61. The crisis in the dark matter problem becomes a historically unparalleled failure in the scientific method

This year, Pavel Kroupa was asked to hold a Golden Webinar in Astrophysics on the dark matter problem. This contribution provides the link to the recording of this presentation which has now become available on YouTube. In this presentation, Pavel Kroupa argues that the dark matter problem has developed to become the greatest crisis in the history of science, ever. This contribution also provides links to recordings available on YouTube of previous related talks by the same speaker from 2010 (the Dark Matter Debate with Simon White in Bonn) and 2013 (in Heidelberg). This might allow some insight into how the debate and the research field have developed over the past dozen or more years.

1) Golden Webinar: “From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?”

On April 9th, 2021, Prof. Pavel Kroupa presented a talk in the Golden Webinars in Astrophysics series on “From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?”. The talk is now available on Youtube:

The slides to the talk without the fictitious story can be downloaded here:

If you are interested in other talks presented during The Golden Webinars in Astrophysics series, you can find the record of those already presented on their Youtube Channel, and here is a list of upcoming talks. The Golden Webinars are provided as a free public service and have no registration fees.

2) The vast polar structures around the Milky Way and Andromeda

In November 2013, Prof. Pavel Kroupa presented “The vast polar structures around the Milky Way and Andromeda” in the Heidelberg Joint Astronomical Colloquium. In the talk he discussed the failures of the Standard model of cosmology and the implications for fundamental physics.

A blog entry from 2012 on the vast polar structure (VPOS) of satellite objects around the Milky Way can be found here.

3) Bethe-Kolloquium “Dark Matter: A Debate”

In November 2010, Prof. Simon White (Max Planck Institute of Astrophysics, Garching) and Prof. Pavel Kroupa (University of Bonn) debated on the concept and existence of dark matter during the Bethe Colloquium in Bonn. Their presentations and the subsequent debate are available here:

a) Presentations by Prof. White and Prof. Kroupa

Summary of both presentations:

b) The Debate

The German-language television channel 3sat produced a TV report on the Bethe Colloquium, which can be also found on Youtube (available only in German):

Part I

Part II

In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.

58. The tidal stability of Fornax cluster dwarf galaxies in Newtonian and Milgromian dynamics

(Guest post by Indranil Banik and Elena Asencio, August 2nd, 2021)

A directly-related presentation by Elena Asencio is available here:

The tidal stability of Fornax cluster dwarf galaxies in Newtonian and Milgromian gravity

The slides of the presentation can be downloaded here:

A large number of dwarf galaxies in the Fornax cluster (Figure 1) appear to be disturbed, most likely due to tides from the cluster gravity. In the standard cosmological model (ΛCDM) , the observable structure of the dwarfs is barely susceptible to gravitational effects of the cluster environment, as the dwarfs are surrounded by a dark matter halo. Because of this, it is very hard to explain the observations of the perturbed Fornax dwarfs in this theory. However, these observations can be easily explained in MOND, where dwarfs are much more susceptible to tides due to their lack of protective dark matter halos and the fact that they become quasi-Newtonian as they approach the cluster center due to the external field effect.

Figure 1: Fornax galaxy cluster. The yellow crosses mark all the objects identified in the Fornax deep survey (FDS) for this region of the sky, the black circles are masks for the spikes and reflection haloes, and the red crosses mark the objects that pass the selection criteria to be included in the FDS catalog. Image taken from Venhola et al. 2018.

The impact of tides on what the dwarfs look like is illustrated in Figure 2, which shows the fraction of disturbed galaxies as a function of tidal susceptibility η in ΛCDM and MOND, with η = 1 being the theoretical limit above which the dwarf would be unstable to cluster tides. Moreover, there is a lack of diffuse galaxies (large size and low mass) towards the cluster center. This is illustrated in Figure 3, which shows how at low projected separation from the cluster center, dwarfs of any given mass cannot be too large, but larger sizes are allowed further away. Figure 3 thus shows a clear tidal edge that cannot be explained by selection effects, since the survey detection limit would be a horizontal line at 1 on this plot such that dwarfs above it cannot be detected. Diffuse dwarf galaxies are clearly detectable, but are missing close to the cluster center. Another crucial detail in Figure 3 is that dwarfs close to the tidal edge are much more likely to appear disturbed, which is better quantified in Figure 2 in the rising fraction of disturbed galaxies with tidal stability η. The tidal edge is also evident in Figure 2 in that the dwarfs only go up to some maximum value of η, which should be close to the theoretical stability limit of 1. This is roughly correct in MOND, but not in ΛCDM.

