Viscosity in cosmic fluids
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Regular Article - Theoretical Physics
Viscosity in cosmic fluids Pravin Kumar Natwariya1,2,a , Jitesh R. Bhatt1,b , Arun Kumar Pandey3,c 1
Theoretical Physics Division,Physical Research Laboratory, Ahmedabad 380 009 Gujarat, India Department of Physics, Indian Institute of Technology Gandhinagar,, Palaj,Gandhinagar 382 355 Gujartat, India 3 Department of Physics and Astrophysics, University of Delhi, Delhi 110 007, India
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Received: 19 February 2020 / Accepted: 10 August 2020 / Published online: 24 August 2020 © The Author(s) 2020
Abstract The effective theory of large-scale structure formation based on 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 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.
1 Introduction In order to study large scales structures in the Universe, there are two important length-scales: one is comoving Hubble −1 . Here, scale H−1 and the another is the non-linear scale kNL −1 kNL describes the scales at which gravitational collapse takes place; it is typically considered to be of the order of the size of a Galactic cluster, i.e., ∼ a few Mpc. The Universe is homogeneous at a scale of ∼ 200 Mpc, and there are roughly 153 homogeneous patches within the Hubble volume. The dynamics of the perturbations can be analyzed in terms of a parameter k = kNL /k, where k is the inverse length scale. The hierarchy between these two scales is quantified by the parameter k 1 which is responsible for the success of a e-mails: [email protected]; [email protected] (corresponding author) b e-mail:
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c e-mail:
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linear perturbation theory in describing the observed large scale structures (LSS) (for a recent review see [1] and also [2]). The dark energy (cosmological constant ) plus cold dark-matter (CDM) model, (i.e. CDM) is highly successful in predicting the large scale structure of the Universe. The model is consistent with observations from the length scales typically of the order of ∼ 1 Mpc (i.e., intergalactic scale) to the scale of the horizon (∼ 15,000 Mpc) [1]. In this model, structure formation in the dark-matter (DM) sector occurs more rapidly than the baryonic matter. The structure formation in the dark sector provides a gravitational p
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