tag:blogger.com,1999:blog-7315236707728759521.post730729180163243946..comments2024-10-10T19:41:07.273-06:00Comments on Dispatches From Turtle Island: Notable Recent Physics ResearchAndrew Oh-Willekehttp://www.blogger.com/profile/02537151821869153861noreply@blogger.comBlogger9125tag:blogger.com,1999:blog-7315236707728759521.post-49303180899328584492013-11-08T19:26:01.826-07:002013-11-08T19:26:01.826-07:00I'm still working all this out, but I can now ...I'm still working all this out, but I can now say two things:<br /><br />Sterile neutrinos produced by the Dodelson-Widrow mechanism have a distribution of velocities that deviates slightly from a perfect thermal (equilibrium) distribution. However, their cosmological effects could be imitated by a truly thermal distribution of dark matter particles, if the DM particle had a slightly different mass than the Dodelson-Widrow neutrino. This "other mass that would have looked the same" is the "thermal mass". <br /><br />54 m/s is a bound on the RMS variance of CDM particle speeds. It just means that they would all be almost at rest with respect to each other - if gravity hadn't caused them to clump; remember that it's a counterfactual quantity, the velocity dispersion of CDM today *if there had been no small-scale structure formation*, and not a statement about how fast they should be moving in the real world of today. <br /><br />So bizarrely, neither quantity refers directly to present-day reality. Mitchellhttps://www.blogger.com/profile/10768655514143252049noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-43220375951960201172013-11-07T19:39:16.302-07:002013-11-07T19:39:16.302-07:00Mitchell,
In Table II, the observed number of subh...Mitchell,<br />In Table II, the observed number of subhalo counts is 18, and in Figure 2, one sees that, if the mass is greater than 14 keV, then the model predicts more subhalo counts than observed.<br /><br />CDM models consistently overpredict the amount of dark matter at the center of galaxies.<br /><br />All I was point out is that the paper by Vega et al as well as this paper pretty much rule out CDM.<br /><br />The only reason that I'm hesitant to say that this is 100% true is due to the fact that researchers haven't found a 2-14 keV dark matter particle and due to the 54 m/s RMS speed minimum that Horiuchi found.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-39638919358054052222013-11-07T13:03:02.735-07:002013-11-07T13:03:02.735-07:00Eddie - there's no way that paper rules out CD...Eddie - there's no way that paper rules out CDM, but I can't even figure out what your argument is. Where do you get this maximum of 14 keV from? <br /><br />Horiuchi et al's concept of thermal mass is supposedly explained somewhere in reference 32, perhaps near its equation 5, but so far I cannot parse what is being said there. In particle physics, a "thermal mass" is often some sort of effective mass, e.g. a particle being slowed down in a thermal medium as if it were heavier, but this seems to be something else. Mitchellhttps://www.blogger.com/profile/10768655514143252049noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-14153839556561221872013-11-07T06:13:57.418-07:002013-11-07T06:13:57.418-07:00Andrew and Mitchell,
Thanks for looking into this....Andrew and Mitchell,<br />Thanks for looking into this.<br />I was confused on what value to use for T in their "sqrt(T/m)" equation.<br />I'm especially confused now that I read the paper you link to in this post, which states that there's a difference between the rest mass and what they are calling "thermal mass." So, now I'm not even sure what to use for m (rest mass or this thermal mass?).<br /><br />The Horiuchi paper seems to be setting a limit of ~9 keV (assuming that there is only one type of dark matter particle.) But what's great about the Horiuchi paper is that one can also infer a maximum rest energy of ~14 keV. Though, I didn't see them put a constraint on maximum rest mass. Their data clearly rules out Cold Dark Matter, but they didn't focus on that in their conclusions.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-69200429770566729022013-11-06T21:54:22.642-07:002013-11-06T21:54:22.642-07:00This paper is tricky to interpret.
