Dark Matter and Dark Energy in the Early Universe

Abstract

Less than 5% of the current energy content of the Universe is contained in Standard Model (SM) particles; the remaining 95% is made up of dark matter and dark energy. Both dark matter and dark energy have only been detected through their gravitational interactions, and their properties require the introduction of new, beyond-SM physics. A promising regime to search for new physics is in high-energy environments like that of the Universe's first second. We investigate how a theory of modified gravity that aims to explain dark energy behaves in the early Universe and how the production method of dark matter in the early Universe could effect the formation of structure. The dark energy model we consider is chameleon gravity, in which a light scalar field that couples to the trace of the stress-energy tensor in such a way that its mass depends on the ambient density, and makes it difficult to detect in high-density environments. We consider a chameleon field with a quartic potential and show that the scale-free nature of this potential allows the chameleon to avoid the problems encountered by other chameleon theories during the Universe's first second. We then determine how producing dark matter particles with relativistic velocities via the decay of heavier particles impacts the dark matter velocity distribution function and the growth of structure. We find that the free streaming of these dark matter particles can prevent structure formation on subgalactic scales. Therefore, current observations of small-scale structure put an upper limit on the velocity of the dark matter particles at their creation. Finally, we investigate whether these limits can be relaxed in the presence of scattering interactions between the dark matter and SM particles.