Dark Matter: A debate – afterwards while on safari

Fleeing  the  European continent to go back to Australia on safari for some satellite-galaxy hunting in Canberra with my friend Dr. Helmut Jerjen, I had a little time on my Quantas  flight and in Singapore and Perth to reflect upon the debate, and I note the following:

Simon White gave an excellent presentation of the impressive agreement of standard cosmology, i.e. the LCDM model, showing some of the available data on large scales and the cosmic background radiation map (his slides are available on his website).  Somebody in the audience during or after the debate was over, mentioned an interesting observation  (unfortunately I do not recall who this was):

In order to get Einstein’s theory of general relativity to fit the data one needs to postulate unknown physics, namely inflation, dark  matter, dark energy.

(This I had indeed stressed in my presentation, therewith putting the LCDM success story implied by Simon on a different if not dis-satisfying footing.)

But, the unknown person continued:

Can one then, after introducing these unknowns to make the theory fit, argue that the LCDM model is correct because it fits the data? Is this not a circular argument?

Perhaps the LCDM model finds support in that various different lines of argument lead to similar values for the numbers which define the precise model. But,  Mr. Unknown has raised a point central to how science advances:

Rather than demonstrating how excellently LCDM does on large scales, a cleaner argument that the LCDM model describes physical reality is as follows:

Accept that the LCDM model is adjusted to fit the data on large scales. Once it is fixed, it can be used to make predictions in a different regime. This different regime is on scales smaller than 8 Mpc, where the model makes very precise predictions how the cold dark matter must be distributed for it to be a valid description of nature and where we have truly exquisite observational data.  This distribution is seen in the form of galaxies and how they cluster. And this is where the observational data, unfortunately, are in highly significant conflict with the model such that they exclude the LCDM model. This holds true despite the often invoked uncertainties and complexities in dealing with the physics of normal matter (for example, observations clearly tell us that galaxies are simple objects obeying simple scaling laws such that true physics describing their structure must be simple as stressed by Disney et la., 2008, Nature).

For example, in the debate after the two presentations, Simon White attempted to address the small scale problem by showing an excellent fit of the LCDM model to one of our satellite galaxies, namely Fornax. Fornax is far away, at 140 kpc, so far in fact, that it must be in dynamical equilibrium.  Dynamical equilibrium means that the stars are orbiting within the galaxy such that the whole galaxy is not changing its appearance. Thus, when a star moves to the right, another one moves to the left such that they compensate each other statistically.  So getting a good description of it with the LCDM model would indeed, as Simon White stresses, be a great success.

However, this is wrong, and it was surprising that Simon White did not note this. Indeed it is also surprising that I did not jump at this logical inconsistency, perhaps because it was such a self-evident failure that I dared not point this out in fear of causing an unpleasant situation. The LCDM fit is unphysical because Fornax has a complex inner structure: As is evident on slide 49 of my presentation, Fornax has a twisted and dislocated inner structure, such that it simply cannot be in dynamical equilibrium. Dynamical equilibrium is, however, one of the fundamentally important assumptions that go into modeling the data via the LCDM model. In the LCDM model, any complex structure would disappear on a short time-scale of about a hundred million years, as I indeed had put much emphasis on during my presentation. That is, the appearance of the little galaxy would be changing significantly on this astronomically short time-scale. Simon White must have missed this point, or simply ignored it.

Unfortunately, Simon did not address all the other failures I had put up, nor did I return to them during the debate – well, they had been stated in my presentation already. But, given the astrophysical literature, it is evident that there are no remedies to save the LCDM model, given its current ingredients.

Now, one attempt to advance from here is to add an additional  Dark Unknown, a Dark Force which acts only between cold dark matter particles, as discussed by Peebles & Nusser (2010, Nature), or an additional Dark Force which acts only between dark matter and normal matter, as discussed by  Kroupa et al. (2010, Astron. & Astrophys). These speculative forces, about which we know absolutely nothing, are none of the other three already known forces (electromagnetic, weak and strong) nor Einsteinian gravity.

Therefore, the failure of LCDM on scales smaller than 8 Mpc is due to it being wrong. Note here that it is wrong even if one adds the above dark forces, since with these forces the LCDM model becomes a different one with different properties on large scales. This is where MOND or another alternative (e.g. MOG) comes in. MOND is not a dark force, but merely a simple modification of either gravity or inertial mass, depending on its interpretation.

