Puzzles!

By Sean Carroll | April 20, 2012 9:30 am

Science keeps advancing, in fits and starts. It was a good week for intriguing results from experiments.

The first bit of news, which has been the subject of the most internet buzz, is a new paper by Chilean astronomers C. Moni Bidin, G. Carraro, R. A. Mendez, and R. Smith, which claims that there’s no evidence for dark matter in the dynamics of stars near the Sun. If this were true, it would imply something funny going on with the distribution of nearby dark matter, which could have significant implications for direct searches here on Earth (see below). It wouldn’t really be much of a threat to the idea of dark matter itself, since there’s plenty of evidence for dark matter elsewhere. But it might mean that the distribution in the Milky Way was very different from the kinds of models we like to use, for example by being much lumpier.

We just heard a great physics colloquium here at Caltech by Katie Freese, who talked about this result very briefly. Her opinion matched those of the skeptics in Ron Cowen’s article linked above: this paper makes a lot of assumptions, some of the a bit dubious, and we would need to see something much more solid before we become convinced. The biggest issue is that they don’t actually measure the DM distribution near the Sun; they try to measure it in a region between 1500 and 4000 parsecs below the galactic plane (which is actually pretty far away), and then fit to a model and extrapolate to what we should have nearby. This kind of procedure relies on our understanding of the vertical structure of the galactic disk, which isn’t all that great. So it’s definitely an intriguing result, one that should be taken seriously and followed up by other surveys, but nothing to lose sleep over just yet.

The second bit of news is another puzzling absence: a lack of neutrinos that were predicted to be produced by gamma-ray bursts. These bursts are among the most energetic events in the post-Big-Bang universe, and for a long time were a major mystery to astrophysicists. More recently, a consensus had grown up that GRB’s (as they are called) are associated with intense beams of particles created by newly-born supernovae. That’s a model that seems to fit most of the data, anyway, and it also makes a pretty good prediction for the production of associated neutrinos. But a new paper by the marvelous IceCube experiment has thrown a spanner into the works: they should have been able to see those neutrinos, and they don’t.

IceCube consists of a series of thousands of detectors arrayed within a cubic kilometer of Antarctic ice, and looks for flashes of light associated with high-energy particles passing through. Recently they have been keeping an eye out for signs of neutrinos that should be associated with GRB’s that have been detected by the Swift and Fermi satellites — but no luck. It’s a puzzle that will send GRB theorists back to the drawing board. One of the funny aspects of the story is that particles from GRB’s are a leading candidate to serve as the origin of high-energy cosmic rays, but that seems to be out the window now. It’s still possible that the cosmic rays come from active galactic nuclei, but there’s another group of theorists who have something new to chew on.

The final bit of news is even dicier, and hasn’t received any internet buzz at all yet — I only heard about it through Katie Freese’s talk. Maybe because there was no press release and the shocking claim is hidden within the guts of a technical paper with a boring title. The paper is by our friend Juan Collar and Nicole Fields. Recall that the DAMA dark matter experiment looks for an annual modulation due to the fact that the Earth moves through the dark-matter “wind” at different velocities during different times of the year. And they see a signal — very strongly — but many people have questioned whether what they are seeing is really due to dark matter. Juan has been leading another experiment, CoGeNT, which has been trying to check DAMA’s results — and has found a very tentative signal that seems to agree with them (which wasn’t what most people were expecting).

One of the reasons for the skepticism is that there are other experiments, which aren’t tuned specifically to look for annual modulations, but nevertheless should be sensitive to dark matter at the level implied by DAMA and CoGeNT — and they see nothing. More recently, some of these experiments have started looking for annual modulations — and they see nothing. Here for example is a recent paper by the CDMS experiment that says exactly that.

But the new paper by Collar and Fields claims that CDMS have analyzed their own data incorrectly. They argue that (1) CDMS isn’t really sensitive to the kind of annual modulation purportedly seen by CoGeNT, and (2) if you look carefully there is actually a statistically significant (more than 5 sigma!) bump at low energies, consistent with the kind of low-mass dark matter particle you would need to explain the annual modulations.

My impression is that the CDMS folks are unmoved by this argument. It’s certainly always very hard to analyze the data from somebody else’s experiment. This kind of controversy comes down to very particular aspects of data collection, analysis, and sources of systematic error. It’s way over my head, so I have no professional opinion about who is right. But at the very least it’s a reminder (as if we needed one) that the dark-matter-detection game is heating up, and big news might be creeping up on us. The universe loves puzzles.

