His final word about whether multiverses exist.
The groundbreaking physicist Stephen Hawking left us one last shimmering piece of brilliance before he passed away: his final paper which is detailing his last theory on the origin of the Universe, co-authored with Thomas Hertog from KU Leuven.
The paper, which was published on the 2nd of May in the Journal of High Energy Physics, claims that the Universe is far less complex than current multiverse theories suggest.
The paper is based on a concept which is called eternal inflation, which was first introduced in 1979 and published in 1981.
After the Big Bang, our Universe has undergone a period of exponential inflation. After that, it slowed down, and the energy transformed into matter and radiation.
But, according to the theory of eternal inflation, some bubbles of the space stopped inflating or slowed on a stopping trajectory, in that way creating a small fractal dead-end of static space.
For the moment, in other bubbles of space, as of quantum effects, inflation never stops, leading to some infinite number of multiverses.
According to this theory, everything that we see in our observable Universe is contained in just one of these bubbles – in which inflation has stopped, permitting for the formation of stars and galaxies.
The usual theory of eternal inflation actually predicts that globally our Universe is like an endless fractal, with a mosaic of various pocket universes which are separated by an inflating ocean. The local laws of physics and chemistry can differ from one pocket universe to another, and together they would form a multiverse. But, I have never really liked the multiverse. If the scale of different universes in the multiverse is large or infinite, the theory cannot be tested.
Even one of the original architects of the eternal inflation figure has denied it in the recent years.
The physicist at Princeton University, Paul Steinhardt, has gone on record saying that the theory took the problem that it was meant to solve – in order to make the Universe universally consistent with our observations – and just shifted it onto a new model.
Stephen Hawking and Thomas Hertog are now saying that the eternal inflation model is something wrong. This is because the theory of general relativity of Einstein breaks down on quantum scales.
The issue with the usual account of eternal inflation is that it assumes an existing background universe which evolves according to the theory of general relativity of Einstein and treats the quantum effects as small fluctuations around this. However, the dynamics of eternal inflation wipes out the separation that is between classical and quantum physics. As a consequence of that, the theory of Einstein breaks down in eternal inflation.
The new theory is actually based on string theory, one of the frameworks which attempt to regulate general relativity with quantum theory by replacing the point-like particles in particle physics with tiny, as well as vibrating one-dimensional strings.
In string theory, the holographic principle actually proposes that a volume of space can be described on a lower-dimensional boundary so that the Universe is like a hologram in which physical reality in 3D spaces can be mathematically decreased to 2D projections on their surfaces.
The research has also developed a variation of the holographic principle which projects the time dimension in the eternal inflation, which permitted them to describe the concept without having to rely on general relativity.
Then, this permitted them to mathematically reduce eternal inflation to a timeless state on a spatial surface at the start of the Universe – a hologram of eternal inflation.
When we trace the evolution of our universe backward 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, Stephen Hawking and one other researcher, the physicist named James Hartle, proposed what is known as the ‘no boundary theory’ or the ‘Hartle-Hawking state.’ They had proposed that before the Big Bang there was space, but there was no time. So the Universe, when it starts, expanded from a single point, but does not have a boundary.
According to that new theory, the early Universe did have some boundaries, and that has permitted Hawking and Hertog to derive more reliable predictions about the structure of the Universe.
We predict that the universe, on the largest scales is reasonably smooth and globally finite. So, it is not a fractal structure.
It is a result which does not deny multiverses but reduces them to a much tinier range – which actually means that multiverse theory may be easier to test in the future if the work can be substituted and confirmed by some other physicists.
Hertog also plans to test it by looking for gravitational waves which could have been caused by eternal inflation.
These waves are too big to be detected by LIGO, but future gravitational wave interferometers like space-based LISA, and future studies of the universal microwave background, may also reveal them.
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