UNC-CH Physics and Astronomy Preliminary Defense

Carisa Miller

“Dark Matter and Dark Energy in the Early Universe”

Two of the most pursued questions in cosmology are the nature and composition of what we appropriately call dark energy and dark matter. The variety of theories and models within both of these topics is extensive, and we present our investigations into single, well-motivated scenarios within each topic.

Many explanations for dark energy posit the existence of a new light scalar field, often one coupled to matter, and so able to mediate a long-range force. Stringent constraints on fifth forces necessitate a mechanism by which to screen these effects locally. The chameleon mechanism achieves this screening by increasing the mass of the field in regions of increasing density. For the runaway potentials commonly considered in chameleon theories, the field’s coupling to matter and the hierarchy of scales between Standard Model (SM) particles and the energy scale of such potentials result in catastrophic effects in the early Universe when SM particles become nonrelativistic. Perturbations with trans-Planckian energies are excited, and the theory suffers a breakdown in calculability at the relatively low temperatures of Big Bang Nucleosynthesis. We consider a chameleon field in a quartic potential and show that the scale-free nature of this potential allows the chameleon to avoid many of the problems encountered by runaway models and it is able remain a well-behaved effective field theory at nucleosynthesis.

It is commonly assumed that dark matter is thermally produced in the early universe during a period of radiation domination. The continuing lack of detection of particles which can fit this model has prompted many interesting modifications, notably the inclusion of a matter-dominated era following inflation and prior to reheating, during which decay of the dominating particle is the primary source of production of dark matter and SM radiation. A typical assumption in analyses which consider this scenario is that the dark matter is produced nonrelativistically from the decay process. A more natural assumption, which does not require fine-tuning between the masses of the parent and daughter particles, is that both the dark matter and the SM particles are born relativistic. We investigate the effects of imparting dark matter particles with relativistic velocities, and their implications for structure growth.