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CosmoMC finding best-fit params
 
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yajing huang



Joined: 21 Jan 2015
Posts: 8
Affiliation: Johns Hopkins University

PostPosted: March 02 2017  Reply with quote

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
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1185
Affiliation: University of Sussex

PostPosted: March 02 2017  Reply with quote

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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