The study, developed by Hawking months before he died, was published posthumously in the Journal of High Energy Physics. In it, the innovative physicist brilliantly details his latest theory on the origin of the universe, co-authored with Thomas Hertog of the KU Leuven University in Leuven, (Belgium).
He argues that the universe is much less complex than current multiverse theories suggest, as it is based on a concept called eternal inflation, first introduced in 1979 and published in 1981.
After the Big Bang, the universe experienced a period of exponential inflation; then it slowed down in speed and energy and turned into matter and radiation.
However, according to the eternal inflation theory, some regions of space stopped igniting or slowed down on a stopping trajectory , creating a small fractal stalemate of static space.
Meanwhile, in other bubbles in space, due to quantum effects, inflation never stops, leading to an infinite number of multiverses . Thus, everything we see in our observable universe, according to this theory, is contained in one of these bubbles, in which inflation has stopped, allowing the formation of stars and galaxies.
“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 inflated ocean,” explained Hawking. “If the scale of the different universes in the multiverse is large or infinite, the theory cannot be proven.”
According to this work by Hawking and Hertog, the eternal inflation model is incorrect. This is because Einstein’s theory of general relativity breaks down on quantum scales.
“The problem with the usual explanation for eternal inflation is that it assumes an existing background universe that evolves according to Einstein’s theory of general relativity and treats quantum effects as small fluctuations around this,” Hertog explained. “However, the dynamics of eternal inflation blurs the separation between classical and quantum physics. As a consequence, Einstein’s theory breaks down into eternal inflation.”
The new theory is based on string theory, one of the frameworks that attempts to reconcile general relativity with quantum theory by replacing point particles in particle physics with tiny vibrating one-dimensional strings.
In string theory, the holographic principle proposes that a volume of space can be described in a lower dimensional boundary; so the universe is like a hologram, in which physical reality in three-dimensional spaces can be mathematically reduced to 2D projections on its surfaces.
The researchers developed a variation of the holographic principle that projects the dimension of time into eternal inflation, allowing them to describe the concept without having to rely on general relativity.
This then allowed them to mathematically reduce eternal inflation to a timeless state on a space surface at the beginning of the universe, a hologram of eternal inflation.
“When we trace the evolution of our universe back in time, at some point we hit the threshold of eternal inflation, where our familiar notion of time ceases to have meaning ,” Hertog said.
According to the new theory, the early universe had a limit, which allowed Hawking and Hertog to derive more reliable predictions about the structure of the universe.
“We predict that our universe, on the largest scales, is reasonably smooth and globally finite. Therefore, it is not a fractal structure,” Hawking said.
It is a result that does not disprove multiverses, but rather reduces them to a much smaller range, meaning that multiverse theory may be easier to test in the future , if the work can be replicated and confirmed by other physicists.
Reference: SW Hawking et al. A smooth exit from eternal inflation ?, Journal of High Energy Physics (2018). DOI: 10.1007 / JHEP04 (2018) 147, https://arxiv.org/abs/1707.07702
