The Accidental Universe

By 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.

Two 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 process.

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.

Some 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 universe.

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.

These various 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.

Out 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.

String theory 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.

Physicists 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.

Neither eternal 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.