Dude! Where's my baryons?

Now, I like dark matter and dark energy as much as the next person (ok, maybe not quite as much as Mark). Still, I simply don't have the temperment to spend the majority of my mental energy on ideas that are so speculative that, while interesting, they're probably wrong. I'm not trying to disrespect the theorists, but it takes a certain mindset to enjoy the type of process Sean described. If you read the three part series, nowhere will you see the evidence that Sean or his collaborators actually believe that inflation had a preferred direction. They just decided it would be cool to explore, and will shed no tears if inflation turns out to be a nice creamy vanilla of isotropy. Me, I'm enough of pragmatist that I'm happiest spending my time working on stuff that actually exists. So, no Nobel Prize for me, but I'm ok with that. What this has been leading me to think about over these past few weeks is where the normal matter actually wound up. Astrophysicists usually refer to "normal matter" as "baryons" -- stuff like neutrons and protons which carry most of the mass of the non-exotic universe -- and astronomically speaking, there's not that many places it can wind up. Most of the baryons in the Universe start out as hot ionized gas (i.e. a plasma of free electrons and positively charged nuclei), and as time goes on, some of this gas cools into colder phases (neutral atomic or molecular gas, which are fairly easy to detect), some of which further cools into dense stars (which then heat up again due to nuclear fusion in the core) or even into solids (dust or rocky planets). So, by the present day, normal matter is either in gas (of various temperatures, densities, and ionization states) or in stars, with a smattering of solids and a tiny pinch of liquids. Although these cooling and transformation processes are a rich field of "gastrophysics" (as the term goes), the overall landscape of baryons is sculpted by the dark matter, to first order. Dark matter has more than 80% of the mass of the Universe, so it dominates the gravitational forces in more than 99% of the volume of the Universe. Only when the barons become extremely concentrated (like in stars, or in the centers of galaxies) does normal matter significantly shape where stuff winds up. In principle. In reality, however, the baryons actually spend quite a bit of time bossing each other around, in spite of dark matter's best attempts to impose the kind of order a theorist would prefer. Not only does gas cool through interactions with itself (i.e. particle A gets near particle B, and an energetic photon ensues, running off with some of the particles' energy), it also heats back up, through supernovae, which spew superheated gas outwards which in turn shock-heats any cool gas unfortunate enough to get in its way, and possibly through jets emitted by accreting black holes in the centers of galaxies. (Astrophysical theorists are all a twitter about this last idea these days, but I haven't bought into it yet.) In other words, the observable Universe contains a level of complexity that should make any theorist shudder. Now, if you're not predisposed to embracing such complexity, why might you care? Well, back when I was a grad student (you know, when dinosaurs roamed the earth and the iPhone did not yet exist), light emitted by galaxies traced mass. If you knew where the galaxies were, you knew where the dark matter was. Theorists stuck in a simple numerical constant (a "bias factor") to allow some wiggle room, but mapping the observable Universe onto the dark sector was something we assumed we could do easily. Unfortunately, we've since come to appreciate how drastically baryonic process change where, when, and if stars form, and thus alter the numbers and properties of galaxies. These effects are worst in the smallest dark matter halos, where the gravitational binding energies approach the typical energies of baryonic heating by supernovae. Thus, when you count galaxies, you're no longer counting dark matter halos in a simple one-to-one mapping. Worse still, it's not even clear that most of the baryons wind up in galaxies at all. In the most massive gravitationally bound dark matter halos (i.e. those that surround clusters of galaxies), everything looks OK. If you add up the mass in stars and gas in the cluster, and compare it to the mass of the dark matter halo (which you've inferred from some combination of kinematics, gravitational lensing, or gas pressure gradients), you get about the ratio you expect from concordance cosmology. Basically, the cluster is massive enough that no matter what happened to the baryons, they got stuck in the gravitational potential well of the cluster, so they're all still there when you count them. On the scale of galaxies like the Milky Way, however, our best accounting says that maybe 20% of the normal matter that exists actually winds up as stars or easily detectable cool gas. So, for typical galaxies, 80% of the normal matter just doesn't seem to be around. It's probably in some hot phase that we have a hard time detecting with current instrumentation, but its not clear if it came into the galaxy and then got shot back out, or just never made it in. On the scale of the lowest mass galaxies (which are particularly interesting, because they give you the most leverage on the small scale power spectrum), we frankly don't know what the f*@$ is up. Really. I've spent the past week or so working hard on this problem, and the lowest mass galaxies are just a huge freakin' mess. We typically estimate the mass of the dark matter by using the motions of gas and stars and then assuming that if the whole mess isn't flying apart, then there must be some amount of mass holding the galaxy together. However, these low mass guys are so puny and pathetic that (1) you can't figure out if their motions are actually rotational or just localized gas physics pushing stuff around and (2) you can't figure out what fraction of the galaxyies' total motion is oriented along the line of sight. And don't even get me started on interactions with larger galaxies. Thus, you measure a characterstic range of internal velocities for these guys, and it tells you squat about the characteristic velocities of the dark matter halo the galaxy lives in. It's enough to make one go drink some beer. Which I will now do.