Non-linear structure formation with elastic interactions in the dark sector

¹ Departamento de Física Fundamental and IUFFyM, Universidad de Salamanca 37008 Salamanca, Spain
² Department of Theoretical Physics, University of the Basque Country UPV/EHU 48080 Bilbao, Spain
³ Institute of Theoretical Astrophysics, University of Oslo N-0315 Oslo, Norway

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 October 2025
Accepted: 9 February 2026

Abstract

Cosmological models in which dark matter interacts with dark energy via pure momentum transfer and no energy exchange (i.e. elastic) provide compelling scenarios for addressing the apparent lack of structures at low redshift. In particular, it has been shown that measurements of S₈ show a statistically significant preference for such elastic interactions. In this work we implement a specific realisation of these scenarios in an N-body code to explore the non-linear regime. We include two populations of particles to describe interacting dark matter and non-interacting baryons, respectively. On linear scales we recover the structure suppression obtained from Boltzmann codes, while non-linear scales exhibit enhanced matter power. We find that fewer massive halos form at low redshift due to elastic interactions and that dark matter halos are more compact than in the standard model. Furthermore, the dark-matter-to-baryon density profile ratio is not constant. Finally, we corroborate that baryons efficiently cluster around dark matter halos and thus trace the dark matter velocity field well despite the interaction. This shows that the interaction is insufficiently strong to disrupt virialised structures.