Thursday, March 06, 2014

Two fresh dark matter stories

Randall, Reece link DM and dinosaurs; strengthening DM signal in Central Milky Way

I want to mention two developments related to dark matter. First, Lisa Randall and Matthew Reece of Harvard have finally released a preprint – to appear in Physical Review Letters – linking extinctions and dark matter:
Dark Matter as a Trigger for Periodic Comet Impacts
As the "comments" (an entry in the arXiv form) point out, there are no dinosaurs in the paper so let me offer you a compensation.

Holy crap, we forgot to install a thermonuclear missile shield above Chick-Ku-Klux-Club in the Yucatan Peninsula (65 megayears before Christ).

At least one of the authors has intensely thought about various extinctions etc. at the same moment when she or he was writing the paper ;-), so the "no dinosaurs" comment is much less off-topic than some people might think.

They take one thing for granted, namely a periodicity of 35 million years in the crater record on the Earth's surface. And they try to link it to a model involving the galactic midplane, a hypothetical dark disk in that plane, and tidal effects on the Oort cloud (a far "Ukraine" of the Solar System; just to be sure, if you happen to be brainwashed by the idea that Ukraine has no permanent link to Russia, "Ukraine" does mean "borderland" or "march" [of Rus'] in the Slavic languages, and even Ukrainian scholars agree with that).

I am a non-expert and confused by the periodicities in similar things. I know that comets and craters are different things than galactic cosmic rays but I still don't fully understand why they should exhibit such a very different behavior when it comes to the periodicity. Note that the periodicity of the galactic cosmic rays used by Shaviv and Veizer is about 140 million years.

At any rate, it is a new paper linking terrestrial traces (in this case, numbers of craters) with some celestial cycles (linked to the inner structure of our galaxy). So when it comes to the basic ideas, I do think that Shaviv-Veizer should be cited by Randall-Reece and it's a mistake that it is not.

The second fresh paper on dark matter I want to mention is
The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter
by Daylan, Finkbeiner, Hooper, Linden, Portillo, Rodd, and Slatyer. A good popular story was printed in
Case for Dark Matter Signal Strengthens (by Wolchower, Simons Foundation's Quanta Magazine, copy in The Guardian)
and in Wired (by Adam Mann).

One looks somewhere at the galactic center using the Fermi gamma-ray telescope. She sees tons of frequencies and tries to maximally accurately subtract the radiation from all the known sources (stars). Something is left, especially gamma rays with energies \(1\)-\(3\GeV\). The question is whether this excess is due to some exciting new physics (dark matter particle in this case) or some relatively mundane astrophysics ("millisecond pulsars" is currently the #1 favorite buzzwords of those who want to prefer this conservative explanation).

They drew the map of "where the excess is coming from" in some more detail, with a more careful geographic subtraction, and the result is that it started to look more like dark matter and less like millisecond pulsars etc. That's why Finkbeiner, a long-term skeptic when it comes to the dark matter interpretation of similar signals, joined the large list of authors (although he's still more skeptical than some co-authors).

The features supporting the dark-matter interpretation include the apparent spherical shape of the DM halo needed to explain that (although it could be elongated a priori); and the extension of the source up to 10° from the Galactic center (where no millisecond pulsars seem to be located) which still seems to agree with a distribution expected for dark matter (they assume a generalized NFW halo profile everywhere).

Even if the photons are created from dark matter by some annihilation, it is hard to determine what the dark matter particle is (and by which process it decays to the gamma rays and something else). Their favorite explanation is a dark matter WIMP particle in the \(31\)-\(40\GeV\) interval that decays to the \(b\bar b\) quark pair. I suppose that the two hadrons containing the bottom quark (or antiquark) then continue to decay so that the "few\(\GeV\) photons" appear among the final products of the decay.

If the dark-matter explanation is real, there is a chance – one could even call it a prediction – that the same excess should be seen in the dwarf galaxies orbiting our Milky Way. Jennifer Siegal-Gaskins of Fermi (and Caltech) leaks the opinion that the excess could indeed be there, too. Dan Hooper says that a confirmation of this rumor by a big excess would make this game over. Well, he seems to be convinced that DM is the only possible explanation already now. ;-) It's not surprising given the purely numerical statistical significance: it's a whopping 40 standard deviations, well enough above 5 sigma! But catches could still be there. The number "40 sigma" results from a comparison of \(\chi^2\) of fits with and without the dark matter halo (imprinted via their assumed decays).

