Time-dependent couplings in the dark sector: from background evolution to non-linear structure formation
Abstract
We present \na complete numerical study of cosmological models with a \ntime-dependent coupling between the dark energy component driving the \npresent accelerated expansion of the Universe and the cold dark matter \n(CDM) fluid. Depending on the functional form of the coupling strength, \nthese models show a range of possible intermediate behaviours between \nthe standard ΛCDM background evolution and the widely studied \ncase of interacting dark energy models with a constant coupling. These \ndifferent background evolutions play a crucial role in the growth of \ncosmic structures and determine strikingly different effects of the \ncoupling on the internal dynamics of non-linear objects. By means of a \nsuitable modification of the cosmological N-body code GADGET-2, we have \nperformed a series of high-resolution N-body simulations of structure \nformation in the context of interacting dark energy models with variable \ncouplings. Depending on the type of background evolution, the halo \ndensity profiles are found to be either less or more concentrated with \nrespect to ΛCDM, contrarily to what happens for constant coupling \nmodels where concentrations can only decrease. However, for some \nspecific choice of the interaction function, the reduction in halo \nconcentrations can be larger than in constant coupling scenarios. We \nalso find that different types of coupling evolution determine specific \nfeatures in the growth of large-scale structures, like peculiar \ndistortions of the matter power spectrum shape or different \ntime-evolutions of the halo mass function. Furthermore, also for \ntime-dependent couplings, baryons and CDM develop a bias already on \nlarge scales, which is progressively enhanced for smaller and smaller \nscales, and the effect can be significantly larger compared to constant \ncoupling scenarios. The same happens to the baryon fraction of haloes, \nwhich can be more significantly reduced below its universal value in \nvariable coupling models with respect to constant coupling cosmologies. \nIn general, we find that time-dependent interactions between dark energy \nand CDM can in some cases determine stronger effects on structure \nformation as compared to the constant coupling case, with a \nsignificantly weaker impact on the background evolution of the universe, \nand might therefore provide a more viable possibility to alleviate the \ntensions between observations and the ΛCDM model on small scales \nthan the constant coupling scenario.