The Accidental Universe
Alan P. Lightman
Harper's Magazine, December 2011
Edited by Andy Ross
Theoretical physicists used to agree that the universe is generated from a
few mathematical truths and principles of symmetry, perhaps with a few
parameters. It seemed that we were closing in on a vision of our universe in
which everything could be calculated, predicted, and understood.
theories in physics, eternal inflation and string theory, now suggest that
the fundamental principles from which the laws of nature derive may lead to
many different universes with many different properties. According to
current thinking, we are living in one of a vast number of universes. We are
living in an accidental universe.
Thirty years ago, Alan Guth
proposed a major revision to the Big Bang theory called inflation. Our
universe began as a nugget of extremely high density and temperature about
14 billion years ago and has been expanding and cooling ever since. Assuming
that for the first tiny fraction of a second of its life our universe
inflated very rapidly, before slowing to the expansion of the standard BB
model, solves some problems in BB theory.
Eternal inflation is a
revision of inflation theory developed by Andrei Linde, Paul Steinhardt, and
Alex Vilenkin. In regular inflation theory, the very rapid expansion of the
infant universe is caused by an energy field that is temporarily trapped in
a condition that does not represent the lowest possible energy for the
universe as a whole. In the theory of eternal inflation, the energy field
has different values at different points of space. Each local minimum starts
a new Big Bang, essentially a new universe. Thus, the original, rapidly
expanding universe spawns a multitude of new universes, in a never-ending
The multiverse idea explains the "fine-tuning" problem that
if the values of some of the fundamental parameters of our universe were a
little larger or a little smaller, life could not have arisen. Given the
anthropic principle that the universe must have the parameters it does
because we are here to observe it, the multiverse includes countless
different universes with different properties, some of them consistent with
the emergence of life and some not, and we live in a universe that permits
life because otherwise we wouldn't be here to ask the question.
physicists remain skeptical. Others have reluctantly accepted the anthropic
principle and the multiverse idea as together providing the best explanation
for the observed facts. But if the multiverse idea is correct, then the
historic mission of physics to explain all the properties of our universe in
terms of fundamental principles is futile. Our universe is what it is
because we are here.
The most striking example of fine-tuning, and
one that practically demands the multiverse to explain it, lies behind the
unexpected discovery, little more than a decade ago, that the expansion of
the universe is accelerating. Galaxies are flying away from each other as if
repelled by antigravity. Physicists call the energy associated with this
repulsion dark energy. The amount of dark energy in every cubic meter of
space is tiny, but it adds up to three quarters of the total energy of the
Theorists have several hypotheses about the identity of
dark energy. According to quantum physics, empty space is a pandemonium of
subatomic particles rushing about and then vanishing before they can be
seen. Dark energy may also be associated with a force field called the Higgs
field, which may explain particle masses. And in the models proposed by
string theory, dark energy may be associated with the way extra dimensions
of space are compacted so that we do not notice them.
hypotheses give a fantastically large range for the theoretically possible
amounts of dark energy in a universe. The amount of dark energy in our
universe is very near zero compared with what it could be. If the
theoretically possible positive values for dark energy were marked out on a
ruler stretching from here to the sun, with zero at one end and the maximum
value at the other, the value of dark energy actually found in our universe
would be closer to the zero end than the width of an atom.
Yet if the
amount of dark energy in our universe were only a little bit different than
what it actually is, then life could never have emerged. A little more and
the universe would accelerate so rapidly that the matter in the young cosmos
could never pull itself together to form stars and thence form the complex
atoms made in stars. And, going into negative values of dark energy, a
little less and the universe would decelerate so rapidly that it would
recollapse before there was time to form even the simplest atoms.
of all the possible amounts of dark energy that our universe might have, the
actual amount lies in the tiny sliver of the range that allows life. As
before, the multiverse can explain such fine-tuning. A vast number of
universes may exist, with many different values of the amount of dark
energy. Some of them permit the emergence of life. We are here, so our
universe must be such a universe. We are an accident.
predicts the possibility of the multiverse. String theory postulates that
the smallest constituents of matter are not subatomic particles like the
electron but extremely tiny one-dimensional strings of energy. These
elemental strings can vibrate at different frequencies, like the strings of
a violin, and the different modes of vibration correspond to different
fundamental particles and forces. String theories typically require seven
dimensions of space in addition to the usual three, which are compacted down
to such small sizes that we never see them. There are a vastly many ways to
fold up the extra dimensions in string theory, each one corresponding to a
different universe with different physical properties.
originally hoped that from a theory of these strings, with very few
additional parameters, they would be able to explain all the forces and
particles of nature. String theory would then be the ultimate realization of
the Platonic ideal of a fully explicable cosmos. But string theory predicts
a huge number of possible universes with different properties, perhaps
10^500 of them. The number might as well be infinite.
inflation nor string theory has anywhere near the experimental support of
many previous theories in physics. Physicists who are adjusting to the idea
of the multiverse must not only accept that basic properties of our universe
are accidental and incalculable but also believe in the existence of all the
other universes. Theologians are accustomed to taking some beliefs on faith.
Scientists are not.
AR This is a short cut of a long piece
that deserves reading in full.