See Jester's text at Resonaances for more info.

Tracy Slatyer who is now at MIT faculty talks about her surprise that the new data would indeed sharpen the picture. Her preferred WIMP mass is \(35\GeV\). The particle is sometimes referred to as a "hooperon", not to be confused with a "hyperon", but "tracon" could be good, too. Juan Collar of CoGeNT etc. who would defend some "dark matter direct discovery" claims that no longer look too plausible now says that such a particle may be detected by similar underground experiments if the sensitivity increases 100-fold.

The rest of Wolchower's article is about the sterile-neutrino-like X-ray excess and the possibility that both excesses could actually be genuine, something that could actually be compatible e.g. with the eXciting dark matter models.


  1. This looks like a nice lecture series, and they very wisely disabled the comments ... ;-)

    Thanks for the fun review of Brian Greene's lecture and in particular the Q&A part of it ... :-D

  2. From knowing nothing about dark matter, I am now entertainingly confused. Seems like we need a swarm of interstellar probes to determine the gravity map of the Solar neighborhood.

    Might as well, additionally use the probes as a giant synthetic array radio telescope.

  3. What's left after subtraction is more likely the error term.

  4. It would be more interesting and inspiring for the sake of science if my humble correspondent tried to consider the no-dark-matter-hypothesis as a serious and rigorous possibility that the so called dark matter is just a bad understanding of how gravity actually works, rather that persisting in the idea that dark matter is composed by any kind of exotic, unknown particles.


  5. The freaking comments aren't disabled Dilatonius.

  6. 1) arXiv:1310.4009, 0906.0668.

    2) Vacuum trace chiral anisotropy selective to fermionic matter (quarks; ECKS gravitation) alters Noetherian coupling of exact vacuum isotropy to angular momentum conservation. It leaks 1.2×10^(-10) m/s^2 Milgrom acceleration. Dark matter curve-fits the Tully-Fisher relation.

    3) 90 days in existing apparatus, a geometric Eotvos experiment, validates. Contrast visually and chemically identical, single crystal test masses in enantiomorphic space groups, e.g., P3(1)21 vs. P3(2)21 alpha-quartz. 40 grams net as 8 single crystal test masses compare 6.68×10^22 pairs of 9-atom enantiomorphic unit cells.

    4) Parity violations, symmetry breakings, chiral anomalies, Chern-Simons repair of Einstein-Hilbert action disappear. String theory becomes predictive, SUSY and dark matter vanish, matter versus antimatter content and biological homochirality are intrinsic. Good times.

    5) Look. 40 years of theory are empirically sterile and increasingly desperate. Randall was better than this.

  7. It would be more interesting and inspiring for the sake of science to consider the no-dark-matter-hypothesis as a serious and rigorous possibility that the so called dark matter is just a bad understanding of how gravity actually works, rather that persisting in the idea that dark matter is composed by any kind of exotic, unknown particles.


  8. See these 20 fun black humor cartoons on Ukraine, mostly anti-Russia but not always.

    Arrow right on the screen to move.

  9. Really? I didn't know that. I must look into more detail on the shifts happening in the european union

  10. Well, to explain why LEP has not seen a 35 GeV particle going to b bbars one has to pull in quantum number conservation from higher mass creation than the masses explorable by LEP.

    The most economical and studied is supersymmetry, and if this dark matter analysis holds then it could be considered, in tandem with LEP results as the first proof of supersymmetry :)

  11. former Phys.SE memberMar 7, 2014, 9:34:00 PM

    To the average user (which I was), that thread and others like it made it near impossible to answer anything as I did not know what was acceptable or not. I love physics, really I do, but the atmosphere there is like walking on egg shells

  12. Yes, the problem is that the site is not (no longer) run by physicists for physicists (and serious enthusiasts), but by a power driven SE political clique of users (and including most moderators) who are more interested in enforcing their stance, suppressing other people and bossing them around.

    Those dilletants are so arrogant, that they no longer feel the need to listen to what real experts, such as professors, say or think concerning for example the on topicness and importance of mathematical questions in theoretical physics

    The pompous political clique gives a damn about the opinion of experts and generally people who know much more physics than they do, but keeps brutally migrating away mathematically inclined questions that are of paramount importance in theoretical physics away to math SE, where they are at best answered in a way to formal way to be of use for physicists, but mostly ignored.

