I just received the following workshop announcement:

11th Birmingham-Nottingham Extragalactic Workshop - 1st Announcement

“Semi-analytic models - are we kidding ourselves?“

Refeshingly honest conference title aside, this is a terrific topic for a workshop. Semi-analytic galaxy formation models are extremely useful tools, which consist of (1) an underlying prescription for the growth of dark matter halos and (2) a set of knobs for grafting complicated baryonic physics onto those halos. The first step is well-understood analytically, and has been reliably calibrated with N-body simulations. The second step, however, contains a lot of crafty juju (how much gas winds up inside the dark matter halos? At what rate does that gas cool? How and when does that gas convert into stars? How does the formation of stars and the subsequent supernovae affect the surrounding gas? How do mergers between dark matter halos change the spatial distribution of the stars and the temperature structure of the gas?). The developers of these models are smart folks, and make reasonably well-informed assumptions about all of these baryonic processes, leading to results that are decent matches to the ensemble properties of the galaxy populations. (Note that I didn’t say “predict results” — these models are ex post facto in most usages.) However, just because the models can be tuned to produce a rough statistical match to observations in no way means that the input assumptions are correct or unique descriptions of what actually happens. Moreover, there are serious discrepancies between the models and the observed properties of very low mass galaxies — when the models are tuned to match the properties of relatively massive galaxies, they predict that the low mass galaxies are red and gas poor, whereas the observations say they’re blue and gas rich. It’s great that the community is looking at these issues head on, given the usefulness the semi-analytic galaxy models.

I’m usually not one to go all blog-happy about the latest press-release, but a recent one happened to be about a paper I had just read, so, I’ll dive on in.

To give a little background, one game that astronomers like to play is “Find the oldest galaxy around at some time”. There are a number of reasons this game has so many eager practitioners. First, the oldest galaxies at any time are probably the locations of the some of the very first collapsed structures in the universe, which make them cool. Second, the most massive galaxies we see today seem to be those that that host the oldest stellar populations — turning this around, if you hunt for the oldest things at a given time, you’re hopefully finding the most massive things at that time. Finally, it’s very tough to figure out how the population of galaxies you see at one time is related to the one you see at the present day (i.e., just because a galaxy is blue and distorted 10 Gigayears (Gyr) ago, doesn’t mean it’s not red and symmetric today). However, if you stick to studying massive galaxies that already looked old 10 Gyr ago, it’s probably a reasonably good guess that they’re comparable to the predecessors of the most massive old galaxies today, giving you one of the few cases where you can match up the two populations with reasonable confidence.

Which brings us to the latest paper by Pieter van Dokkum and his collaborators, on the spectroscopic and HST/NICMOS follow-up of 9 massive red galaxies from the MUSYC NIR imaging survey. These galaxies are at redshifts of 2.5-ish, corresponding to about 2-3 Gyr after the Big Bang for WMAP cosmology. They are very red, and have spectra consistent with a dormant stellar population (i.e. one that stopped forming stars roughly a Gyr previously). An earlier paper by Kriek et al estimated masses for these galaxies (assuming reasonable models of the kinds of stars and dust that are in the galaxies), and found that not only are the galaxies dormant, they’re also extremely massive.

So, now you’ve got yourself a bunch of massive galaxies, that in spite of being no more than 2-3 Gyr old, already finished all their star formation a gigayear previously. Pretty nifty, and what you’d hope to find as the precursors of the old massive galaxies we see today. That’s all well and good, but the problem is that the galaxies are just too dang small. They’re tiny. The most massive galaxies today are typically much larger than the Milky Way, but the galaxies van Dokkum is reporting on are much much smaller. (On the plot at right, the solid scale bar shows a size of 10 kpc. On the plot below, the big solid dots are van Dokkum’s sample, and the little dots are the galaxies seen today in the Sloan Digital Sky Survey.)

