Groundbreaking physicist Stephen Hawking left us one last shimmering
piece of brilliance before he died: his final paper, detailing his last theory on the origin of the Universe, co-authored with Thomas Hertog from KU Leuven.
The paper, published in the Journal of High Energy Physics in May, puts forward that the Universe is far less complex than current multiverse theories suggest.
It's based around 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, and the energy converted into matter and
radiation.
However, according to the theory of eternal inflation,
some bubbles of space stopped inflating or slowed on a stopping
trajectory, creating a small fractal dead-end of static space.
Meanwhile,
in other bubbles of space, because of quantum effects, inflation never
stops - leading to an infinite number of multiverses.
Everything
we see in our observable Universe, according to this theory, is
contained in just one of these bubbles - in which inflation has stopped,
allowing for the formation of stars and galaxies.
Visualisation of the inflating multiverse. (A. Linde/Stanford University)
"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," Hawking explained.
"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."
Even one of the original architects of the eternal inflation model has disavowed it in recent years.
Paul Steinhardt, physicist at Princeton University, has gone on record
saying that the theory took the problem it was meant to solve - to make
the Universe, well, universally consistent with our observations - and
just shifted it onto a new model.
Hawking and Hertog are now
saying that the eternal inflation model is wrong. This is because
Einstein's theory of general relativity breaks down on quantum scales.
"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," Hertog explained.
"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."
Hawking's last theory is based on string theory,
one of the frameworks that attempts to reconcile general relativity
with quantum theory by replacing the point-like 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 on a lower-dimensional
boundary; so the universe is like a hologram, in which physical reality
in 3D spaces can be mathematically reduced to 2D projections on their
surfaces.
The researchers developed a variation of the holographic
principle that projects the time dimension in eternal inflation, which
allowed 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 spatial surface at the
beginning of the Universe - a hologram of eternal inflation.
"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," said Hertog.
In 1983, Hawking and another researcher, physicist James Hartle, proposed what is known as the 'no boundary theory'
or the 'Hartle-Hawking state'. They proposed that, prior to the Big
Bang, there was space, but no time. So the Universe, when it began,
expanded from a single point, but doesn't have a boundary.
According to the new theory, the early Universe did have a
boundary, and that's 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. So it is not a fractal structure," Hawking said.
It's
a result that doesn't disprove multiverses, but reduces them to a much
smaller range - which means that multiverse theory may be easier to test
in the future, if the work can be replicated and confirmed by other
physicists.
Hertog plans to test it by looking for gravitational waves that could have been generated by eternal inflation.
These waves are too large to be detected by LIGO, but future gravitational wave interferometers such as space-based LISA, and future studies of the cosmic microwave background, may reveal them.
The team's research was published in the Journal of High Energy Physics, and can be read in full on arXiv. Good luck.
