So, job done? Not quite. The problem with the causal patch measure
is that the result depends on the vacuum energy of the universe the
calculation starts with. And such arbitrariness is anathema to
physicists.

A hologram of the multiverse

While Bousso was working on his observer’s-eye view of the
multiverse, cosmologist Alexander Vilenkin of Tufts University in
Boston was formulating another approach to the global picture.
Vilenkin, too, had become dissatisfied with past approaches to
measure making, and had decided there had to be a better way.
Together with Jaume Garriga of the University of Barcelona in Spain,
Vilenkin thought there might be some clues in an earlier
breakthrough made by Argentinean physicist Juan Maldacena at the
Institute for Advanced Study in Princeton.

Maldacena had been working with string theory to build model
universes when he made a startling discovery. He found a model in a
bizarrely shaped universe with five dimensions that is exactly
equivalent to a simpler model on its four-dimensional boundary. This
is a classic example of what is known as the “holographic
principle”, the idea that for a space in any number of dimensions,
all the physics inside that space can be encoded on its outer
boundary in much the same way that a two-dimensional hologram on a
credit card can encode all the information about a 3D object.

Vilenkin and Garriga figured the entire multiverse must similarly
have a holographic image living on its boundary
(arxiv.org/abs/0905.1509). In the case of the multiverse, though,
the boundary is not a frontier in space, but in time, infinitely far
into the future. Could it hold a uniquely defined measure for the
multiverse?

Bousso was intrigued. While he believed his causal patch measure was
more promising, he decided to see what would happen if he tried to
derive a measure for the multiverse by studying its boundary
instead. “I wanted to figure out a straightforward way of
transferring what we had learned from Maldacena to the multiverse,”
he says.

It turns out that zooming in on part of the boundary is equivalent
to selecting different, finite slices of time in the interior of the
multiverse (see diagram). To see how it works, imagine you are
standing in a dark room with your back against one wall and facing
another wall. You switch on a flashlight, which illuminates a large
oval on the far wall. As you walk towards the wall ahead, the
illuminated oval shrinks. The further away you move from the back
wall where you started, the smaller the area of illumination
becomes. In other words, there is a clear relationship between areas
on your future boundary and distance from your starting point. In a
similar way, a particular area on the boundary of the multiverse is
associated with a particular time inside it.

What is so powerful about this approach is that it sidesteps the
problem Einstein raised about time being relative to different
observers. Here the boundary tells you which bubble universes
existed at a particular time. Knowing this, you can start comparing
universes and calculating the probability of finding one with a
particular value of dark energy, for instance.

As Bousso studied this measure, something astonishing came into
focus. The global measure he had discovered using the holographic
representation of the multiverse and its future boundary turns out
to be exactly equivalent to the causal patch measure he had already
derived by simply considering what a single observer can see. The
two dramatically different approaches turned out to be two different
ways of looking at the same underlying reality: one considers an
ensemble of possible histories for a single observer; the other, the
entire infinite history of an infinite number of disconnected bubble
universes.

“That was really stunning,” says Bousso. “It was amazing to me when
I realised that the two measures reproduce the exact same
probabilities.”

Their equivalence turns out to be extremely useful, as weaknesses in
one measure are strengths in the other, and vice versa. “They are
like two people on crutches holding one another up,” Bousso says.

So while in the causal patch measure your answers depend strongly on
the universe in which your observers start out, the global measure
does not suffer from this ambiguity. In the multiverse, bubbles
beget bubbles beget bubbles, so that initial conditions are quickly
lost in the crowd and no longer matter when it comes to calculating
probabilities. In fact the global picture actually defines what the
starting vacuum for the causal patch approach should be.

On the other hand, while the global picture suffers from the problem
of “duplicate information” (see “What black holes can teach us”),
Bousso’s causal patch measure successfully circumvents this.

The implications might be immense. The two equivalent measures have
not only provided a prediction for dark energy in our own universe
that closely matches observations, they were both inspired in
different ways by the holographic principle. This suggests that the
holographic principle is profoundly significant, and could lead us
to a theory of quantum gravity–the long-sought theory of
everything that mirrors the dynamics of the multiverse. “By thinking
about the measure problem, we seem to be learning, perhaps
unexpectedly, about another, equally deep mystery, namely how to
formulate the quantum gravity theory of the multiverse,” says
Bousso.

Even Ellis is impressed by Bousso’s results, if not exactly sold on
the multiverse. “It is a useful and intriguing kind of consistency
test based in fascinating but speculative physics,” he says. And
there is another far-reaching consequence. If Bousso’s equivalence
holds, then not only can the resulting measure be used to make real,
testable predictions, they can also make calculations in the
multiverse without ever referring to unobservable universes lurking
beyond our cosmic horizon. Everything we need to know about the
multiverse might be right here in our own universe.

What black holes can teach us

When Stephen Hawking calculated that black holes radiate away energy
and eventually evaporate, he left a nagging question: what happens
to the information about all the stuff that has fallen in? If it
escaped back into the universe, it would have to be travelling
faster than the speed of light, violating Einstein’s theory of
relativity. If it vanished from the universe, it would be violating
a fundamental tenet of quantum mechanics. This conundrum became
known as the black hole information loss paradox (New Scientist, 28
October 2006, p 36).

The answer comes from the idea known as the holographic principle,
which says that the physics inside a region of space-time is
equivalent to the physics on the region’s boundary. You can think of
a black hole as equivalent to a hot gas of ordinary particles on the
boundary of the universe. And since a hot gas of ordinary particles
never loses information, neither can a black hole.

The lesson from the holographic picture is that no observer should
ever see information disappear from the universe. If Alice is
watching from a distance as an elephant falls into a black hole, she
will see it approach the black hole’s event horizon, at which point
it is incinerated by the Hawking radiation, which sends it streaming
back towards her as a sad, scrambled heap of ashes. Meanwhile, Bob,
who falls into the black hole along with the elephant, sees the
elephant cross the horizon safely, and live happily for some time
before hitting the singularity in the black hole’s core.

According to the holographic principle, both stories must be true.
But how can the elephant be in a heap of ashes outside the horizon
and alive and well inside the black hole? It would seem the elephant
has been cloned, but the laws of physics prohibit such duplication
of information.

Cosmologist Raphael Bousso explains the paradox results from the
mistaken idea that we can describe what’s happening both inside and
outside the horizon simultaneously, when in reality no single
observer can ever see both at once. In other words, for physics to
make sense, you must restrict your description of the universe to
what a single observer can see. It’s a profoundly different approach
from the old idea that we can describe the entire universe from an
observerless, God’s-eye-view.

Talking about the multiverse as if it can all be directly observed
at once, Bousso says, leads to an even greater nonsense than trying
to simultaneously describe what’s happening inside and outside a
black hole horizon.

Amanda Gefter is an editor in New Scientist’s Opinion section, based
in Boston