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. Fig.1: The Matter of the
Bullet Cluster 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. 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?
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. 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.
M. Markevitch
et.al
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.
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
et.al. “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 et.al. (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: www.cxc.harvard.edu/cdo/xclust11/pres/Russell_Helen.pdfAnother 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., et.al.
(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. 5) Moni Biden, C.,
et.al. Kinematical
and chemical vertical structure of the Galactic thick disk II. A lack
of dark
matter in the solar neighborhood |
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