Page 20: the ...This paper is tricky to interpret. <br /><br />Page 20: the temperature T0 is not the actual present-day DM temperature, it is (apparently) what the temperature would be if the DM hadn't clumped together because of gravity. <br /><br />Page 21: the bound on "DM temperature" is not so far from a corresponding (counterfactual?) temperature for baryonic matter. So perhaps these bounds are less constraining than it seems? <br /><br />It also seems that the bounds become weaker as the DM mass gets smaller, because the bound is for T0/M. <br /><br />Page 26: an absolute lower-bound of 1 keV for DM particles which interact according to equation 49. I wonder if there is a DM model in which the mass is *caused* to be on the edge of the bound, much as the Higgs boson mass is caused to be at the edge of criticality in some models? <br /><br />But all these comments should be treated as suspect until we really understand the paper.Mitchellhttps://www.blogger.com/profile/10768655514143252049noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-23212656467872496772013-11-06T07:51:08.488-07:002013-11-06T07:51:08.488-07:00Thanks for the tip. I'll look into it. I hav...Thanks for the tip. I'll look into it. I haven't looked at the paper. I have seen multiple papers rule out dark matter with a rest mass of less than 1 keV on various grounds.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-83281165146603326762013-11-06T07:44:50.869-07:002013-11-06T07:44:50.869-07:00Andrew,
Thanks for the summary of recent papers. I...Andrew,<br />Thanks for the summary of recent papers. I'm glad to see that there is a science journalist who covers this material without assuming that supersymmetry & WIMP-cold-dark-matter have already been proven, such as is the case for most journalists at Scientific American and New Scientist.<br /><br />My question is related to the discussion of warm-dark-matter in your post. I've been loosely following this research, and I see that you have been covering in detail for a while. My question is: there was a recent paper (covered by New Scientist) that states that the RMS speed of dark matter must be less than 54 m/s.<br />http://www.newscientist.com/article/dn24361-speed-limit-found-for-sluggish-dark-matter.html#.UnpS3_nku88<br />In the paper, the authors rule out dark matter with a rest mass of less than 1 keV. However, it seems to me that if there is a limit of 54 m/s, then this likely rules out dark matter with rest masses less than scales ~GeV.<br />Are you familiar with the paper with the 54 m/s RMS speed limit? And if so, does this rule of the possibility of ~2 keV warm dark matter? (which de Vega et al. has shown to fit some astrophysical data really well)<br />Thanks for any additional thoughts on this subject of warm-dark-matter.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-2810395137081175102013-11-05T19:00:45.449-07:002013-11-05T19:00:45.449-07:00The abstract states: "we derive unitarity bou...The abstract states: "we derive unitarity bound on the Higgs gravitational coupling ξ in Einstein frame, which is stronger than that inferred from the current LHC Higgs measurements. We further study ξ-dependent weak boson scattering cross sections at TeV scale, and propose a new LHC probe of the Higgs-gravity coupling ξ via weak boson scattering experiments."<br /><br />If I am awry, I have done so in reliance on a misleading abstract.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-77442437242948419412013-11-05T17:01:21.896-07:002013-11-05T17:01:21.896-07:00"a recent paper looks at the role of gravity ..."a recent paper looks at the role of gravity in Higgs boson and weak boson scattering. The paper predicts scattering effects that can be discerned at the TeV scale and hence that should be possible to observe at the LHC."<br /><br />These aren't effects that anyone would expect to actually see. The paper concerns the xi parameter of Higgs inflation, which says how strong the coupling between Higgs and curvature is. For Higgs inflation, usually xi is around 10^4. For an LHC-detectable effect, xi needs to be about 10^15, an order of magnitude already noticed by Atkins and Calmet (ref 8 in this paper). I would be surprised if that magnitude of xi wasn't already ruled out by something in cosmology. <br /><br />So I think the value of the paper is more the calculations that were carried out, than the predictions that were made. New calculations can be a prototype for further calculations in different regimes or with different assumptions. Mitchellhttps://www.blogger.com/profile/10768655514143252049noreply@blogger.com