Our Place in the Multiverse

The biggest problem with the multiverse—the plethora of neighboring cosmoses that some cosmologists propose exists—is that even if it is real, it is, by definition, unobservable. There may be a vast range of different physical laws on display in different universes, but a single observer sees only the properties of the universe she lives inside. Two major unresolved issues are how many of these parallel universes could also give rise to observers—and whether any observations within a single cosmos can tell us something about our place in the larger multiverse.

And, unfortunately, there is very little physical knowledge to go on when it comes to working out the answer, says Eugene Lim of King’s College London, UK. Undaunted by the lack of tools to help, Lim and his colleague Richard Easther of the University of Auckland, New Zealand, are considering the minimal sort of environment that could give rise to observers. At the simplest level, they are assessing the effect of switching off different physical laws, and then calculating whether galaxies would be able to form in such modified universes. "If you can form galaxies then maybe you have the environment required for an observer to exist: you can use that as a placeholder," says Lim.

Cosmic Event Horizon

The pair are then considering how much information a hypothetical observer in that universe could possibly collect, and whether we can learn anything about our place in the multiverse as a result. Keeping track of a universe’s information content is tricky because matter is constantly being lost beyond our cosmic ’event horizon’—the universe’s edge, beyond which we cannot see, which moves outwards as the universe expands. Lim and Easther’s evaluation of the maximum information available to an observer thus rests on two theories: eternal inflation—the idea behind the mechanical existence of a multiverse—and string theory, a candidate theory for physics on the smallest scales, which may tell us how such parallel worlds could be populated with various particles and forces.

Inflation was first raised as an idea in the early 1980s by Alan Guth, now at MIT, to resolve two dilemmas. The first dilemma is called the flatness problem, and arose because astronomers were puzzled by why the density of matter in the universe is so precisely close to a critical density needed to make space flat (rather than causing it to curve back inwards on itself, or bend outwards in a saddle-configuration). This critical density conveniently allows galaxies, stars and planets—and observers such as ourselves—to exist.

The second problem is called the isotropy problem, and arose when astronomers observed that the temperature of the cosmic microwave background radiation—the relic of the primordial fireball of the big bang—is extremely uniform to a precision of one part in 100,000, despite coming from parts of the fireball that seemed to be unlinked, at the beginning of time.

Inflation resolves the dilemmas mechanically, allowing matter and space to expand at an enormously fast rate for a short period early in our cosmic history. The exponential expansion would have naturally stretched the universe into a flat geometry, solving the first dilemma. The isotropy problem is resolved because inflation would have stretched out a tiny patch of the cosmos, which initially had only a tiny temperature variation, across our entire sky. This explains why the universe we observe today looks so uniform in temperature in every direction.

But inflation, while solving these problems, seems to be too much of a good thing. Whilst in some places inflation stops as needed, and the inflationary energy converts into matter and radiation, producing a conventional universe such as our own, cosmologists realised that in other places it keeps going. Our universe, then, only encompasses one of the regions where inflation ended locally. The pattern of break-up into local universes continuously repeats in neighbouring regions outside our cosmos, producing a fractal structure of ’bubble universes’ or ’pocket universes’ in a vast inflating multiverse.

This raises some key questions for Lim: "Which of these pocket universes do we live in, and why?" he asks. "Can we learn something from this particular scenario?" The answer may come from bringing string theory to the table.

String Landscape

String theory predicts the existence of 10⁵⁰⁰ different versions of spacetime, each representing a different universe, with different values for the fundamental constants such as the cosmological constant (the feature thought by many physicists to be causing our universe’s expansion to accelerate today) or gravity’s strength. This ’landscape’ of different universes can populate the bubble universes in the inflating multiverse with different characteristics. Lim and Easther hope that combining eternal inflation with string theory will help them to understand how the different universes are distributed in a probabilistic manner.

Only universes that have a cosmological constant lying within a certain range can give rise to intelligent observers. If the constant is too large, say, then the expansion of the early universe would accelerate so wildly that the cosmos would be ripped apart before stars, galaxies and life could form. So one question is, how common are universes with a cosmological constant that could give rise to observers within the wider multiverse? "Maybe we can explain why we live in a universe with such a small cosmological constant," says Lim.

There is a hitch though: to be able to understand the distribution of universes, with a particular cosmological constant, you need to make a number of observations of that distribution, ideally from different universes. "If you focus on our universe being only one drawing from this distribution, it is clear that we can never reconstruct this distribution," says Lim. "But maybe we can actually make several drawings from the distribution."

To make several drawings, Lim and Easther suspect there may be clues available in the cosmic microwave background radiation that link to other pocket universes. Or via the observed fundamental constants. "We are not yet sure how that works but we are trying to be creative on the possibilities by considering what a drawing means," says Lim. If successful, it will allow them to make sound predictions about the multiverse, a notoriously difficult problem.

Guth says he is looking forward to reading the results from Lim and Easther’s project. "In trying to use empirical science to draw conclusions about a possible multiverse, it is clearly essential to take into account the selection effects implied by our existence, but it is much less clear how to do this," says Guth. "Easther and Lim are attempting to approach this question with logical precision, and I am optimistic that they will make significant progress in clarifying these issues."

"Quantifying minimal requirements for observers is a surprisingly powerful approach," adds Raphael Bousso, a theoretical physicist at the University of California, Berkeley. "I’m pleased that they are pursuing this challenge."