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The Bullet Cluster: Evidence for Dark Matter or Not?

Doug Marett (2013)

   Gravitational lensing studies of the bullet cluster 1E 0657-56 in Carina have recently been suggested to be the best evidence to date for the existence of that elusive hypothetical substance, dark matter. [1] The object is a collision of two galaxy clusters and their associated hot interstellar gas. The most commonly cited graphic is the NASA composite image below. The blue hues show the putative dark matter masses, separated from the visible mass in pink. The x-ray emitting hot gas is shown in red. The explanation given by NASA is that in this collision the dark matter and interstellar gas have separated, as is evidenced by the gravitational lensing profile of the cluster, which shows the largest lensing in the blue areas, separated from the hot gas and matter.


The Matter of the Bullet Cluster
Composite Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.;
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

The lensing contours purported to be due to dark matter are shown below from the Wikipedia image. It is because the lensing is found separated from the visible mass of the cluster, and because the lensing is attributed to gravity, that the conclusion is drawn that this must be dark matter that has separated from the two clusters.

Fig. 2:

This conclusion is premature in our opinion for the following reasons:

       1)      It is possible that lensing of light around galaxies and clusters is due to simple refraction by the changing refractive index of the interstellar gas clouds that it passes through, or by Fresnel dragging of starlight by the high velocity plasma in the cluster.

       2)      There is no proof that dark matter actually exists – it was conjured up as a book-keeping substance to account for the huge errors in the mass of galaxies encountered when using conventional physics, such as the virial theorem and the Keplerian rotation curve. Some recent mathematical treatments that take into account the anisotropic mass distribution of galaxies achieve the correct answer without resorting to dark matter or MOND, as is described in "The 100 year wrong turn in cosmology."      

 3) The dark matter hypothesis for the bullet cluster is contradicted by the cold dark matter ΛCDM model. Mastropietro & Burkert (2008) have shown that an initial relative velocity of the two colliding clusters would need to be around 3000 km/s in order to explain the observed shock velocity, X-ray brightness ratio and morphology of the main and sub-cluster. However, Jounghun and Eiichiro (2010) have shown that such a high infall velocity is incompatible with the predictions of the cold dark matter ΛCDM model. The probability that such an event could occur is roughly one in 10 billion! The lower velocity simulations of, for example Milosavljevic et. al. (2007) and Springel& Farrar (2007), that could be compatible with ΛCDM, do not reproduce the weak lensing data of the Bullet cluster. What this means is that it is pretty much impossible that the lensing effect seen in the Bullet cluster can be due to dark matter, based on the cold dark matter models! To quote Jounghun and Eiichiro:  

“The bullet cluster 1E0657-56 is not the only site of violent cluster mergers. For example, there are A520 (Markevitch et al. 2005) and MACS J0025.4-1222 (Bradac et al. 2008)… it seems plausible that there may be more clusters like 1E0657-56 in our universe. This too may present a challenge to ΛCDM.”

So let’s look at an alternative explanation for what might be going on.

Refraction by interstellar gas:

   As is well known, lensing is far more commonly caused by light passing from a medium of one refractive index to another, as is shown below for an air/glass interface.

 Fig. 3: Normal bending light due to a refractive index change:

However, lensing can also occur due to a change in the density of a gas, due to a temperature gradient for example. This is the mechanism of what is called a spinning pipe gas lens – if you heat the wall of a pipe and then spin it, the air inside forms a temperature driven density gradient that can then lens light.

Fig. 4: A heated spinning pipe lens  - a Graded Refractive INdex (GRIN) lens

The process with hot air gradients created in rotating pipes is well described at the reference here. Quoting Mafusire and Forbes: “Using air as an example, it is possible to bend light by a process of continuous refraction as opposed to stepwise refraction in the case of solid state optics. This is achieved by effecting a continuous change of the refractive index from a high value for lowest temperature (or high density) to a low value for the highest temperature (or lowest density). Since the density changes gradually from a low to a high value, the refractive index is graded as well. In fact, this effect is well known in nature: the mirage effect where light refracts away from a hot surface, usually a road or desert sand, is precisely due to continuous refraction of light.” 

