## Varying Helium fraction

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
Sunny Vagnozzi
Posts: 55
Joined: August 15 2016
Affiliation: Kavli Institute for Cosmology (KICC), University of Cambridge

### Varying Helium fraction

Hi Antony and all,

a quick question regarding varying the Helium mass fraction. Have I understood correctly that, if I want to do this, I should first of all switch off bbn_consistency, i.e. in params_CMB_defaults.ini

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bbn_consistency = F

and then just set the prior ranges for the parameter y_he as I would usually do, e.g.

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param[yhe] = 0.24 0.20 0.28 0.01 0.01

(the numbers I set are just for indicative purposes), where setting bbn_consistency to False is necessary to avoid bbn_consistency otherwise overwriting my prior on the Helium mass fraction? On a related note, say I were using only Planck data (TTTEEE+lowl+lowE), what would be reasonable values for the minimum and maximum ranges for yhe, and the proposed step size? I see the Planck public chains used

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param[yhe] = 0.245 0.1 0.5 0.006 0.006

is that what you would recommend?

Cheers,
Sunny

Antony Lewis
Posts: 1719
Joined: September 23 2004
Affiliation: University of Sussex
Contact:

### Re: Varying Helium fraction

The Planck chain settings should be fine.