From Mars to the Multiverse

Powerful instruments had led to astonishing progress in tracing the emergence of atoms, galaxies, stars and planets from a mysterious 'beginning' 13.8 billion years ago. An exciting development  has been the realisation that many other stars are orbited by retinues of planets -- some resembling our Earth (and capable of harboring life).

Looking further afield, observers can probe galaxies and the massive black holes at their centres back to an epoch only a billion years after the 'Big Bang'.  Indeed we can trace pre-galactic history with some confidence back to a nanosecond after the 'Big Bang'.

But the key parameters of our expanding universe -- the expansion rate, the geometry and the content -- were established far earlier still, when the physics is still conjectural but is being constrained, especially by precision measurements of the microwave background. These advances pose new questions: in particular, was our 'Big Bang' the only one? If there are others, would the physical laws within them be the same as in ours -- or (as some string theorists conjecture) would the laws be different? The structure of our universe and the emergence of stars, planets and life within it are dependent on the values of a few basic microphysical and cosmological numbers. I will discuss the consequences of tweaking the values of these numbers. This exercise is of course a necessary strand of cosmological research if there are multiple vacuum states or if 'eternal inflation' leads to a multiplicity of big bangs. However, even those who are allergic to the 'multiverse' may have their intuition developed by exploring these alternative scenarios -- just as some historians explore counterfactual scenarios, such as what might have happened if the Brits had fought harder in 1776. This illustrated lecture will attempt to address such issues.

Speaker: Martin Rees, Royal Society