Dark matter annihilation in the early universe injects energy into the primordial plasma and increases the ionization fraction. Planck is looking for imprints of that in the CMB temperature and polarization spectrum. The relevant parameters are the dark matter mass and <σv>*feff, where <σv> is the thermally averaged annihilation cross section during the recombination epoch, and feff ~0.2 accounts for the absorption efficiency. The new limits are a factor of 5 better than the latest ones from the WMAP satellite, and a factor of 2.5 better than the previous combined constraints.
What does it mean for us? In vanilla models of thermal WIMP dark matter <σv> = 3*10^-26 cm^3/sec, in which case dark matter particles with masses below ~10 GeV are excluded by Planck. Actually, in this mass range the Planck limits are far less stringent the ones obtained by the Fermi collaboration from gamma-ray observations of dwarf galaxies. However, the two are complementary to some extent. For example, Planck probes the annihilation cross section in the early universe, which can be different than today. Furthermore, the CMB constraints obviously do not depend on the distribution of dark matter in galaxies, which is a serious source of uncertainty for cosmic rays experiments. Finally, the CMB limits extend to higher dark matter masses where gamma-ray satellites lose sensitivity. The last point implies that Planck can weigh in on the PAMELA/AMS cosmic-ray positron excess. In models where the dark matter annihilation cross section during the recombination epoch is the same as today, the mass and cross section range that can explain the excess is excluded by Planck. Thus, the new results make it even more difficult to interpret the positron anomaly as a signal of dark matter.
9 comments:
Hello,
Where did you get the plot from? Can you share the link?
Thanks.
You can find it in the legendary French press release: http://www.insu.cnrs.fr/node/5108
“The hardest thing of all is to find a black cat in a dark room, especially if there is no cat.”
― Confucius
Hi Jester, thanks for an interesting post. Perhaps there is no cat, but we would like to learn this the hard experimental way, not the presumptive philosophical way as some Confucius lovers would want.
We could also learn why there would eventually be no black cat - neither in the sky nor in the mine - the hard theoretical way, thinking more deeply about the dark room (space-time/quantum vacuum) and considering motivated mimetic dark matter models for instance (arxiv.org/abs/1409.2471).
You can find here: http://arxiv.org/abs/0905.0003
the method adopted by the Planck collaboration to derive this result.
CVL is mentioned there as "is the zone ultimately allowed to probe by
a cosmic variance limited experiment with angular resolution comparable to Planck"
So Planck won't get exclusion limits better than the CVL line.
Thus, Planck seems to exclude entirely thermal relic warm dark matter with a WIMP-like annihilation cross-section.
Given the multiple problems with thermal relic cold dark matter with a WIMP-like annihilation cross-section pointed out by Warm Dark Matter proponents, the combined exclusion would seem to rule out all thermal relic dark matter with a WIMP-like annihilation cross section.
The lambda CDM model (which uses a definition of Cold Dark Matter that includes both thermal relic WDM and thermal relic CDM), requires a thermal relic.
So, if there is thermal relic dark matter it must not have weak force annihilation cross-sections and must instead be truly sterile with respect to the weak, strong and EM forces, although it might have self-interactions that do not lead to DM annihilation or at a different cross-section.
CDM in the absence of self-interaction doesn't seem to work at all to fit the galaxy scale data, so at this point you either have thermal relic WDM as keV scale sterile fermions that interact only via gravity and fermi contact forces, or there is no thermal relic DM at all. Such particles wouldn't be produced at the LHC or would be to light to detect in current direct dark matter detection experiments.
Other studies also place very strict bounds on purely bosonic DM.
Axions escape this issue because it is not a thermal relic form of DM, but have other issues.
Of course, none of this rules out a fifth force or force modification approach.
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