CATEGORIZED UNDER: Science, Top Posts
  • eric gisse

    …huh? There was a silly crank argument by “Adam” which disappeared once I hit “submit comment” ???

  • X

    As the number of incorrect dark matter papers goes to infinity, how fast does the look-elsewhere multiplier grow? I’m thinking you need to show about 8 sigma of signal to be taken seriously at this point.

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  • KenF

    I am no physicist, so take this for what it’s worth (nothing.) However, let me say, Popper gave you guys some guidance that you usually follow pretty well. This “dark matter” stuff is starting to look unfalsifiable. It’s out there because it has to be out there, though we don’t know what it is and can’t find it. Sounds like luminiferous ether to me.

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  • Torbjörn Larsson, OM

    @ KenF:

    Sark matter is not only testable (tested by standard cosmology, say), it isn’t only predicting structure formation from the universe scale (the Bolshoi simulation) down to spiral galaxies (the Eris simulation; watch the youtube, it is beautiful) – the first successful simulation of spiral galaxies, self consistent _and showing that dark matter was the missing key_ to understand galaxies – it is highly observable. Gravity lensing images shows it in the same way that AFM images shows surfaces at atom scale.

    As a comparison, no one had seen an atom directly until a few years ago when you could trap individual ions in EM traps and make them blink. Yet people accepted them from seeing brownian motion predicted, in the same way that dark matter now explains how galaxy matter moves.

    Ironically, just a few months after dark matter reigns in all the scales it is expected to (galaxies being the last bit), people has the audacity, or perhaps the sheer ignorance, to claim that dark matter “is starting” to look weak. How sad.

  • anon

    Never trust anyone trying to reanalyze the data of an experiment that he or she is not part of, especially if that analysis is based on scanning in the plots from other papers. I’m sure CDMS will reply at some point, but to be honest Juan seems a bit out to lunch on this.

  • anon2

    I beg to disagree, anon, that analysis looks rather straightforward. What is odd to me is why CDMS stopped their new analysis at 5 keV, having gone down to 2 keV before.

  • marshall

    It is disappointing that the Bidin et al paper doesn’t not appear to consider MOND (Modified Newtonian Dynamics) or TeVeS (a Tensor-Vector-Scalar theory of gravity, the relativistic version of MOND).

    The way to think about Dark Matter is it represents a problem with physics, namely excess force in the dynamics of galactic sized and larger objects. We don’t know if the problem is with quantum field theories or with general relativity. The first possibility leads to theories such as Cold Dark Matter (CDM) or Weakly Interactive Massive Particles (WIMPs); the second to something like MOND / TeVeS. As literally pretty much all we know about Dark Matter is that there is excess force, I don’t think that either approach can be ruled out at present. (There is the Bullet Cluster and a couple of other similar clusters showing a separation of dark matter and mass, but these can be explained by the vector field of the TeVeS theory.)

    So, it’s disappointing that they didn’t consider the gravitational alternative. It’s not clear from the paper whether or not MOND would survive this test. Unlike CDM or WIMP, MOND effects should be present at all places in the disk, so the real question is, are they compatible with these observations?

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  • KenF

    @Torbjörn Larsson Obviously the question is whether dark matter is just a theoretical construct that has utility in models or whether it is some kind of stuff that is actually out there. Obviously, there is a great deal of overlap between these two categories, but it seems to me people are a little too willing to attribute stuff-ness to a theoretical construct. Also, since as I said I’m not a physicist, so I really can’t evaluate the evidence at all. This is just my uninformed reaction.

  • John Merryman

    Here is an interesting addition:

    http://phys.org/news/2012-04-fermi-gamma-rays-unearth-clues.html

    “According to Porter, the new analysis leads to several conclusions. For example, it shows that the density of cosmic rays is higher than anticipated in the outer regions of the galaxy and beyond the central galactic plane. In addition, the total amount of gamma radiation from cosmic ray electrons due to interactions with infrared and visible light – which consist of photons of much lower energy than gamma rays – is larger than previously thought.”

    So no halo of dark matter is detected, but there is a halo of excess cosmic rays and gamma radiation.

    Consider that when mass converts to energy, it expands, from striking a match, to nuclear explosions. What would be the effect of energy/light turning into mass? Wouldn’t it contract, leaving a vacuum? Could gravity be not simply an effect of mass, but energy turning into/being absorbed by mass? The flat rotational curve of the galaxy would be due to this being a steady process of increasing density, as the amount of vacuum created is proportional to the amount of mass created.

