Many visitors to will be familiar with the timeline of the Big Bang, the idea that our universe started from some kind of ‘gigantic explosion’ some 13.6 billion years ago. But this is not the version of reality accepted by most cosmologists: their story is far, far stranger.

In the beginning (and probably way before 13.6 billion years ago) there was no space, time, matter or energy. All that existed was a totally empty geometry containing no points whatsoever – a particular solution to Einstein’s field equations. Quantum principles still applied though and the totally-empty universe was able to tunnel to a peculiar metastable state called the false vacuum.

At this point the new universe (at c. 10-52 meters) was 17 orders of magnitude smaller than the Planck length .

The false vacuum was permeated with a field called the inflaton field with a strange property: while in its metastable state, before it decays away, its energy density stays constant as the universe expands. Imagine expanding our ultra-tiny new universe by hand to double its size, then at constant energy density the energy doubles too. This is energy you have to put in as you stretch our universe against its resistance. This shows that the pressure of the false vacuum is negative, it’s a suction.

At first sight, this suction would tend to make the new universe do anything but expand – but there’s a catch. In Einstein’s theory gravitational effects are caused by pressure as well as mass-energy, and when the pressure is negative, gravity turns repulsive. The net effect is that the baby universe expands exponentially with a doubling time of around 10-37 seconds. This creates an enormous increase in size: after perhaps 100 doublings taking just 10-35 seconds the universe has grown to a meter in size. This effect is called inflation and the huge increase in inflaton field energy is exactly counterbalanced by the corresponding and negative gravitational potential energy. In fact the total energy of the universe has always been precisely zero.

What stops the inflationary process in its tracks is the decay of the inflaton field. Recall that we started with a metastable vacuum state, the false vacuum. Like a radioactive substance, the false vacuum has a half-life, somewhere between 10-30 to 10-35 seconds. Once the inflaton field has completed its decay at a particular location, the false vacuum is replaced by the true vacuum, the one we live in today. What happened to that enormous energy density, spread throughout the inflating universe? It thermalizes, at 10-12 seconds turning into a quark-gluon plasma of the kind briefly generated in the Large Hadron Collider today. From this point on, the ultra-hot early universe continues expanding according to the Big Bang theory you’re already familiar with.

But there’s something else. Turn your attention back to the original vanishingly-small, subatomic universe, lately emerged from a random quantum tunneling event and permeated with the false vacuum. At each point in this tiny, but violently expanding volume there is some probability of the inflaton field decaying. As it does so it continues to expand of course – that’s inflation – but the inflation quickly comes to an end in a burst of “normal” particles which form our own universe. But what about those regions of field which with low probability didn’t get round to decaying yet? There is nothing to stop them expanding exponentially so of course they will. Exponential expansion is so powerful that the overall universe expands with unimaginable speed faster than the inflaton field can decay away: a phenomenon called eternal inflation.

The universe we see around us has an optical boundary of around 13 billion light years but due to the continuing expansion of our universe those remotest galaxies are now around 46 billion light years away. However, the universe which resulted from our local inflation decay event is not restricted to the parts we can see. Inflation-pioneer Alan Guth calls the result of a local inflation field decay event a ‘pocket universe’; it’s the one which started out within that ‘meter-diameter’ volume as a region of the inflaton field there decayed away. In fact our own pocket universe today is most likely to be hundreds of times bigger than the part of it we can observe.

So the picture of our universe in modern inflationary cosmology is like raisins in a cake. Surrounding our own pocket universe (think of it as a raisin) there is a region of the ‘cake’ where the false vacuum has still not decayed and is doubling in size every 10-37 of a second even while parts of it begin to collapse to form new pocket universes. Further out there are other pocket universes like our own, which cohered from the false vacuum at an earlier time. In the overall universe, inflation will never stop, and almost all the pocket universes ‘out there’ have only just formed.

This is perhaps the strangest view of the origins and nature of our own universe. But despite all the popular (although obsolete) discussion about “The Big Bang”, the inflationary universe described here is the one which best matches what we observe and which most cosmologists now accept.


Some physicists have suggested that each of the c. 10500 members of the string theory ‘landscape’ is realized in one pocket universe or another. This means that each pocket universe emerges with its own false vacuum variant and its own variant of the laws of physics. If this is the case, it’s purely blind accident that the specific laws of physics are as we find them in our own (pocket) universe – they’re merely constrained to be consistent with intelligent life, the anthropic principle.

The pocket universes, each with their own laws of physics, are ‘quarantined’ by the ever-growing zones of uncollapsed inflation between them.

Further Reading

1. Eternal inflation and its implications, (PDF), Alan Guth, 2007.

2. The Inflationary Universe, Alan Guth, (1998).

3. A Universe from Nothing: Why There Is Something Rather Than Nothing, Lawrence Krauss, (will be published in Jan 2012).