CosmoMC finding best-fit params

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
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yajing huang
Posts: 9
Joined: January 21 2015
Affiliation: Johns Hopkins University

CosmoMC finding best-fit params

Post by yajing huang » March 02 2017

Hi,

I was trying to use CosmoMC action=2 to find best-fit parameters for WMAP data. To make the results more reliable, I ran 4 minimizations at the same time as suggested in the CosmoMC README. I got the best-fit points without warning.

However, when I looked closer at each set of parameters given by the minimizations, although their Chi2 were very close, the parameters differed from one another quite significantly (at some fractions of a sigma). I think it's possible that the best-fit points I got were just local minimums. If so, is there some way to avoid them?

Below is the setting I used for the input file:

#when estimating best fit point (action=2 or estimate_propose_matrix),
#required relative accuracy of each parameter in units of the covariance width
max_like_radius = 0.0001
max_like_iterations = 100000
minimization_points_factor = 2
minimize_loglike_tolerance = 0.0005
minimize_separate_fast = T
#if non-zero do some low temperature MCMC steps to check minimum stable
minimize_mcmc_refine_num = 20
minimize_refine_temp = 0.01
minimize_temp_scale_factor = 5
minimize_random_start_pos = T


Is there anything I can do to make the results from action=2 more reliable?

Thanks a lot !

Yajing

Antony Lewis
Posts: 1352
Joined: September 23 2004
Affiliation: University of Sussex
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Re: CosmoMC finding best-fit params

Post by Antony Lewis » March 02 2017

I doubt they are local minima. The likelihood surface has degeneracies, so e.g. you can shift along the Ωm h3 degeneracy with little change in likelihood. Instability in the parameters is just telling you the best fit point is actually closer to a best fit line in parameter space. (plus numerical error of course)

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