Sky and Telescope - August 2018 - 12

NEWS NOTES

Hawking Takes on the
Multiverse
STEPHEN HAWKING'S LAST cosmol-

ogy paper, published posthumously
with Thomas Hertog (KU Leuven,
Belgium) in the Journal of High Energy
Physics, might solve the cosmological
problem of eternal inflation.
A lot of evidence suggests that, just
a tiny fraction of a second after the Big
Bang, the infant universe experienced
inflation, a brief, fantastic growth
spurt. Our pocket of space - what we
call the observable universe - experienced a quantum fluctuation and
spontaneously exited inflation. But
according to most theories, inflation
wouldn't have stopped everywhere at
once. Beyond what we can observe,
most of space will keep stretching at
an exponential rate forever.
Eternal inflation implies that our
cosmos is one of an infi nite variety of
pocket universes, known as the multiverse. Each one follows an infi nite
variety of physical laws. But an infi nite
multiverse hypothesis may pose a problem: Hawking argued it isn't testable.
The problem, Hawking and Hertog
write, could stem from the fact that
previous theories have treated quantum effects as fluctuations in a universe dominated by relativity. Instead,
the scientists argue, relativity should
break down in the early universe.
To show this, they fi rst simplified
the math, formulating a universe without matter or energy and with an overall saddle-shaped geometry. We don't
live in such a universe, but it's a common starting point for new cosmological theories. As is often done in string
theory, they then treat this universe as
a hologram: 3D equations reduce to 2D
quantities on a surface. (This doesn't
mean our universe is a hologram, just
that we can mathematically treat it like
one.) Rather than reducing a spatial
dimension, though, Hawking and
Hertog removed time. Relativity is no
longer necessary in a timeless universe;
quantum theory reigns.
12

AUGUST 2 018 * SK Y & TELESCOPE

When Hawking and Hertog solved
the equations that govern this universe, they came to an astounding
conclusion: The universe that emerges
from inflation is finite. We still live in
a multiverse, but now it's one with limited possibilities, which makes it more
easily testable.
However, Alexander Vilenkin (Tufts
University) urges caution in inter-

preting these results, noting that the
simplified mathematical treatment,
and the conclusions Hawking and
Hertog derive from it, might not apply
to our real, more complicated universe.
"I am sure Thomas Hertog will try to
go beyond this model," Vilenkin adds,
"but it is hard to tell how successful
this is going to be."
■ MONICA YOUNG

This view of
the rock slab
dubbed Old
Soaker spans
1.2 m (4 ft).

SOLAR SYSTEM

Mud Cracks on Mars Reveal Lake's History
NASA'S CURIOSITY ROVER spotted polygonal shapes on the surface of

rocks on Mars in January 2017. Now, analysis confirms that these are
cracks in dried mud, which shed light on the history of the lake that once
filled Gale Crater.
Nathaniel Stein (Caltech) and colleagues found that the cracks formed
during fluctuations in the water level of a lake that existed there about
3.5 billion years ago. The results appear in Geology.
Curiosity landed on the northern side of the crater in 2012 and has
slowly been making its way south. Close to the crater's center, it took pictures of a rock slab nicknamed Old Soaker, as well as a similar nearby slab
dubbed Squid Cove. A polygonal network of cracks in the red mudstone on
these slabs' surfaces looks like what you'd expect from wet sediment that
contracted as it dried.
The cracks are filled with sediment that has a chemical composition
matching the underlying rock. Most likely, the cracks were filled in shortly
after they formed as the lake level rose again. Sediment suspended in the
water would have slowly settled into the cracks.
"These cycles of rise and fall likely happened many times before the
region finally dried out for good," Stein says.
The history of Martian water will continue to unfold as the rover
encounters ever-younger material on its way up the Murray Formation, the
mountainous region in the crater's center.
■ JULIE FREYDLIN

N ASA / JPL- CA LTECH / MSSS

COSMOLOGY



Sky and Telescope - August 2018

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