It is remarkable how brilliantly MOND has been performing since its conception in 1983 by Mordehai Milgrom. In fact, in his most recent paper, Milgrom (“MD or DM? Modified dynamics at low accelerations vs dark matter”, 2010, Proceedings of Science) writes in his abstract:

Some of the complaints leveled at MOND are: (i) “MOND was designed to fit rotation curves; so no wonder it is so successful in predicting them”. This is both incorrect and quibbling: The first ever MOND rotation curve analysis was undertaken more then four years after the advent of MOND. And, even if MOND, epitomized by a very simple formula, could have been designed to predict hundreds of rotation curves, it would still be a great achievement. (ii) “MOND outperforms CDM only on small, galactic scales, where formation physics is anyhow very messy, but falls behind in accounting for `simpler’, large-scale phenomena”. Quite contrarily, all the salient MOND predictions on galactic scales follow as unavoidable, simple, and immediate corollaries of the theory – independent of any messy formation scenario – just as Kepler’s laws, obeyed by all planetary systems, follow from an underlying theory, not from complex formation scenarios. To think, as dark-matter advocates say they do, that the universal MOND regularities exhibited by galaxies will one day be shown to somehow follow from complex formation processes, is, to my mind, a delusion. What is left for MOND to explain on large scales is a little in comparison, and has to await a full fledged relativistic MOND theory. (iii) “The `bullet cluster‘ shows that MOND still requires some matter that is dark”. Yes, it has long been known that MOND does not fully remove the mass discrepancy in the cores of galaxy clusters. Some additional still-dark matter is needed. But this need not be THE “dark matter”; a small amount of the still-missing baryons, in some dark form (dead stars? cold gas clouds?), or perhaps (sterile?) neutrinos, could fit the bill.

Finally it serves to be useful to note the following statement from the paper by Peebles & Nusser (“Nearby galaxies as pointers to a better theory of cosmic evolution“, 2010, Nature, p.568):

The variety of problems we have considered in the interpretation of the present baseline motivates serious consideration of adjustments of the fundamental theory.

Prof. Jim Peebles at Princeton University is one of the leading cosmologists who had actively worked in developing the LCDM model.


The 18th November was very special. Not only because we had such a debate in Germany, which is otherwise overall well on-track with the LCDM model with most major professorships having been filled with its adherents. More importanly, the generally well educated public in Germany is interested, and for me having so many cameras around was a new experience which clearly had an effect on how the scientific debate proceeded. I would like to sincerely thank Prof. Gerhardt Hensler from Vienna, Prof. Robert Sanders from Groningen and Prof. Tom Shanks from Durham for following my call to join-in with the debate. Prof. Hensler is an expert on star-formation and gas-dynamical processes in galaxies. Prof. Sanders is an expert on gravitational dynamics and the astrophyics of galaxies. Prof. Shanks is an expert on observtional cosmology andextragalactic astronomy. The presence at the debate of my long-term supporting colleague Prof. Klaas S. de Boer from Bonn was also central. Their expertise was essential during the debate, and their active participation also demonstrates that here are a substanial number of  scientists who see major problems with the LCDM model, such that, with adequate funding support, significant progress in cosmology can be hoped for.

by Pavel Kroupa and Marcel Pawlowski (27.11.2010): “Dark Matter: a debate – afterwards  while on safari” in “The Dark Matter Crisis – the rise and fall of a cosmological hypothesis” on SciLogs. Written in Perth. See the overview of topics in  The Dark Matter Crisis.

Reactions to the Dark Matter Debate and Another One in German TV

While the official video podcast of the Bethe Colloquium “Dark Matter, a Debate” is not available yet, there nevertheless have already been some reactions send via email or posted on astronomy blogs. Some are based on the live blog of the debate. In addition to that, there will be a discussion about Dark Matter in German television this thursday.

