Professor Stephen Hawking passed away on March 14. Before he died, he and his colleague Professor Thomas Hertog worked on a theory about what happens to the universe after the Big Bang. The idea was first presented last year in Cambridge, during the celebration of Hawking’s 75th birthday, and now it has been published in the Journal of High Energy Physics.
This theory takes on a widely discussed idea: eternal inflation. After the Big Bang, the universe experienced for a few instants a period of extremely accelerated expansion dubbed cosmic inflation. In the eternal inflation hypothesis, scientists have suggested that most of the universe will continue to experience this inflation and some areas, like the visible universe, are actually the exceptions to the rule. Hawking and Hertog disagree with the standard approach.
“The problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein’s theory of general relativity and treats the quantum effects as small fluctuations around this,” Professor Hertog, from KU Leuven, said in a statement. “However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein’s theory breaks down in eternal inflation.”
Eternal inflation also creates a multiverse from the Big Bang. All the regions where cosmic inflation stopped become pocket universes that have a fractal volume, a region that is too detailed to be three-dimensional but doesn’t possess the characteristic of a four-dimensional cosmos.
“We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure,” Professor Hawking said in an interview before his death. “The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean.”
“The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can’t be tested.”
Hawking and Hertog approached eternal inflation in a different way. They used string theory, one of the potential ways to reconcile relativity and quantum physics, and they used the holographic principle as well. This states that all the information of our 3D universe can be encoded on a 2D sphere at its boundary.
Armed with those, the scientists were able to tweak eternal inflation, turning it into a timeless state, which creates a small range of possible universes with no complicated fractal multiverse. And on top of that, it might be testable.
Hertog is now looking for the implication of their eternal inflation interpretation on smaller scales. He hopes that our telescopes might be able to see some of the consequences of the theory in the cosmos. Gravitational waves are considered a particularly interesting candidate and they might provide crucial information in the coming decades that confirm or disprove the theory of eternal inflation.