Blogs / Cosmic Variance

Lead paragraph from the Times Online UK about the latest LHC snafu:

The rehabilitation of the beleaguered Large Hadron Collider was on hold tonight after the failure of one of its powerful cooling units caused by an errant chunk of baguette.

Speaking of successful NASA/DOE collaborations, there’s an interesting new paper on astro-ph claiming that the Fermi gamma-ray satellite has found evidence for a gamma-ray excess in the vicinity of the galactic center — similar to what you might expect from high-energy electrons produced by annihilations or decays of dark matter.

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze
Authors: Gregory Dobler, Douglas P. Finkbeiner, Ilias Cholis, Tracy R. Slatyer, Neal Weiner

Abstract: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal in the inner Galaxy with the same spatial morphology as the microwave haze observed by WMAP, confirming the synchrotron origin of the microwaves. Using spatial templates, we regress out pi0 gammas, as well as ICS and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess towards the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with ICS from a hard population of electrons. The morphology and spectrum are consistent with it being the ICS counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition to confirming that the microwave haze is indeed synchrotron, the distinct spatial morphology and very hard spectrum of the ICS are evidence that the electrons responsible for the microwave and gamma-ray haze originate from a harder source than supernova shocks. We describe the full sky Fermi maps used in this analysis and make them available for download.

In English: if the dark matter is a weakly-interacting massive particle (WIMP), individual WIMPs should occsasionally annihilate with other WIMPs, giving off a bunch of particles, including electron/positron pairs as well as high-energy photons (gamma rays). Indeed, searching for such gamma rays was one of the primary motivations behind the Fermi mission (formerly GLAST). And it makes sense to look where the dark matter is most dense, in the center of the galaxy. But it’s a very hard problem, for a simple reason — there’s lots of radiation coming from the center of the galaxy, most of which has nothing to do with dark matter. Subtracting off these “backgrounds” (which would be very interesting in their own right to galactic astronomers) is the name of the game in this business.

But Doug Finkbeiner at Harvard has for a while now been suggesting that there was already evidence for something interesting going on near the galactic center — not in the form of high-energy photons, but in the form of low-energy photons. The so-called WMAP haze is alleged to be radiation emitted when high-energy electrons are being accelerated by magnetic fields, leading to low-energy photons (synchrotron radiation). And Finkbeiner and collaborators claim that a careful analysis of data from WMAP (whose primary mission was to observe the cosmic microwave background) reveals exactly the kind of radiation you would expect from annihilations near the galactic center.

If that model is right, it gives us some guidance about what to look for in the gamma rays themselves, which Fermi is now observing. And according to this new paper, this is what we see.

That’s one of many images, and has been extensively processed; see paper for details. The new paper claims that there is an excess of gamma rays, and that it has just the right properties to be arising from the same population of electrons that gave rise to the WMAP haze. These much higher-energy photons arise from inverse Compton scattering — electrons bumping into photons and pushing them to higher energies — rather than synchrotron emission. So we’re not talking about gammas that are produced by dark-matter annihilations, but ones that might arise from electrons and positrons that are produced by such annihilations. The authors pointedly do not claim that what we see must arise from dark matter, or even delve very deeply into that possibility.

There have been speculations that the microwave haze could indicate new physics, such as the decay or annihilation of dark matter, or new astrophysics. We do not speculate in this paper on the origin of the haze electrons, other than to make the general observation that the roughly spherical morphology of the haze makes it difficult to explain with any population of disk objects, such as pulsars. The search for new physics – or an improved understanding of conventional astrophysics – will be the topic of future work.

That’s as it should be; whether or not the gamma-ray haze is real is a separate question from whether dark matter is the culprit. But on a blog we can speculate just a bit. Therefore I’m going to go out on a limb and say: maybe it is! Or maybe not. But a wide variety of promising experimental techniques are attacking the problem of detecting the dark matter, and we’ll be hearing a lot more in the days to come.

It’s well known that dark energy is a mystery — both for scientists, and apparently for funding agencies who are trying to figure out how best to learn more about this stuff that makes up about 73% of the energy of the universe. I haven’t been paying close attention to the ins-and-outs of this saga (there are more rewarding ways to give yourself an ulcer), but last I had heard the National Academy of Sciences had given very high priority to a satellite observatory meant to pin down the properties of dark energy. This was the JDEM idea — Joint Dark Energy Mission, where “joint” indicates a partnership between NASA and the Department of Energy. (They don’t always play well together, but the Fermi satellite is a notable recent success.)

