### pivot scale k_0 of cosmomc

Posted:

**December 08 2008**It seems that the default pivot scale k_0 of cosmomc is 0.005/Mpc, while the WMAP team set it to 0.002/Mpc. any particular reason for such choices?

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Posted: **December 08 2008**

It seems that the default pivot scale k_0 of cosmomc is 0.005/Mpc, while the WMAP team set it to 0.002/Mpc. any particular reason for such choices?

Posted: **December 08 2008**

I think cosmomc default is 0.05/Mpc.

If the pivot scale is much larger (smaller k) there are big degeneracies between the spectral index and amplitude when you have running (basically because the data is pinning things at a scale much smaller than 0.002/Mpc). The likelihood is easier to explore using a pivot choice where the parameters are more independent; 0.05/Mpc is however not an optimized choice, but inherited as a default from CMBFAST (I think).

On the other hand the tensor ratio is effectively measured on a larger scale.

If the pivot scale is much larger (smaller k) there are big degeneracies between the spectral index and amplitude when you have running (basically because the data is pinning things at a scale much smaller than 0.002/Mpc). The likelihood is easier to explore using a pivot choice where the parameters are more independent; 0.05/Mpc is however not an optimized choice, but inherited as a default from CMBFAST (I think).

On the other hand the tensor ratio is effectively measured on a larger scale.

Posted: **December 10 2008**

See Table 4 and section 5.5 of Komatsu et al. for a little discussion about this.

Posted: **December 12 2008**

There is actually a very nice paper by Cortes, Liddle, and Mukherjee

(http://arxiv.org/abs/astro-ph/0702170) in which they determine the pivot scale at which spectrum degeneracies are minimized, k = 0.017 h Mpc^-1. Their analysis was done for WMAP3.

(http://arxiv.org/abs/astro-ph/0702170) in which they determine the pivot scale at which spectrum degeneracies are minimized, k = 0.017 h Mpc^-1. Their analysis was done for WMAP3.