    Those dimwits simply dont get the relationship between maths and physics in theoretical physics. By their aggressive ignorance they are tearing the subject appart on Physics SE

    The aggressive ignorance and political overmoderation on Physics SE leads to tons of other problems I complained enough here, such that I can no longer ask questions their either, because they would not be save from the attacks of the dominating rascals. Strictly speaking, not even the best, by the community well received questions are save from the ravaging orcs ...

    BTW Emilio Pisanthy had written a very nice answer in this thread, but strangely it is gone now ...
    Maybe @CentralCharge15 can retrieve it somehow?

  13. CentralCharge15

    This is umbelievable how fast the political clique has succeeded in overthrowing all reasonable agreements physicists on Physics SE agreed upon before the SE aggression during the last election.

    The first victim of these agreements was the notion that well defined study material / reference questions are OK. Manishearth was blatantly lying and distorting the truth about the history of the books policy to hunter, there never existed any community agreement saying that such questions are not OK, rather David Z simply started to shoot them down just like that and political oportunists agreed with him because they wanted to enforce a certain use of tag wikis invented on MSO by all means ...

    Now they are going after the agreement that as the old Astronomy site was merged into Physics SE, all astronomy questions are allowed by definition

    How much money are you willing to bet that what they do next is overthrowing the notion that all questions of the former TP.SE are on topic on Physics SE? And maybe they will start this with declaring mathematical physics off topic, this they are practically alread doing by migrating such questions away against the will of the experts still there ...
    Happily TP.SE questions are well and alive on PhysicsOverflow ... ;-)

    Brandon Enright ist the most overreaching pompous dilletant highly enjoying his power I have ever seen. Now he feels even entitled to reject tag wiki description of topics he has not the slightest clue about, such as topological field theory for example

    and he is definitively the worst reviewer and a nuisanse due to his casting tons of inappropriate closevotes which have then be canceled by leave opens of more reasonable people.

  14. It seems Ron has been banned in quora too :(

    see this discussion :

  15. Yes, I know ...

    It is the typical superficial nice-nice spirit overly present in the American society, which does not tolerate speaking to openly the truth, questioning facts the community has reached a consensus on, etc ...
    Unfortunately places like SE, Quora, etc are dominated by this American nice-nice by all means attitude, and not with the more open to robust discussions and confrontations if needed European or German attitude.

    I am happy to report that Ron is well and happy on our private PhysicsOverflow beta site. He contriutes great ideas concerning the site and we are having a good and productive time. Maybe you will be surprised when the site finaly goes public ... ;-)



    (at work, can not log in to discus here, darn!)

  16. CentralCharge15

    an additional point in favor of bringing PhysicsOverflow online publicly is that the ruling trolling despotes on Physics SE are getting highly efficient in driving good and reasonable people away now

    In dgh's recent chat discussion we learn that David Z likes not only to unilateraly shoot down questions for completely unimportant non-physics linguistic reasons on a physics site, but he also prefers to delete correct stuff for the same formal/linguistic points of view instead of just improving it by a trivial edit for example.

    I have stopped counting how many times I have seen reasonable up to very good folks saying "please delete me" in their profile just this year ...

  17. Where can we go and what assurances are there that the same nonsense will not occur?

  18. A PhysicsOverflow will emerge here

    we are in technical private beta now.

    I expect that we will be able to avoid the detoriations observed on Physics SE, as the site will be run by physicists for physicists (and serious and knowledgeable enough people). PhysicsOverflow will exclusively serve the needs of the physics community, instead of having to fullfill the obscure goals, rules and guidelines of a mysterious online company, which are orthogonal to a free academic high-level community. There will be no external forces controlling the site, so such things as the brutal raid of the SE Overlords and their disciples in Decemper 2012 (which induced the downfall of the site), will not be possible.

  19. What happened in Dec 2012?

  20. Wow, I managed to stay a record of 10 days without doing anything on Physics.SE! Before, I would get uneasy without visiting SE for even a day, thinking about how many questions would have been wrongly closed, how many questions have been kicked out of the reopen queue, how many users have been unjustly banned, etc. ! Finally, that stupid site is off my mind.