# A new cosmological theory points to a smaller, simpler, finite universe

Shortly before his death, Stephen Hawking left us his final legacy – a theory developed with physicist Thomas Hertog that tackles the problem of multiple, infinite universes.

Current Big Bang theories predict that it gave rise to not one but many universes, called the multiverse. The common perception of the multiverse is that it’s like an infinite fractal composed of little diverse pocket universes. But the multiverse holds a limitless number of possibilities. And this is where the problem lies. Since anything is possible somewhere in this collection of universes, the theory fails to predict very much about our own universe. In a paper published in the ‘Journal of High Energy Physics’, the late Stephen Hawking and Thomas Hertog from KU Leuven present a model that reduces the boundless multiverse to a more manageable range of possible universes. The paper was submitted for publication a few days before Hawking’s death in March. The physicists’ new model, developed with partial support from the EU-funded HoloQosmos project, deals with a concept called eternal inflation. According to the eternal inflation theory, for a fraction of a second after the Big Bang, space-time must have expanded at an enormous rate. Once begun, this rapid expansion – called inflation – is believed to go on forever. But in some regions inflation stops, forming local, pocket universes with stars and galaxies. According to this theory, everything in our observable universe is contained inside one of these pockets. In their paper, the authors now suggest that the current eternal inflation model of the Big Bang is wrong. This is because it bases the evolution of the existing universe on Einstein’s general relativity theory, which breaks down at the Big Bang. “We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure,” said Hawking in an interview with Cambridge University last year. Hawking and Hertog used string theory as a basis for their new model. Their approach to eternal inflation centres around a string theory principle called holography. According to this principle, the physical reality we perceive in three dimensions can be written on a two-dimensional surface, like a hologram. The researchers developed a variation of the holographic principle to project the time dimension in eternal inflation. This approach allowed them to describe eternal inflation without using general relativity. Instead, they reduced it mathematically to a timeless state on a spatial surface at the beginning of time. Their new theory implies “a much more manageable global structure of the universe, in which regions can differ from each other, but not at all as much as in the old theory of the multiverse,” Hertog explained in an interview given to the European Research Council. “I think the key point about our model is not so much that constant density surfaces in the universe are finite, but rather that the variation in the multiverse is restricted. In other words, that the range of different pocket universes is much smaller. That makes the cosmology based on our new theory a lot more predictive, a lot stronger as a scientific theory and therefore ultimately, we hope, testable.” Hertog’s next step is to test the theory by looking for gravitational waves that may have been generated at the Big Bang. By developing a holographic framework for quantum cosmology, HoloQosmos (Holographic Quantum Cosmology) is striving to revolutionise the current perspective on cosmology. For more information, please see: CORDIS web page

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