Do we live in a Multiverse?Big Ideas — POSTED BY David Luke on April 12, 2010 at 12:11 pm
WHEN cosmologist George Ellis turned 70 last year, his friends held
a party to celebrate. There were speeches and drinks and canapés
aplenty to honour the theorist from the University of Cape Town,
South Africa, who is regarded as one of the world’s leading experts
on general relativity. But there the similarity to most parties
By Amanda Gefter for the NewScientist
For a start, Ellis’s celebration at the University of Oxford lasted
for three days and the guest list was made up entirely of
physicists, astronomers and philosophers of science. They had
gathered to debate what Ellis considers the most dangerous idea in
science: the suggestion that our universe is but a tiny part of an
unimaginably large and diverse multiverse.
To the dismay of Ellis and many of his colleagues, the multiverse
has developed rapidly from being merely a speculative idea to a
theory verging on respectability. There are good reasons why.
Several strands of theoretical physics–quantum mechanics, string
theory and cosmic inflation–seem to converge on the idea that our
universe is only one among an infinite and ever-growing assemblage
of disconnected bubble universes.
What’s more, the multiverse offers a plausible answer to what has
become an infuriatingly slippery question: why does the quantity of
dark energy in the universe have the extraordinarily unlikely value
that it does? No theory of our universe has been able to explain it.
But if there are countless universes out there beyond our cosmic
horizon, each with its own value for the quantity of dark energy it
contains, the value we observe becomes not just probable but
Despite the many virtues of the multiverse, Ellis is far from alone
in finding it a dangerous idea. The main cause for alarm is the fact
that it postulates the existence of a multitude of unobservable
universes, making the whole idea untestable. If something as
fundamental as this is untestable, says Ellis, the foundations of
science itself are undermined.
One of the guests at Ellis’s party doesn’t see it that way. Raphael
Bousso of the University of California, Berkeley, has also been
grappling with the multiverse, and in the past few months he has
found a way round the troubling problem of unobservable universes.
At a stroke, he has transformed the multiverse from a theory so
problematic that it threatens to subvert science, into one that
promises predictions we can test. His insights are steering
physicists along the path to their ultimate goal of uniting quantum
mechanics and gravity into one neat theory of everything.
Bousso’s achievement is all the more impressive because he has
succeed where so many others have tried and failed. The problem they
all encountered boils down to this: like quantum mechanics and
thermodynamics, multiverse cosmology is an exercise in statistics.
Given a universe within the multiverse, you cannot predict what its
key characteristics will be–how much dark energy it contains, say.
The best you can do is calculate the probability that it looks the
way it does based on how likely it is that a universe with its
particular set of characteristics will occur in the multiverse.
Calculating probabilities, though, requires a “measure”–a
mathematical tool that tells you how to define relative
probabilities. And finding the right measure for the multiverse is
far from easy.
The trouble is that in an infinite multiverse, everything that can
happen will happen–an infinite number of times. In such a set-up,
probability loses all meaning. “How do you compare infinities?” asks
Andrei Linde of Stanford University in California.
Prior to Bousso’s work, the favoured approach was to pick a snapshot
of the multiverse at a particular time and calculate the
characteristics of all the bubble universes inside, noting how many
different values for the amount of dark energy crop up. From there,
you extrapolate the relative probabilities to the multiverse as it
develops over time with its infinite number of bubble universes.
Unfortunately, there’s a nasty hole in this approach, in the shape
of the phrase “at some particular time”: according to Einstein’s
theory of relativity, it renders the whole exercise utterly
meaningless. The problem arises from Einstein’s insight that clocks
run differently for different observers. Two events that are
simultaneous for me are not simultaneous for you, so there are an
infinite number of ways you can slice up the multiverse. None is
more “true” than any other, so there’s no reason to choose one time
slicing over another–and different slices can yield dramatically
Implicit in previous approaches was the idea that the multiverse can
be described from an observerless, God’s-eye-view, and Bousso
realised that this was what lead to all those intractable
infinities. So he decided to calculate probabilities based on what
any one observer can see from within their own universe.
Quantum mechanics tells us that the vacuum of space is not empty;
instead, it crackles with energy. It also tells us that, sooner or
later, any given universe will decay spontaneously into another one
with lower energy. Indeed, most cosmologists envisage our big bang
as precisely such an event, during which the vacuum we live in
emerged from a higher-energy vacuum that constituted a universe
before ours. What matters here, though, is that there are a plethora
of possible universes that can be produced in this way–each one
with its own probability. By adding up these probabilities, Bousso
was able to work out the various probabilities of the observer
ending up in a universe with a particular set of characteristics.
Using this approach, Bousso was able to derive probabilities for
things like the amount of dark energy in any particular universe,
without ever have to resort to a God’s-eye point of view, or
speculation about what might be happening in disconnected bubble
universes beyond our view. He calls this approach the causal patch
measure, and the important thing is that it works. He has used it to
predict the value of the dark energy we ought to see in our own
universe, and it turns out to be remarkably close to the observed
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
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
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,”
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
“That was really stunning,” says Bousso. “It was amazing to me when
I realised that the two measures reproduce the exact same
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
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
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
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
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