Now, via Dan Vergano’s Twitter feed, I see a story in Nature News to the effect that things have become murky once again. The proposals got too expensive, so NASA turned to the European Space Agency for help, but ended up giving away things the DOE thought were in their domain, so they threatened to take their toys and go home, giving up on the idea of a satellite altogether.

The story is complicated by disagreement over how important it is to measure the dark energy equation-of-state parameter, the number characterizing how quickly the energy density changes (if at all). It’s frequently said that “we know nothing” about dark energy, but that’s not true; we know that it’s smoothly distributed and nearly-constant in density through time. We even have a very natural candidate for what it is: the vacuum energy. There is of course the problem that the vacuum energy is much smaller than it should be, but that problem is there whether it’s strictly zero or just really small. Other models still have that problem, and tend to add other fine-tunings on top. It would be great, and we would certainly learn a lot, if the dark energy were not simply vacuum energy; but right now we have no compelling reason to think it’s not, so it’s a bit of a long shot.

This past weekend saw the first beam particles in the LHC since the magnet quench incident of September 2008. Protons and lead ions were threaded in two directions around part of the ring before being dumped, and everything worked without a hitch. The graphs show the ion beam spot entering Collision Point 2 before being dumped.

The LHC machine commissioning will pick up where it left off more than a year ago, and the plan is, if all goes well, to collide beams of protons in the experiments at a center of mass energy of 7 TeV (3.5 TeV per beam) before the end of the year. The luminosity will not be large at first, but should increase steadily with time until next fall, when the long shutdown to retrofit the remaining magnets with new quench detection and helium pressure relief systems begins. By that point the experiments hope to have accumulated upwards of 200 pb^-1 of integrated luminosity. This initial data sample is sorely needed to shake down the detectors and start tuning up the event reconstruction and analysis. And who knows, maybe we’ll see something totally unexpected. (Please, no black hole comments!)

The next main milestone will be beam circulating around the whole ring and captured by the RF system. That should happen by mid-November. Fingers crossed!

I am just back from Princeton where we held the annual meeting of the GRE Physics Committee of Examiners, a group of six, ahem, distinguished professors (we have grey hair) who sit around a conference table working through hundreds of potential and actual Physics GRE problem. Each year new exam forms are completed, new questions added to the pool, statistics reviewed, and a good time is generally had by all.

This was my last meeting – I have served on the committee for six years. The membership rotates roughly every two years. I had been an external reviewer and problem writer for a couple years, and was then asked to serve on the committee. I am sworn to secrecy about a lot of the details, for good reason, but let me try to tell you from my perspective as an exam writer how to study for this dreaded event in your physics education.

Firstly, there’s the format. The exam is 100 questions long, and you have 170 minutes to do it. This is, therefore, different from just about every other physics exam you have had in college, where you have, say, four to six problems in an hour-long exam. The GRE Physics problems (or “items” in assessment world jargon) are short, to-the-point questions, and just about all of them are short calculations, if any, and take little time once you see what to do. Writing such questions is a difficult thing to do, let me tell you. We are continually amazed how, after about six levels of review, we can find issues of clarity, reasoning, and even sometimes basic physics correctness in the items submitted to the pool. All the committee members spend a lot of time each year reviewing hundreds of problems, looking for flaws, but more often than you would think the face-to-face meeting in Princeton with the ETS folks reveals something previously overlooked. It’s a really interesting process.
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I’m in the middle of jetting hither and yon, talking to people about the arrow of time. (Wouldn’t it be great if I had a book to sell them?) Right now, as prophesyed, I’m at the Quantum To Cosmos Festival at the Perimeter Institute. They’re extremely on the ball over here, so every event is being recorded by the ultra-professional folks at TVO, and instantly available on the web. So here is the talk I gave on Saturday night — a public-level discussion of entropy and how it connects to the history of our universe.

Yes, that’s a pretty suave picture of me on the image capture. What can I say? I’m just one of those lucky folks with an effortless magic in front of the camera.

If you prefer to get your talks about entropy unadulterated by voice and motion, and don’t mind a more technical presentation, I’ve put the slides from my recent Caltech colloquium online. These are aimed basically at grad students in physics, so there is an equation or two, and the caveats are spelled out more clearly. But the punchline is the same.

A recent essay in the New York Times by Dennis Overbye has managed to attract quite a bit of attention around the internets — most of it not very positive. It concerns a recent paper by Holger Nielsen and Masao Ninomiya (and some earlier work) discussing a seemingly crazy-sounding proposal — that we should randomly choose a card from a million-card deck and, on the basis of which card we get, decide whether to go forward with the Large Hadron Collider. Responses have ranged from eye-rolling and heavy sighs to cries of outrage, clutching at pearls, and grim warnings that the postmodernists have finally infiltrated the scientific/journalistic establishment, this could be the straw that breaks the back of the Enlightenment camel, and worse.

