The Bullet Cluster is a spectacular piece of cosmological evidence showing a cluster of galaxies that has passed through an even bigger cluster. Interpretations of this image claim that it proves the existence of dark matter. This essay questions this interpretation. It also suggests a novel idea that does not involve dark matter or modifying the laws of gravity.
The Bullet Cluster
The Bullet Cluster is a gravitationally bound group of about 40 galaxies that has evidently passed straight through an even bigger cluster of galaxies.
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There are dozens of images on the Bullet Cluster on-line – this is one of the best. The cluster on the right is called the Bullet Cluster and has evidently passed straight through the larger cluster on the left. Here is another image of the same thing …
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 (artifically) blue color indicates density of gravitational masses as inferred from analysing images of background objects that have been weakly lensed through and around the whole ensemble. The (artifically) pink areas show hot gases as revealed through X-ray telescopes. The conventional interpretation is as follows.
There are three types of matter in galaxies. Most of it is dark matter that is as yet undiscovered but is deduced to be cold and interacting with matter only through gravity. Most of the rest is gaseous and this interacts electro-magnetically with other things, especially itself and other gases like itself. The remaining matter consists of stars, planets, rocks, ice, dust and black holes (call this the hard matter).
When galaxies collide there are in fact very few collisions between the hard matter bits, simply because they are very small proportion of the overall volume. The gases on the other hand interact with each other electro-magnetically and ‘stick to each other’ a bit. Hence they get left behind while the hard matter moves on. The Bullet Cluster has left most of its gases behind in a ‘bow-wave’ pattern behind the hard matter still moving off to the right. There is more gaseous matter than hard matter but the gravitational lensing is coming from the areas colored blue, not the areas colored pink. Hence there must be a lot of dark matter and this has travelled with the hard matter rather than get caught up with all the gaseous interactions. If there was no dark matter the gravitational lensing should align more closely with the areas of heated gas in the middle.
Gravitational Lensing
Gravitational lensing has its origins in the fact that gravity bends starlight, in accordance with a key prediction of General Relativity. Eddington’s 1919 expedition to Principé to observe background stars during a total eclipse of the Sun provided the first proof of this. The huge increase in astronomical evidence in recent decades presents hundreds of examples of galaxies and clusters of galaxies surrounded by multiple distorted images of sources of light positioned far behind them. This is well explained by gravitational lensing and gravitational lensing has become an increasingly important part of astronomy. See for example the overview by Joachim Wambsganss (Wambsganss, 1998) and more recently the description and images on the Hubble Telescope website (Hubblesite, 2023).
Discussion
One question to ask about this event is why does it (and dozens of less dramatic events like it) exist at all? Students are generally taught that the expanding Universe is a bit like a loaf of raisin bread. Galaxies are the raisins and the intergalactic space is like the dough. But the raisins don’ t collide with each other so why should galaxies? The glib answer is that galaxies are affected by gravity, raisins less so. But if intergalactic space is expanding at the Hubble rate of expansion shouldn’t intergalactic attractions be getting weaker?
A second question is why is the bow wave so well defined? The large cluster is colliding just as fast as the small cluster, relative to each other, but it doesn’t have a neat bow wave. Bow waves in air or water are caused when waves are set up orthogonal to the line of motion. What is going on here?
A third question is how much gas and dust is left behind and how much stays with each cluster? The standard interpretation seems to require that nearly all of the gas is stripped away from the hard matter and ends up in the hot gas regions revealed by X-ray imaging. But what if this is over estimated? It is not unreasonable to imagine that a lot of the dust and uncharged particles in the cluster might actually stay with their host galaxies. Since the ‘evidence for dark matter claim’ is based on the amount of gravitational lensing around the central bright areas being too small compared to the amount of gravitational lensing on the left and right of the image a revision to the modeling for what happens to all the baryonic matter could affect the interpretation.
There are also questions that can be asked about the gravitational lensing. Think carefully about this. The lensing environment is very different from normal. Gravitational lensing was first established for stars and gradually extended to galaxies. What we have here is hot ionised gases that have been stripped out of the cluster of galaxies. The electro magnetic aspects cannot be ignored. The assumption that just the mass of the gases is responsible for lensing is questionable. Furthermore the whole environment is turbulent and possibly chaotic.
