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

Clipping the Cosmos II: Cosmological information from non-linear scales

We present a method for suppressing contributions from higher-order terms in perturbation theory, greatly increasing the amount of information which may be extracted from the matter power spectrum. In an evolved cosmological density field the highest density regions are responsible for the bulk of the nonlinear power. By suitably down-weighting these problematic regions we find that the one- and two-loop terms are typically reduced in amplitude by ~70 per cent and ~95 per cent respectively, relative to the linear power spectrum. This greatly facilitates modelling the shape of the galaxy power spectrum, potentially increasing the number of useful Fourier modes by more than two orders of magnitude. We provide a demonstration of how this technique allows the galaxy bias and the amplitude of linear matter perturbations sigma_8 to be determined from the power spectrum on conventionally nonlinear scales, 0.1<k<0.7 h/Mpc.Comment: 13 pages, 11 figures. To appear in Physical Review

Cosmic explosions, life in the Universe and the Cosmological Constant

Galactic Gamma-Ray Bursts (GRBs) are copious sources of gamma-rays that can pose a threat to complex life. Using recent determinations of their rate and the probability of GRBs causing massive extinction, we explore what type of universes are most likely to harbour advanced forms of life. For this purpose we use cosmological N-body simulations to determine at what time and for what value of the cosmological constant ($\Lambda$) the chances of life being unaffected by cosmic explosions are maximised. We find that $\Lambda-$dominated universes favour the survival of life against GRBs. Within a $\Lambda$CDM cosmology, the parameters that govern the likelihood of life survival to GRBs are dictated by the value of $\Lambda$ and the age of the Universe. We find that we seem to live in a favorable point in this parameter phase space which minimises the exposure to cosmic explosions, yet maximises the number of main sequence (hydrogen-burning) stars around which advanced life forms can exist.Comment: Accepted version in Physical Review Letter