    It would be difficult to test and measure, but it would be the wave collapse as something more than math.

  • http://tgd.wippiespace.com/public_html/ Matti Pitkanen

    This war between dark matter believers and non-believers is not very productive.

    For a more constructive approach to galactic dark matter see http://matpitka.blogspot.com/2012/04/standard-views-about-cosmic-rays-and.html. No spherical halo but galaxies along long string like objects (magnetic flux tube) like pearls in string. String like object carrying the dark matter. 1/rho gravitational acceleration giving constant velocity spectrum. Free motion galaxies along string as a killer prediction.

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  • JollyJoker

    For the lack of neutrinos; wouldn’t neutrino mass mean the neutrinos arrive years later then the gamma given how far away GRBs typically are? Did they actually use the upper bounds on neutrino mass and still manage to rule out their production in GRBs?

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  • David Brown

    The work of Fritz Zwicky, Vera Rubin, Milgrom, McGaugh and Kroupa shows that something is wrong with the current Newton-Einstein theory of gravity.
    http://en.wikipedia.org/wiki/Pavel_Kroupa
    If dark matter particles really exist then something weird would have to be obscuring most of the inertial mass-energy of the dark matter particles.

  • Ben

    @marshall [9]: First of all, let me also be very careful, as many others are, about the result of Moni Bidin et al. This result is based on 400 stars: that is definitely not a lot in the era of large surveys… Then, to get their result, they are making 10 assumptions (very nicely listed in their section 2), all of them being wrong to some extent. But that’s fine, in galactic dynamics one always has to make zeroth order approximations, but one should in turn be very careful about the result one gets. In their section 4, they try to relax a few of their hypotheses to show the robustness of the result, but they do not relax all of them, and in particular they do not relax the assumption that the mean Vr=0 and the mean Vz=0 everywhere (i.e. they assume that there is not net radial and no net vertical motion at any position), while recent studies have shown the contrary (see, e.g., http://arxiv.org/abs/1203.6861): to my mind, this is unfortunately probably a killer for all such studies based on Jeans equations in axisymmetric equilibrium… The future is in direct comparisons of data with simulations taking into account perturbations from spiral arms, associated instabilities, and effects from mergers. That said, not an easy task… Well, then, with all these caveats in mind, assuming the result of Moni Bidin et al. would nevertheless be correct, to the question of how MOND would perform: straightforward MOND would simply be ruled out, as it definitely predicts a dark matter-like effect close to the Galactic plane (see http://arxiv.org/abs/0904.3893). Less conventional versions of MOND such as the dipolar dark matter theory would nevertheless be more flexible (see e.g. Section 7.9 of http://arxiv.org/abs/1112.3960) and might perhaps pass the test with some awkward contorsions, just as normal CDM could with similarly inelegant tricks. But the most probable outcome of all this will probably be that there is a DM effect after all, and that these observations are probably a challenge to neither CDM nor MOND.

  • KWK

    @JollyJoker,
    Good question, but in fact the neutrino mass will have negligible effect on the arrival time of the neutrinos. The bigger question is precisely when the neutrinos are generated/emitted relative to the photons–in some theories, the neutrinos could arrive quite a bit earlier! But the time window searched by IceCube is sufficiently large that they can say with confidence that they should have seen *some* neutrinos.
    Which is too bad; I was really hoping that GRBs were the answer for high energy cosmic rays, and that we’d have our first astrophysical neutrino source detections by now. But as always, nature does what it does, regardless of our feelings about it…

  • Chris

    Are neutrinos from GRB emitted in the direction of the beam or isotropically? Maybe the intensity isn’t as high as they think and that’s why they haven’t seen them.

  • http://juanrga.com Juan Ramón González Álvarez

    Any serious search for dark matter has returned null results A short list is:

    Particle dark matter: evidence, candidates and constraints 2005: Phys. Rep. 405(5–6), 279–390. Bertone, G; Hooper, D; Silk, J.

    First Results from the XENON10 Dark Matter Experiment at the Gran Sasso National Laboratory 2008: Phys. Rev. Lett. 100, 021303-1–5. Angle, J.; et al. (XENON Collaboration).

    Constraining Dark Matter Models from a Combined Analysis of Milky Way Satellites with the Fermi Large Area Telescope 2011: Phys. Rev. Lett. 107, 241302-1–6. Ackermann, M.; et al. (The Fermi-LAT Collaboration).

