The late professor Stephen Hawkings final theory on the origin of the universe has been published – and it turns existing theories about the origins of the universe on their head.
Thousands lined Cambridge's streets to bid farewell to the world renowned physicist who died on March 14 aged 76 – but his work lives on.
His latest theoretical work, a collaboration with Professor Thomas Hertog from KU Leuven, has been published today (May 3) in the Journal of High Energy Physics .
It is based on string theory and turns science on its head, predicting the universe is finite, rather than infinite as previously suggested, and far simpler than many current theories about the big bang say.
Professor Hertog, whose work has been supported by the European Research Council, first announced the new theory at a conference at Cambridge University in July of last year, organised on the occasion of Professor Hawkings 75 birthday.
The theory was submitted for publication before Hawkings death earlier this year.
What the new theory means
Cambridge University have explained the meaning of the new theory.
It has explained how 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,” said Hawking 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 cant be tested. ”
In their new paper, Hawking and 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 Einsteins theory of general relativity and treats the quantum effects as small fluctuations around this,” said Hertog.
“However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einsteins 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,” said Hawking.
The theory of eternal inflation that Hawking and Hertog 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.
Hawking and 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 Einsteins 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”, said Hertog.
Hawkings 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 were saying that there is a boundary in our past,” said Hertog.
Hertog and Hawking 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.
Stephen Hawking factfile
- Born 8 January 1942 in Oxford
- Won a place at Oxford University to read natural science in 1959, before studying for his PhD at Cambridge
- By 1963, was diagnosed with motor neurone disease and given two years to live
- Outlined his theory that black holes emit "Hawking radiation" in 1974
- Published his book A Brief History of Time in 1988, which has sold more than 10 million copies
- His life story was the subject of the 2014 film The Theory of Everything, starring Eddie Redmayne
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,” said Hawking.
This makes the theory more predictive and testable.
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 – ripples in spacetime – generated at the exit from eternal inflation constitute the most promising “smoking gun” to test the model.
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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.