The Biggest Argument for Dark Matter Just Died

[00:00] Physicists have been arguing about dark matter for  almost a century. And now, after all this time,   their most favorite evidence for dark matter  disappeared. This isn't just an interesting  

[00:12] development from a scientific perspective. It's  also a good illustration for what's going wrong   in this research area. I have a brief summary.  Dark matter is supposedly invisible stuff that  

[00:24] makes up 80% of the matter in the universe and  that makes itself noticeable only through its   gravitational pull. But decades of attempts to  measure even a single one of the dark matter   particles failed. The alternative explanation  for the observations is that Einstein's theory  

[00:41] of gravity is wrong and must be replaced by a  different theory which is called modified gravity.   Astrophysicists have invented dark matter to  explain multiple observations that just don't  

[00:54] fit to Einstein's theory. Galaxies rotate  too fast. Galaxy clusters move too fast.   Gravitational lenses are too strong. Structures  form too quickly and so on. But their proudest   piece of evidence has been the Bullet Cluster.  The Bullet Cluster is a collision of two big  

[01:12] galaxy clusters about 3.7 billion lightyears  away from us in the constellation Carina. The   reason the bullet cluster is so interesting  is that we have two different ways to measure it. The one is a measurement of electromagnetic  radiation in the X-ray range. From this one can  

[01:29] infer the distribution of hot gas in the clusters.  This is shown in red in this image overlaid over   the visible light from the galaxies. The other  observation is the distribution of all gravitating  

[01:41] mass inferred from gravitational lensing. This  is shown in blue in this image. You can clearly   see that the total gravitating mass seems to  be elsewhere than the hot gas of the cluster.   Astrophysicists interpret this to say that these  two galaxy clusters passed through each other like  

[01:59] this. And there is a component of each cluster's  mass that just passed through the other cluster.   It's dark and basically collisionless. That's the  Dark Matter. The normal matter, on the other hand,   did collide and was slowed down in this process.  The bullet cluster is relevant because not only  

[02:16] do astrophysicists claim that it's evidence for  dark matter, they also claim that it's evidenced   against the alternative: Modified Newtonian  Dynamics, MOND for short. This is because in MOND

[02:28] one changes the way that matter creates  a gravitational pull. At low accelerations,   gravity is then stronger than Einstein said. But  in MOND gravity still pulls in the same direction,  

[02:40] namely that created by the normal matter. The  Bullet Cluster seems to say no that doesn't work.   The news is now that a group of researchers from  the University of Bonn say that the observations of  

[02:53] the bullet cluster are fully compatible with  MOND after all. They claim that the standard   explanation of the bullet cluster observations  undercounted ordinary but "dark" matter such as  

[03:05] white dwarfs, neutron stars, and black holes left  behind by an early generation of massive stars.   They used the most recent data from the James Webb  Space Telescope and compared it with a model in  

[03:18] which the galaxies contain more massive stars than  previously assumed. They find that with the new   data, realistic amounts of ordinary matter suffice  to make the observations compatible with MOND.  

[03:30] This doesn't rule out dark matter, but it means  that the Bullet Cluster no longer rules out MOND.   Or does it? Well, that's where the problem start.  The Bullet Cluster has never been good evidence  

[03:42] either for dark matter or against MOND. This is  because to fit the observations, the two clusters   must have collided at enormously high velocity.  Where the clusters would get such high velocity  

[03:54] is difficult to explain in a universe with dark  matter. It's actually easier to explain with MOND.   The other issue is that no one in their right mind  expects MOND a phenomenological model developed to  

[04:07] describe time independent situations to correctly  describe a strongly dynamical situation like this.   For this you need a more complicated modification  of gravity like for example John Moffett's TeVeS  

[04:20] and he showed ages ago that this does indeed  fit with bullet cluster observations. That said,   either way you look at it, the bullet cluster  is just one specific instance and a statistical  

[04:32] outlier at this, which is not good evidence one  way or another. And this brings me to today's   main point, namely the question of why so many  astrophysicists nevertheless constantly refer  

[04:44] to it as if it tells us anything. The reason is  almost certainly that it's a deceptively simple   shut up argument. They began using this in 2008  with the first observations and initially I saw  

[04:56] this mostly for science communication purposes. At  this point I think astrophysicists still realized   it's a nice argument but scientifically not a good  one. But then over the years they began to believe   it themselves. Especially younger people no longer  question it or even look at the evidence because  

[05:13] anyone who questions the evidence for dark matter  is a crank anyway. You see the problem? Now look,   you'll know that I'm neither a fan of dark  matter nor of MOND. These ideas both have too  

[05:26] many shortcomings. But the problem is that even  after all these years, these two camps are still   basically not talking to each other. Each side  has dug themselves into a hole where they insist  

[05:40] they are right and the other side is wrong.  And that's why we're stuck. And that's why I   give this paper an 8 out of 10 on the [ __ ]  meter because I think pretty much all papers that   have been written about the Bullet Cluster are  a waste of time. Subscribe for more grumpy takes  

[05:56] on the miserable status of physics. A few years  ago, I received some ugly letters from a debt   collection company. They threatened to sue me for  ordering something online and then not paying it.  

[06:11] Turned out that someone had stolen my private  data. I managed to get out of this. But since   then, I've become very protective of my  personal information, which is why I've   signed up to Incogni who've been sponsoring  this video. Each time you open a website,  

[06:28] it'll try to collect data about who you are  and where you are and what other websites   you've visited. Many firms then make money by  selling your private information to data brokers.  

[06:40] Most countries have laws against that and you  can ask for your data to be removed. But doing   this takes up a lot of time. Incogni automates the  process of getting you out of those databases.  

[06:52] You sign up and they'll contact the big sinners,  request that your personal details be removed.   They'll keep on doing that and if you want send  you updates about the progress they're making.  

[07:04] They now also have a custom removal feature  where you can submit specific websites. I'll   be using this a lot. Incogni has really made my  life better. If that sounds like something you  

[07:18] could need to use my link, Sabine or the  custom link in the info because the first   100 people to use it will get 60% off. Stay  safe. Thanks for watching. See you tomorrow.