Could  a similar density gradient of interstellar gas be responsible for the lensing seen in the bullet cluster? 

An examination of the Bullet Cluster for evidence of interstellar gas gradients sufficient to cause lensing of visible light

    If we examine the structure of the bullet cluster, we find immediately that it is one of the hottest galaxy cluster collisions known! The collision formed a shockwave of interstellar gas which is visible in the x-ray emissions – in fact, it is considered a textbook example of a bow shock. [2] An image showing the position of the bow shock on the right side beyond the so-called “bullet feature is shown below:

Fig. 5: The shock front of the Bullet cluster

The formation of the shocked gas is described here. In this image below it can be seen that the border of the bow shock (the outer extreme of the blue area) appears to pass through the centers of the so-called gravitational lensing effect described in the literature.

 Fig. 6: The shock front of the bullet cluster overlaid on the lensing map

In Fig. 7, we show the estimated position of the shock front, and how it lines up with the bulls-eyes of the gravitational lensing map. This is assuming that the shockwave is symmetrical and aligned with a position equidistant between the two clusters, which is consistent with the temperature data to be discussed below. The x-ray brightness is shown as the red contours, adapted from here.

  Fig. 7: Alignment of shock front with the gravitational lensing map

M. Markevitch describe the shock front, saying that the x-ray brightness edge (at least on the right side) is the edge of this front. There is a temperature or density jump across the front. Further, they describe the front as having “two brightness edges whose shapes indicate spherical gas density discontinuities in projection.” The projected gas temperature map from Markovich’s paper is shown below. It can be clearly seen that the gas temperature it at a maximum at either antipode of the shock front, and that the gas temperature decreases towards the center between the two clusters, and also out into space in front of the shock wave.

 Fig. 8: Projected gas temperature map for the bullet cluster overlaid on the x-ray brightness contours, from M. Markevitch

If a refractive index gradient of the interstellar gas exists, it is likely greatest starting from the edge of the shock front on either side, and diminishing out into space in a spherical pattern, similar to the temperature pattern shown above. This approximates the pattern seen in the lensing map of Fig. 6 and 7. Quoting M. Markevitch “the gas density and temperature jumps at the shock are related…the shock propagates in front of a cooler gas bullet…” The cluster gas is extremely hot, which is perhaps part of the reason why this phenomenon is occurring. Considering the extremely high gas temperatures and the large gradients across the shock front, it would seem plausible that visible light could be lensed across this hot interstellar gas gradient by simple refraction, and that gravitational lensing is not necessarily involved.  It is already known that interstellar gas can disperse, refract and even Faraday rotate electromagnetic waves, as was discussed in our previous article “The 100 year wrong turn in cosmology.” In Bruce Draine’s Physics of the Interstellar and Intergalactic Medium P.117 he discusses the Fiedler effect, where interstellar gas dramatically refracts radio signals from an extragalactic radio source.  It is known that gravitational lensing is supposed to involve lots of aberrations but not chromatic aberration, and that normal uncorrected lenses usually  have some degree of chromatic aberration.  Thus follows the argument that the two phenomenon can be distinguished in theory at least by an examination of the degree of chromatic aberration seen in the lensing phenomenon.  It is unclear from our review of the literature to date whether large scale GRIN lenses of interstellar gas demonstrate appreciable chromatic aberration, or whether the bullet cluster lensing phenomenon has been proven to be perfectly achromatic. We will continue to look for this evidence one way or the other. 

