What value is expected for baryon to photon density?

[arroy_0205]

It is generally stated that observed baryon to photon density (about 6.1x10^(-10)) is too small. What is the reason? What ratio is expected?

 

[AnTiFreeze3]

http://lambda.gsfc.nasa.gov/product/map/pub_papers/firstyear/basic/wmap_params_table.pdf

This paper confirms your number of 6.1 * 10^(-1).

http://pdg.lbl.gov/2012/reviews/rpp2012-rev-cosmological-parameters.pdf

"A typical collection would be baryons, photons, neutrinos, and
dark matter (given charge neutrality, the electron density is guaranteed to be too small
to be worth considering separately and is included with the baryons)."

This would appear to answer your question as to whether or not it is commonplace to claim the ratio to be too small.

As for your question about what ratio is expected, I think my first link should suffice, but as to why this is so, I am not knowledgeable enough to know. I just used google and spent twenty minutes or so reading through papers.

"Even the baryon density, now measured to an accuracy of a few percent, lacks an underlying theory able to predict it even within orders of magnitude. Precision cosmology may have arrived, but at present many key questions remain to motivate and challenge the cosmology community." - (second link)

This article was updated June 18th, 2012, so that would lead to suggest some uncertainty in regards to our understanding of baryons and photons.