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4.1. Power-law behaviour

If the inflationary paradigm survives the tests above, it will be time to decide which of the existing inflation models actually fits the data. In most models, to a good approximation the density perturbations are given by a power-law and the gravitational waves are at best marginally detectable by PLANCK. Accurate measures of these two quantities have the potential to exclude nearly all existing inflation models.

At present, the spectral index is very loosely constrained; in general the limits are probably around 0.8 < n < 1.3, though if specific assumptions are made (e.g. critical matter density or significant gravitational waves) this can tighten. As it happens, this entire viable range is fairly well populated by inflation models, which means that any increase in observational sensitivity has the power to exclude a significant fraction of them.

A benchmark for future accuracy is the PLANCK satellite; recently a detailed analysis, including estimates of foreground removal efficiency, concluded that it would reach a 1-sigma accuracy on n of around ±0.01 [22]. By contrast, the 1-sigma error from the MAP satellite is predicted to be in the range 0.05 to 0.1, which in itself may not significantly impact on inflationary models, though it may be powerful in combination with probes such as the power spectrum from the Sloan Digital Sky Survey.