Very large scale structures in growing neutrino quintessence

TL;DR

This paper explores how growing neutrino quintessence models lead to large-scale neutrino structures that influence cosmic evolution and could impact CMB anisotropies, using numerical methods to simulate non-linear formation of neutrino lumps.

Contribution

It introduces a numerical method to simulate the non-linear formation of neutrino lumps in growing neutrino quintessence models, demonstrating their potential to mimic large dark matter structures.

Findings

Neutrino lumps form at supercluster scales (~200 Mpc) in models with ~2 eV neutrino mass.

Neutrino-induced gravitational potentials are smaller than linear extrapolations suggest.

Neutrino structures could modify CMB anisotropy spectra at small angular scales.

Abstract

A quintessence scalar field or cosmon interacting with neutrinos can have important effects on cosmological structure formation. Within growing neutrino models the coupling becomes effective only in recent times, when neutrinos become non-relativistic, stopping the evolution of the cosmon. This can explain why dark energy dominates the universe only in a rather recent epoch by relating the present dark energy density to the small mass of neutrinos. Such models predict the presence of stable neutrino lumps at supercluster scales (~200 Mpc and bigger), caused by an attractive force between neutrinos which is stronger than gravity and mediated by the cosmon. We present a method to follow the initial non-linear formation of neutrino lumps in physical space, by integrating numerically on a 3D grid non-linear evolution equations, until virialization naturally occurs. As a first application, we show results for cosmologies with final large neutrino average mass ~2 eV: in this case, neutrino lumps indeed form and mimic very large cold dark matter structures, with a typical gravitational potential 10^{-5} for a lump size ~10 Mpc, and reaching larger values for lumps of about 200 Mpc. A rough estimate of the cosmological gravitational potential at small k in the non-linear regime, Phi_nu = 10^{-6} (k/k_0)^{-2}, 1.2x10^{-2} h/Mpc < k_0 < 7.8x10^{-2} h/Mpc, turns out to be many orders of magnitude smaller than an extrapolation of the linear evolution of density fluctuations. The size of the neutrino-induced gravitational potential could modify the spectrum of CMB anisotropies for small angular momenta.