CosmoFit.OmegaBLikelihood¶

class CosmoFit.OmegaBLikelihood(cosmology, version='bbn2024')[source]¶

Bases: GaussianPriorLikelihood

A BBN constraint on the physical baryon density omega_b h^2.

This is the piece that makes a BAO-only fit able to say anything about H0 at all. BAO measures D/r_d, and r_d depends on omega_b and omega_m – so with r_d computed rather than left free (see SoundHorizon), a BBN prior on omega_b closes the loop and turns BAO into an absolute distance measurement. That is exactly how the DESI “BAO + BBN” constraints are produced, and it is a genuinely CMB-independent route to H0.

Versions: "bbn2024" (default; Schoeneberg 2024, the conservative PDG-abundance value DESI adopts, 0.02218 +- 0.00055) and "cooke2018" (0.02166 +- 0.00019 – three times tighter because it propagates only the deuterium measurement error, not the nuclear-reaction-rate spread; a deliberate choice, not a better default).

Parameters:: version (str)

__init__(cosmology, version='bbn2024')[source]¶

Parameters:: version (str)

Methods

`__init__`(cosmology[, version])
`chi2`()	Chi-square statistic.
`log_likelihood`()	Return log-likelihood.
`model`()	Predicted value of the constrained quantity.
`predictions`()	Alias for model().
`residuals`()	Data minus model residual (a length-1 vector).
`summary`()	Return a summary of the likelihood evaluation.

Attributes

`n_data`	Number of data points.
`name_and_size`	Name together with the number of data points.
`observable`