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Nature 443, 145-146 (14 September 2006) | doi:10.1038/443145a; Published online 13 September 2006

Our place in the Multiverse

Joseph Silk1

Was the appearance of a Universe that can support life inevitable?

BOOK REVIEWEDMany Worlds In One

by Alex Vilenkin

Hill & Wang: 2006. 248 pp. $24

The smallest person in the world, an Indian called Gul Mohammed, had a height of just 57 centimetres. The tallest, an American called Robert Wadlow, measured 2.72 metres. But the observed distribution of human heights fills only a small part of the range in between. Why? The average is about 1.63 metres, a respectable height for much of the world's population. Where are the giants of fairy-tale fame? And where are the Lilliputians?

We know the answers. Our genes control the supply of the growth hormones that spur our bones to elongate. Genetic abnormalities can reduce the abundance of these hormones in dwarfs, and oversupply leads to giantism. The environment, most notably nutrition, also plays a part, and gravity constrains our height — we are taller after sleeping, and astronauts gain height in space. Genetic evolution, with help from physical constraints, has narrowed the height range to the observed distribution.

Here we can reconcile observation with theory using known physics and biology; there is no need to invoke another explanation, such as a Grand Designer. But does the same kind of reasoning apply when scientists discuss our place in the Universe? In his stimulating new book, Many Worlds in One, cosmologist Alex Vilenkin invokes the anthropic principle in his interpretation of the Universe we observe: it is the way it is because we are here to observe it.

There are many possible universes that are inhospitable to our existence. The latest theories of quantum gravity count some 10500 realizations of the universe, in which the various fundamental constants of nature differ. In this Multiverse, all universes are equally real, although we can only hope to explore our own one. Given the staggering array of alternatives, it is exceedingly unlikely that our observed universe should even exist.

Our place in the Multiverse

J. BAUM/SPL

Bubble vision: theories of quantum gravity suggest that the Multiverse may contain 10500 universes.

Take the mystery of dark energy, for instance, which dominates the energy density in the Universe. Our best theories predict a value for the amount of dark energy that is too large by a factor of 10120. It is a tautology to assert that our existence selects an appropriate universe from the ensemble of all universes. After all, we can only observe a universe of a certain size, old enough for stars and planets, and for life to have developed. But it is physics, or at least metaphysics, to state, as the physicist Robert Dicke first did, that the Universe must be old enough for stars to have synthesized carbon, a necessary condition for our presence. It is one further logical step to assert that the values of all of the fundamental constants of nature, which may vary throughout the Multiverse, are determined by our presence. This is the anthropic principle in its weakest form. It is simply observational selection, with the caveat that our presence is not guaranteed.

A strong version of the anthropic principle claims that intelligent life is inevitable somewhere in the Multiverse. But let us put that aside, if only because strong anthropic arguments are weakened by the inclusion of a possibly infinite age for the Universe. A great deal can happen over a long time in a universe that perpetually renews itself by eternal inflation. The weak anthropic principle, preferred by many of my colleagues, selects only the small subset of 'pocket universes' within the Multiverse that allow galaxies to form and life to develop. There is then a high probability of finding only a small but non-zero value for dark energy today, which is what we observe.

At least three rival hypotheses could explain the values of the fundamental constants of nature. First, the selection could have been made by a Grand Designer. This has great appeal to proponents of the intelligent design of the Universe. Vilenkin argues forcefully that there is no need to invoke such a concept, although ultimately it reduces to a question of personal belief. The second option appeals to currently unknown physics. The height distribution of human beings can be understood by known rules, so there is no need to invoke another explanation; perhaps we simply do not yet know the rules for navigating in the Multiverse. It may be that it takes an infinite time to populate the plenitude of potential universes. The ultimate voyage through the quantum foam that characterizes the Multiverse to arrive in our Universe may take so long that it could only have happened once. If so, it makes no sense to talk of probabilities for our Universe to appear.

The third option, and to my mind the most likely, is that there was no selection at all: we are here because we are here. This is what must happen in an infinite Multiverse. Some versions of quantum gravity appeal to the complexity of the initial conditions to assert that there were an infinity of landscapes and universes in the Multiverse. If this were the case, the game is over. The dice were rolled and our Universe was inevitable, somewhere in the Multiverse. And here we are.

Remarkably, we can test this hypothesis. Future experiments will measure the curvature of space with exquisite precision. If the curvature turns out to deviate from flatness, we would come to a conclusion unprecedented in human thought. A slightly closed Universe would prove the finiteness of space. A slightly open Universe would go a long way towards demonstrating that space is infinite, at least in standard cosmologies. If this were the case, we would no longer need to invoke any version of the anthropic principle; it would simply be redundant.

I thoroughly recommend Many Worlds in One. Vilenkin has made some major contributions to the Multiverse hypothesis. Here he illuminates the current issues with clarity and elegance, yet the stories he tells are accessible to non-specialists.

  1. Joseph Silk is in the Department of physics, University of Oxford, Oxford OX1 3RH, UK.
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