pivot scale k_0 of cosmomc

 Posts: 13
 Joined: January 21 2008
 Affiliation: Max Planck Institute for Astrophysics
pivot scale k_0 of cosmomc
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?

 Posts: 1543
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
Re: pivot scale k_0 of cosmomc
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.
pivot scale k_0 of cosmomc
See Table 4 and section 5.5 of Komatsu et al. for a little discussion about this.

 Posts: 18
 Joined: August 08 2006
 Affiliation: IPMU, U. of Tokyo
pivot scale k_0 of cosmomc
There is actually a very nice paper by Cortes, Liddle, and Mukherjee
(http://arxiv.org/abs/astroph/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/astroph/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.