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Old 06-02-2008, 03:59 PM
bryanmcc bryanmcc is offline
Join Date: Apr 2000
Location: St. Petersberg, FL USA
Posts: 428
Originally Posted by Chronos
It's been known for quite a while that there's baryonic matter out there that we can't easily see, but that's not very exciting, so the exotic stuff gets all the press.
Yes, but why is it known? It's perfectly reasonable that the definition of dark matter has changed since its existence was first deduced from gravitational effects, when the term was a catch-all for the unknown. But how do we now know that there's twice as much baryonic matter than we see, if we don't see it? It's not from its gravitation, unless the gravitation of baryonic matter can be distinguished from that of dark matter, which doesn't seem likely. It's not from its radiation, because then it wouldn't be invisible. So presumably it's a consequence of either some cosmological model(s), or there is some other reason to infer its presence...

...Ok, I read the article MikeS linked to. Apparently the percentages of the various nucleons produced is determined by the value of the baryon density parameter in the early universe, and by calculating backwards from observed percentages, the original baryon density can be determined. (Of course, this has a circular logic built into it: It is observed by averaging lots of oxygen/nitrogen-poor galaxies, say, that the percentages are 75%H/24%He/1%other, and those percentages are used to find the value of the baryon density that produces those results, which then, when plugged into the model predict twice the observed value of baryonic matter. So we'd better hope that the half of the missing baryonic matter doesn't contain percentages enough different from 75/24/1 to throw off the model that generated them!)

But still, that seems reasonably clear. One remaining question: The nucleosynthesis article mentions that "While the abundance predictions have traditionally been used to fix the correct value for eta, there are different possibilities for measuring that number. Most notably, the presence of particles like protons and neutrons in the early universe leaves a slight, but measurable imprint on the cosmic background radiation." So apparently WMAP was measuring eta directly--what imprint on the CMB was WMAP measuring to determine eta?