Ok fine, you might say. A lot can happen in the intervening 10 Gyr. However, not that much can happen, since these galaxies are already pushing the mass limit of the most massive galaxies we see today. You therefore can’t grow the galaxies by padding them out with new stars from accreted satellite galaxies, without taking them over the limit (i.e. looking at the plot on the upper left, accreting new galaxies would take the galaxies up, but would also move them to the right). Even if you could pull this off, the centers would still be too dense. You also can’t magically puff the galaxies up over time. You could imagine perhaps driving a bunch of mass out of the center of the galaxy, maybe through stellar winds, but it would take an absurd amount of mass loss, even if you believed you could get all that mass out of such a dense honking galaxy. You could also imagine trying to disrupt the galaxies somehow by puffing them up kinematically, maybe through violent mergers. However, the problem here is that the galaxies are waaaaaay too dense. If the galaxies really are as dense as the plot above suggests, then they’re going to be almost impossible to disrupt. It would be like trying to disrupt an iron ball by dropping it into a bowl of pudding.

My take on this is that some of the assumptions that went into making the plot above are wrong, because it’s very hard to imagine hiding the descendants of incredibly massive, incredibly dense galaxies somewhere in the local universe. The authors understand this, and argue that it might be a combination of issues related to measuring the radius (loss of diffuse light at large radii, radial gradients in the conversion of light to stellar mass), and possibly the initial mass function (the great “here be dragons” of all extragalactic astronomy).

I actually think the errant step might wind up being the assumed conversion of light to mass. The standard lore is that when you observe galaxies in the NIR, the light is dominated by old red giant branch stars, giving you a reasonably robust conversion from light into stellar mass. However, at a redshift of 2.5, there’s no way that any star in the galaxy is older than 3 Gyr. In this case, most of the red light from the galaxy will be coming from asymptotic giant branch stars, which are notoriously difficult to model. Thus, the calibration of light to mass can easily be off. AGB stars can also give you a nice red spectrum like those observed, and can be potentially centrally concentrated in response to a central burst of star formation. I think the authors have done the best that they could with their analysis, but I suspect we may have run into some of the real limits of our current modeling.

While deeply held feelings about string theory (”Genius!” “Total Bunk!”) may sometimes drive us apart, all of us can certainly get behind the theory that chocolate is a net good. However, in spite of its appeal as a tasty eatable (with or without bacon), it’s actually a bit of a pain to work with. If you’ve ever tried to use chocolate in its melted form, you’ve probably discovered that chocolate has a number of peculiarities that frequently thwart your best culinary efforts. For example, if your melted chocolate becomes contaminated with an errant drop of water, the chocolate siezes up. If you try to reharden chocolate that’s been melted (say, in making chocolate covered strawberries), you’re frequently left with a matte finish and crumbly texture that in no way resembles the dark glossy chocolate you began with.

The reasons for this should be familiar to any solid state physicist (or at least, they were to the one who made my wedding cake and first clued me in). Cocoa butter, one of the dominant ingredients in chocolate, contains several triglycerides that lock into a crystal form when cooled. However, there is not just one form that the triglycerides can lock into, but six of them (β(I) through β(VI)). Each successive form is more stable and has a higher melting point. Almost all commercial chocolate is in the β(V) form — from what I can tell, you only get to sample β(VI) in the afterlife, if you’ve been very, very good. When chocolate goes all wrong, it is usually a failure of the melted and cooled chocolate to recrystallize into the β(V) state. Similar problems can affect commercial chocolate suppliers as well, leading to chocolate that develops that unsightly chalky film we associate with old chocolate. Even previously stable β(V) chocolate can wind up with the same unsightly film after temperature fluctuations break down the crystal structure, or melt and reharden a thin layer on the surface. Given the commercial implications, there’s been some solid technical work on the structure of the magical β(V) form, which has been studied with x-ray diffraction using synchrotron radiation (more technical data here).

