Natwariya, Pravin KumarPravin KumarNatwariyaBhatt, Jitesh R.Jitesh R.BhattPandey, Arun KumarArun KumarPandey2025-08-312025-08-312020-08-0110.1140/epjc/s10052-020-8341-82-s2.0-85089775898https://d8.irins.org/handle/IITG2025/24075The effective theory of large-scale structure formation based on Λ CDM paradigm predicts finite dissipative effects in the resulting fluid equations. In this work, we study how viscous effect that could arise if one includes self-interaction among the dark-matter particles combines with the effective theory. It is shown that these two possible sources of dissipation can operate together in a cosmic fluid and the interplay between them can play an important role in determining dynamics of the cosmic fluid. In particular, we demonstrate that the viscosity coefficient due to self-interaction is added inversely with the viscosity calculated using effective theory of Λ CDM model. Thus the larger viscosity has less significant contribution in the effective viscosity. Using the known bounds on σ/ m for self-interacting darkmatter, where σ and m are the cross-section and mass of the dark-matter particles respectively, we discuss role of the effective viscosity in various cosmological scenarios.trueArticlehttps://link.springer.com/content/pdf/10.1140/epjc/s10052-020-8341-8.pdf143460521 August 202013767arJournal14