Bullet Cluster as an argument against dark matter

Original author: Sabine Hossenfelder
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A long time ago, two galactic clusters collided in the back of the universe. Their collision broke apart galaxies and left behind two modified heaps of stars and a gas, flying apart from each other, whose fate was unknown.

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Four billion years later, a curious group of water-based humanoid life forms is trying to figure out the collision of galaxies. They direct their telescopes to the remnants of the clusters and are surprised at their strange shape. They call them "bullet cluster".

Three clustered data types can be seen in the image of the cluster. First, the stars and galaxies in the optical mode. Secondly, the highlighted red regions, showing the distribution of hot gas, obtained from x-ray measurements. Thirdly, the blue regions, showing the curvature of space-time, obtained from gravitational lensing, which bends the shape of galaxies behind the cluster.

Bullet Cluster plays an important role in humanoid understanding of the universe. Their previous generation has already found that their explanation of the gravitational attraction of matter does not match the observations. The outer stars of many galaxies moved faster than necessary, which meant that the gravitational pull was stronger than their theories said. Galaxies in clusters also moved very quickly. The humanoids concluded that their theory that gravity caused space-time curvature needs to change.

Some of them argued that they still understood gravity correctly. They believed that there was an additional type of invisible, "dark matter", interacting so weakly that it has no consequences, except for additional gravitational attraction. They even tried unsuccessfully to catch elusive particles. Experiment after experiment did not give anything. Decades passed. And yet they claimed that dark matter particles simply interacted even less. They built increasingly larger experiments to catch them.

Dark matter was a convenient invention. It can spread in the right quantities where necessary, and thus justify all the data on galaxies and their clusters. But, although dark matter matched the data well, itdid not explain why the modification of gravitational attraction is so systematic . On the other hand, it was difficult to work with modifications of gravity, especially with the dynamics of the early Universe, which was much easier to explain with dark matter particles.

In order to move science, curious scientists needed to separate two hypotheses: altered gravity or dark matter particles? They needed observation, eliminating one of two ideas, irrefutable proof - the Bullet cluster.

The theory of dark matter particles has been dubbed the “consistent model” (as well as ΛCDM). It depended heavily on computer simulations optimized to match the observable structures of the Universe. From them, scientists could infer the frequency with which galactic clusters should collide and the typical relative speed with which they should collide.

From X-ray observations, it became known that the speed of collisions of galaxies in the Poole cluster was about 3000 km / s . But such high speeds almost never met in computer simulations based on dark matter particles. Scientists estimated the probability of a collision that occurred in the cluster area, in one chance out of 10 billion, and concluded: the collision we observe is incompatible with the agreed model . That is how Bullet Cluster becamestrong evidence in favor of altered gravity .

However, after a few years, some inventive humanoids optimized computer simulations describing dark matter, and received a more optimistic prediction of the probability of finding something like a Bullet cluster,4 , 6 × 10 - 4 . Soon after, they once again clarified the probability and got6 , 4 × 10 - 6 .

Anyway, the Bullet cluster remained a surprisingly unlikely event for the theory of dark matter. Conversely, it was very easy to place it in theories of altered gravity, in which collisions at relatively high speedsoccur much more often.

It may sound like a story from a parallel universe - but this is true. The Bullet Cluster is not an irrefutable proof of the theory of dark matter, as reported to you by the media. It can be explained with the help of theories of altered gravity. And it is difficult to explain with the help of dark matter.

Why then do we so rarely read about the difficulties presented by a cluster for dark matter? Because non-fiction media do not like things more complicated than a simple explanation with the notion “scientific consensus”. Is it not obvious that visible matter is separated from the center by gravitational attraction?

But the change of gravity is carried out by entering additional fields mated with gravity. There is no reason why in the dynamic system these fields should focus in the same place where there is normal matter. One would expect that the altered gravity would also be nonholonomic , which would lead to the delocalization observed in this and other collisions.

The real test of altered gravity will not be the Bullet cluster, but a correct description of the early Universe, an explanation of the abundance of particles and temperature fluctuations in relic radiation. Bullet Cluster - just a distraction maneuver, used in the media. This is a simple explanation. But simple explanations are almost always wrong.

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