One result is very definite by now: neither the Bullet nor the Train Wreck clusters support (nor do they prove) the existence of cold or warm dark matter. And, they certainly do not disprove MOND. Quite on the contrary, according to current knowledge, they falsify the concordance cosmological (or LCDM) model.
The Bullet cluster consists of two clusters of galaxies that have penetrated each other leaving behind a slab of gas while the now seperating clusters retain matter as revealed through gravitational lensing. Assuming General Relativity (GR) to be valid the lensing measurements tell us that collisionless dark matter must be present in the separating clusters. But, it has been shown that the relative velocity of the two clusters need to be so large that the observed constellation ought to not occur in the real universe if it were described by GR, i.e. by the concordance cosmological model. Instead, it turns out that MOND-based models can readily account for the large relative velocity and the lensing signal as long as both clusters contain some hot dark matter or, alternatively, gas in cold clouds that cannot be detected. The Train Wreck cluster shows the opposite behaviour: assuming GR to be valid, the putative cold or warm dark matter has separated from the galaxies in this other collision of galaxy-clusters. The core of dark matter is evident from gravitational lensing (assuming GR to hold). This is inexplicable within GR because there is no known physical mechanism known for separating the dark matter from the galaxies as it does not dissipate like gas. In MOND-based models, the train wreck is also a challenge, but in principle it may perhaps be possible to separate the hot-dark-matter cluster core and the galaxies, and/or to obtain spurios lensing signals suggesting matter concentrations where there are none. Thus, the train wreck may, in the end, turn out to be a case supporting MOND-based models over GR-based ones.
As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK had been contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are
C) What are the three best reasons for the failure of the LCDM model?
D) What about the Bullet cluster? And what about the Train-Wreck cluster Abell 520? (this contribution)
E) Why is the main stream community so reluctant to go along with accepting the failure of LCDM?
This contribution deals with Question D, while an upcoming contribution will concentrate on the remaining question. Beyond that we will keep posting on issues of relevance for the paradigm change.
The full question posed was "And how do you respond to the bullet cluster results which seems to point to a center of mass that does not match luminosity via weak gravitational lensing"
We augment the answer with a brief discussion of the Train-Wreck cluster Abell 520. Our pevious contributions on this issue are:
- "But the Bullet cluster ... " - proof of Cold or Warm Dark Mattr in galaxy clusters is but a myth (30.07.2010)
- The Bullet Cluster and galaxy clusters in modified-gravity theories (04.08.2010)
- The Train Wreck Cluster - an "anti-Bullet-Cluster": disproof of Cold or Warm Dark Matter (11.08.2010)
A brief background to galaxy cluster dynamics:
In the Einsteinian/Newtonian theory of gravitation, galaxy clusters require about ten times as much mass in cold (or warm) dark matter than is present in normal matter.
Assuming MOND, the observed gravitational lensing and the observed kinematics in galaxy clusters merely require about a factor of two to perhaps three in additional mass. In MOND, the problem of missing mass in galaxy clusters is therefore significantly reduced. It may be completely removed if the missing mass is normal matter which is in undetectable cold gas. Or, the missing mass may be in agreement with particle physics because neutrinos oscillate and thus must have a mass. This implies the existence of additional neutral particles such as sterile neutrinos. If sterile neutrinos have a mass near 11 eV (see below) then the dark-matter problem in MOND-galaxy-clusters and in MOND-cosmological models disappears (Angus & Diaferio 2012). Such dark matter is "hot", i.e. after the Big Bang the particles had relativistic (extremely high) velocities, and so such dark matter cannot agregate into galaxies but can be captured into galaxy-cluster-sized gravitational bodies which have sufficiently deep potential wells for the hot dark matter to not be able to escape.
This is nicely explained by Sanders (2003) in his research paper "Clusters of galaxies with modified Newtonian dynamic., and in his review of MOND (Sanders 2009), and in the recent 160 page "Modified Newtonian Dynamics: A Review" by Famaey & McGaugh (2012).
Answer to the Bullet cluster:
According to Tom Shanks (private communication 2010), the data reduction to get the actual weak-lensing matter distribution map is very complex and relies on subtraction of a background. There is some freedom and it is difficult to extract a signal. See also the comment by "JR" quoted below.
But, accepting the data reduction which has been published, the
Bullet cluster surprisingly turns out to be a counter-argument against the validity of the LCDM
model. In LCDM the required velocities of the two clusters is too high
(about 3000km/s), a velocity which does not occur. Now, in a MONDian
universe, such velocities occur rather naturally, and so with some hot
dark matter (in fact the same as I wrote above, 11eV particles) one gets
beautiful agreement with the observations!
The Bullet Cluster (1E 0657-56) is often perceived to be a disproof of Milgromian dynamics because even in Milgromian dynamics DM is required to explain the observed separation of the weak lensing signal and the baryonic matter. In actuality, the Bullet Cluster is, if anything, a major problem for the LCDM model because the large relative cluster–cluster velocity at the mass scale of the two observed clusters required to provide the observed gas shock front cannot be attained in the LCDM model, as shown by Lee & Komatsu (2010) in their research paper "Bullet Cluster: A Challenge to ΛCDM Cosmology" and as verified and deepened by Thompson & Nagamine (2012) in their research paper "Pairwise velocities of dark matter haloes: a test for the Λ cold dark matter model using the bullet cluster". Thomposn & Nagamine "conclude that either 1E 0657-56 is incompatible with the concordance ΛCDM universe or the initial conditions suggested by the non-cosmological simulations must be revised to give a lower value of" the relative velocity.
