Four billion years later, a curious group of water-based humanoid life-forms tries to make sense of the galaxies’ collision. They point their telescope at the clusters’ relics and admire its odd shape. They call it the “Bullet Cluster.”
In the below image of the Bullet Cluster you see three types of data overlaid. First, there are the stars and galaxies in the optical regime. (Can you spot the two foreground objects?) Then there are the regions colored red which show the distribution of hot gas, inferred from X-ray measurements. And the blue-colored regions show the space-time curvature, inferred from gravitational lensing which deforms the shape of galaxies behind the cluster.
|The Bullet Cluster. |
[Img Src: APOD. Credits: NASA]
The Bullet Cluster comes to play an important role in the humanoids’ understanding of the universe. Already a generation earlier, they had noticed that their explanation for the gravitational pull of matter did not match observations. The outer stars of many galaxies, they saw, moved faster than expected, meaning that the gravitational pull was stronger than what their theories could account for. Galaxies which combined in clusters, too, were moving too fast, indicating more pull than expected. The humanoids concluded that their theory, according to which gravity was due to space-time curvature, had to be modified.
Some of them, however, argued it wasn’t gravity they had gotten wrong. They thought there was instead an additional type of unseen, “dark matter,” that was interacting so weakly it wouldn’t have any consequences besides the additional gravitational pull. They even tried to catch the elusive particles, but without success. Experiment after experiment reported null results. Decades passed. And yet, they claimed, the dark matter particles might just be even more weakly interacting. They built larger experiments to catch them.
Dark matter was a convenient invention. It could be distributed in just the right amounts wherever necessary and that way the data of every galaxy and galaxy cluster could be custom-fit. But while dark matter worked well to fit the data, it failed to explain how regular the modification of the gravitational pull seemed to be. On the other hand, a modification of gravity was difficult to work with, especially for handling the dynamics of the early universe, which was much easier to explain with particle dark matter.
To move on, the curious scientists had to tell apart their two hypotheses: Modified gravity or particle dark matter? They needed an observation able to rule out one of these ideas, a smoking gun signal – the Bullet Cluster.
The theory of particle dark matter had become known as the “concordance model” (also: ΛCDM). It heavily relied on computer simulations which were optimized so as to match the observed structures in the universe. From these simulations, the scientists could tell the frequency by which galaxy clusters should collide and the typical relative speed at which that should happen.
From the X-ray observations, the scientists inferred that the collision speed of the galaxies in the Bullet Cluster must have taken place at approximately 3000 km/s. But such high collision speeds almost never occurred in the computer simulations based on particle dark matter. The scientists estimated the probability for a Bullet-Cluster-like collision to be about one in ten billion, and concluded: that we see such a collision is incompatible with the concordance model. And that’s how the Bullet Cluster became strong evidence in favor of modified gravity.
However, a few years later some inventive humanoids had optimized the dark-matter based computer simulations and arrived at a more optimistic estimate of a probability of 4.6×10-4 for seeing something like the Bullet-Cluster. Briefly later they revised the probability again to 6.4×10−6.
Either way, the Bullet Cluster remained a stunningly unlikely event to happen in the theory of particle dark matter. It was, in contrast, easy to accommodate in theories of modified gravity, in which collisions with high relative velocity occur much more frequently.
It might sound like a story from a parallel universe – but it’s true. The Bullet Cluster isn’t the incontrovertible evidence for particle dark matter that you have been told it is. It’s possible to explain the Bullet Cluster with models of modified gravity. And it’s difficult to explain it with particle dark matter.
How come we so rarely read about the difficulties the Bullet Cluster poses for particle dark matter? It’s because the pop sci media doesn’t like anything better than a simple explanation that comes with an image that has “scientific consensus” written all over it. Isn’t it obvious the visible stuff is separated from the center of the gravitational pull?
But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Indeed, one would expect that modified gravity too should have a path dependence that leads to such a delocalization as is observed in this, and other, cluster collisions. And never mind that when they pointed at the image of the Bullet Cluster nobody told you how rarely such an event occurs in models with particle dark matter.
No, the real challenge for modified gravity isn’t the Bullet Cluster. The real challenge is to get the early universe right, to explain the particle abundances and the temperature fluctuations in the cosmic microwave background. The Bullet Cluster is merely a red-blue herring that circulates on social media as a shut-up argument. It’s a simple explanation. But simple explanations are almost always wrong.