Many Worlds in One: The Search for Other Universes. Alex
Vilenkin. viii + 235 pp. Hill and Wang, 2006. $24.
Cosmology is built out of physics. As physics becomes more unified,
sophisticated and inclusive, so does the reach of cosmological
thought, and eventually that of real experimental data. Modern
cosmology began nine decades ago with Einstein's general theory of
relativity; models of our expanding universe based on that theory
connected with Edwin Hubble's observations of real galaxies in the
1920s. The addition of quantum physics in the following decades
extended those ideas to include the behavior of the early universe
when most energy was in the form of radiation. This Big Bang theory
was confirmed with the discovery in the 1960s of vestiges of this
radiation, still shining from all directions in the sky, and with
corroboration of the predicted primordial abundances of elements.
Around 1980, the importation of ideas of symmetry breaking from
quantum field theory led to the concept of cosmic inflation, a
process that both kick-started the Big Bang's expansion and
introduced small quantum fluctuations, which eventually became
galaxies. This extraordinary synthesis is well on its way to
being precisely confirmed by detailed observations of the cosmic
microwave background that faithfully record those fluctuations.
In Many Worlds in One, Alex Vilenkin offers an engaging
personalized tour of cosmology, sharing his intimate view of
developments in the field over the past quarter of a century.
Cosmologists have a daily urge to explore in ways that sometimes
seem to take them beyond the edge of practical common sense, but by
clearly explaining what they are thinking and why, Vilenkin makes it
possible for the reader to accompany them. He enlivens the
scientific narrative by including colorful sketches of friends,
colleagues and the heroic figures of earlier generations. Indeed,
the book recalls the informal, lighthearted, inviting style of
popular books by another Russian émigré physicist,
George Gamow, who introduced the ideas of the Big Bang to a broad
readership half a century ago.
"The search for other universes" referred to in the
subtitle is mostly a search in the world of ideas rather than in any
experiment or observation; certainly no data appear here from other
universes! The ongoing fusion of quantum field theory with string
theory, a quantum version of general relativity, now promises to
remake and extend cosmology once again, this time into the
metacosmology of the multiverse. In this larger arena, seemingly
crazy ideas—a universe arising from nothing, lots of big
universes fitting into one small one, properties of physics itself
varying in many if not all respects from one universe to
another—emerge naturally out of well-controlled mathematical
arguments. Like some of the outlandish concepts of earlier
generations, these proposals may eventually come under, and survive,
real experimental scrutiny.
The emphasis here is on the biggest of big pictures. The ideas of
string theory and inflation lead naturally to a generous and
expansive view of everything, where zillions of versions of zillions
of types of universes are being created all the time. Much of
physics in any particular universe does not derive from any
mathematical principle (as many physicists have long hoped it would)
but by random selection from many choices in this large ensemble. In
the vast "landscape of string theory," a few places form
universes that become both big and hospitable—in the sense of
allowing interesting things to arise, such as atoms, molecules and
life. (In much of the landscape, quarks and electrons have masses
that do not even allow stable atomic nuclei to exist, much less any
biochemistry to happen. At an opposite extreme, a tiny pocket of
universes may be so like one another, and so like ours, that you and
I, existing in multiple copies, could never notice the subtle
differences between the various versions of ourselves.) The
extravagant multiplicity of possibilities may be dizzying or
disturbing to some and comforting to others, but in the end, it's
just another physical hypothesis, like the other
cosmologies that came before.
Where does the multiverse idea lead? It is not clear what shape
cosmology will take as the fusion of quantum strings, quantum fields
and gravity becomes a mature branch of physics. Quite possibly,
space and time will themselves end up being derived as
"emergent" concepts rather than fundamental ones. The many
worlds may really exist but in some kind of mathematical space we do
not know how to describe yet. As of now, it is not even known how to
define probabilities of universes, much less calculate them. These
ideas near the edge of understanding should be treated as
provisional until they are confronted in some way with real data.
It is amazing to step back and look at what has been achieved in the
past century with the bold approach of scientific cosmology: a
quantifiable and intelligible account of our universe that unifies
the physics of the very large with the physics of the very small and
survives a battery of precise, deeply probing experimental tests.
Given the solid factual basis of what has become standard cosmology,
it does not seem too ambitious to make a few straightforward
extrapolations and deduce a much larger universe of universes behind
it. We can even predict new kinds of things to look for to test some
of these models; for example, gravitational-wave observatories now
in development may have the ability to "hear" spacetime
vibrations from the first emergence of space and time, or from
cosmic superstrings left over from that era. Further experimental
clues may come from study of the mysterious phenomena of dark matter
and dark energy. In some leading models, the densities of dark
matter and dark energy are random variables in the multiverse
ensemble, their properties determined by selection. These models
make predictions that can be tested, either by direct detection of
dark matter in the laboratory or by precise studies of the history
of cosmic expansion that expose the nature of dark energy. So no
matter how crazy they might seem, these mind-boggling concepts count
as scientific ideas.
Even if we never figure out any more about the beginning of time or
our place in the great landscape of multiverses, observational
cosmology has already quietly revealed a profound property of our
universe: the evolution of complexity from a very simple initial
state. That is, direct data show that the Big Bang in the distant
past was a simple, smooth system extremely close to (local,
microscopic) thermodynamic equilibrium. This "heat birth"
is much like the "heat death" that thermodynamics
textbooks used to scare us with, except that it happens at the
beginning and not at the end of things.
Only much later, after many millions of years, did richly detailed
structure emerge (via an apparently entropy-defying information
growth enabled by gravity and the cosmic expansion) to become
galaxies, stars, planets and life. All of those interesting things
came to pass well after the Big Bang, from just the physics we
already know about. Moreover, that transformation happened in plain
sight: Deep telescopic observations, looking far back in time, will
progressively unveil a direct view of the cosmic equivalent of the
origin of life. It would be fair for cosmologists to claim that they
have already solved in principle one of the greatest mysteries of
creation—how we got here—and that they now merely await
better telescopes and better computer simulations to resolve the details.
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