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FQXI ARTICLE

December 17, 2017

Our Place in the Multiverse

Calculating the odds that intelligent observers arise in parallel universes—and working out what they might see.

FQXi Awardees: Richard Easther, Eugene Lim

September 22, 2017

Your cosmos, or mine?

What might observers in parallel universes see when they look into the sky?

Credit: NASA

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:

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

Which of these pocket

universes do we live

in, and why?

universes do we live

in, and why?

- Eugene Lim

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 10

Peeling back the cosmic microwave background

Are there clues about parallel worlds hidden in radiation from our universe’s infancy?

Credit: European Space Agency

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."

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