Given the above, when cooking with chocolate, one’s goal is to coax the cooled chocolate back into the β(V) form if one wants the end product to look glossy, be solid at room temperature, and have a nice crisp snap when bitten. The traditional mechanism for this is known as tempering (video here). Traditional tempering involves carefully controlling the temperature of the chocolate as it cools, so that the chocolate favors the preferred crystalline state. However, there is a vastly simpler mechanism, namely, seeding the crystal. If you take a lump of unmelted commercial chocolate, toss it into your bowl of melted chocolate, and stir for a bit, you’ll melt the new lump while cooling the melted chocolate. The cooling chocolate will then prefer the same crystal structure as the melting lump, such that when it hardens completely, you’ll find it in the luscious β(V) state.

PS. I can verify that the above works exactly as advertised. Last weekend I made the wedding cake above for the same solid state physicist who made mine a decade ago. (The cake was alternately described as looking like the Heatmiser’s hair, Mordor, and Garrett Lisi’s E₈ symmetry group, so you can imagine it was a pretty techie crowd). Making the thin chocolate sheets from which I cut the decorations, I got huge swaths of perfectly glossy chocolate. Occasionally, though, there’d be a small section with a matte surface, that was clearly a different crystalline form. Science. It works, bitches.

Remember E = mc²? It’s the one equation that you are allowed to include in your popular-physics book (unless you’re George Gamow, who couldn’t be stopped). Mark gave a nice explanation of why it is true some time back, and I babbled about it some time before that. For a famous equation, it tends to be a bit misunderstood. A profitable way to think about it is to divide both sides by the speed of light squared, giving us m = E/c², and take this as the definition of what we mean by mass. The mass of some object is just the energy it has in its rest frame — according to special relativity, the energy (not the mass!) will be larger if the object is moving with respect to us, so the mass of an object is essentially the energy intrinsic to its state, rather than that imparted by its motion. Energy is the primary concept, and mass is derived from it. Interestingly, the dark energy that makes up 70% of the energy of the universe doesn’t really have “mass” at all, since it’s not made up of objects (such as particles) that can have a rest frame — it’s a smooth field filling space.

All of which is to say that the mainstream media have let us down again. C. Clairborne Ray, writing in the New York Times, attempts to explain whether a spinning gyroscope weighs more than a stationary one, and answers “The weight stays the same; there is no known physical reason for any change.” Actually, there is! The spinning gyroscope has more energy than the non-spinning one. As a test, we can imagine extracting work from the spinning gyroscope — for example, by hooking it up to a generator — in ways that we couldn’t extract work from the stationary gyroscope. And since it has more energy, it has more mass. And the weight is just the acceleration due to gravity times the mass — so, as long as we weigh our spinning and non-spinning gyroscopes in the same gravitational field, the spinning one will indeed weigh more.

Admittedly, it’s a very tiny difference — the energy will increase by an amount proportional to the speed of the spinning gyroscope, divided by the speed of light, that quantity squared, which is really tiny. Nothing you’re going to measure at home. (I’m guessing it’s never even been measured in any laboratory, but I don’t know for sure.) And the article is correct to emphasize that there is no difference in mass that depends on the direction of spin of the gyroscope — that would violate Lorentz invariance, which is something worth looking for in its own right, but would be a Nobel-worthy discovery for anyone who found it.

You may have heard some of the buzz about a new result concerning the direct detection of dark matter particles in an underground laboratory. The buzz originates from a new paper by the DAMA/LIBRA collaboration; David Harris links to powerpoint slides from Rita Bernabei, leader of the experiment, from her talk at a meeting in Venice.

The new experiment is an upgrade from a previous version of DAMA, which had already been on record as having recorded a statistically significant signal of the form you would expect from the collision of weakly interacting massive particles (WIMP’s) with the detector. The experiment uses a challenging technique, in which their focus is not on eliminating all possible backgrounds so as to isolate the dark-matter signal, but to look at the annual modulation in that signal that would presumably be caused by the Earth’s orbital motion through the cloud of dark matter in the Solar System: you expect more events when we are moving with a high velocity into the dark-matter wind. Other workers in the field have not been shy about expressing skepticism, but the DAMA team has stood their ground; as Jennifer notes in her report from the recent APS Meeting, the DAMA collaboration home page currently features a quote from Kipling: “If you can bear to hear the truth you’ve spoken/ twisted by knaves to make a trap for fools,/ ……………you’ll be a Man my son!”

