Professor Stephen Hawking's last theory on the origin of the universe, which he developed with KU Leuven's Professor Thomas Hertog, was published in the Journal of High Energy Physics in 2018.
The theory, which was submitted for publication before Hawking's death in 2018, is based on string theory and predicts that the universe is limited and significantly simpler than many current big bang theories claim.
Professor Hertog, whose research is funded by the European Research Council, originally unveiled the new idea at a conference commemorating Professor Hawking's 75th birthday at the University of Cambridge in July 2017.
Modern big bang theories suggest that our local universe arose from a brief burst of inflation — in other words, the world expanded at an exponential pace for a fraction of a second after the big bang. However, it is usually assumed that once inflation begins, it will continue in some areas indefinitely. Quantum effects are supposed to be able to keep inflation going in some parts of the universe indefinitely, resulting in worldwide inflation. Our universe would subsequently be reduced to a hospitable pocket universe, a place where inflation has stopped and stars and galaxies have developed.
In a 2017 interview, Hawking noted, "The typical theory of everlasting inflation predicts that our universe is like an infinite fractal, with a mosaic of distinct pocket universes separated by an inflating ocean." "From one pocket world to the next, the local laws of physics and chemistry might alter, forming a multiverse." The multiverse, on the other hand, has never been a favourite of mine. The theory cannot be tested if the scale of distinct universes in the multiverse is huge or infinite.
Hawking and Hertog argue in their work that this description of everlasting inflation as a hypothesis of the great bang is incorrect. "The difficulty with the traditional view of eternal inflation is that it assumes an existing backdrop universe that evolves according to Einstein's general theory of relativity and interprets quantum phenomena as minor fluctuations around this," Hertog explained. "However, the everlasting inflation dynamics eliminate the distinction between classical and quantum physics." As a result, Einstein's theory collapses, resulting in everlasting inflation."
"On the biggest scales, we expect that our universe is reasonably smooth and globally finite." "It's not a fractal structure," Hawking explained.
String theory is a school of theoretical physics that aims to reconcile gravity and general relativity with quantum physics, in part by characterising the fundamental constituents of the universe as small vibrating strings. Hawking and Hertog's idea of everlasting inflation is based on string theory. Their method is based on the holography notion from string theory, which states that the cosmos is a huge and complicated hologram in which physical reality in particular 3D areas can be reduced to 2D projections on a surface using mathematics.
In everlasting inflation, Hawking and Hertog proposed a form of this holography notion to project out the time dimension. They were able to describe perpetual inflation without relying on Einstein's theory as a result of this. Eternal inflation is reduced in the new theory to a timeless state specified on a spatial surface at the beginning of time.
"If we track the evolution of our universe backwards in time, we eventually arrive at the threshold of everlasting inflation, when our familiar concept of time loses all meaning," Hertog explained.
Hawking's previous 'no boundary theory' predicted that if you travel back in time to the beginning of the universe, the world would shrink and close off like a sphere, but this new hypothesis differs from his previous work. "Now we're saying there's a line in our past," Hertog explained.
Hertog and Hawking used their new theory to make more accurate predictions about the universe's overall structure. They projected that the universe formed by eternal inflation on the past boundary would be small and significantly simpler than the endless fractal structure promised by the old eternal inflation hypothesis.
If their findings are validated by additional research, they will have far-reaching ramifications for the multiverse concept. "We're not down to a single, unique world," Hawking remarked, "but our findings suggest a considerable reduction of the multiverse, to a far smaller spectrum of conceivable universes."
This improves the theory's predictability and testability.
Hertog now intends to investigate the ramifications of the new hypothesis on smaller scales, which are within the capabilities of our space telescopes. The most potential "smoking gun" to test the idea, he says, is primordial gravitational waves — ripples in spacetime — formed at the end of unending inflation. Because our universe has been expanding since the beginning, any gravitational waves would have extremely long wavelengths, far beyond the detection range of the present LIGO detectors. However, they could be detected in future tests detecting the cosmic microwave background by the planned European space-based gravitational wave observatory, LISA.
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