Thursday, April 19, 2012

Dark Matter Missing In Milky Way

In our Milky Way galaxy, in the vicinity of the solar system, the difference between the dark matter effects which are observed, and conventional estimates of its amounts, show that all or most of the dark matter conventionally predicted to be present in the general region of the solar system by leading dark matter theories isn't there. The results are consistent with zero and not consistent with results more than about 20% of the predicted value.

Theories predict that the average amount of dark matter in the Sun’s part of the galaxy should be in the range 0.4-1.0 kilograms of dark matter in a volume the size of the Earth. The new measurements find 0.00±0.07 kilograms of dark matter in a volume the size of the Earth. The new results also mean that attempts to detect dark matter on Earth by trying to spot the rare interactions between dark matter particles and “normal” matter are unlikely to be successful.

“Despite the new results, the Milky Way certainly rotates much faster than the visible matter alone can account for. So, if dark matter is not present where we expected it, a new solution for the missing mass problem must be found. Our results contradict the currently accepted models. The mystery of dark matter has just become even more mysterious. . . .” concludes Christian Moni Bidin.

Citation: C. Moni Bidin, G. Carraro, R. A. Méndez and R. Smith, “Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood", The Astrophysical Journal (upcoming)

Their abstract ends with this highly pertinent conclusion:

Only the presence of a highly prolate (flattening q >2) DM halo can be reconciled with the observations, but this is highly unlikely in CDM models. The results challenge the current understanding of the spatial distribution and nature of the Galactic DM. In particular, our results may indicate that any direct DM detection experiment is doomed to fail, if the local density of the target particles is negligible.

A "prolate DM halo" would be one that spews up and down from the black hole at the center of the galaxy while being thin in the radial direction of the disk (which would be favored in what I have dubbed a "black hole barf" scenario).

Now, this isn't by any means the first experimental data set that is inconsistent with Cold Dark Matter theory, although this experiment's methodology is particularly direct and compelling.

As I've noted previously at this blog in multiple posts, the accumulating evidence tends to favor a combination of significant undercounts of ordinary visible matter in previous studies, possible failures to consider general relativistic effects properly, unmodeled dynamics associated with black holes at galactic cores, and some quantity of "warm dark matter."

As I explained in a September 20, 2011 post:

"WDM refers to keV scale DM particles. This is not Hot DM (HDM). (HDM refers to eV scale DM particles, which are already ruled out). CDM refers to heavy DM particles (so called wimps of GeV scale or any scale larger than keV)." . . . For comparison sake, an electron has a mass of about 511 keV. So, a keV scale sterile neutrino would be not more than about 3% of the mass of an electron, and possibly close to 0.2% of the mass of an electron, but would have a mass on the order of 1,000 to 50,000 times that of an ordinary electron neutrino, or more.

The trouble is that any weak force or electromagnetic force interacting particle at this mass scale should have been discovered in particle accelerator experiments by now.

Weak force decays of heavier particles predict very specific branching fractions for neutrinos and those data are a good fit to a universe which has only three generations of left handed neutrinos with masses of under 45 GeV. To a first order approximation, weak force decays are "democratic", i.e. every energetically permitted fundamental particle possibility in a W or Z boson decay that conserves certain other quantum quantities is equally likely to appear. Hence, an extra kind of electromagetically neutral fundamental particle, regardless of its mass, would have a branching fraction equal to the branching fraction of ordinary neutrinos, and this is well within the range of the accuracy of current branching fraction determinations from experiment which are consistent with the Standard Model.

All known fundamental particles that have a non-zero rest mass interact via the weak force, and no known fundamental particles that have a zero rest mass interact via the weak force. A "sterile neutrino" would be the sole exception to this rule if it existed.

Any electromagnetically interacting light fundamental particles other than electron, muon, tau, the six quarks, and the W boson would have been even more obvious in the experiments. While electrically neutral neutrinos are inferred from "massing mass" in reactions, electrically charged particles are detected more or less directly.

Glueballs, a well motivated but hypothetical composite particle made entirely of gluons which would be electrically neutral, would be too heavy and probably wouldn't be sufficiently stable. All but a few of the hypothetically possible composite particles that include quarks have been observed, and from what we can infer about the strong force physics that bind them and the masses of these composite particles, there cannot be any composite particles with masses in keV range or any mass range that is even remotely close. There are simply no indications in the data that there are missing strong force interacting particles either. And, none of the hypothetical particles predicted by string theory or supersymmetry appear to be light enough.

On the other hand, the universal prediction of all theories of gravity that predict a particle the mediates the gravitational force the way that the known Standard Model bosons do for electromagnetism (photons), the strong force (gluons) and the weak force (W bosons and Z bosons) predict a zero mass, spin-2 graviton, which would not be a good fit for dark matter (in addition to the fact that such a graviton would hypothetically reproduce general relativity). And, the tentatively discovered Higgs boson is both too ephemeral and too heavy to be a good fit.

This leaves as dark matter candidates either (1) one or more lighter than electron fundamental particles that don't interact with electromagnetism, the weak force or the strong force (a class of particles genericallly known as "sterile neutrinos"), or (2) some sort of composite particle phenomena, presumably heavier than its component parts and hence made only out of neutrinos, such as a "neutrino condensate."

Any other fundamental particles that could have escaped detection in high energy physics experiments would have to be too heavy to fit the data - they would be, almost by definition, cold dark matter. Yet, it is becoming increasingly clear that cold dark matter theory is wrong.

In some sterile neutrino and composite dark matter scenarios, dark matter interacts with some previously unknown fundamental force that has no observable effect on ordinary matter but interacts with dark matter or dark matter components in some way.

Of course, the possibilities aren't really so constrained, because the estimates regarding the possible mass of a warm dark matter particle themselves assume certain properties of those particles, and if wark dark matter doesn't have those properties, the mass estimates could be wildly wrong.

In the current experimental and theoretical environment, my inclination is to expect that: (1) there is no undiscovered fundamental fermion that gives rises to dark matter phenomena which we observe, (2) the amount of dark matter in the universe is greatly overestimated, and (3) if there are dark matter particles at all, they are probably some sort of keV scale composite particle comprised of neutrinos in some manner, either with a new fundamental force or arising from a new understanding of the fundamental forces. I also don't rule out entirely the possibility that some subtle modification to the equations of gravity could explain the observed dark matter effects, not withstanding the seemingly damning evidence of the bullet cluster.

This latest report also suggests that direct detection of dark matter may be impossible anywhere remotely close to Earth because there is little, if any, dark matter anywhere in Earth's vicinity for many light years around.

Since dark matter phenomena increasingly looks like one of the very few gaps in physics in which beyond the standard model physics with interesting phenomenologically detectable effects are even possible, this is a pretty glum picture. No more fermions. Maybe one more class of undiscovered bosons. And, that's it.


Maju said...

I've always suspected that dark matter made little sense. It's not something invisible (mythology) but an error in the calculations (science). Something in the theory needs to be fixed.

Andrew Oh-Willeke said...

Probably more a product of insufficient data than errorneous calculations, although there may be some of that too. Part of the issue with calculations is that until we had massive increases in computing power, it was impossible to get even estimates without making lots of simplifying assumptions.