The idea of such a coupling has been addressed and developed by several authors within
MaVaNs theories first [356, 714, 135, 12, 952, 280, 874, 856, 139, 178, 177] and more recently
within growing neutrino cosmologies [36, 957, 668
, 963
, 962
, 727
, 179
]. It has been shown that
neutrinos can play a crucial role in cosmology, setting naturally the desired scale for dark energy.
Interestingly, a coupling between neutrinos and dark energy may help solving the ‘why now’
problem, explaining why dark energy dominates only in recent epochs. The coupling follows the
description illustrated in Section 1.4.4 for a general interacting dark-energy cosmology, where now
.
Typically, in growing neutrino cosmologies, the function is such that the neutrino mass grows
with time from low, nearly massless values (when neutrinos are non-relativistic) up to present masses in a
range in agreement with current observations (see the previous section of this review for latest bounds on
neutrino masses). The key feature of growing neutrino models is that the amount of dark energy
today is triggered by a cosmological event, corresponding to the transition from relativistic to
non-relativistic neutrinos at redshift
. As long as neutrinos are relativistic, the coupling
plays no role on the dynamics of the scalar field, which follows attractor solutions of the type
described in Section 1.4.4. From there on, the evolution of dark energy resembles that of a
cosmological constant, plus small oscillations of the coupled dark energy-neutrino fluid. As a
consequence, when a coupling between dark energy and neutrinos is active, the amount of dark
energy and its equation of state today are strictly connected to the present value of the neutrino
mass.
The interaction between neutrinos and dark energy is a nice and concrete example of the significant
imprint that dynamical coupled dark energy can leave on observables and in particular on structure
formation and on the cosmic microwave background. This is due to the fact that the coupling,
playing a role only after neutrinos become non-relativistic, can reach relatively high values as
compared to gravitational attraction. Typical values of are order
or even more
such that even the small fraction of cosmic energy density in neutrinos can have a substantial
influence on the time evolution of the quintessence field. During this time the fifth force can be of
order
times stronger than gravity. The neutrino contribution to the gravitational
potential influences indirectly also dark matter and structure formation, as well as CMB, via the
Integrated Sachs–Wolfe effect and the nonlinear Rees–Sciama effect, which is non-negligible at the
scales where neutrinos form stable lumps. Furthermore, backreaction effects can substantially
modify the growth of large scale neutrino lumps, with effects which are much larger than in the
dark matter case. The presence of a fifth force due to an interaction between neutrinos and
dark energy can lead to remarkably peculiar differences with respect to a cosmological constant
scenario.
Here, we just recall some of the typical features that can arise when such an interaction is active:
Investigation of structure formation at very large scales (order ) as well as cross
correlation with CMB are crucial in order to disentangle coupled neutrino-quintessence cosmologies from a
cosmological constant scenario. Detection of a population of very large-scale structures could pose serious
difficulties to the standard framework and open the way to the existence of a new cosmological interaction
stronger than gravity.
http://www.livingreviews.org/lrr-2013-6 |
Living Rev. Relativity 16, (2013), 6
![]() This work is licensed under a Creative Commons License. E-mail us: |