# Do we live in a Multiverse?

Big Ideas — POSTED BY David Luke on April 12, 2010 at 12:11 pmWHEN 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

ends.

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

inevitable.

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.

## Comparing infinities

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

different results.

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

value (arxiv.org/abs/hep-th/0702115)

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

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