To help provide some insight and context, we’ve solicited the help of a true expert in the field — Juan Collar of the University of Chicago. I got to know Juan back in my days as a Midwesterner, and a trip to his bustling underground experimental empire was always a highlight of anyone’s visit to the UofC physics department. You can hear him talk about his own work in this colloquium at Fermilab; he’s agreed to post for us about his views on the new DAMA result, and more general thoughts on what it takes to search for 25% of the universe. I promise you won’t be bored.

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My dear friend Sean has me blogging: hey, I’ll try anything once. On the subject of the recent DAMA results no less, as per his request. I am normally a bit of a curmudgeon but… Sean, you really want the worst of me out there permanently on the internets, don’t you?

I’ll try to keep this to the point. A bard I am not, nor the subject invites any poetry. I have therefore chosen brief eruptions of flatulence as the metric and style for this piece. The result of indigestion, you see. I’ll start with the most negative, so as to end up on a brighter note:

• The modulation is undeniable by now. I don’t know of any colleagues who doubted these data were blatantly modulated already back in 2003, when “the lady” (DAMA) decided to keep mum for a while. However, to conclude from something this mundane that the experiment “confirms evidence of Dark Matter particles in the galactic halo with high confidence level” or that there is “an evidence for the presence of dark matter particles in the galactic halo at 8.2 sigma confidence level” is simply delusional. There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right? (Occam is turning in his grave, rusty razor still in hand. He is thinking a remake of that opening scene in “Un chien andalou”, with help from this little lady. I am channeling him loud and clear).
• Someone should take the DAMA folks aside for a beer, make them see the following. If one day soon we are all convinced that this effect was DM-induced (see below for what that will really take), they will be recognized for one of the greatest discoveries in the history of science, without them having to look desperate or foolish today. Or making the rest of us in the field do, by association: thanks DAMA, for cheapening the level of our discourse to truly imbecilic levels. (Sean, if you edit this I will scratch the paint off your car. I may not write blogs, but I do read them: I know how to hurt you).
• Deep breath. Having cleared the air some (or just made it toxic, whatever), it is not DAMA’s fault that there is a penury of signatures in this field of ours, laboratory searches for particle dark matter. The one possible exception to this is a detector with good recoil directionality and sufficient target mass to be truly competitive, but we don’t know of a good enough way to do this as of today (“good enough” folds in the price tag). People are still trying. The diurnal modulation in the DM signal that would be sensed by such a device is wickedly rich in features, extremely hard for nature to imitate with anything else. The annual modulation resides on the other side of this spectrum of complexity. It is the poor man’s smoking-gun to DM “evidence”. Inspected carefully, it is disappointingly feeble: different models of the halo can shift the phase of this modulation completely, turning expected maxima into minima and vice-versa, changing the expected amplitude as well. Add to this the fact that essentially every possible systematic effect able to pass for a “signal” can be yearly-modulated, for one reason or another. That’s the ones we can presently think of, and the ones yet to be proposed. To grow convinced that we have observed dark matter in the lab we’ll require a number of entirely different techniques, using a variety of targets, all pointing at the same WIMP (mass, cross sections), with additional back-up information from accelerator experiments and from gamma-ray satellite observations (so-called indirect searches). All of those lines crossing at one point, so to speak. This I (for one) will call “evidence”. I know of no single existing or planned DM experiment, including those I participate in, that would be able to make anything close to a bulletproof claim on its own. My advice to any overambitious individuals looking for a quick kill is to look elsewhere in physics. WIMP hunting is not it, no matter how important the discovery of these particles might be.

