cosmology, and people just worried about other questions for the next 20 years.

And then in the 1980s, there was a merging of particle physics and cosmology, when the theory of inflation was invented. Inflationary theory also didn’t deal with the beginning of the Universe, but it took us back further towards it. People said, let’s just assume the Universe began, somehow. But, we’re going to assume that when it began, it was full of a weird sort of energy called inflationary energy.

This energy is repulsive – its gravitational field is not attractive, like ordinary matter – and the main property of that energy is that it causes the Universe to expand, hugely fast. Literally like dynamite, it blows up the Universe.

This inflationary theory became very popular. It made some predictions about the Universe, and recent observations are very much in line with them. The type of predictions it made are rather simple and qualitative descriptions of certain features of the Universe: it’s very smooth and flat on large scales, and it has some density variations of a very simple character.

Inflationary theory predicts that the density variations are like random noise – something like the ripples on the surface of the sea – and fractional variation in the density is roughly the same on all length scales. And these predictions of inflation have been broadly confirmed by observation. So people have become very attracted to inflation and many people think it’s correct. But inflationary theory never really dealt with the beginning of the Universe. We just had to assume the Universe started out full of inflationary energy. That was never explained.

My own work in this subject started about ten years ago, when I moved to Cambridge from Princeton. There I met Stephen Hawking, who, with James Hartle, developed a theory about how the Universe can begin. So I started to work with Stephen, to do calculations to figure out what this theory actually predicted. Unfortunately, we quickly reached the conclusion that the theory predicted an empty Universe. Indeed, this is perhaps not so surprising: if you start with nothing, it makes more sense that you’d get an empty Universe rather than a full one. I’m being facetious, of course, but when you go through the detailed math, Hawking’s theory seems to predict an empty Universe, not a full one.