Abstract:
Exploring the Early Universe with Accelerators and Astrophysics
The nature of dark matter, a fundamental ingredient of our universe, remains unknown. I will show how theoretical insights about the first moments after the Big Bang can guide our search for this mysterious substance. By considering the two limiting cases where dark matter has a coupling to Standard Model particles or interacts solely through gravity, I will illustrate the breadth of theoretical models, and experimental and observational methods required to test them.
If dark matter is a new fundamental particle that has even feeble interactions with familiar matter, the hot plasma of the early universe will bring it into thermal equilibrium. This simple process can produce the correct relic abundance of dark matter, and it implies specific masses and couplings for the new particle. I will describe a new small-scale accelerator experiment that will definitively test this hypothesis in a wide range of particle physics models, and highlight the broad physics case for this programme.
While accelerators are a powerful probe of dark matter with significant couplings to familiar matter, such non-gravitational interactions can be either too feeble for detection or absent altogether. In this case, the dark matter abundance and its spacial distribution at small scales can carry the imprints of the early universe, which, in turn, suggest novel gravitational probes of its nature. I will illustrate this in the context of a cosmological history where the energy content of the universe is dominated by a non-relativistic particle that decays before Big Bang Nucleosynthesis. I will show that dark matter density perturbations grow much faster during this epoch, leading to the formation of dense clumps, or minihalos, with characteristic masses below the Earth mass. These small-scale structures can be searched for with observations of pulsars and photometric monitoring of highly magnified extragalactic stars.