Stephen Hawking's final work suggests the universe is simpler than we thought
Cambridge thereotical physicist and Prof Thomas Hertog reduce the multiverse and say there is boundary in our past
Professor Stephen Hawking’s final work has been published – and it suggests that the universe is finite and far simpler than many current models suggest.
The Cambridge theoretical physicist worked in collaboration with Prof Thomas Hertog from KU Leuven on the theory.
Submitted before Prof Hawking’s death on March 14 and now published in the Journal of High Energy Physics, it uses string theory to explain the nature of our universe.
Modern theories suggest our local universe came into existence with a brief burst of inflation – meaning that a tiny fraction of a second after the Big Bang, the universe expanded at an exponential rate. It is widely believed that there are regions where this inflation never stops due to quantum effects. This means that globally, inflation is eternal and the observable part of our universe is like a bubble or pocket – a region in which inflation has ended and stars and galaxies formed.
In an interview last autumn, Prof Hawking said: “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.”
Prof Hertog announced the new theory at a conference in Cambridge last year to mark Prof Hawking’s 75th birthday.
It says the 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,” said Prof Hertog. “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.”
Prof Hawking added: “We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure.”
Their alternative is based on string theory – a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, partly by describing the fundamental constituents of the universe as tiny vibrating strings.
They employ the string theory concept of holography, which suggests that the universe is a large, complex hologram – meaning physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.
Prof Hawking and Prof Hertog’s variation of this concept of holography projects out the time dimension in eternal inflation. This enables them to describe eternal inflation without relying on Einstein’s theory. They reduce eternal inflation to a timeless state defined on a spatial surface at the beginning of time.
Prof Hertog said: “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.”
In 1983 Prof Hawking’s famous ‘no boundary theory’ with physicist James Hartle predicted that if you go back to the beginning of the universe, the universe shrinks and closes off like a sphere. The new theory changes this.
“Now we’re saying that there is a boundary in our past,” said Prof Hertog.
They used their new theory to develop more reliable predictions about the global structure of the universe. They predicted that the universe emerging 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. If confirmed, their theory 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 Prof Hawking.
This means the concept is more testable and Prof Hertog now plans to study its implications on smaller scales within reach of space telescopes.
He believes that primordial gravitational waves – ripples in spacetime – generated at the exit from eternal inflation constitute the most promising lead to test the model.
Due to the expansion of our universe, these gravitational waves have very long wavelengths, outside the range of current LIGO detectors. But they might be detected by the planned European space-based gravitational wave observatory, LISA, or seen in future experiments measuring the cosmic microwave background.
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