But the high relative velocities between the two sub-clusters in the Bullet cluster arise naturally and abundantly in a Milgromian cosmology:
Assuming the Milgromian framework to be the correct description of effective gravitational dynamics, it has been shown that the Bullet Cluster lensing signal can be accounted for in it. Ibn their researhc paper, "Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry", Angus, Famaey & Zhao (2006) state "In particular, we can generate a multicentred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution."
If a Milgromian cosmology is allowed to have a hot DM component then the Bullet Cluster is indeed well explainable. In the research paper on "The collision velocity of the bullet cluster in conventional and modified dynamics", Angus & McGaugh (2008) they summarise:
"We consider the orbit of the bullet cluster 1E 0657-56 in both cold dark matter (CDM) and Modified Newtonian Dynamics (MOND) using accurate mass models appropriate to each case in order to ascertain the maximum plausible collision velocity. Impact velocities consistent with the shock velocity (~ 4700kms-1) occur naturally in MOND. CDM can generate collision velocities of at most ~3800kms-1, and is only consistent with the data, provided that the shock velocity has been substantially enhanced by hydrodynamical effects. "
Using a new cosmological N-body code for MOND, Angus & Diaferio (2011) find "As a last test, we computed the relative velocity between pairs of haloes within 10 Mpc and find that pairs with velocities larger than 3000 km s-1, like the bullet cluster, can form without difficulty.
We know that neutrinos oscillate, therefore they must have a mass. That mass is small. This makes them a form of hot DM that we most definitely know to exist. In order to explain the oscillations, particle physics suggests the possible existence of more massive, sterile neutrinos, which interact by gravity. If they exist they might be massive enough to account for the missing mass in galaxy clusters in MOND (and they can fit the first three acoustic peaks in the CMB). A research paper discussing the possible role of sterile neutrinos for dark matter has been published by Dodelson & Wildrow (1994).
Taking this ansatz, Angus, Famaey & Diaferio (2010) demonstrate, in their research paper "Equilibrium configurations of 11 eV sterile neutrinos in MONDian galaxy clusters" that consistency in solving the mass-deficit in galaxy clusters and accounting for the CMB radiation power spectrum is achieved if sterile neutrinos (SN) have a mass near 11 eV. They write “we conclude that it is intriguing that the minimum mass of SN particle that can match the CMB is the same as the minimum mass found here to be consistent with equilibrium configurations of Milgromian clusters of galaxies"
The Train Wreck cluster:
The Train-Wreck cluster (Abell 520) has been shown to be incompatible with the LCDM model because the putative C/WDM particles have separated from the galaxies such that a core of DM is left behind and away from the concentrations of galaxies, as Mahdavi et al. (2007) find in their research paper "A Dark Core in Abell 520". There is no known physical mechanism which can separate cold or warm dark matter from galaxies to the extend required by the Train Wreck.
Jee et al. (2012) return to the Train Wreck with their research paper "A Study of the Dark Core in A520 with the Hubble Space Telescope: The Mystery Deepens", confirming the problem. They speculate on a possible solution such as DM possibly having a self-interaction property, and interestingly they avoid discussion of any alternative theory of gravitation.
The Train Wreck remains not understood.
As pointed out by Kroupa (2012), in a MONDian cosmological model with hot dark matter (HDM) it is conceivable, at least in principle under certain conditions, for the self-bound galaxy-cluster-sized HDM core of the whole cluster to dissociate itself from the baryonic matter in galaxies since the galaxies do not reside in HDM halos. Each individual galaxy would remain on the baryonic Tully-Fisher relation, as is observed to be the case for all disk galaxies and as is required to be the case if MOND is correct (e.g. Famaey & McGaugh 2012). And, in MOND-based models it may perhaps be possible to obtain spurios lensing signals suggesting matter concentrations where there are none.
Finally, in the comment "Scientific Polemicism" to our previous contribution "The Train Wreck Cluster - an "anti-Bullet-Cluster": disproof of Cold or Warm Dark Matter, "JR" writes on the 18.10.2010 at 14:45:
The main reason why most scientists remain sceptical about the Abell 520 "train wreck" results is that different groups analysing the *same data* obtain different mass maps (see Okabe & Umetsu 2008). Now that's a train wreck! The same cannot be said for the bullet cluster, where - to the best of my knowledge - all authors currently agree on the lensing mass maps. This does not mean, of course, that the bullet is right and Abell 520 is wrong - we should remain open minded about both. But I am particularly sceptical of the Abell 520 results because of a well-known problem with lensing mass reconstruction: the monopole degeneracy. This was illustrated beautifully in recent work by Liesenborgs et al. (2008) who show that the monopole degeneracy can lead to phantom peaks in the mass distribution (see their Figure 3). Their work focused on strong rather than weak lensing, but weak lensing suffers from exactly the same problems.
A ring of dark matter?:
Milgrom & Sanders (2008) analyse in their research paper "Rings and Shells of "Dark Matter'' as MOND Artifacts" the recent detection using weak lensing of a ring of dark matter around a galaxy cluster. They write in their abstract:
We consider the possibility that this pure MOND phenomenon is in the basis of the recent finding of such a ring in the galaxy cluster Cl 0024+17 by Jee et al. (2007). We find that the parameters of the observed ring can be naturally explained in this way; this feature may therefore turn out to be direct evidence for MOND.
By Pavel Kroupa and Marcel Pawlowski (15.04.2012): "Question D: What about the Bullet cluster? And what about the Train Wreck cluster Abell 520" on SciLogs. See the overview of topics in The Dark Matter Crisis.