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[CosmoMC] Rombint failed to converge (Out of range finding H0)
 
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Mason Ng



Joined: 17 May 2017
Posts: 22
Affiliation: The University of Auckland

PostPosted: September 11 2017  Reply with quote

I'm actually running it with ModeCode, but the error is still from CosmoMC. I set param[w] = 0.33333 (radiation dominated; i.e. just after inflation).

However, I get the error as written in the title. This is probably a basic question - I looked at http://cosmocoffee.info/viewtopic.php?t=2593 which brought up the idea of finding the starting value of theta that commensurates with w -
how can I derive an 'intelligent' start value for theta?

In the radiation-dominated universe, I can write

D_A = \frac{1}{H_0(1+z)} \int_0^z \frac{dz}{\sqrt{\Omega_r(1+z)^4}}, where
a = \frac{1}{1+z} = \sqrt{2H_0\sqrt{\Omega_r}}; I'm not sure where to integrate to though. Additionally, I'm wondering where the equation of state parameter w comes in. Knowing DA, I was wondering if there was something I need to consider for getting the sound horizon distance rs. From there, I want to get θ. Thanks in advance.

For now, I have (conservatively)

Code:
H0_min = 1
H0_max = 150
param[theta] = 1.0411 1.00 1.08 0.0004 0.0004
param[w] = 0.33333 (but plan to put a prior on it).


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Antony Lewis



Joined: 23 Sep 2004
Posts: 1271
Affiliation: University of Sussex

PostPosted: September 17 2017  Reply with quote

Theta for any model that fits the CMB data will always be close to the LCDM best-fit value (by construction). Better to use a more sensible value for w (or other degenerate parameters).
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