There have been many reports floating around recently about the discovery of a good chunk of the “missing” matter of the Universe. First, some preliminary accounting of the Universe’s matter:
74% dark energy
22% dark matter
4% ordinary (or baryonic) matter
OK, easy enough. The recent reports have been stating that only ~50% of “known” baryonic matter had actually been observed, and a chunk of the missing portion has been discovered as a diffuse plasma between galaxy clusters.
Also easy enough. However, what I haven’t seen explained is how 50% of “known” baryonic matter could be missing. Isn’t missing, invisible matter that we can deduce must be there due to its gravitational influence on galactic rotation curves, etc., exactly what defines dark matter? How is it that we know that the universe is comprised of 22% dark matter, plus 2% baryonic matter that is also, well, dark, but is not “dark”?
I suspect that this has something to do with how WMAP used the CMB to come up with the 74/22/4 breakdown, but it beats me how it’s explained…
It’d help if you could link to some of the reports you’re talking about — there’s probably some key phrases in them that us scientifically-inclined folks could use to better interpret what they’re talking about.
That said, there are ways to get at the absolute amount of “baryonic matter” in the Universe. The CMB angular correlation studies of WMAP are indeed one of them. Another is Big Bang nucleosynthesis, which basically treats the early Universe like a big fusion reactor and figures out how much of the various isotpoes of helium, lithium, and so forth should have been created in the first few minutes of the Universe’s creation. Too many baryons would cause the relative abundances of isotopes to shift around.
Nowadays, when people refer to “dark matter”, they mostly mean stuff which just plain can’t interact significantly with light, not just stuff which can interact but happens not to be. This is the 22% you’re referring to. More properly, it ought to be called “non-baryonic dark matter”, but what can you do?
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?
