Hi,
I was trying to use CosmoMC action=2 to find bestfit 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 bestfit 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 bestfit 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 nonzero 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
CosmoMC finding bestfit params

 Posts: 9
 Joined: January 21 2015
 Affiliation: Johns Hopkins University

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 Joined: September 23 2004
 Affiliation: University of Sussex
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Re: CosmoMC finding bestfit params
I doubt they are local minima. The likelihood surface has degeneracies, so e.g. you can shift along the Ω_{m} h^{3} 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)