    And this recent Solar System study just confirms the previous results: no dark matter exists here out. Moreover, a simple literature search reveals that dark matter models cannot explain the data. E.g. dark matter cannot explain the observed shock velocity in the Bullet clusters, whereas MOND can (MNRAS 2007, 383 417) and so on.

    Sean Carroll claims: “There’s plenty of evidence for dark matter elsewhere.

    The problem here is this ‘evidence’ is essentially the same kind of ‘evidence’ that did that 19th century astronomers postulated the existence of unseen mass, a new planet which was named Vulcan.

    The first discovery of Vulcan was announced on 2 January 1860 during a meeting of the Académie des Sciences in Paris. Several re-discoveries and confirmations were done in posterior decades, somehow as discoveries of the hypothetical Dark Matter are announced in our days. This quote from a book of history must be relevant: «For the people of the late 19th century, Vulcan was real. It was a planet. It had theoretical credibility and had actually been seen. Even textbooks accorded it a chapter».

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  • JollyJoker

    @KWK,

    I thought the detection of extragalactic neutrinos would be enough to get the neutrinos to arrive after the visible light. The neutrinos from SN1987A arrived three hours before the light, having energies around 10 MeV. Neutrinos from 10M light years away should travel a factor of 1 / 3 x 10^10 slower than light to arrive at the same time as the light, assuming they’re emitted 3 hours before.

    Not sure I can use that number as is for a Lorentz factor, is that roughly correct? In that case, neutrinos of > 0.3meV should arrive after the light. I’m not sure what the current upper bound on mass is, but that has to be higher.

  • Kevin

    Props to my undergrad professor, Heidi Newberg, for being a voice of reason and caution in the Nature article.

    #9, marshall: You might take a look at Sean’s previous post on MOND. He notes, for example, “Even with MOND, you still need dark matter.”

    #23, JollyJoker: You must also take into account the fact that neutrinos interact much more sparingly than photons do. Though the photons nominally travel at a slightly higher speed than the neutrinos, the neutrinos will zip right through most matter in their paths (because their only interactions come through the short-range, low cross section weak force), while photons can scatter off just about anything. This gives the photons a longer effective path length, more than making up the difference in speed. (This is certainly a simplified picture of supernova radiation, but conveys the main idea.)

  • JollyJoker

    @ Kevin,

    How much slower than c does the light from a supernova move then?

  • JollyJoker

    Also, if light moves much slower than c, more than making up the difference in speed, doesn’t that in itself screw up the expectation that photons and neutrinos arrive simultaneously?

  • Kevin

    You can calculate the effective speed of light in a medium as c/n, where n is the index of refraction. The exact value of n is dependent on both the wavelength of light and the physical properties of the interstellar medium. The interstellar medium is going to vary quite a bit over the course of a photon’s trip from a supernova to Earth, so I don’t think we could meaningfully specify a single speed.

    However, there is another wrinkle about which I forgot earlier: neutrinos are actually produced earlier in the process of a supernova explosion than visible light. Neutrinos are produced right away, when the core of the star collapses. That collapse creates a massive amount of pressure, which leads to a shock wave. Visible light is produced only when the shock wave exits the star, which does not happen instantaneously. (The shock wave will travel at some speed above the speed of sound in the star, which varies based on density and material properties.) It’s this effect that actually accounts for most of the time lag between photons and neutrinos from a supernova like SN1987A.

    The expectation is not that they will arrive simultaneously, for all of these reasons. Photons should arrive only a short amount of time after neutrinos based on current knowledge, though, if GRBs come from supernovae. That behavior is what IceCube did not observe.

  • JollyJoker

    @ Kevin,

    My point is that the the delay for neutrinos with mass over typical distances for GRBs (billions of LY) could amount to years. I had no idea intergalactic space could slow down light that much, but surely there’s no reason to believe that would match the neutrino delay so closely?

    I’m actually starting to believe the slowdown of light in space you’re talking about is too little to be in any way relevant. Many GRBs last less than a minute and contain a fairly wide range of frequencies that should travel at different speeds. This doesn’t seem to happen, or the burst would be smeared out when it arrives here. A minute over a billion light years is one part in 10^14 -10^15.

  • Kevin

    When you say the delay could amount to years, that entirely depends on the mass and energy of the neutrinos. There’s a good discussion of actual measurements of neutrino speed on Wiki, and you can see the deviation from light speed is very small for typical neutrino energies.