Reactions to the Dark Matter Debate

Before the debate, a number of people labeled it as a “MOND vs. Dark Matter” debate, which is simply a wrong statement. This is not the question we as scientists have to ask today. The real issue at hand is the question whether we understand LCDM as being falsified or not. This is independent of the possible existence of an alternative and its successes or failures. Unfortunately, in his report of the debate, Daniel Fisher, who was there himself, also presents it as one between a Dark Matter and a MOND advocate. He shares his impressions (in German) on his Blog Skyweek Zwei Punkt Null.

In contrast to that, Nando Patat, astronomer at ESO, who posts his thoughts on the matter in a post titled “We do not understand nature, we measure it“, puts more emphasis on the sociological points Pavel Kroupa raised. He even backs up the statement that it is difficult to go against the mainstrem by refering to one of his papers about very old stars, which was “brutally but superficially rejected”. This is in-line with reports by other scientists who for example had observing proposals rejected because they suggested observing disk galaxies at a high redshift. Because the  time-allocation commission “knew” that these cannot have formed in a LCDM universe, the observing time was not granted.

On her Blog “One Small Step”, Sarah Kendrew, a Post-Doc in Heidelberg, posts a good introduction to the background of the debate. She also mentiones that Pavel Kroupas main point was not to fighting for a particular alternative theory. She writes: “I get the impression that a large part of Kroupa’s argument is actually sociological: he’s calling for research into paradigm-challenging cosmologies like MOND to be given more attention (without smirking) and funding, rather than creating ever more patches to cover the holes in concordance cosmology.” Concerning Simon Whites position, she makes an interesting statement: “White acknowledges the problems yet doesn’t think that a radical new line of thought is needed either […]. Given his views on the future of astronomy […] I would have thought he’d be all in favour of setting bright students’ creative minds loose on a problem like this.”

As a last point, there was an email by a colleague. He shares some of his thoughts about the debate, which he attended himself. However, he asked us not to mention his name because he does not have a permanent position yet. He is afraid that articulating his point of view publicly will reduce his chances to ever get one. Of course we follow his request by citing him anonymously and would like to thank him for his trust in us which he showed by sending this email.

Simon White’s statement that Pavel Kroupa is arguing with a “proof through assertion” is an insolence, as it supposes that the arguments lack any evidence. Consistently, in the following Simon White did not really adress the problems of Dark Matter. Instead, he showed where the standard model works. Ignoring criticism is bad scientific practice. In addition to this, Simon White tried to reduce Pavel Kroupa’s arguments to a pro-MOND position, while the failures of the concordance cosmology have nothing to do with the question whether MOND is valid or not. While Simon White initially said that alternatives have to be investigated, he later argued that he gets a lot of letters each day proposing alternative cosmologies. This made Professor Klaas de Boer protest, as it puts theories like MOND on the same level as, for example, crackpot-ideas of non-academics claiming to have disprooven Einstein.

But there was also criticism concerning Pavel Kroupas presentation during the debate. His statement that he does not see any possibility to verify LCDM was perceived as a too fixed position. He might also have insisted more on discussing the small-scale problems of LCDM and that they can not simply be dismissed by stating that the physics at these scales is too complicated. This might have made the debate more ground-breaking, as a lot of people were hoping to see if there are proper counter-arguments to the findings presented in the recent paper. Furthermore, he could have reacted to Simon White’s claim that Fornax fits excellently in LCDM, using the isophote-plots of the satellite galaxy he showed in his talk: If the galaxy were embedded in a dark matter halo, it could not be too asymmetric but would mostly be spherically and non-structured, which is not the case. During the debate, Pavel Kroupa did not raise this contradiction between Fornax and the LCDM model.

Another Debate (on TV)

This Thursday (November 25th 2010), at 9pm, there will be another discussion about dark matter in the TV show “scobel”. It will be broadcasted on 3sat, a TV station in German providing a common program for Germany, Austria and Swizerland. The title is: “Dark Matter – New Studies Question its Existence“. Guests in the show will be Arnold Benz, Gerhard Hensler and Simon White. A TV team of 3sat was filming at the Dark Matter Debate between Simon White and Pavel Kroupa last week, so this will probably be the first chance to see some footage of the event. Furthermore, Pavel Kroupa and Robert Sanders were interviewed for the show. Unfortunately, it is in German. It will be available online in the 3sat mediathek afterwards.

by Pavel Kroupa and Marcel Pawlowski (24.11.2010): “Reactions to the Dark Matter Debate and Another One in German TVin “The Dark Matter Crisis – the rise and fall of a cosmological hypothesis” on SciLogs. See the overview of topics in  The Dark Matter Crisis.

