# Sigma_8 and n_s Cosmological Parameters

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1. Jan 20, 2016

### SteveDC

Hi, I'm doing a project on galaxy clustering and in a lot of papers where they list the cosmological parameters they've used, they list σ8 and ns. Can anyone quickly tell me what these parameters are referring to?

Thanks

2. Jan 20, 2016

### bapowell

$n_s$ is the spectral index of the scalar power spectrum ($P(k) \sim k^{n_s -1}$). $\sigma_8$ is the amplitude of the power spectrum on the scale of 8 Mpc/h.

3. Jan 20, 2016

### Chalnoth

$\sigma_8$ is the current amplitude of the matter power spectrum on that scale (as inferred by linear cosmological evolution). This parameter basically encodes information about how much large-scale structure has formed.

A Definition of $\sigma_8$ Aug 25, 2017