Figure 2: Fraction of disturbed galaxies for each tidal susceptibility bin in MOND (red) and ΛCDM (blue). Larger error bars in a bin indicate that it has fewer dwarfs. The bin width of the tidal susceptibility η is 0.5 in MOND and 0.1 in ΛCDM (each data point is plotted at the center of the bin). Notice the rising trend and the maximum η that arises in each theory.

Figure 3: Projected distances of Fornax dwarfs to the cluster center against the ratio Re/rmax, where Re is the dwarf radius containing half of its total stellar mass, and rmax is the maximum Re at fixed stellar mass above which the dwarf would not be detectable given the survey sensitivity. The dwarfs are classified as “disturbed” (red) “undisturbed” (blue). The black dashed line shows a clear tidal edge – at any given mass, large (diffuse) dwarfs are present only far from the cluster center. This is not a selection effect, as the survey limit is a horizontal line at 1 (though e.g. some nights could be particularly clear and allow us to discover a dwarf slightly above this).

We therefore conclude that MOND and its corresponding cosmological model νHDM (see blog post “Solving both crises in cosmology: the KBC-void and the Hubble-Tension” by Moritz Haslbauer) is capable of explaining not only the appearance of dwarf galaxies in the Fornax cluster, but also other ΛCDM problems related to clusters such as the early formation of El Gordo, a massive pair of interacting galaxy clusters. νHDM also better addresses larger scale problems such as the Hubble tension and the large local supervoid (KBC void) that probably causes it by means of enhanced structure formation in the non-local universe. These larger scale successes build on the long-standing success of MOND with galaxy rotation curves (“Hypothesis testing with gas rich galaxies”). MOND also offers a natural explanation for the Local Group satellite planes as tidal dwarf galaxies (“Modified gravity in plane sight”), and has achieved many other successes too numerous to list here (see other posts). Given all these results, the MOND framework appears better suited than the current cosmological model (ΛCDM) to solve the new astrophysical challenges that keep arising with the increase and improvement of the available astronomical data, which far surpass what was known in 1983 when MOND was first proposed.

In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.

56. From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?

(by Pavel Kroupa, 4th of April, 2021, 11:11)

Update (April 15th): After receiving some queries, the slides to the talk w/o the fictitious story can be downloaded here

On April 9th, 2021, I will give this public talk:

If interested, you can join the public lecture by registering here.

The talk, held via zoom, is on April 9that 11:00 Chilean Time (CLT = UTC-4),  8am Pacific Daylight Time (PDT = UTC-7),11am Eastern Daylight Time (EDT = UTC-4), 17:00 Central European Summer Time (CEST = UTC+2)

The Golden Webinars are provided as a free public service and have no registration fees. They are recorded and made available for later viewing via youtube.

In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.

43. Pavel Kroupa on ” The vast polar structures around the Milky Way and Andromeda “

In case you, like me, have missed Pavel Kroups’s recent talk at the Joint Astronomical Colloquium in Heidelberg, you now have the opportunity to watch a movie of the event and download the slides. The movie is quite long (more than an hour), but it is worth watching it to the end. While the talk is titled “The vast polar structures around the Milky Way and Andromeda”, Pavel talks about much more, starting with tidal dwarf galaxies and ending with a discussion of indications for an alternative model of gravity.

This presentation is very similar and in most parts identical to Pavel’s presentations held at Monterey at the conference “Probes of Dark Matter on Galaxy Scales” and in Durham at the “Ripples in the Cosmos” conference. The latter talk resulted in quite a discussion on Peter Coles’ (aka Telescoper) blog “In the Dark”, following his criticism of Pavel’s talk as being “poorly argued and full of grossly exaggerated claims”. The video of a very similar presentation now offers everybody the opportunity to develop their own opinion on the issue. Given the numerous questions Pavel got during his talk and afterwards, people must have thought that it was worth the effort to argue with him, in contrast to Peter’s opinion.


See the overview of topics in The Dark Matter Crisis.

39. Question E: The Dark Matter Crisis continues: on the difficulties of communicating controversial science

(Continuation of the series A-E)

There has been an unsuccessful attempt to close down The Dark Matter Crisis. Here is the story (and an email by Jim Peebles): UPDATE: The guest post is now online.

