**Professor Stephen Hawking’s theory about the Big Bang, which he worked on in collaboration with Katholieke Universiteit Leuven’s Professor Thomas Hertog, has been published this week in the Journal of High-Energy Physics (arXiv.org preprint).**

Modern theories of the Big Bang predict that our local **Universe** came into existence with a brief burst of inflation — in other words, a tiny fraction of a second after the Big Bang itself, the **Universe** expanded at an exponential rate.

It is widely believed, however, that once inflation starts, there are regions where it never stops. It is thought that quantum effects can keep inflation going forever in some regions of the **Universe** so that globally, inflation is eternal.

The observable part of our **Universe** would then be just a hospitable pocket **universe**, a region in which inflation has ended and stars and galaxies formed.

“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,” Professor Hawking explained in an interview last autumn.

“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.”

In their new paper, Professor Hawking and Professor Hertog say this account of eternal inflation as a theory of the Big Bang is wrong.

“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 said.

“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.”

“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.

The theory of eternal inflation that the team put forward is based on string theory: a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, in part by describing the fundamental constituents of the **Universe** as tiny vibrating strings.

Their approach uses the string theory concept of holography, which postulates that the **Universe** is a large and complex hologram: physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.

Professor Hawking and Professor Hertog developed a variation of this concept of holography to project out the time dimension in eternal inflation. This enabled them to describe eternal inflation without having to rely on Einstein’ theory.

In the new theory, eternal inflation is reduced to a timeless state defined on a spatial surface at the beginning of time.

“When we trace the evolution of our **Universe** backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning,” Professor Hertog said.

Professor **Hawking’s** earlier ‘no boundary theory’ predicted that if you go back in time to the beginning of the **Universe**, the **Universe** shrinks and closes off like a sphere, but this new theory represents a step away from the earlier work.

“Now we’re saying that there is a boundary in our past,” Professor Hertog said.

The physicists used their new theory to derive more reliable predictions about the global structure of the **Universe**.

They predicted the **Universe** that emerges from eternal inflation on the past boundary is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.

Their results, if confirmed by further work, would have far-reaching implications for the multiverse paradigm.

“We are not down to a single, unique **universe**, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes,” Professor Hawking said.

This makes the theory more predictive and testable.

Professor Hertog now plans to study the implications of the new theory on smaller scales that are within reach of our space telescopes.

He believes that primordial gravitational waves generated at the exit from eternal inflation constitute the most promising ‘smoking gun’ to test the model.

The expansion of our **Universe** since the beginning means such gravitational waves would have very long wavelengths, outside the range of the current LIGO detectors. But they might be heard by the planned European space-based gravitational wave observatory, LISA, or seen in future experiments measuring the cosmic microwave background.

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S.W. Hawking & T. Hertog. A smooth exit from eternal inflation? *Journal of High Energy Physics* 2018: 147; doi: 10.1007/JHEP04(2018)147