LambdaCDM model with Broken Scale Invariance confronted with observations

We studied in details a class of spatially flat models with cold dark matter (CDM), a cosmological constant and a broken-scale-invariant (BSI) steplike primordial spectrum of adiabatic perturbations in which the inflaton potential has a rapid change of its first derivative. The matter power spectrum can account for a feature at k = 0.05 hMpc^{-1}. Best fits with the observational data are obtained for currently favoured cosmological parameters (Omega_Lambda ~= 0.7, h ~= 0.7) and an inverted step (less power on large scales). These models can produce a gravitational waves spectrum of arbitrary amplitude. Observations allow models with a large contribution: C^T₁₀ / C^S₁₀ <= 1. Using polarisation measurements on the precision level scheduled for Planck, it is possible the extract the ``step'' parameters with very high accuracy.