As regular readers of our blog know, and first-time readers may be able to guess from this blog name, Pavel and I mostly write about the problems and shortcomings of the dark matter hypothesis. One aspect of our research is to test dark matter models on cosmologically small scales such as the Local Group of galaxies. Over the past years, our research and those of others has revealed that numerous model expectations of the dark matter hypothesis are not met by observations. This led us to the conclusion that we should consider a paradigm shift in how we understand the dark matter phenomenon. Maybe, we thought, a modification of the laws of gravity, one possible approach being Mordehai Milgrom’s MOdified Newtonian Gravity (MOND), could solve these issues.

Doing research that identifies shortcomings in a widely-held assumption and that is skeptical of a mainstream hypothesis is certainly a very interesting and rewarding endeavor for a scientist. It is closely connected to the fundamental scientific method of falsification and holds potential for groundbreaking discoveries. However, working on a controversial scientific topic also has its downsides. For one, papers criticizing basic assumptions are less attractive to be cited in mainstream publications. And before publication, controversial science already faces a more challenging peer-review process. For example Ashutosh Jogalekar explains in his blog The Curious Wavefunction:

“[…] reviewers under the convenient cloak of anonymity can use the system to settle scores, old boys’ clubs can conspire to prevent research from seeing the light of day, and established orthodox reviewers and editors can potentially squelch speculative, groundbreaking work.”

In addition to these ‘formal’ scientific interactions via academic publishers, there is also communication amongst scientists. For instance, early PhD students, who are still in the process of learning about the business of doing science, may be looking for advice from mentors and other more experienced scientists. Unfortunately, when the talk comes to controversial areas of science, students are often discouraged from getting involved in non-mainstream research (note, however, Avi Loeb‘s opposite advice). This begins with the commonly expressed belief that such research might “hurt your career”, but sometimes even more direct warnings are made. For example, a few years ago a professor told me that he would never hire someone who has published even a paper on MOND. A fellow PhD student got a similar piece of “advice” while visiting a different university, where one scientist advised him that he should only publish results which are negative for MOND, but nothing in support of it.

For people who are just starting in science, especially, such comments may be alarming. Graduate students do not yet know much about the job market. They therefore tend to believe what the ‘old boys’ tell them. To researchers who have a bit more experience, such warnings are often incomprehensible since they know by then (if they didn’t already initially) that it is entirely unscientific to withhold research results that do not fit a pre-determined picture.

The difficulties of working in a controversial field of research do not stop here. Communicating such science to a wider audience can also result in problems. While the public is generally very interested in the challenges faced by prevailing theories, there are difficulties to overcome. One of them is the question of how to differentiate completely unscientific things (the paranormal, creationism, …), from actual, albeit controversial, science.

A promising approach to overcome this difficulty is to discuss controversial science publicly. This way, the public can follow and be part of the debate, learn that arguments are backed by references to peer-reviewed research and see that hypotheses need to be tested through comparison with observational data—essentially the public gets to view the scientific process as it is applied in any branch of research. By demonstrating that scientists stick to facts, respond to opposing arguments and do not resort to emotionally driven rhetoric, we can adequately demonstrate the strengths of science.

The strength of the scientific method over dogmatic beliefs should always prevail in order to be able to contemplate the possibility of paradigm shifts. This is indeed a complex idea to explain, and presenting research results as absolute truth is something scientists should be prepared not to do. Unfortunately, this is not always the case. Sometimes, some people profess the ideas they subscribe to as the scientific or absolute truth. Such claims of absolute truth completely contort the nature of science. It is certainly going too far when science bloggers, in an attempt to protect their preferred mainstream theory, demand that a scientists’ blog be closed because their views differ. Scientists who publish their research in scientific journals, who go through the peer-review process and who in the end publish slightly unorthodox but nonetheless valuable ideas, should not be censored from the science blogosphere.

Unfortunately, this is what happened to our blog, The Dark Matter Crisis.

A popular science blogger demanded that SciLogs.com discontinue our blog and has, for a short time, succeeded. We would like to use this occurrence as an example of the reactions and difficulties faced when doing online communication of controversial science topics. The incident demonstrates why debate in science must be based on objective facts and not be driven by personal opinions. It illustrates the dangers of mixing scientific convictions with personal goals and emotions.

Why we started the Dark Matter Crisis blog

In late 2009, Pavel and I wrote an invited article for the German popular science magazine Spektrum der Wissenschaft about dwarf galaxies as tests of cosmology. During the process, Spektrum asked us to also start an accompanying science blog on SciLogs.eu, to provide a place for discussions that might arise due to the controversial nature of our work. We were very hesitant initially, but after talking to students and colleagues we agreed to start a blog. What convinced us to blog was the possibility to get in touch with readers, which would allow immediate feedback and discussions, and the ability to continuously provide current information about our active field of research. When the Spektrum article was published in July 2010, the blog The Dark Matter Crisis went online, too. We blogged on it for about two years, and then agreed to move The Dark Matter Crisis to the new SciLogs.com network. The first article on the SciLogs.com blog was published on January 3, 2013.

