Whenever a discussion about the problems of the Cold Dark Matter Hypothesis and possible alternatives like Modified Newtonian Gravity (MOND) emerges, one argument you can be sure to hear soon is “But the Bullet Cluster ...”. It is the same whether you discuss with scientists or other people interested in astronomy. But can the Bullet Cluster be considered as a proof of Cold or Warm Dark Matter? No, because that conclusion rests on further assumptions and is in itself not logically valid. Furthermore, the problems of the Dark Matter Hypothesis are independent of the Bullet Cluster, making it a false argument in many discussions. Even worse, the collision velocity of the Bullet Cluster seems to be incompatible with the concordance cosmology. At the same time, alternative gravity theories, while often said to fail in explaining galaxy clusters, can account for them rather naturally.
In this first post in a series of three we will discuss the Bullet Custer as a "smoking gun" for Dark Matter. The other two parts about "The Bullet Cluster and galaxy clusters in modified-gravity theories" and "The Train Wreck Cluster - an 'anti-Bullet-Cluster': disproof of Cold or Warm Dark Matter?" will follow in a few days.
What is the “Bullet Cluster”?
It all started in 2006 with a paper titled “A direct empirical proof of the existence of dark matter” and a press release with the no-less lurid headline “NASA Finds Direct Proof of Dark Matter”. Right in the title the authors claimed that their discovery would immediately settle the question whether there is Dark Matter (DM) or not. Naturally, the spectacular announcement was adopted by the majority of the media and could well be one of the most successful press releases in astronomy. Since then, the picture of the Bullet Cluster (BC) has been shown countless times:
What do we see in this picture? There are two clusters of galaxies (left and right). Overlaid are blue and pink colors. The two pink clumps in the middle shows where x-ray observations find the hot gas, the Bullet Cluster got it's name from the bullet-like shape of the gas on the right. The gas usually sits in the center of a galaxy cluster, but is here shifted to the point between the clusters. Some time ago the two galaxy clusters have passed through each other, making their gas collide. As gas interacts electromagnetically, it is slowed down when it collides, like two streams of air that can not pass through each other unhindered in opposite directions. That is why the gas is a bit behind the galaxies. Those do only interact through gravity and therefore pass each other unhindered without being slowed-down like the gas. This is maybe similar to two swarms of flies that can fly through each other.
The blue blobs were derived in a bit more complicated manner using the gravitational lens effect. To put it simple, since Einstein we know that matter deforms space-time. This leads to a bending of light rays when they come close to a large amount of matter, like a galaxy cluster. Thus, when the light of distant galaxies passes a massive cluster of galaxies before it reaches us, we will see a deformed image of the distant galaxies. The effect can be calculated and astronomers are able to trace it back. From the distorted shapes of distant galaxies behind a galaxy cluster they can infer the distribution of mass in that galaxy cluster. The heavier a galaxy cluster is, the more it bends the light and the more it distorts background galaxies. For the Bullet Cluster, the blue blobs in the picture above show the distribution of mass as inferred from the gravitational lensing effect assuming General Relativity to be valid. One can see that it follows the distribution of galaxies, not the gas.
If the hot gas would be the most massive part of a galaxy cluster, the mass found through gravitational lensing would have to be centered on it. But as this is not the case, it is said that the majority of matter in the galaxy cluster has to be close to the galaxies. Because the visible mass in the galaxies is not enough to account for the velocity dispersion of a galaxy cluster (assuming Newtonian Dynamics, i.e. General Relativity), it is conjectured that there is Dark Matter, which by definition only interacts through gravity, too. Thus distributions of DM can pass through each other just like the galaxies and the majority of mass should be found close to the galaxies in such a cluster collision.
This is why the Bullet cluster is often said to be the “smoking gun” of the Standard Cosmological Model. It behaves just like it is expected. But is it really that simple? Does this proof the existence of Dark Matter? No, it doesn't.
Observations and Interpretations
In the most-often given description of the Bullet Cluster, what is observation and what is interpretation get mixed up. The observations tell us that the hot gas component and the lensing mass have an offset. One good conclusion from this is: The visible, hot gas can not make up the majority of mass in the system. But a wrong conclusion is: The Cold Dark Matter Hypothesis is right.
While we can argue that the majority of mass has to be close to the galaxies, we can not immediately conclude that it has to be in the form of Dark Matter as a new type of particles. Actually, we can only say that the majority of gravity, or even more specific, the major bending of space-time, happens close to the galaxies. Whether the reason is missing mass or a different law of gravity is not that easy to distinguish (there is gravitational lensing in modified gravities, too). The whole, most-mentioned conclusion is therefore based on one important, but never mentioned assumption: that gravity is best described by Newtons law. In addition to that, it supposes that other, known forms of dark matter (e.g. neutrinos) can not be the reason. Without those assumptions, the case of the Bullet-Cluster is not decided at all.