Slides of the Dark Matter Debate and of Robert Sanders' MOND Talk

On November 18th 2010, Pavel Kroupa and Simon White met in Bonn for a special Bethe Colloquium: “Dark Matter, a Debate”. While the video podcast is not available yet, there is a replay of the live blog. Furthermore, Pavel Kroupa’s presentation slides can be downloaded as a pdf file.

UPDATE Dec 8th: Simon White’s slides are now available at his website, too.



In addition to the debate on Thursday, Robert H. Sanders gave a talk about Modified Newtonian Dynamics (MOND) the following Friday. His abstract reads:
Here I review the empirical basis of MOND.  I emphasize the success of the algorithm in predicting galaxy-scale phenomenology, not only the form of rotation curves but also galaxy scaling relationships and general trends. This success constitutes a fundamental challenge for dark matter that clusters on the scale of galaxies. I discuss the problems for MOND on larger scale and suggest possible resolutions of these issues.
We are happy that he agreed to make his slides public as well, for which we would like to thank him a lot. You can download his pdf file here. If you are looking for more information on the dark matter hypothesis, you might want to know that Robert Sanders wrote the book “The Dark Matter Problem: A Historical Perspective”, which was published by Cambridge University Press this year.
by Pavel Kroupa and Marcel Pawlowski (22.11.2010): “Slides of the Dark Matter Debate and of Robert Sanders’ MOND Talk” 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.

Live Blog of the Dark Matter Debate in Bonn

Starting at 3pm local time (Germany), Andreas (AHW) and Marcel (8minutesold) will try to live blog from the debate about Dark Matter between Simon White and Pavel Kroupa in Bonn. As there is no live video stream available, this seems to be the only live coverage of the event. You can follow the report in the field below and even send in a comment or two.


<a href="http://www.coveritlive.com/mobile.php/option=com_mobile/task=viewaltcast/altcast_code=6dba8e29be" mce_href="http://www.coveritlive.com/mobile.php/option=com_mobile/task=viewaltcast/altcast_code=6dba8e29be" >Dark Matter, a Debate</a>

Dark Matter: A debate at the Bethe Center for Theoretical Physics at Bonn University

On July 15th, 2010, Pavel Kroupa held an invited colloquium on “Local Group galaxies as critical tests of the contemporary cosmological model and its failure” at the Helmholtz-Institut fuer Strahlen und Kernphysik of the University of Bonn. The venue had to be shifted to a larger lecture theatre.
The Bethe Center for Theoretical Physics invited Prof. Dr. Simon White (Max Planck Institue for Astrophysics, Garching) and on July 16th Pavel Kroupa (AIfA, Bonn) to hold the Bethe Colloquium on “Dark Matter: a debate“.
It will take place on November 18th, 2010, at 3:15 pm, in Lecture Hall 1 of the Physikalisches Institut, Nussallee 12, 53115 Bonn. It is open for all interested. However, it is not likely to be easily accessible for the lay person. The abstract of the Nov.18th Bethe Colloquium reads:
The subject of this months Bethe Colloquium concerns a question at the interface of cosmology, astrophysics and elementary particle physics: the possible existence of Dark Matter. The existence of Dark Matter is the most prominent proposal to account for the discrepancy between measurements of the mass of galaxies, clusters of galaxies and the entire universe, and measurements based on the mass of the visible matter. So far the existence of Dark Matter is inferred from gravitational effects on visible matter and background radiation and not through direct detection. In his talk Professor S. White introduces the dark matter paradigm and explains its virtues. An alternative proposal to explain the observed discrepancies is introduced by Professor P. Kroupa. He argues that these effects could be due to a modification of the laws of gravity without the need of Dark Matter. The talks are followed by a discussion.”
The web-site of the Bethe Colloquia can be found here, and the poster advertising the debate is provided below. A higher resolution pdf version of the poster can be downloaded here. Uni-Bonn-TV will prepare a videopodcast of the event.
 See the overview of topics in  The Dark Matter Crisis.