A particularly cogent observation in that discussion is whether the dark matter effects beyond MOND predictions observed in galactic clusters must be non-baryonic (i.e. not made out of ordinary atoms) to fit the data. According to Stacey McGaugh, one of the most knowledgeable academics in the area of modified gravity theories, the answer to that question is no.
Clusters you certainly could fit just with baryons. They’re rare systems. If that is the only place we need dark baryons, then do the integrals. You can satisfy the residual mass discrepancy in clusters in MOND without making much dent in the BBN missing baryon budget.
Do I *like* such a solution? Certainly not. Neither do I like that fact that clusters are the only systems that come close to having the right baryon content in LCDM. Why are galaxies missing more than half of their baryons? Dwarfs > 90%? I can imagine how this might happen, but the solutions are comparably contrived. The more basic point is that I am not willing to condemn a theory for needing some dark baryons if its competitor also needs dark baryons.
To this Rainer Plaga clarifies:
90% of all cosmic baryons are presently undetected, right? Only a fraction of the baryonic matter we see directly is in clusters (O(a few percent), let’s say 10%) So why can’t a small fraction, say O(2%), of all the cosmic dark baryons be in the form of e.g. jupiters in the central parts of clusters? They and stars would then dominate the cluster mass and be dissipationless —> no problem with the bullet cluster in MOND. . . . In [Sander's paper] http://arxiv.org/abs/astro-ph/0703590 he states about cluster dark matter in MOND: “For example, there are more than enough undetected baryons to make up the missing dark component; they need only be present in some non-dissipative form which is difficult to observe.”
There are also a couple of other pro-MOND points made there which aren't new to me, but are notable.
First, MOND produces remarkably accurate estimates of the dynamics of galactic scale and smaller system with a single parameter. Dark matter theories take many more parameters to get that quality of fit. Clearly, dark matter theories are missing something that produces much more regular behavior than the model requires, but the model does not explain why this is the case. (The debate doesn't address the more general point that a galaxy's general structure seems to be pretty much a direct product of the size of its black hole so that the single parameter of black hole size can nearly completely describe the range of observed galaxies, but there is a gap in the distributions for medium sized galaxies with masses intermediate between ellipical and spiral galaxies.)
Second, MOND itself, as originally proposed by Milgrom, is a straw man. Everyone agrees that it does not reproduce well established general relativity phenomena. Bekenstein (TeVeS) and Moffat have produced relativistic theories for the modification of gravity that address the theoretical shortcomings of the original theory, and no one is claiming that it isn't possible that someone could come up with somewhat better relativistic gravity modification theories.
Third, none of the main proponents of dark matter theories or modified gravity theories disputes that a cosmological constant in the equations of general relativity (or the modified analog) fully describe all observed dark energy phenomena.
The discussion does miss, or only superficially treat, a few key points, however:
1. Considerable evidence falsifies the cold dark matter version of dark matter theory with heavy (ca. 10GeV to 100 GeV) weakly interacting massive particles (called WIMPs), particularly evidence from the large scale structure of the universe. "Warm dark matter" theories (with 1-13 keV size dark matter particles) appear to be a better fit to the data than either "cold dark mater" (with heavier, slower moving dark matter particles) or "hot dark matter" (with electron neutrino sized dark matter particles moving at close to the speed of light).
2. The most commonly modeled spherical distribution of dark matter in spiral galaxies has also recently been falsified both with models of its effects and with detailed Milky Way rim astronomy measurements that rule out such large quantities of dark matter in the galactic rim in the vicinity of the solar system (a result confirmed by some, but not all, direct dark matter detection experiments). The observed dark matter distribution in spiral galaxies appears to be close to cones around the axis of the galaxy through the central black hole of the galaxy.
3. Particle accelerator experiments like those at the large hadron collider (LHC) have ruled out all of the most plausible and well motivated fundamental particle candidates that interact via the weak force for dark matter with masses in a range consistent with cold dark matter or warm dark matter. There are also no known stable composite particles with the right mass for warm dark matter or cold dark matter theories. (Sterile neutrinos are a top candidate for warm dark matter theories, neutrino condensates could also conceivably be a warm dark matter fit, and I wouldn't rule out the impact of ultra fast outflows aka black hole barf from galactic cores because the galactic bulge shape which is strongly influenced by ultra fast outflows appears to be more indicative of dark matter halo shape than the entire galaxy's shape including its rim).
4. A fairly recent discovery revealed that the amount of ordinary baryonic matter in elliptical galaxies (one of two main galaxy types, spiral galaxies like our own are the other) was dramatically undercounted and that therefore estimates of the amount of dark matter in the universe as a whole. The new data suggest an overall dark matter v. ordinary matter ratio closer to 50-50, than the canonical account in which there is far more dark matter than ordinary matter. There also appears to be a significant uncount of the amount of ordinary matter in the form of very dim stars (ca. 0.18 times the mass of the sun) in galactic clusters.
5. There is a legitimate reason to doubt the correctness of estimates of the gravitational effects in spiral galaxies due to unmodified general relativity, which is often approximated with purely Newtonian gravity in computer models. Published papers estimating the phenomenological difference between Newtonian gravity and general relativity in a spiral galaxy vary widely. Some see no effect, some see this accounting for most of the effects otherwise attributable to dark matter. The full GR calculation is difficult to carry out and requires simplifying assumptions. The hard part is to know whose simplifying assumptions involve the smallest sacrifice of accuracy and most closely approximate the actual structure of these galaxies.
6. In the last few years we have learned more about the impact of matter ejected from black holes at the centers of galaxies (ultra fast outflows aka black hole barf), and about the fine detail of the make up galactic clusters than we knew in the past. Black holes eject more matter in different directions and at different speeds than previously known and this has more of an influence of the shape of the bulge at galactic centers than previously known. This ejected matter may also help to explain why there appears to be a maximum size for central black holes of galaxies which appears to produce a corresponding maximum size for individual galaxies generally (galactic clusters have multiple distinct galaxies and some amount of other stuff in between).
I have references to published research or lay level articles describing published research on all of these points in previous posts at this blog tagged "dark matter", but will probably not manage to get links to those sources today.
For what it is worthy, my current subjective belief (and heck, I'm just a lawyer in Denver who was an undergraduate math major with a blog who reads way too many physics journal articles), is that there is no undiscovered dark matter particle or modification to gravity, even though the effects attributed to those causes undeniably exist and lots of data seem to fit each of these theories. I personally think that instead of new fundamental physics:
(I) dark matter is some combination of dim baryonic matter that forms the regular patterns observed in the MOND equation because the process by which galaxies are formed, which highly constrains their structure on what amounts to a pretty much linear continum, and that ultra fast outflows play a critical part in this structure,
(II) neutrino condensates may also be critical to explaining the observed dark matter/MOND phenomena (and that the number of slow moving neutrinos may be dramatically underestimated; neutrino oscillation and weak force interactions may produce neutrinos that have exhausted sufficient kinetic energy to keep them at an equilibrium at the same energy scale as the cosmic background radiation), and
(III) the general relativistic corrections to Newtonian approximations of gravity, correctly calculated with more observationally accurate mass distributions than are known today for most of the universe, probably account for any remaining discrepencies.
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