Since I am quoted (in a rather non-committal way) in the essay, it’s my responsibility to dig into the papers and report back. And my message is: relax! Western civilization will survive. The theory is undeniably crazy — but not crackpot, which is a distinction worth drawing. And an occasional fun essay about speculative science in the Times is not going to send us back to the Dark Ages, or even rank among the top ten thousand dangers along those lines.

The standard Newtonian way of thinking about the laws of physics is in terms of an initial-value problem. You specify the state of the system (positions and velocities) at one moment, then the laws of physics tell you how it will evolve into the future. But there is a completely equivalent alternative, which casts the laws of physics in terms of an action principle. In this formulation, we assign a number — the action — to every possible history of the system throughout time. (The choice of what action to assign is simply the choice of what laws of physics are operative.) Then the allowed histories, the ones that “obey the laws of physics,” are those for which the action is the smallest. That’s the “principle of least action,” and it’s a standard undergraduate exercise to show that it’s utterly equivalent to the initial-value formulation of dynamics.

In quantum mechanics, as you may have heard, things change a tiny bit. Instead of only allowing histories that minimize the action, quantum mechanics (as reformulated by Feynman) tells us to add up the contributions from every possible history, but give larger weight to those with smaller actions. In effect, we blur out the allowed trajectories around the one with absolutely smallest action.

Nielsen and Ninomiya (NN) pull an absolutely speculative idea out of their hats: they ask us to consider what would happen if the action were a complex number, rather than just a real number. Then there would be an imaginary part of the action, in addition to the real part. (This is the square-root-of-minus-one sense of “imaginary,” not the LSD-hallucination sense of “imaginary.”) No real justification — or if there is, it’s sufficiently lost in the mists that I can’t discern it from the recent papers. That’s okay; it’s just the traditional hypothesis-testing that has served science well for a few centuries now. Propose an idea, see where it leads, toss it out if it conflicts with the data, build on it if it seems promising. We don’t know all the laws of physics, so there’s no reason to stand pat.

NN argue that the effect of the imaginary action is to highly suppress the probabilities associated with certain trajectories, even if those trajectories minimize the real action. But it does so in a way that appears nonlocal in spacetime — it’s really the entire trajectory through time that seems to matter, not just what is happening in our local neighborhood. That’s a crucial difference between their version of quantum mechanics and the conventional formulation. But it’s not completely bizarre or unprecedented. Plenty of hints we have about quantum gravity indicate that it really is nonlocal. More prosaically, in everyday statistical mechanics we don’t assign equal weight to every possible trajectory consistent with our current knowledge of the universe; by hypothesis, we only allow those trajectories that have a low entropy in the past. (As readers of this blog should well know by now; and if you don’t, I have a book you should definitely read.)

To make progress with this idea, you have to make a choice for what the imaginary part of the action is supposed to be. Here, in the eyes of this not-quite-expert, NN seem to cheat a little bit. They basically want the imaginary action to look very similar to the real action, but it turns out that this choice is naively ruled out. So they jump through some hoops until they get a more palatable choice of model, with the property that it is basically impotent except where the Higgs boson is concerned. (The Higgs, as a fundamental scalar, interacts differently than other particles, so this isn’t completely ad hoc — just a little bit.) Because they are not actually crackpots, they even admit what they’re doing — in their own words, “Our model with an imaginary part of the action begins with a series of not completely convincing, but still suggestive, assumptions.”

Having invoked the tooth fairy twice — contemplating an imaginary part of the action, then choosing its form so as to only be relevant where the Higgs is concerned — they consider consequences. Remember that the effect of the imaginary action is non-local in time — it depends on what happens throughout the history of the universe, not just here and now. In particular, given their assumptions, it provides a large suppression to any history in which large numbers of Higgs bosons are produced, even if they won’t be produced until some time in the future.

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A philosophy professor of mine used to like to start a new semester by demanding of his class, “How many facts are in this room?” No right answer, of course — the lesson was supposed to be that the word “fact” doesn’t apply directly to some particular kind of thing we find lying around in the world. Indeed, one might go so far as to argue that what counts as a “fact” depends on one’s theoretical framework. (Is “spacetime is curved” a fact? What if spacetime isn’t fundamental in quantum gravity?)

