Neutrino lumps and the Cosmic Microwave Background
  The interaction between the cosmon and neutrinos may solve the "why now
problem" for dark energy cosmologies. Within growing neutrino quintessence it
leads to the formation of nonlinear neutrino lumps. For a test of such models
by the integrated Sachs-Wolfe effect for the cosmic microwave background (CMB)
we estimate the size and time evolution of the gravitational potential induced
by these lumps. A population of lumps with size of 100 Mpc or more could lead
to observable effects on the CMB anisotropies for low angular momenta. The
linear approximation is found to be invalid for the relevant length scales.
Quantitative estimates depend strongly on the details of the transition between
the linear and nonlinear regimes. In particular, important backreaction effects
arise from the nonlinearities of the cosmon interactions. At the present stage
the uncertainties of the estimate make it difficult to constrain the parameter
space of growing neutrino models. We explicitly discuss scenarios and models
that are compatible with the CMB observations.
