Nonlinear growing neutrino cosmology

TL;DR

This paper demonstrates that in Growing Neutrino Quintessence models, nonlinear neutrino structures form rapidly but are stabilized, potentially allowing cosmic neutrino background detection, and addresses the timing of Dark Energy's influence.

Contribution

It shows that nonlinear structure formation in Growing Neutrino Cosmology does not spoil the evolution and links Dark Energy onset to neutrino mass transition.

Findings

Neutrino forces can be up to 10^6 times gravity.

Neutrino fluctuations become nonlinear at redshift z ≈ 2.

Large-scale neutrino lumps may be observable.

Abstract

The energy scale of Dark Energy, $\sim 2 \times 10^{-3}$ eV, is a long way off compared to all known fundamental scales - except for the neutrino masses. If Dark Energy is dynamical and couples to neutrinos, this is no longer a coincidence. The time at which Dark Energy starts to behave as an effective cosmological constant can be linked to the time at which the cosmic neutrinos become nonrelativistic. This naturally places the onset of the Universe's accelerated expansion in recent cosmic history, addressing the why-now problem of Dark Energy. We show that these mechanisms indeed work in the Growing Neutrino Quintessence model - even if the fully nonlinear structure formation and backreaction are taken into account, which were previously suspected of spoiling the cosmological evolution. The attractive force between neutrinos arising from their coupling to Dark Energy grows as large as $10^6$ times the gravitational strength. This induces very rapid dynamics of neutrino fluctuations which are nonlinear at redshift $z \approx 2$. Nevertheless, a nonlinear stabilization phenomenon ensures only mildly nonlinear oscillating neutrino overdensities with a large-scale gravitational potential substantially smaller than that of cold dark matter perturbations. Depending on model parameters, the signals of large-scale neutrino lumps may render the cosmic neutrino background observable.