The deceased physicist Stephen Hawking made a great contribution to cosmology during his lifetime, but failed to solve all the secrets of the universe. Now one of the last papers he has ever worked on has been published – and it brings some new ideas about the size and shape of the cosmos. But what does research say and how important are the results?
Our conventional image of cosmology is that our universe began with a big bang and then had a period of very rapid expansion, known as inflation. This created the seeds of the large, smooth (ish) universe in which we live today. The problem is that inflation has a problem with stopping.
This is because inflation in the early Universe was actually determined by the strange rules of quantum mechanics that led to unexpected behaviors. For example, quantum mechanics allows particles to come into existence from the vacuum at random and then disappear again, a property known as quantum fluctuation. In the early universe, quantum fluctuations could have caused some parts of the universe to suddenly inflate faster than others.
This means that most of the explicit descriptions of inflation have the characteristic that inflation may end in some regions, it continues in others, so that the universe as a whole "blooms forever". That is, we should live in a tiny pocket of a "multiverse" of parallel universes in which inflation has ceased and stars have formed. However, the entire multiverse constantly creates new and bloating universes that grow like a fractal.
Such a universe would be very uneven – and that's the aspect Hawking's paper looks at. If we thought of the universe as a balloon, it would be even inflation if the balloon were inflated quickly. But the perpetual inflation would look like a small spot on the balloon would suddenly cease to be elastic and would not expand. As the rest of the balloon is still expanding, this would result in a very spiky surface overall.
The theories of eternal inflation and the multiverse are powerful, as they can quite conveniently explain the existence of conscious beings like us, when coupled with the "anthropic principle." Essentially, this means that the universe, to exist, must be fairly close to the universe. For example – if planets do not form, then it's hard to see how life might exist, or at least parody Star Trek – life as we know it.
If there is only one universe, it would be very unlikely that it would become just like our own. However, if there are an infinite number of universes, it makes sense that at least one of them would contain life.
Taming the Infinity
But is that enough of an explanation of the cosmos? After all, we have never seen one of these infinite parallel universes. In the new work, published in the Journal of High Energy Physics, Hawking and his colleague Thomas Herzog from the Belgian University of KU Leuven use a toy model for our universe to study its structure.
They use a technique known as holography – which mathematically reduces three-dimensional spaces to two-dimensional projections on a surface – to try to calculate what the universe looks like. Hiding a whole dimension makes it much easier to do such modeling. Considering the possibility of random quantum fluctuations, they consider the likelihood that the multiverse is like a spiky balloon while being smoother in its shape and that the universe prefers to be smooth. This suggests that perpetual inflation may not be the preferred outcome that the scientists originally thought.
Instead, the duo argues that the multiverse is not infinite, though there is likely to be more than one universe. But the existence of a smaller number of possible parallel universes is preferable to an infinite number – it means that we could try to pinpoint what and where they are. We could also investigate if they left any traces of the radiation left over from the Big Bang, which makes the theory much easier to test.
While this is just a toy model that directly addresses the notion of perpetual inflation within a quantum, the cosmological environment is new and intriguing. Whether Hertog can now express these ideas in a prediction for the search for signs of other universes remains to be seen.
Although this is Hawking's latest release, at the end of his life he also worked on many other theories. So he tried together with the cosmologists Malcolm Perry from the University of Cambridge and Andy Strominger from Harvard University to find out if physical information in a black hole could permanently disappear – a question that have raised his previous research. Further work from this collaboration will be released, so we have not heard the last of Hawking yet.