# Stephen Hawking's Final Theory About Our Universe Will Melt Your Brain

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.
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