3.4.1 Uncertainties in the power spectrum
As mentioned in the introduction, some shortcomings on galactic and sub-galactic scales of the currently favored model of hierarchical galaxy formation in a universe dominated by CDM have recently appeared. The significance of these discrepancies is still being debated, and "gastrophysical" solutions involving feedback mechanisms may offer a possible way out. Other models have been constructed in an attempt to solve the apparent small-scale problems of CDM at a more fundamental level, i.e. by reducing small-scale power. Although the "standard" ACDM model for structure formation assumes a scale-invariant initial power spectrum of density fluctuations with index n =1, the recent WMAP data reveal strong evidence for a departure from scale invariance, with a best-fit value n = 0.951+°019. Furthermore, the 1-year WMAP results favored a slowly varying spectral index, dn/dln k = -0.031+°;°16, i.e. a model in which the spectral index varies
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