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 today in
the Journal of High Energy Physics, 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.

"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."

The new 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 actually began, expanded from a very 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 actually 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 has been published in the Journal of High
Energy Physics, and can be read in full on arXiv. Good luck.