SN Ia contours - Union2.1 data (CosmoMC October 2012 version)

[Gudmundur Kari Stefansson]

I am trying to replicate the [tex]\Omega_\Lambda[/tex] vs [tex]\Omega_m[/tex] contours expected from the Union2.1 SN Ia data (see the blue contours in the left figure below). I am using the October 2012 version of CosmoMC with the additional Union2.1 module which can be gotten from the Supernova Cosmology Project website (Here: http://supernova.lbl.gov/Union/).

I first tried with the normal parameter values in parameters.ini with little luck - the 2D contours were observed to be perpendicular to the desired result (similar to the gray intersection area in the left figure).

I attributed this to [tex]\Omega_K[/tex] being assumed zero and having too low of a starting-width for both [tex]\Omega_b h^2[/tex], and [tex]\Omega_c h^2[/tex]. I thus changed the original parameter values to;

param[omegak] = 0 -0.7 0.7 0.05 0.001
param[omegabh2] = 0.0223 0.005 0.1 0.05 0.001
param[omegadmh2] = 0.105 0.01 0.99 0.1 0.01

Such calculations frequently yielded the error:
"Reionization_zreFromOptDepth: Did not converge to optical depth"
which I seemed to have solved by making a tighter constraint on [tex]\tau[/tex] (as was proposed in another post here on CosmoCoffee), setting;

param[tau] = 0.088 0.01 0.2 0.001 0.003

Subsequent runs seem to be going in the right direction, but it seems like the chain runs into some barrier (see the arrow in the right figure).



So I want to ask, should I try to experiment more with changing the parameters above, or are there other parameters that I should change to get over this sharp barrier?

Any help is greatly appreciated!

[Antony Lewis]

Probably the H0 prior? The April 2013 version has an alternative background Omegam parameterization that makes it easier to do this kind of thing.