Continue reading ‘Guest Post: Juan Collar on Dark Matter Detection’

One beautiful Fall day seventeen years ago I wandered into an office and my life profoundly changed. I was an undergraduate at Princeton, and was looking for a thesis advisor. Jadwin Hall was an intimidating place. Plenty of names familiar from my textbooks. Nobel laureates scattered about. And we were expected to just barge into their offices, and ask to work with them.

One office door was always open. As you walked by you could peek in, and see its occupant hard at work. Hunched over his notebook, scribbling away. Or standing by his bookcase, deep in thought. Most often at the blackboard, chalk in hand. This was John Archibald Wheeler, one of the legends of modern physics. He did foundational work on quantum mechanics, collaborating with Niels Bohr on some of the earliest work in nuclear fission. He invented the S-matrix. He played important roles in both the Manhattan project (atomic bomb) and the Matterhorn project (Hydrogen bomb). He made major contributions to general relativity, co-authoring with Charlie Misner and Kip Thorne the bible of the field. He was legendary for his way with words, coining such terms as wormholes, quantum foam, black holes, and the wave function of the Universe (the Wheeler-DeWitt equation). He trained generations of students; one of his first was Richard Feynman.

Fortunately, being a relatively clueless 20-year old, I was only dimly aware of these things. I was interested in gravity and cosmology, and I had heard Wheeler knew a thing or two about such topics. So I waltzed in, and asked if he had any projects I could work on. I staggered out of his office four hours later, laden with books, a clearly defined project in my hands. For the ensuing two years I spent essentially every weekday with Wheeler. Each morning I would rush over to his office, always to be greeted the same way: “What’s new?” I would have been up late the night before, desperately trying to find something interesting with which to answer that question. We would then spend hours working together, going over my results, scrutinizing my calculations, poring through the literature, brainstorming new ideas. Wheeler gave me a direct and personal introduction to the joys of research. We would break for lunch, and walk up to the faculty club. I often had trouble keeping up with him. He would always take the stairs (”No time to wait for an elevator!”). He would hook his arm into the banisters, and swing around, practically leaping from one flight to the next. This was 1990; Wheeler was 79 years old.

We would often work all afternoon (with the occasional interruption, the nuisance of having to leave for my class lectures). Every evening I would walk with him from Jadwin up across the full length of campus, to catch his bus. We would pass the corner of Ivy lane and Washington road, where he had scratched 137 into the concrete when they were pouring the sidewalk. We would pass Jones Hall, where he used to discuss relativity with Einstein. We would continue on through campus, crossing in front of Nassau Hall. Wheeler would insist we walk diagonally to the far gate, instead of exiting through the more convenient FitzRandolph Gate. An Undergraduate was not meant to exit FitzRandolph Gate until graduation, and Wheeler didn’t want to be responsible for what might occur were I to break tradition.

For two years I sat at the feet of the master, and I absorbed as much as I could. I learned about science, and about life. Wheeler had broad interests. We would often discuss biology, or history, or poetry. Over the ensuing years we kept in touch. We collaborated together on Wheeler’s last published paper.

Yesterday I spent a couple of hours at Wheeler’s bedside. I tried to say thank you. But it was impossible to convey how much he means to me, and how grateful I am to him. In that moment when I crossed the threshold to his office, I was embarking on a new path. I am still on that path, and every day I am grateful to him for showing me the way.

John Wheeler died this morning.

Today’s Bloggingheads dialogue features me and writer John Horgan — I will spare you a screen capture of our faces, but here is a good old-fashioned link.

John is the author of The End of Science, in which he argues that much of modern physics has entered an era of “ironic science,” where speculation about unobservable things (inflation, other universes, extra dimensions) has replaced the hard-nosed empiricism of an earlier era. Most of our discussion went over that same territory, focusing primarily on inflation but touching on other examples as well.