Nevertheless, people sometimes use the word. A recent post by PZ reminded me of how it comes up especially in arguments over evolution, which is occasionally accused of being “just a theory.” I’ve tried to make my own view clear — when we as scientists use these words, we shouldn’t pretend they have some once-and-for-all meanings that were handed down by Francis Bacon when he was putting the finishing touches on the scientific method. Rather, we should be honest about how they are actually used. “Theory,” in particular, isn’t cleanly separate from words like “law” or “hypothesis” or “model,” and doesn’t have any well-defined status on the spectrum from obviously false to certainly true. And “fact” — well, that’s a word scientists hardly use at all. We use words like “data” or “evidence,” but the concept of a “fact” simply isn’t that useful in scientific practice.

But you know what would really be useful here? Some facts! Or at least some data. There’s one repository of professional scientific communication that I know very well — SPIRES, the high-energy physics literature database run by SLAC. (My hypothesis guess is that any other field would turn up similar results.) I don’t know an easy way to search entire papers, but it’s child’s play to search the titles. So let’s ask it — how often do scientists (as represented by high-energy physicists) use the word “fact”?

find t fact or t facts
120 records

Okay, they clearly use the word sometimes. What about some competitors?

find t data
9909 records

Ha! Now that’s the kind of word scientists like to use. And the others?

find t evidence
4396 records

find t observation or t observations
10924 records

You get the picture. Scientists prefer not to talk about “facts,” because it’s hard to tell what’s a fact and what isn’t. Science looks at the data, and tries to understand it in terms of hypothetical models, which rise or fall in acceptance as new data are gathered and better theories are proposed. Just for fun:

find t theory
42285 records

find t model
45977 records

find t hypothesis
578 records

find t law
1293 records

So I’m happy to say evolution is “true,” or is “correct,” but I’ll leave “facts” to Joe Friday.

General relativity, Einstein’s theory of gravity and spacetime, has been pretty successful over the years. It’s passed numerous tests in the Solar System, scored a Nobel-worthy victory with the binary pulsar, and gets the right answer even when extrapolated back to the first one second after the Big Bang. But no scientific theory is sacred. Even though GR is both aesthetically compelling and an unquestioned empirical success, it’s our job as scientists to keep probing it in different ways. Especially when it comes to astrophysics, where we need dark matter and dark energy to explain what we see, it makes sense to put Einstein to the most stringent tests we can devise.

So here is a new such test, courtesy of Rachel Bean of Cornell. She combines a suite of cosmological data, especially measurements of weak gravitational lensing from the Hubble Space Telescope, to see whether GR correctly describes the behavior of large-scale structure in the universe. And the surprising thing is — it doesn’t. At the 98% confidence level, Rachel finds that general relativity is inconsistent with the data. I’m not sure why we haven’t been reading about this in the science media or even on other blogs — it’s certainly a newsworthy result. Admittedly, the smart money is still that there is some tricky thing that hasn’t yet been noticed and Einstein will eventually come through the victor, but this is serious work by a respected cosmologist. Either the result is wrong, and we should be working hard to find out why, or it’s right, and we’re on the cusp of a revolution.

Here is the abstract:

A weak lensing detection of a deviation from General Relativity on cosmic scales
Authors: Rachel Bean

Abstract: We consider evidence for deviations from General Relativity (GR) in the growth of large scale structure, using two parameters, γ and η, to quantify the modification. We consider the Integrated Sachs-Wolfe effect (ISW) in the WMAP Cosmic Microwave Background data, the cross-correlation between the ISW and galaxy distributions from 2MASS and SDSS surveys, and the weak lensing shear field from the Hubble Space Telescope’s COSMOS survey along with measurements of the cosmic expansion history. We find current data, driven by the COSMOS weak lensing measurements, disfavors GR on cosmic scales, preferring η < 1 at 1 < z < 2 at the 98% significance level.

Let’s see if we can’t unpack the basic idea. The real problem in testing GR in cosmology is that any particular kind of spacetime curvature can be a solution to Einstein’s theory — all you need are the right sources of matter and energy. So in order to do a real test, you need to have some confidence that you understand what is creating the gravitational field — in the Solar System it’s the Sun and planets, in the binary pulsar it’s two neutron stars, and in the early universe it’s radiation. For large-scale structure things are a bit less clear — there’s ordinary matter, and dark matter, and of course dark energy.

Nevertheless, even though there are some things we don’t know about dark matter and dark energy, there are some things we think we do know. One of those things is that they don’t create any “anisotropic stress” — basically, a force that pulls different sides of things in different directions. Given that extremely reasonable assumption, GR makes a powerful prediction: there is a certain amount of curvature associated with space, and a certain amount of curvature associated with time, and those two things should be equal. (The space-space and time-time potentials φ and ψ of Newtonian gauge, for you experts.) The curvature of space tells you how meter sticks are distorted relative to each other as they move from place to place, while the curvature of time tells you how clocks at different locations seem to run at different rates. The prediction that they are equal is testable: you can try to measure both forms of curvature and divide one by the other. The parameter η in the abstract is the ratio of the space curvature to the time curvature; if GR is right, the answer should be one.