                                                     Fresnel Dragging by High Speed Plasma in the Shock Zone

    Another possibility is that the light from the galaxies behind the bullet cluster are being lensed by a Fresnel dragging process. Let’s assume we have a high velocity explosion-like radial expansion of plasma similar to what is happening in the bullet cluster. We should expect at least some Fresnel dragging by the expanding medium. An article discussing Fresnel dragging of light in interstellar space is here. The effect may be relevant because of the extremely high plasma velocity at the shock front of 4500 km/s, which could deviate the angle of starlight passing through it.

This might explain why the lensing observed is coincident with the shock wave fronts. Fresnel dragging of light passing through an rapid radially expanding plasma should also mimic a dispersive lens similar to weak gravitational lensing. A very nice animation of the evolution of the shock fronts as they expand radially outward from the collison is available here.  A screen shot of this animation by Volker Springel and Glennys Farrar is shown below:

The Chandra data x-ray map of the lensing field:

   The temperature profile of the bullet cluster gas suggests that the temperature is hottest near the center of the lensing effect and drops towards the periphery. This is the opposite of the lens shown in Fig. 4 – therefore, for visible light this kind of interstellar gas lens might be dispersive rather than focusing if the plasma density is inversely correlated with temperature.  A matter lens for visible light needs to be dispersive in order to mimic Weak lensing.  It then follows that a plasma with a hot center and a cooler periphery could act as a dispersive GRIN lens  if it has the correct pressure or density gradient profile - if the density is reversed, then a GRIN lens of plasma will be focusing.  

  The more detailed temperature map below obtained from the Chandra x-ray center website shows the x-ray emission temperature across the bullet cluster. Now viewed from a different orientation, the blue circles correspond approximately with the position of the weak lensing centers. These positions also correspond to the hot centers (white areas) of the x-ray temperature map. This at least suggests that the lensing effect is associated with the temperature of the hot gas, and not necessarily associated with some unseen dark matter.  In this detailed image, the  temperature transition can be seen to be more graded from the center to the periphery around each shock zone.  We note that in the original paper from 2006 by Clowe et. al. that generated the gravitational lensing map shown in Fig. 2., that their argument was that there are no cluster sized concentrations of galaxies or hot plasma near the lensing structures. This appeared to be justification that hot plasma was not responsible for the effect. To quote from their paper: 

“There is no evidence, however, in our deep imaging for additional cluster sized concentrations of galaxies or of plasma hotter than T~0.5 keV (the lower bound of the Chandra energy band) near the observed lensing structures.”

However the temperature map of the plasma from M. Markevitch  (Fig. 8) appears to show plasma temperatures near the lensing structure centers of 16-24 keV. The temperature map from the Chandra x-ray website in Fig. 9 shows temperatures in the same regions at or higher than 16 keV. These regions are the hottest regions in the cluster. Considering the extremes of temperature and velocity of the plasma in the "lensing" zone, and the small amount of observed lensing used to construct the map, it would seem plausible that some more conventional optical explanation involving refraction or Fresnel dragging might be in operation, accounting for part, if not all, of the observed effect.  Conventional weak gravitational lensing by the matter of the hot plasma itself could also be involved.  After all, the studies are based on the bending of extremely distant light sources passing through what is effectively an explosion of hot gas that is travelling at up to 1.5% of the speed of light, a highly abnormal and energetic situation.  

Fig. 9: X-ray temperature image of the bullet cluster, and an image of the bow and upstream shock.

Adapted from:

Another detailed temperature map of the bullet cluster with the X-ray brightness contours overlaid is here.

Useful References Critical of the Dark Matter Hypothesis and Concordance Cosmology:

1) Clowes, R., (2012) A structure in the early Universe at z ˜ 1.3 that exceeds the homogeneity scale of the R-W concordance cosmology.

2) Kroupa, P.; Pawlowski, M.; Milgrom, M. (2013) The Failures of the Standard Model of Cosmology Require a New Paradigm.

3) Kroupa, P. (2012) The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology.

4) Karachentsev, I.D. (2012) Missing Dark Matter in the Local Universe.

5) Moni Biden, C., Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood