[00:00] Physicists have been arguing about dark matter for&nbsp; almost a century. And now, after all this time,&nbsp;&nbsp; their most favorite evidence for dark matter&nbsp; disappeared. This isn't just an interesting&nbsp;&nbsp;
[00:12] development from a scientific perspective. It's&nbsp; also a good illustration for what's going wrong&nbsp;&nbsp; in this research area. I have a brief summary.&nbsp; Dark matter is supposedly invisible stuff that&nbsp;&nbsp;
[00:24] makes up 80% of the matter in the universe and&nbsp; that makes itself noticeable only through its&nbsp;&nbsp; gravitational pull. But decades of attempts to&nbsp; measure even a single one of the dark matter&nbsp;&nbsp; particles failed. The alternative explanation&nbsp; for the observations is that Einstein's theory&nbsp;&nbsp;
[00:41] of gravity is wrong and must be replaced by a&nbsp; different theory which is called modified gravity.&nbsp;&nbsp; Astrophysicists have invented dark matter to&nbsp; explain multiple observations that just don't&nbsp;&nbsp;
[00:54] fit to Einstein's theory. Galaxies rotate&nbsp; too fast. Galaxy clusters move too fast.&nbsp;&nbsp; Gravitational lenses are too strong. Structures&nbsp; form too quickly and so on. But their proudest&nbsp;&nbsp; piece of evidence has been the Bullet Cluster.&nbsp; The Bullet Cluster is a collision of two big&nbsp;&nbsp;
[01:12] galaxy clusters about 3.7 billion lightyears&nbsp; away from us in the constellation Carina. The&nbsp;&nbsp; reason the bullet cluster is so interesting&nbsp; is that we have two different ways to measure&nbsp;it. The one is a measurement of electromagnetic&nbsp; radiation in the X-ray range. From this one can&nbsp;&nbsp;
[01:29] infer the distribution of hot gas in the clusters.&nbsp; This is shown in red in this image overlaid over&nbsp;&nbsp; the visible light from the galaxies. The other&nbsp; observation is the distribution of all gravitating&nbsp;&nbsp;
[01:41] mass inferred from gravitational lensing. This&nbsp; is shown in blue in this image. You can clearly&nbsp;&nbsp; see that the total gravitating mass seems to&nbsp; be elsewhere than the hot gas of the cluster.&nbsp;&nbsp; Astrophysicists interpret this to say that these&nbsp; two galaxy clusters passed through each other like&nbsp;&nbsp;
[01:59] this. And there is a component of each cluster's&nbsp; mass that just passed through the other cluster.&nbsp;&nbsp; It's dark and basically collisionless. That's the&nbsp; Dark Matter. The normal matter, on the other hand,&nbsp;&nbsp; did collide and was slowed down in this process.&nbsp; The bullet cluster is relevant because not only&nbsp;&nbsp;
[02:16] do astrophysicists claim that it's evidence for&nbsp; dark matter, they also claim that it's evidenced&nbsp;&nbsp; against the alternative: Modified Newtonian&nbsp; Dynamics, MOND for short. This is because in&nbsp;MOND
[02:28] one changes the way that matter creates&nbsp; a gravitational pull. At low accelerations,&nbsp;&nbsp; gravity is then stronger than Einstein said. But&nbsp; in MOND gravity still pulls in the same direction,&nbsp;&nbsp;
[02:40] namely that created by the normal matter. The&nbsp; Bullet Cluster seems to say no that doesn't work.&nbsp;&nbsp; The news is now that a group of researchers from&nbsp; the University of Bonn say that the observations of&nbsp;&nbsp;
[02:53] the bullet cluster are fully compatible with&nbsp; MOND after all. They claim that the standard&nbsp;&nbsp; explanation of the bullet cluster observations&nbsp; undercounted ordinary but "dark" matter such as&nbsp;&nbsp;
[03:05] white dwarfs, neutron stars, and black holes left&nbsp; behind by an early generation of massive stars.&nbsp;&nbsp; They used the most recent data from the James Webb&nbsp; Space Telescope and compared it with a model in&nbsp;&nbsp;
[03:18] which the galaxies contain more massive stars than&nbsp; previously assumed. They find that with the new&nbsp;&nbsp; data, realistic amounts of ordinary matter suffice&nbsp; to make the observations compatible with MOND.&nbsp;&nbsp;
[03:30] This doesn't rule out dark matter, but it means&nbsp; that the Bullet Cluster no longer rules out MOND.&nbsp;&nbsp; Or does it? Well, that's where the problem start.&nbsp; The Bullet Cluster has never been good evidence&nbsp;&nbsp;
[03:42] either for dark matter or against MOND. This is&nbsp; because to fit the observations, the two clusters&nbsp;&nbsp; must have collided at enormously high velocity.&nbsp; Where the clusters would get such high velocity&nbsp;&nbsp;
[03:54] is difficult to explain in a universe with dark&nbsp; matter. It's actually easier to explain with MOND.&nbsp;&nbsp; The other issue is that no one in their right mind&nbsp; expects MOND a phenomenological model developed to&nbsp;&nbsp;
[04:07] describe time independent situations to correctly&nbsp; describe a strongly dynamical situation like this.&nbsp;&nbsp; For this you need a more complicated modification&nbsp; of gravity like for example John Moffett's TeVeS&nbsp;&nbsp;
[04:20] and he showed ages ago that this does indeed&nbsp; fit with bullet cluster observations. That said,&nbsp;&nbsp; either way you look at it, the bullet cluster&nbsp; is just one specific instance and a statistical&nbsp;&nbsp;
[04:32] outlier at this, which is not good evidence one&nbsp; way or another. And this brings me to today's&nbsp;&nbsp; main point, namely the question of why so many&nbsp; astrophysicists nevertheless constantly refer&nbsp;&nbsp;
[04:44] to it as if it tells us anything. The reason is&nbsp; almost certainly that it's a deceptively simple&nbsp;&nbsp; shut up argument. They began using this in 2008&nbsp; with the first observations and initially I saw&nbsp;&nbsp;
[04:56] this mostly for science communication purposes. At&nbsp; this point I think astrophysicists still realized&nbsp;&nbsp; it's a nice argument but scientifically not a good&nbsp; one. But then over the years they began to believe&nbsp;&nbsp; it themselves. Especially younger people no longer&nbsp; question it or even look at the evidence because&nbsp;&nbsp;
[05:13] anyone who questions the evidence for dark matter&nbsp; is a crank anyway. You see the problem? Now look,&nbsp;&nbsp; you'll know that I'm neither a fan of dark&nbsp; matter nor of MOND. These ideas both have too&nbsp;&nbsp;
[05:26] many shortcomings. But the problem is that even&nbsp; after all these years, these two camps are still&nbsp;&nbsp; basically not talking to each other. Each side&nbsp; has dug themselves into a hole where they insist&nbsp;&nbsp;
[05:40] they are right and the other side is wrong.&nbsp; And that's why we're stuck. And that's why I&nbsp;&nbsp; give this paper an 8 out of 10 on the [&nbsp;__&nbsp;]&nbsp; meter because I think pretty much all papers that&nbsp;&nbsp; have been written about the Bullet Cluster are&nbsp; a waste of time. Subscribe for more grumpy takes&nbsp;&nbsp;
[05:56] on the miserable status of physics. A few years&nbsp; ago, I received some ugly letters from a debt&nbsp;&nbsp; collection company. They threatened to sue me for&nbsp; ordering something online and then not paying it.&nbsp;&nbsp;
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