# A Definition of $\sigma_8$

1. Aug 25, 2017

### Einj

Hi everyone,
apologies if this question has been asked already. My search didn't give any results.

Can anyone tell me the definition of $\sigma_8$ in terms of the power spectrum? A reference where I could find it is perfect too!

Thanks a lot

2. Aug 25, 2017

### George Jones

Staff Emeritus
3. Aug 25, 2017

### Einj

Thanks a lot! Does that simply mean $\sigma_8=\mathcal{P}(k=1/8 \,h/Mpc)$? Where $\mathcal{P}$ is the power spectrum.

4. Aug 25, 2017

### George Jones

Staff Emeritus
I have attached one page from the second edition of "Fundamentals of Cosmology" by James Rich. Take $r=8$ in equation (7.32).

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• ###### sigma 8.pdf
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5. Aug 25, 2017

### kimbyd

And just to clarify, this is the matter power spectrum. There are other power spectrums that are used in cosmology.

6. Aug 26, 2017

### Einj

All clear now! Thank you so much everyone!