55. “A Philosophical Approach to MOND” wins prestigious award

It is with delight we learned today that David Merritt’s book on “A Philosophical Approach to MOND” published by Cambridge University Press won the Prose Award for Excellence in Physical Sciences and Mathematics. Other authors also competing for this price were Peebles and Weinberg. 

I had written a review of this book which can be read here.

Note also that in 2013 David published a noteworthy text book on “Dynamics and Evolution of Galactic Nuclei” (with Princeton Series in Astrophysics).


This is an opportunity to recall how I personally stumbled into this whole problem concerning dark matter (see also this article on Aeon): My research up until the mid1990s was based on stellar populations, although in Heidelberg we had also measured, for the first time, the actual space velocity of the Magellanic Clouds (in 1994 and 1997). These were my first endeavours into the extragalactic arena. I had heard a fabulous lecture by Simon White who was visiting Heidelberg, showing movies of structure formation in the LCDM model they had just computed in Garching. I personally congratulated Simon for this most impressive achievement.  One could see how major galaxies were orbited by many dwarf satellite galaxies and how all of that formed as the Universe evolved. I had also noted from photographs that when two gas-rich galaxies interact, they expel tidal arms in which new dwarf galaxies form. These new dwarf galaxies are referred to as tidal dwarf galaxies.

The Tadpole Galaxy recorded with the Hubble Space Telescope’s Advanced Camera for Surveys. Evident are the new dwarf galaxies in the 100 kpc long tidal tail.

In the 1990’s the community had largely discarded satellite dwarf galaxies being tidal dwarfs because it was known that they cannot have dark matter (this goes back to Barnes & Hernquist,1992,  later confirmed by Wetzstein, Naab & Burkert 2007).  So it was thought that tidal dwarfs just dissolve and play no important role.  The observed satellite galaxies of the Milky Way have large dynamical M/L ratios, going up to 1000 or more. This proved they can contain a 1000 times more mass in dark matter than in stars and gas. So obviously they cannot be tidal dwarfs. I very clearly remember Donald Lynden-Bell exclaiming in Cambridge, when I was still visiting regularly, that his suggestion that the satellites came from a broken-up galaxy cannot thus be correct, since they contain dark matter. Then I made my discovery (truly by pure chance) published in Kroupa (1997), which made me think that what the celebrated experts are telling me seemed not to be quite right. After this publication I was told more than once this work made me un-hireable.
 
I had then noted (Kroupa et al. 2005), that the disk of satellites (DoS, including the newer once which Donald had not known) is in conflict with them being dark-matter substructures, as these ought to be spheroidally distributed around the Milky Way galaxy. 
 
We  argued (to my knowledge for the first time in print, in Kroupa et al. 2010 and in Kroupa 2012 ) that the disk of satellites can only be understood if they are tidal dwarfs. I had also come to the conclusion that my chance discovery above is unlikely to be able to explain the high M/L values of all the satellite galaxies as they would all need to be quite strongly affected by tidal forces which poses a problem for those further than 100 kpc from the Milky Way because their orbital periods begin to approach a Hubble time. And if they are tidal dwarfs (which they must be given they make a disk of satellites),  then this implies we need non-dark-matter models, i.e. , we need to change the law of gravitation to account for the high M/L values these little galaxies display.  Subsequently I was quite fevering (with PhD student Manuel Metz and later Marcel Pawlowski) each time a new satellite was discovered to see where it lay (I used to run to their offices whenever some survey reported a new satellite), and ultimately what the proper motions are doing: if the satellite galaxies form a pronounced disk of satellites then they must be orbiting only within this disk (Pawlowski & Kroupa 2013). I was (this was already in the 2000s) also interested if  John Moffat’s “modified gravity” (MOG) might explain the large M/L ratios, and John Moffat visited me in Bonn. But it turns out that MOG is falsified while Milgromian gravitation (MOND) is, as far as one can tell, the at the moment only possible gravitational theory we can use which accounts for all data and tests so far performed.  Oliver Mueller, Marcel Pawlowski  et al. (2021) affirm that the Milky Way is not unique in having a disk of satellites system. Observing disks of satellites around larger galaxies is not a “look elsewhere effect” since the very-nearest large galaxies are looked at, rather than finding such DoSs around some host galaxy in a very large ensemble of observed galaxies. I think the disk-of-satellites or satellite-plane problem is the clearest-cut evidence why we do not have dark matter. 
 
The (negative) test for the existence of dark matter particles (warm, cold, fuzzy) via Chandrasekhar dynamical friction is the other (Kroupa 2015).
 
Plus, with all the other tests performed in strong collaboration with Indranil Banik (notably Haslbauer et al.  2019a, Haslbauer et al. 2019b,  Haslbauer et al. 2020 and Asencio et al. 2021) it materialises that the tests all lead to mutually highly consistent results – we do not have the situation that one test is positive (for dark matter), the other not. They all turn out to be consistently negative. Indranil Banik concludes correctly (Feb.5th, 2021): “There are so many lines of evidence that no single one is critical any more.”
 
I am personally deeply impressed how everything seems to fall into place (quite nearly everything) once one uses MOND (which is based on a Lagrangian etc.).  Apart from completely naturally resolving the Hubble Tension and easily accounting for massive high-redshift galaxy clusters like El Gordo (see also this account on Triton Station), the DoSs or satellite planes form naturally (as shown independently by Banik et al. 2018 and by Bilek et al. 2018) and these tidal tail dwarf galaxies have large M/L values due to the correct law of gravitation (e.g. this amazing prediction by McGaugh 2016 of the velocities of stars in one of the satellite galaxies and verification thereof by Caldwell et al. 2017).
 
But, just like with the standard model of particle physics, there definitely is a deeper layer to MOND which we have not yet discovered; a more fundamental theory, which may well be the quantum vacuum which also explains particle masses. Milgrom had already published seminally on this issue.
 
The huge success of MOND comes not only in it naturally account for the data on scales of a few 100 pc to a Gpc, but also that it is a “progressive research programme“, with the standard dark-matter based models being “degenerative“.  For details, see David Merritt’s book above. 
 

In The Dark Matter Crisis by Pavel Kroupa. A listing of contents of all contributions is available here.