Simulating Momentum Exchange in the Dark Sector

Low energy interactions between particles are often characterised by elastic scattering. Just as electrons undergo Thomson scattering with photons, dark matter particles may experience an analogous form of momentum exchange with dark energy. We investigate the influence such an interaction has on the formation of linear and nonlinear cosmic structure, by running for the first time a suite of N-body simulations with different dark energy equations of state and scattering cross sections. In models where the linear matter power spectrum is suppressed by the scattering, we find that on nonlinear scales the power spectrum is strongly enhanced. This is due to the friction term increasing the efficiency of gravitational collapse, which also leads to a scale-independent amplification of the concentration and mass functions of halos. The opposite trend is found for models characterised by an increase of the linear matter power spectrum normalisation. More quantitatively, we find that power spectrum deviations at nonlinear scales ($k \approx 10\, h/$Mpc) are roughly ten times larger than their linear counterparts, exceeding $100$ for the largest value of the scattering cross section considered in the present work. Similarly, the concentration-mass relation and the halo mass function show deviations up to $100$ and $20$, respectively, over a wide range of masses. Therefore, we conclude that nonlinear probes of structure formation might provide much tighter constraints on the scattering cross section between dark energy and dark matter as compared to the present bounds based on linear observables.Comment: 12 pages, 11 figures, 2 tables. Submitted to MNRA

Observing Baryon Oscillations with Cosmic Shear

A cosmic shear survey, spanning a significant proportion of the sky, should greatly improve constraints on a number of cosmological parameters. It also provides a unique opportunity to examine the matter power spectrum directly. However, the observed lensing signal corresponds to a weighted average of the power spectrum across a range of scales, and so the potential to resolve the baryon oscillations has been somewhat neglected. These features originated prior to recombination, induced by the acoustics of the photon-baryon fluid. Recent galaxy surveys have detected the imprints, and in the future such measurements may even be used to refine our understanding of dark energy. Without redshift information, cosmic shear is an ineffective probe of the baryon oscillations. However, by implementing a novel multipole-dependent selection of photometric redshift bins, sensitivity is improved by an order of magnitude, bringing the "wiggles" within reach of future surveys. As an illustration, we show that data from surveys scheduled within the next ten years will be able to distinguish a smoothed power spectrum at the two sigma level.Comment: 5 pages, 3 figures, submitted to PR