We see that the “direct proof for the existence of Dark Matter”, is an indirect hint at best, in that it is based on untested assumptions and does not even look at other possible predictions of alternative gravities. But there are more problems to come.
A Proof of Dark Matter?
Even if the cluster can be explained in the standard or concordance cosmological LCDM framework, this does not proof the theory. Because there can be no proof of a scientific theory. For the BC to be a proof of a scientific theory, it would have to rule out each and every alternative explanation, even those of which we can not even think of today. This, of course, is impossible. This fact is well known in the philosophy of science and I guess most scientists know this. Scientific inference does not function without this elementary fact.
Furthermore, there are different possible explanations for the BC. There even are different possible forms of Dark Matter. Not only the currently favored Cold Dark Matter, on which the Concordance Cosmology Model rests, but a model with Hot Dark Matter (where the DM-particles are fast/relativistic because they would be of low mass, like neutrinos) could explain the BC as well. So, please don't state that the Bullet Cluster has proven the LCDM-model right. It has not. And it can not.
The Bullet Cluster, a problem for Dark Matter?
In fact, the Bullet Cluster might not only not be a proof of the DM hypothesis but it actually appears to be a major problem for the concordance model. Mastropietro and Burkert (2008) have found that the two colliding clusters need to have a relative velocity of about 3000 km/s to produce the observed X-ray gas properties. This result was compared to a cosmological simulation named MICE. Such cosmological large-volume simulations show the formation of structure in the universe and are often said to be another important success of the Concordance Cosmological Model. In the MICE simulation, Lee and Komatsu (2010) determined the probability that the Bullet Cluster's velocity could be found in the concordance cosmological model. It is roughly one in ten billion! They ...
“... conclude that the existence of [the Bullet Cluster] is incompatible with the prediction of the ΛCDM model ...”.
This is a paradoxical situation: While the structure formation simulations are used to argue in favor of Dark Matter because they fit so well, and the Bullet Cluster is used in favor of Dark Matter as a “direct proof” or “smoking gun”, putting them both together leads to an incompatibility.
Does the Bullet Cluster matter at all?
So far we have shown that the Bullet Cluster can not be understood as proof for the Dark Matter Hypothesis. We took the argument seriously. But in doing so, we have repeated the same mistake as many people who bring up the BC when they try to dismiss our work on testing the Dark Matter Hypothesis. Why that? Well, usually the discussion follows these lines:
- “Testing the predictions of the Cold Dark Matter Hypothesis on galaxy scales, we have found several serious problems.”
- “But don't you know about the Bullet Cluster? It is the proof that there is Dark Matter!”
Put that way, it is easy to spot the mistake: Even if the Bullet Cluster could only be explained with Dark Matter, the problems on small scales persist. The BC does not tell us anything about the Local Group of galaxies, the two arguments are completely independent. There is a serious problem with the DM Hypothesis and even a thousand Bullet Clusters would not make it go away. In fact this is similar to the hypothetical scenario that someone, for example at the LHC, would find “The Dark Matter Particle”. Even in that case, the current problems of the model would not go away. Rather, this would point at a much more serious issue with our understanding of physics.
In trying to understand the universe, we as good scientists should thus look for alternative explanations that account for all independent observations and discuss these without ideological pre-conceptions.
Solutions to the Bullet cluster in modified Newtonian dynamics (MOND) have indeed been shown to exist (Angus, Famaey & Zhao 2006). The authors conclude
In multicentred models, the convergence map does not always reflect the projected matter in the lens plane in MOND. This cautions simple interpretations of the analysis of weak lensing in the bullet cluster 1E 0657−56 (Clowe et al. 2004; see fig. 7).
Similarly for Modified Gravity (MOG): Brownstein and Moffat (2007) write
The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +/- 3.9keV, in good agreement with the experimental value T = 14.8+2.0-1.7keV. Excellent fits to the 2D convergence κ-map data are obtained without non-baryonic dark matter..." and they uncover a significant disagremenet with the dark-matter based analysis (the baryon fraction is to high in a dark-matter model).
Stay tuned, there is more to be said about galaxy cluster and the Bullet Cluster in modified gravity theories in our next post.
by Anton Ippendorf, Pavel Kroupa and Marcel Pawlowski (30.07.2010): "But the Bullet Cluster ... - Proof of Cold or Warm Dark Matter in galaxy clusters is but a myth" in "The Dark Matter Crisis - the rise and fall of a cosmological hypothesis" on SciLogs. See the overview of topics in The Dark Matter Crisis.