Dark Sector Coupling

My master thesis is still under development. It concerns cosmological models in which dark matter and dark energy interact through an elastic coupling, rather than evolving as two entirely decoupled fluids as in standard ΛCDM. Instead of vanishing separately, the four-divergences of the two components' stress-energy tensors are sourced by a term proportional to their relative four-velocity,

$$\nabla_\mu T^{\mu\nu}_\text{dm} = \bar{\alpha}(u^\nu_\text{de}-u^\nu_\text{dm}) = -\nabla_\mu T^{\mu\nu}_\text{de}$$

Elastic dark matter-dark energy coupling, following Figueruelo et al. (2021)

Because the coupling depends on the relative velocity of the two fluids rather than only their background energy densities, it leaves a distinct imprint on the evolution of vector perturbations, which are otherwise absent or negligible in most dark energy models. My thesis extends the Einstein-Boltzmann code CLASS to include this coupling term in the perturbation equations, and studies how it affects the growth and decay of vector modes and the observables derived from them.

The work is ongoing, and I will update this page with results as the thesis progresses.