Comments

1. #1 Ryano

   February 12, 2011

   1) Do they mention how much longer it will take to detect the daily (sidereal) modulation? From what I figure, the relative signal is ~.45 km/s / 30 km/s ~ 1/67 that of the annual modulation, but they get to fold the data 366 times more. Unless I have had too much wine, that should be a 9 sigma/67 * (366)^.5 ~ 2.5 Sigma detection (actually below that by ~.7 when you consider the latitude of the experiment).

   2) Their error bars in this paper: http://arxiv.org/abs/1007.0595v1 seem a bit too large. I only see ~ 3 1 sigma outliers

2. #2 Steinn Sigurdsson

   February 12, 2011

   Not that I heard, I didn’t make all the talks – too many “TBAs” and some reshuffling due to no-shows.

   CDMS changed its claimed low energy efficiency, but still claims to exclude the DAMA and coGent results.
   There was a strange presentation on how the “best” CDMS detector of the set put really strong limits on DAMA-like results – a strange sort of formal statistical error.

   Xenon10 also backed off, but everyone seems to think Xenon100 will settle the issue.

   The crowd at the meeting were firmly in the “DAMA is wrong” camp.

   The claims, and upper limits, are horribly model dependent, and some of the assumptions are very over simplified – I don’t think any of the halo models are very realistic, and the model errors are easily comparable to the signal – eg if the local halo has significant streaming or partial co-rotation with the disk.

   I found a lot of the astrophysical estimates to be rather naive, a lot of “found this in a review paper and it makes for a convenient assumption”, or worse still. people remembering what they learned as grad students…

   The CDM modelers are really wedded to NFW profiles, it is rather cute. Like some sort of cognitive defect.

3. #3 Shantanu

   February 12, 2011

   Steinn and others,
   Even though one chapter of my thesis was on indirect detection of WIMPS through neutrinos , one concern I currently have is that the limits on WIMP-nucleon scattering cross-section
   are already getting close to 10^-43 cm^2 (which is the weak-interaction scale) or the right
   value of cross-section needed for it to be a cold dark matter candidate and have the right relic abundance (so called WIMP miracle). However once the limits fall below, IMO WIMP
   dark matter is ruled out (its of course easy to cook up complicated models, but that defeats the original purpose)

4. #4 Ben

   February 13, 2011

   As I’m sure Stacy can tell you, CDM modelers are so wedded to NFW profiles that they fight any suggestion that a galaxy halo might not have an NFW profile, because (according to them) that would totally undermine galaxy formation in a CDM model, including Lambda CDM.

   Of course, once a galaxy somewhere is demonstrated to have a not-NFW profile, they will go back to the modeling and find that astrophysical processes (outflows, torques, whatever) affect the halo profile enough to change it, and thereby save Lambda CDM, while providing gainful employment for a new generation of theory postdocs. (And there are some who say that this has already happened!)

5. #5 Andrew Shevchuk

   February 13, 2011

   The highlights of last year’s Paris Cosmology Colloquium (http://arxiv.org/abs/1009.3494) suggest that a consensus against CDM is growing, specifically in favor of WDM. From the above comments, I take it that this view is not sweeping the field right now.

6. #6 Ben

   February 14, 2011

   Studying galactic scale dynamics is a good way to begin to understand the messy, astrophysical process of galaxy formation. (It involves baryons – it must be messy!)

   It is a lousy way to refute CDM, much less in favor of WDM. That’s like trying to refute the idea that you can or can’t stand an egg on end at the solstice, by intensive study of scrambled eggs.

7. #7 Steinn Sigurdsson

   February 14, 2011

   @Shantanu – I think CDMS and Xe## are already claiming cross-sections smaller than that for the favoured mass ranges (~ 60 GeV or the “light” 6-8 GeV).
   Certainly simple scattering through virtual Z_0 seems excluded for favoured mass ranges.
   You can of course always play with alpha, tweak the masses, or just make dark matter more complicated

   @Andrew – you can always improve the fit by adding WDM, it gives you at least two more free parameters.
   Problem with it is to explain the dwarf halos you need sigma ~ 30 km/sec, so it is really tepid rather than warm, and the implied energy scale is ~ 10^-8 which is uncomfortable fine tuning.

   @Ben – the gastronomics can be very helpful, but ultimately it is very hard to see how the baryons can push around that much dark matter by so much. And why the result seems near universal.The enthusiasm with which a single estimated galaxy profile with a slow rise in apparent central density was greeted really made me uneasy – reminiscent of similar enthusiasm of claimed decreases in a frew spiral galaxy rotation curves at large radii.
   Rosie also made a very convincing case that dwarfs with radically different star formation histories showed near identical (flat core) central density profiles, which is hard to reconcile.

   It is tempting to conclude there must be additional physics, and there are some hints towards that, but then I am a theorist and we never do learn…

8. #8 Ben

   February 16, 2011

   I didn’t intend to be dismissive of testing CDM. CDM models should be tested and broken wherever possible. However, it takes very strong evidence to motivate adding dark matter or particle physics epicycles, vs. mucking around with baryonic details.

   The flip side is the common story arc: 1. theorist dismisses discrepant observational result; 2. clever people rename result to something catchy (like “missing XXX” or “XXX catastrophe”); 3. theorists resolve such catastrophe with better models, saving CDM; 4. Profit!

   Here’s a paraphrased dialog I had a few years ago with a theorist we’ll call “JP”:

   JP: There are all these systematic effects that could lead observations to claim slowly rising rotation curves instead
   of inner cusps.

   Ben: I just don’t believe some of those systematics are that large. Besides, not a single one of the galaxies shows an inner profile as steep as r^-1. There’s no sign of an NFW halo let alone r^-1.5.

   JP: But given these issues CDM is still consistent with all available data.

   Ben: Look, one of these days someone is going to show us a galaxy with a gold-plated linearly rising rotation curve, no bar, perfectly aligned slit, good resolution, and so on. That nominal inner cusp has less than 1 percent of the halo mass. Are you really then going to throw out an entire theory of structure formation based on what happened to 1% of the mass down in the center where baryons have done their thing enough to form stars?

   JP: No really, this is a crucial test of CDM.

   This dialog isn’t quite fair since I probably paraphrased myself to be more coherent. But I think it will turn out to be true that the theory can be patched with the details of galaxy formation.