You can judge for yourself whether I was persuasive or not, but the case I tried to make was that attitudes along the lines of “that stuff you’re talking about can never be observed, so you’re not doing science, it’s just theology” are woefully simplistic, and simply don’t reflect the way that science works in the real world. Other branches of the wavefunction, or the state of the universe before the Big Bang, may by themselves be unobservable, but they are part of a larger picture that remains tied to what we see around us. (Inflation is a particularly inappropriate example to pick on; while it has by no means been established, and it is undeniably difficult to distinguish definitively between models, it keeps making predictions that are tested and come out correct — spatial flatness of the universe, density fluctuations larger than the Hubble radius, correlations between perturbations in matter and radiation, fluctuation amplitudes on different scales that are almost equal but not quite…)

If you are firmly convinced that talking about the multiverse and other unobservable things is deeply unscientific and a leading indicator of the Decline of the West, nothing I say will change your mind. In particular, you may judge that the question which inflation tries to answer — “Why was the early universe like that?” — is a priori unscientific, and we should just accept the universe as it is. That’s an intellectually consistent position that you are welcome to take. The good news is that the overwhelming majority of interesting science being done today remains closely connected to tangible phenomena just as it (usually!) has been through the history of modern science. But if you instead ask in good faith why sensible people would be led to hypothesize all of this unobservable superstructure, there are perfectly good answers to be had.

The most important point is that the underlying goal of science is not simply making predictions — it’s developing an understanding of the mechanisms underlying the operation of the natural world. This point is made very eloquently by David Deutsch in his book The Fabric of Reality. As I mention in the dialogue, Deutsch chooses this quote by Steven Weinberg as an exemplar of hard-boiled instrumentalism:

The important thing is to be able to make predictions about images on the astronomers’ photographic plates, frequencies of spectral lines, and so on, and it simply doesn’t matter whether we ascribe these predictions to the physical effects of gravitational fields on the motion of planets and photons or to a curvature of space and time.

That’s crazy, of course — the dynamics through which we derive those predictions matters enormously. (I suspect that Weinberg was trying to emphasize that there may be formulations of the same underlying theory that look different but are actually equivalent; then the distinction truly wouldn’t matter, but saying “the important thing is to make predictions” is going a bit too far.) Deutsch asks us to imagine an “oracle,” a black box which will correctly answer any well-posed empirical question we ask of it. So in principle the oracle can help us make any prediction we like — would that count as the ultimate end-all scientific theory? Of course not, as it would provide no understanding whatsoever. As Deutsch notes, it would be able to predict that a certain rocket-ship design would blow up on take-off, but offer no clue as to how we could fix it. The oracle would serve as a replacement for experiments, but not for theories. No scientist, armed with an infinite array of answers to specific questions but zero understanding of how they were obtained, would declare their work completed.

If making predictions were all that mattered, we would have stopped doing particle physics some time around the early 1980’s. The problem with the Standard Model of particle physics, remember, is that (until we learned more about neutrino physics and dark matter) it kept making predictions that fit all of our experiments! We’ve been working very hard, and spending a lot of money, just to do experiments for which the Standard Model would be unable to make an accurate prediction. And we do so because we’re not satisfied with predicting the outcome of experiments; we want to understand the underlying mechanism, and the Standard Model (especially the breaking of electroweak symmetry) falls short on that score.

The next thing to understand is that all of these crazy speculations about multiverses and extra dimensions originate in the attempt to understand phenomena that we observe right here in the nearby world. Gravity and quantum mechanics both exist — very few people doubt that. And therefore, we want a theory that can encompass both of them. By a very explicit chain of reasoning — trying to understand perturbation theory, getting anomalies to cancel, etc. — we are led to superstrings in ten dimensions. And then we try to bring that theory back into contact with the observed world around us, compactifying those extra dimensions and trying to match onto particle physics and cosmology. The program may or may not work — it’s certainly hard, and we may ultimately decide that it’s just too hard, or find an idea that works just as well without all the extra-dimensional superstructure. Theories of what happened before the Big Bang are the same way; we’re not tossing out scenarios because we think it’s amusing, but because we are trying to understand features of the world we actually do observe, and that attempt drives us to these hypotheses.

