Physics in Real and Counterfactual Universes

Over 13.8 billion years, the material emerging from the big bang has transformed into our complex cosmos. But what would the universe look like if a different set of forces and cosmological parameters shaped its evolution?

In this lecture, Martin Rees will describe how the emergence and properties of galaxies, stars, planets and life depend on a few key numbers: the ‘constants’ of microphysics and the parameters that describe our expanding universe. According to some cosmologists and string theorists, domains may actually exist where these constants have different values. It is therefore interesting to explore what range of values would permit the emergence of complexity. Even those who are allergic to this concept of a multiverse may find their insight enhanced by exploring ‘counterfactual’ universes, just as some historians speculate about ‘counterfactual’ scenarios such as what would have happened if the British triumphed during the American Revolutionary War.