Calculating sigma_8 in CAMB

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
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Alexandr Malinovsky
Posts: 3
Joined: December 19 2006
Affiliation: ASC

Calculating sigma_8 in CAMB

Post by Alexandr Malinovsky » December 19 2006

Good day.

May be my question looks very naive, but I really don't understand this. Speaking clearly, when I'm tried to calculate \sigma_{8} in CAMB, using suppled program sigma8.f90, results was very unexpected for me. Namely, with increasing matter density \sigma_{8} increased as well (for example, for \Omega_{m} = 0.25 \sigma_{8} = 0.665; for \Omega_{m} = 0.6 \sigma_{8} = 1.161, and so on), contradictory to my previous experience (say, like old familiar \sigma_{8}\Omega_{m}^0.5 ~ 0.5).

Could anybody do me a favour, and explain me my mistake?

Thank you in advance,
Alexandr.

David Parkinson
Posts: 23
Joined: January 19 2005
Affiliation: KASI
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Calculating sigma_8 in CAMB

Post by David Parkinson » December 19 2006

As the value of [tex]\Omega_m[/tex] increases, the epoch of matter-radiation equality moves further into the past, and so perturbations in the matter, which were frozen in during radiation-domination, have longer to grow. Therefore, you would expect the matter power on small scales (as normalised by [tex]\sigma_8[/tex]) to increase as [tex]\Omega_m[/tex] increases.

The relationship [tex]\sigma_8 \Omega_m^{0.5} \sim 0.5 [/tex] is an observational result (which I think comes from Clusters - please correct me if I am wrong) rather than a physical "law".

Alexandr Malinovsky
Posts: 3
Joined: December 19 2006
Affiliation: ASC

Calculating sigma_8 in CAMB

Post by Alexandr Malinovsky » December 20 2006

Thank you. :) After your explanation whole situation is very clear for me.

Moreover, I understand perfectly now, why the cluster data analysis leads to degeneracy this type (\sigma_{8}\Omega_{m}^{a} ~ b). :)

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