Quarks, Electrons, and Atoms in Closely Related Universes

In a model where a multiverse wavefunction explores a multitude of vacua with different symmetries and parameters, properties of universes closely related to ours can be understood by examining the consequences of small departures of physical parameters from their observed values. The masses of the light fermions that make up the stable matter of which we are made-- the up and down quarks, and the electron-- have values in a narrow window that both allows a variety of nuclei other than protons to exist, and at the same time allows atoms with stable shells of electrons that are not devoured by their nuclei. These fundamental parameters of the Standard Model are good candidates for quantities whose values are determined through selection effects within a multiverse, since a living world of molecules needs stable nuclei other than just protons and neutrons. If the fermion masses are fixed by brane condensation or compactification of extra dimensions, there may be observable fossils of the branching event, such as a gravitational wave background.