How all this affects gravitational lensing cannot be taken lightly. Firstly the light from background sources is going to be absorbed, scattered and deviated in unusual ways. Secondly the gravitational field created in this turbulent environment might not be conducive to the smooth lensing of background light. The stronger parts of the lensing might simply be blurred out of the picture.
Look at the picture again. Perhaps the blue areas encompass the pink areas but the lensing light has been absorbed and scattered in the overlaps.
Q Theory and the Bullet Cluster
Mach’s Principle suggests that the correlation between an absence of rotational effects and being at rest with respect to the ‘fixed stars’ is not a coincidence. The modified version of this principle presented in Essay 1.2 of this series, see (Van de Vusse, 2024) suggests that the relationship is not due to any ‘action at a distance’ but occurs because local inertial effects and the grand scale distribution of matter both have a relationship to an all persuasive local quantum field.
There are dozen of theories involving some sort of persuasive field filling all of space, and dozens of experiments tightly constraining the properties of any such thing. The theories include aethers, quintessence, background geometries and quantised vacuum energy type ideas. The contra-indications come from a host of precise experiments that fail to find evidence of linear movement in or through such field, or indeed any other form of isotropy.
Later essays in this series will investigate whether it is possible to specify a field that is consistent with the evidence. To avoid prejudicing the outcome the conjectured field is given a new name … Q. (The name was chosen because Q is easy to type and pronounce and it was available.)
A variant on the Q field hypothesis is that it is also an aether field. Which would also mean it is the physical reality behind the mathematical concept of curved spacetime. In other words it would literally be the fabric of space-time, and the medium in which gravitational disturbances travel. It would also have dielectric properties and be the medium in which light travels, where ‘light’ covers radio waves, gamma rays and everything in between.
Maxwell, Mach, Michelson, Morley, Hertz, Poincaré, Lorentz, Sagnac and innumerable other great physicists would be delighted if such a thing existed. Whether or not is does is an interesting question.
Q theory is introduced and discussed in later essays. It is mentioned here because it might be relevant to the Bullet Cluster. Unlike most background field ideas Q is not envisaged to be static – at least not on galactic scales of distance and time. On cosmic scales it can have currents, whirls and swirls. Large-scale movements in Q are associated with large-scale patterns in the structure of the Universe. The association is causal.
On a galactic scale there is movement in the Q associated with the large scale rotation of all the stars gases and dust, but also resistance to rotation due to all the surrounding Q. This underpins the Mixed Inertial Rotational Reference Field effect (MiRRFe) conjectured in Essay 1.1.
Gravitational lensing by a massive object, galaxy or cluster of galaxies works by affecting the geometry of Q. The distorted Q tells the background light which way to go.
When clusters of galaxies collide the relationship with the associated Q is completely disturbed. Not so much around the hard matter. The hard matter continues to affect the Q as before and the affected Q continues to bend light as before. It is the gases that become disrupted and turbulent and this affects the Q field, which in turn affects the lensing of any background light.
Whether or not Q theory has merit does not affect the earlier concerns about the claim that the Bullet Cluster has provided definite proof that dark matter exists.
Summary
The Bullet Cluster provides evidence relative to the existence of dark matter, but by itself it is not conclusive. There are questions about what exactly has happened to all the normal (and possibly abnormal) forms of baryonic matter, and also about the apparent dearth of gravitational lensing around the bright gas regions. The best proof of dark matter would come from actually finding some. Decades of searching have not been successful and the list of candidates is almost exhausted.
References
- Van de Vusse, Sjoerd B.A., 2024, Some ideas and experiments for issues affecting modern physics, https://hereticalphysics.com.au
- Wambsganss, Joachim, 1998, Gravitational Lensing in Astronomy, Living Reviews in Relativity, 1998 1(1) 12; doi: 10.12942/lrr-1998-12 or https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5567250/
- Hubblesite, 2023, Gravitational Lensing, www.hubblesite.org/contents/articles/graviational.lensing
Author contact: SBAvan@utas.edu.au
Author’s location: Hobart, Australia