There is a straightforward way, in principle, to measure these two types of curvature. A slowly-moving object (like a planet moving around the Sun) is influenced by the curvature of time, but not by the curvature of space. (That sounds backwards, but keep in mind that “slowly-moving” is equivalent to “moves more through time than through space,” so the curvature of time is more important.) But light, which moves as fast as you can, is pushed around equally by the two types of curvature. So all you have to do is, for example, compare the gravitational field felt by slowly-moving objects to that felt by a passing light ray. GR predicts that they should, in a well-defined sense, be the same.

We’ve done this in the Solar System, of course, and everything is fine. But it’s always possible that some deviation from Einstein shows up at much larger distance and weaker gravitational fields than we have access to in our local neighborhood. That’s basically what Rachel’s paper does, considering different measures of the statistical properties of large-scale structure and comparing them to the predictions of a phenomenological model of the gravitational field. A crucial role is played by gravitational lensing, since that’s where the deflection of light comes in.

And here is the answer: the likelihood, given the data, for different values of 1/η, the ratio of the time curvature to the space curvature. The GR prediction is at 1, but the data show a pronounced peak between 3 and 4, and strongly disfavor the GR prediction. If both the data and the analysis are okay, there would be less than a 2% chance of obtaining this result. Not as good as 0.01%, but still pretty good.

So what are we supposed to make of this? Don’t get me wrong: I’m not ready to bet against Einstein, at least not yet. Mostly my pro-Einstein prejudice comes from long experience trying to come up with alternative theories of gravity that are simultaneously logically sensible and observationally consistent; it’s just very hard to do. But more generally, good scientists naturally have a strong suspicion of any claimed observational result that purports to overthrow an extremely well-established theory. That’s just common sense, not hidebound establishmentarianism; most such anomalies eventually go away.

But that doesn’t mean that you ignore anomalies; you just treat them with caution. In this case, there could be an unrecognized systematic error in the data set, or a subtle error in the analysis. Given 1:1 odds, that’s certainly where the smart money would bet right now. It’s also possible that the fault lies with dark matter or dark energy, not with gravity — but it’s hard to see how that could work, to be honest. Happily, it’s an empirical question — more data and more analysis will either reinforce the result, or make it go away. After all, some anomalies turn out to be frighteningly real. This one is worth taking seriously, to say the least.

This year’s Nobel Prizes in Physics have been awarded to Charles Kao, for fiber optics, and Willard Boyle and George Smith, for charge-coupled devices (CCD’s, which have replaced film as the go-to way to take pictures). Very worthy selections, which are being justly celebrated in certain quarters as a triumph of practicality. Can’t argue with that — as Chad says, things like the internet (brought to you in part by fiber-optic cables) and digital cameras (often based on CCD’s) affect everyone’s lives in tangible ways.

But they are also important for lovely impractical uses! When I hear “fiber optics” and “CCD’s” in the same breath, I am immediately going to think of the Sloan Digital Sky Survey (SDSS), which has provided us with the most detailed map we have of our neighborhood of the universe. Almost a million galaxies, and over 100,000 quasars, baby! How impractical is that?

The SDSS is a redshift survey, which means it’s not sufficient to just snap a picture of all those galaxies; you also want to measure their spectra (i.e., break down their light into individual frequencies) to see how much they have been shifted to the red by the cosmological expansion. And you just want the spectra of the galaxies, not the blank parts of the sky in between them. The Sloan technique was to drill giant plates for each patch of sky, with one hole corresponding to the position of every galaxy to be surveyed. (There were a lot of plates.) This image from the Galaxy Zoo blog.

Then you want to bring that light down to the camera. You guessed it — fiber-optic cables. Thanks, Dr. Kao.

The camera in question was possibly the most complex camera ever built — thirty separate CCD’s, combining for 120 megapixels in total, all cooled to -80 degrees Celsius. Thanks, Drs. Boyle and Smith.

And the result is — well, it’s pretty, but it doesn’t materially affect your standard of living. It’s a map of our local neighborhood in the universe. Extremely useful if you’d like to understand something about the evolution of large-scale structure, for example to pin down the properties of dark matter and dark energy.

Also useful for providing a bit of perspective. It’s technological advances like those honored in this year’s Prize that make it possible for we insignificant sacs of organic matter to stretch our senses out into the universe and understand the much bigger picture of which we are a part.