Ultimately, of course, we do need to make contact with observation and experiment. But the final point to emphasize is that not every prediction of every theory needs to be testable; what needs to be testable is the framework as a whole. If we do manage to construct a theory that makes a set of specific and unambiguous testable predictions, and those predictions are tested and the theory comes through with flying colors, and that theory also predicts unambiguously that inflation happened or there are multiple universes or extra dimensions, I will be very happy to believe in the reality of those ideas. That happy situation does not seem to be around the corner — right now the data are offering us a few clues, on the basis of which invent new hypotheses, and we have a long way to go before some of those hypotheses grow into frameworks which can be tested against data. If anyone is skeptical that this is likely to happen, that is certainly their prerogative, and they should feel fortunate that the overwhelming majority of contemporary science is not forced to work that way. Others, meanwhile, will remain interested in questions that do seem to call for this kind of bold speculation, and are willing to push the program forward for a while to see what happens. Keeping in mind, of course, that when Boltzmann was grounding the laws of thermodynamics using kinetic theory, most physicists scoffed at the notion of these “atoms” and rolled their eyes at the invocation of unobservable entities to explain everyday phenomena.

There is also a less rosy possibility, which may very well come to pass: that we develop more than one theory that fits all of the experimental data we know how to collect, such that they differ in specific predictions that are beyond our technological reach. That would, indeed, be too bad. But at the moment, we seem to be in little danger of this embarrassment of theoretical riches. We don’t even have one theory that reconciles gravity and quantum mechanics while matching cleanly onto our low-energy world, or a comprehensive model of the early universe that explains our initial conditions. If we actually do develop more than one, science will be faced with an interesting kind of existential dilemma that doesn’t have a lot of precedent in history. (Can anyone think of an example?) But I’m not losing sleep over this possibility; and in the meantime, I’ll keep trying to develop at least one such idea.

It doesn’t seem like all that long ago that we were enthusing about the results from the first three years of data from the Wilkinson Microwave Anisotropy Probe satellite. Now the team has put out an impressive series of papers discussing the results of the first five years of data. Here is what the CMB looks like, with galaxy and foregrounds and monopole and dipole subtracted, from Ned Wright’s Cosmology Tutorial:

And here is one version of the angular power spectrum, taken from the Dunkley et al. paper. I like this one because it shows the individual points that get binned to create the spectrum you usually see. (Click for larger version.)

The headline two years ago was “Cosmology Makes Sense.” (That was my headline, anyway — others were not quite as accurate.) This continues to be true — the biggest piece of news isn’t that the results have overturned any foundations, but that the concordance model with dark matter, dark energy, and ordinary matter continues to work. The WMAP folks have produced an elaborate cosmological parameters table that runs the numbers for different sets of assumptions (with and without spatial curvature, running spectral index, etc), and for different sets of data (not just WMAP but also supernovae, lensing, etc). Everything is basically consistent with a flat universe comprised of 72% vacuum energy, 23% dark matter, and 5% ordinary matter. The perturbations are close to scale-free, but still seem to be a little larger on long wavelengths than shorter ones (0.014 < 1-n_s < 0.067 at 95% confidence). Probably the most fun result is that there is, for the first time, evidence from the CMB that neutrinos exist! Good to know.

My personal favorite was the constraint in the Komatsu et al. paper on parity-violating birefringence that would rotate CMB polarization. I was in on the ground floor where birefringence is concerned, so I’m sentimentally attached to it. But it’s also a signature of some very natural quintessence models, so this helps constrain the physics of dark energy as well.

Congratulations to the WMAP team, who have done a great job in establishing some of the pillars of contemporary cosmology — it’s historic stuff.

Researchers at University of Arizona’s Lunar & Planetary Laboratory have just released images of major rockslides on Mars in progress.

That’s just plain cool. Phil at Bad Astronomy has more, as expected!

There really is something out there, bending the light from distant galaxies:

Galaxies, and galaxy clusters, appear to be surrounded by clouds of something invisible which interacts gravitationally. It apparently also causes the rotation of galaxies to deviate from the simple prediction of Newton’s laws assuming that only the visible matter in galaxies is present.