    You are probably right that the slowdown from the interstellar medium is usually negligible. Most of the time lag comes from the fact that light is generated by the shock wave which is slowed within the exploding star itself.

    However, photons above a certain energy (~10^14 eV) will interact with cosmic background radiation and pair produce electrons and positrons, meaning we actually can’t detect those signals. Such photons are, of course, well outside the visible light range. This opacity of space to high-energy photons is another reason to use neutrinos for astronomy. (This has little direct relevance to the above discussion, but I think an it’s interesting point, and it does affect future investigation of GRBs.)

  • KWK

    Regarding the time delay of neutrinos relative to photons, there are probably more careful ways to derive a precise number, but a nice hand-wavy way to think about it is this:
    The (muon) neutrino mass is somewhere in the neighborhood of 0.3eV, give or take a few orders of magnitude (my guess is it’s probably lower, actually), while IceCube is looking for neutrinos in the neighborhood of 10^14 eV. Thus the rest mass energy is about 3 parts in 10^15 of the total energy, and a particle with these characteristics would experience only a negligible a time delay relative to a particle traveling at exactly c. The time window that IceCube uses is enough to compensate for this potential difference.

    And in the usual (“fireball”) theories, neutrinos come from the interactions of jetted material, so the neutrinos would be in the same (or narrower) jets than the photons. If they’re emitted isotropically, then you’re pretty much looking at a whole new theory of high energy neutrinos associated with GRBs, which is why the IceCube result warrants a “back to the drawing board” conclusion.

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  • http://juanrga.com Juan Ramón González Álvarez

    @24. And at least three authors in that previous post explain why the claim against MOND is unfounded. Milgrom has written beautiful stuff explaining by the Bullet cluster results have not falsified MOND, in despite of many misconceptions in the science news.

    Moreover, the same Bullet cluster is problematic for the dark matter hypothesis. See for instance the Astrophysical Journal article Bullet Cluster: A Challenge to ΛCDM Cosmology

    And it has been also shown that MOND can explain the observed shock velocity in the Bullet cluster, whereas dark matter cannot. Check MNRAS 2007, 383, 417. A figure comparing MOND vs the dark matter prediction can be found online here

  • JollyJoker

    @ KWK,

    Ok, thanks. The energy difference between the neutrinos from SN1987A and what IceCube is sensitive to explains why the time window is small enough. (10^10 vs 10^14 eV) Assuming my earlier calculations were correct, that means the average neutrino should be >3 eV to arrive even 3 hours later.

    @ Kevin,

    Thanks for the Wiki link, have to read.

  • http://coraifeartaigh.wordpress.com cormac

    Very informative post Sean, but I’m puzzled by the sentence
    “Recently they have been keeping an eye out for signs of neutrinos that should be associated with GRB’s that have been detected by the Swift and Fermi satellites — but no luck”

    Taking this sentence at face value, if GRB neutrinos have already been detected by SWIFT and FERMI , doesn’t that suggest a shortcoming of the IceCube experiment, rather than any great mystery about GRBs?
    Regards, Cormac

  • http://coraifeartaigh.wordpress.com cormac

    Oops, I see it now – did you mean the GRBs have been detected by SWIFT and FERMI, not the neutrinos? In that case, do we know IceCube can deliver, has it detected other neutrino events successfully? I’m always wary of non-results with new experiments…

    • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean Carroll

      Yes, Fermi and Swift detected the gamma-rays, not the neutrinos. IceCube has detected plenty of atmospheric neutrinos.

  • KWK

    @cormac–that’s right, the GRBs have been detected by Swift and Fermi because of the light they emit, not the neutrinos. And IceCube does see neutrinos all the time (usually those generated by other cosmic rays striking the atmosphere), but what it hasn’t seen–what no detector has seen yet, and part of what makes the whole project interesting–is neutrinos from specific astrophysical sources like Gamma-Ray Bursts or Active Galactic Nuclei.

