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I've been reading that the results from the Planck satellite constrain a number called the "scalar spectral index" to be 0.96 rather than 1 at the 5-sigma level.

This is supposed to be big news, but I don't understand why (my ignorance, of course).

For instance, with an index of 0.96, can we say how many e-foldings occured during inflation? What else does it tell us?

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1 Answer 1

The scalar spectral index (usually denoted '$n_s$') describes how density fluctuations vary with scale. An index of unity means that the variations are the same on all scales. $n_s$ is an input parameter to $\Lambda$CDM, and influences the characteristic size-scales of structure formation (where this small of an adjustment has little effect). Inflationary models, on the other hand, do generally suggest particular values (often ranges) of $n_s$, and $n_s = 0.96$ is still very compatible with current models of inflation.

The plots I've seen definitely state that $n_s$ effects the number of e-folds, but I'm no expert on the subject, and will refrain from conjecturing wildly. I'd recommend taking a look here, and the references there-in.

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