This “dark matter” makes up nearly a quarter of the mass/energy density of the universe, whereas “light matter” (stars, interstellar dust and atoms) make up only a few percent.

Our prejudice is that dark matter has a particle nature. Theories of particle physics beyond the Standard Model these days offer many possibilities for dark matter candidate particles, many of which could be detected via their weak interactions with ordinary matter. Here we mean weak in the sense of interacting by exchanging a W or Z boson with the ordinary matter particles. The common thread among the particles proposed for dark matter is that they are weakly interacting and massive (of the order of hundreds of times the proton mass - one to several hundred GeV). Particles in this general class of dark matter candidates are called WIMPs.

If WIMPs are really there, there are only a few per cubic meter in our galaxy, and most fly straight through ordinary matter without interacting. But, if they can and do interact weakly, occaisionally they could transfer enough energy to a nucleus of ordinary matter that it would be detected, if our equipment is sensitive enough. Also, there is hope that WIMP dark matter particles could be created in the high energy collisions of the Tevatron at Fermilab or soon at the LHC at CERN, and detected indirectly by the fact that they carry away apparently missing energy.

Experimentally, now, in the area of direct detection there is a race on between two main competing technologies for observing the feeble signal of WIMP interactions. Until a few days ago, the most sensitive search for WIMPs was that of the XENON10 collaboration, using a detector of liquid and gaseous xenon as the target for the WIMPs. The detector is kept deep underground at the Gran Sasso Laboratory in Italy. But at the Dark Matter 08 meeting in Marina del Rey last week, the lead in this search was recaptured by the Cryogenic Dark Matter Search collaboration. They use solid crystalline silicon and germanium detectors, cooled to liquid helium temperatures, to sense the nuclear recoil from WIMP interactions, taking extraordinary measures (as all these experiments do) to avoid false signals from cosmic rays, natural radioactivity, and stray neutrons. They did a blind search, making carefully controlled predictions of the number of WIMP signal events they expected to see, and then “opened the box” earlier this month. They expected 0.6 background events, give or take about half an event. The result: nothing. No signal from WIMPs or backgorund or anything!

To “measure nothing” is usually a great experimental challenge. You do have to convince the world that you would have seen something if it had been there, that your apparatus isn’t just mute for some other reason. CDMS have done a great job convincing the world of this, I’d say, and their result is nearly a factor of three more constraining than the previous Xenon 10 result. They show it in this plot of WIMP interaction strength versus WIMP mass:

What the plot is saying is that assuming that all dark matter is a WIMP of a certain mass, there is less than a 10% chance that, if the spin-independent cross section had some value greater than that indicated by the heavy black line, they would have seen no events in the detector. This can be turned around to say that at the 90% confidence level if such WIMPs exist, they must have even feebler interaction strengths than that indicated by the heavy black line.

These exclusion limits are starting to cut deep into the theoretically favored regions indicated on the plot…and the fact that there really are zero events at this stage means that any sort of five sigma discovery could be a long way away.

In the mean time, there are larger xenon based experiments planned, including a larger version of the Gran Sasso one and the LUX experiment, to be based at the nascent deep underground facility DUSEL.

And, perhaps as early as later this year, the LHC experiments CMS and ATLAS will begin to get a glimpse of the results of protons “colliding in anger” as a colleague of mine likes to put it. The Tevatron has seen no hint of WIMP production/decay yet, and so with seven times more energy, the LHC maybe able to produce the heavy particles that decay down to lighter ones which may include WIMPs such as the neutralinos predicted by supersymmetry.

I muse from time to time on the possibility that dark matter may really only interact with “light matter” gravitationally. Perhaps it is composed of an entirely separate sector which does not interact with light matter. It may be several or many particles which interact amongst themselves, forming structures we can only speculate about. If our only probe of dark matter is gravity, there will be a very long road ahead to understand it. I hope the cosmologists among the readers here can offer us more hope that indeed, there are strong reasons to believe dark matter interacts weakly…