  • Shantanu

    Sean, can you report on Verlinde’s colloquium at Caltech?
    Thanks
    shantanu

  • http://coraifeartaigh.wordpress.com Cormac

    Thanks Sean and KWK. In which case this is a very interesting result! I take it IceCube is sensitive to all types of neutrinos…(it’d be funny if it turned out to be similar to the missing solar neutrinos but I’m sure the boffins have that covered)

  • http://www.ioannisxydous.gr/ Ioannis

    An alternative answer to this puzzle:

    http://www.ioannisxydous.gr/

    Best Wishes

    Ioannis Xydous

    Electronic Engineer

  • Jimbo

    Sociological Experimental Challenge: Visit almost any physics dept.’s faculty page, & eyeball the pix. Nearly every faculty member displays a pic ~10 yrs younger than they actually are ! Its easy to ballpark their age, since on avg. most get their PhD ~ 25. Simply subtract the dates.
    Dr.Freeze’s appears ~ 35; Her PhD is in `84 => 25+28 = 53. Dare I say we have a pic ~ 18 yrs out of date ? Male faculty pix exhibit ~ the same intentional time dilation.
    Strikes me this is a blatant recruitment ploy, as bright young kids prefer bright young faculty, in this youth-obsessed culture we have. We need to project a dept. image of Who we Really are, not some air-brushed marketing of who we wish we were, or used to be.

  • forester

    Methodology wars between rival direct DM search teams, continued negative results both coming from both indirect and direct searches, no support for Supersymmetry– the leading particle physics theory that predicts the existence of extra massive weakly interacting particles. All of the above points to the fact the CDM theory is in serious crisis. On the one hand it’s good that scientists are doggedly testing the theory; on the other hand, the continued negative results aren’t very frustrating. After all, despite all of the advances in modern science, we still do not know what accounts for 95% of the mass-energy of Universe!

  • Sesh

    Jimbo #41:

    Interesting comment (though of course completely off-topic). I’d never seen Katie Freese’s homepage, so went to verify your assertion. However I discovered a photo on there which is clearly from June 2011, and looks much the same as the main mugshot. So it would appear that it isn’t an elaborate outdated-photo-recruitment-ploy, just that you aren’t particularly good at guessing a person’s age from their photograph (neither am I).

    Somewhat more disconcerting though is the link on her webpage offering me the opportunity to “download the entire zip file of photos of Dr. Freese”.

  • Jimbo

    Sesh #43; You can argue interpretation all you want. I can get w/in 5-7 yrs at worst, & NoWay that pic is an accurate rendering of a 50-something woman. I suspect the zip file will serve up any dilation you desire. In addition, my observation is true of male faculty 40-60 as well. Ex.#2: PhD in `81 => ~ 56 yrs of age. Pic ~ 37: http://physics.berkeley.edu/index.php?option=com_dept_management&act=people&Itemid=299&task=view&id=361
    :

  • anon2

    @Jimbo: it is simpler than that, really: who has the time to update those pages? Typically, these are taken when a faculty member joins a department, then refreshed every ~20 years by the Chairperson, if he or she gives a hoot.
    (but you are correct about KF’s age, yet she looks pretty much as in that picture: good genes in her case)

  • http://juanrga.com Juan Ramón González Álvarez
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  • Ben

    Sean et al., not sure whether this thread is still read but just in case, I wanted to point out that the main error of Moni Bidin et al has now been spotted: to make a long story short, they neglected what is known as the “asymmetric drift”. Their error is that they assumed that the mean azimuthal velocity of stars is constant as a function of radius at all heights, but this is actually not true. What is constant at z=0 is the circular velocity for a “cold” population i.e. one that has very low velocity dispersion. But in reality, stars are not on perfectly circular orbits, and the more the velocity dispersion is high, the more you have elliptical orbits, the lower the mean azimuthal velocity. The difference between circular velocity and mean azimuthal velocity is known as the asymmetric drift, because it makes the local stellar velocity distribution asymmetric with a kink towards low azimuthal velocities. Now, the crucial point is that the velocity dispersion is *not* constant with radius, and its radial variation also depends on height. So the mean azimuthal velocity is not constant with radius, and its radial variation also depends on height. In reality, the gradient of the mean azimuthal velocity is not zero but about 7km/s/kpc in the plane, growing to 11km/s/kpc at a height of 1kpc, and 40km/s/kpc at a height of 3.5kpc. Moni Bidin et al. had claimed that a gradient of 17 km/s/kpc on the mean azimuthal velocity was needed to be compatible with the expected amount of DM in the solar neighbourhood, but dismissed the possibility by confusing the mean azimuthal veklocity with the circular velocity. But, at their probed heights, this gradient is perfectly compatible with the numbers above. QED This is all explained in more details in today’s rebuttal: http://arxiv.org/abs/1205.4033

    • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean Carroll

      Ben, I hadn’t seen that, thanks for pointing it out.

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  • Ben

    You’re welcome. However, see also my comment 18 from April 21 hereabove about what this means regarding the modified gravity vs. particle dark matter debate. The short answer is: nothing

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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