Time-dependent couplings in the dark sector: from background evolution to non-linear structure formation

Abstract

We present a complete numerical study of cosmological models with a time‐dependent coupling between the dark energy component driving the present accelerated expansion of the Universe and the cold dark matter (CDM) fluid. Depending on the functional form of the coupling strength, these models show a range of possible intermediate behaviours between the standard ΛCDM background evolution and the widely studied case of interacting dark energy models with a constant coupling. These different background evolutions play a crucial role in the growth of cosmic structures and determine strikingly different effects of the coupling on the internal dynamics of non‐linear objects. By means of a suitable modification of the cosmological N‐body code gadget‐2, we have performed a series of high‐resolution N‐body simulations of structure formation in the context of interacting dark energy models with variable couplings. Depending on the type of background evolution, the halo density profiles are found to be either less or more concentrated with respect to ΛCDM, contrarily to what happens for constant coupling models where concentrations can only decrease. However, for some specific choice of the interaction function, the reduction in halo concentrations can be larger than in constant coupling scenarios. We also find that different types of coupling evolution determine specific features in the growth of large‐scale structures, like peculiar distortions of the matter power spectrum shape or different time‐evolutions of the halo mass function. Furthermore, also for time‐dependent couplings, baryons and CDM develop a bias already on large scales, which is progressively enhanced for smaller and smaller scales, and the effect can be significantly larger compared to constant coupling scenarios. The same happens to the baryon fraction of haloes, which can be more significantly reduced below its universal value in variable coupling models with respect to constant coupling cosmologies. In general, we find that time‐dependent interactions between dark energy and CDM can in some cases determine stronger effects on structure formation as compared to the constant coupling case, with a significantly weaker impact on the background evolution of the universe, and might therefore provide a more viable possibility to alleviate the tensions between observations and the ΛCDM model on small scales than the constant coupling scenario.

Testing coupled dark energy with next-generation large-scale observations

“…Various choices of couplings have been investigated in literature [12][13][14][30][31][32][33][34][35][36]. Analysis of the models and constraint on these couplings have been obtained in several ways, including spherical collapse ( [37,38] and references therein), time renormalizazion group [39], N -body simulations [17,40,41] and effects on supernovae, CMB and crosscorrelation of CMB and LSS [7,9,[19][20][21][22][42][43][44]. Refs.…”

“…While β could also be a coupling function β(φ) (see e.g. [16][17][18]), in this paper we restrict ourselves for simplicity to a constant value. A coupled particle of mass m will respond to this interaction by following in the Newtonian regime the scalar-gravitational potential…”

Imprint off(R)gravity on nonlinear structure formation

“…14, 16] -possibly also featuring * E-mail:iachitouv@swin.edu.au direct interactions with other matter fields [17][18][19][20]25]. Another possibility are modifications of General Relativity (GR) on large scales [as e.g.…”

Ghost Dark Energy Model with Non-linear Interaction

“…If Q is positive, dark energy decays into dark matter, while if Q is negative, dark matter decays into dark energy. Some authors have also considered the possibility to have non-linear interaction terms [27,28,29,30,31,32]. Here we take this last idea, with a concrete selection for the interaction term Q, which allows us to find analytical and phenomenologically feasible solutions.…”