The discontinuation of The Dark Matter Crisis

On January 28, we received an email from the SciLogs.com community manager. The email informed us that our blog had been discontinued and that we would no longer be able to update it, although the blog’s archive would remain on the site. The short explanation provided was that the “thesis pushed by The Dark Matter Crisis is now overwhelmingly considered incorrect by the scientific community and as such cannot be considered sound enough to be promulgated by SciLogs.com”.

As we blog mostly about our own and related research, such a justification not only attacks our blogging but also hits at the very heart of our scientific work. Consequently, the first reaction to this email was shock, quickly followed by many questions. Which “theses pushed” by our blog “is now overwhelmingly considered incorrect”? That the currently prevailing hypothesis of cold dark matter has serious problems? This certainly is not considered overwhelmingly incorrect, as there are many scientists working on addressing these problems, both within the framework of standard cosmology (e.g. Mutch et al. 2013, Fouquet et al. 2012), as well as by modifying it (e.g. Lovell et al. 2012, Macció et al. 2012) or even by taking a completely different approach (e.g. Famaey & McGaugh 2012). Also, we were invited to start the blog because of the controversial nature of this topic.

Furthermore, at the time of discontinuation, the SciLogs.com version of The Dark Matter Crisis had only one blog post thus far. The sole post presents the recent discovery of a co-rotating plane of satellite galaxies around Andromeda reported in Ibata et al. (2013, Nature). It discusses possible implications which are right now actively debated among scientists. In fact, that blog post was, as far as I can tell, the only one on the web to provide a detailed explanation as to why the Nature paper might be a threat to Einstein’s theory of gravitation, which was explicitly alluded to by numerous publications, but explained by none (most articles in classical media focussed on the 15-year-old co-author of the study). Surely, it is not the aim of SciLogs.com, as a service to provide information to the public, to censor a blog that was communicating science to the public. Therefore, we concluded that this blog post could not have been the reason for the discontinuation.

But even expanding the scope to the old SciLogs.eu blog, we cannot see where we push a thesis which is not scientifically sound. Our blog posts are full of references to peer-reviewed publications. While we often discuss non-mainstream interpretations, we always remain within the realm of science and discuss an active field of research. For example, we frequently mention alternatives to dark matter which try to explain the missing mass phenomenon by non-Newtonian gravity laws. As an active scientist in this field, one can certainly not say that this is not scientifically sound and “overwhelmingly considered incorrect”. Just looking at the number of citations to the first paper about MOND by Milgrom, shows a citation count that has been constantly rising over the last few years and is currently at 1066.

So, what might have triggered the decision to discontinue our blog?

What Who has triggered our blog’s discontinuation?

Digging around on Twitter revealed several interesting discussions which were obviously related to the discontinuation of The Dark Matter Crisis. It turns out that a former-scientist-turned-blogger, who had spent a few years doing research in cosmology (publishing 5 first-author papers with now 88 citations), demanded the discontinuation.

The blogger (@StartsWithABang) contacted @scilogscom on January 24 by replying to a 15-day old tweet that announced our blog’s move to the new domain. He tweeted “Bummed that @scilogscom is in the business of promoting contrarian scientist viewpoints.”, and asks the SciLogs.com community manager (@notscientific) “[Why] are you allowing @scilogscom to promote contrarian voices that undermine public understanding of [science]?”, adding “You have taken on “Dark Matter Crisis” blog, whose mission is to undermine all of physical cosmology & promote MOND.”

The two agreed to discuss the issue via email, with the blogger adding that he was “*personally* worried that you are promoting clicks & false controversy over quality science content”, and states that he is “very, VERY disappointed about this move that @scilogscom has made”.

By now the SciLogs.com community manager has explained to us what happened after these tweets. He and the publishing director responsible for SciLogs.com unfortunately assumed that the blogger’s criticism was justified. They decided to close our blog without conferring with others or asking us for a statement. After we complained about the discontinuation, they performed an internal investigation, which involved reaching out to astrophysicists and other people, and have realized that discontinuing our blog was a big mistake. We attribute SciLogs.com’s poor judgement to two factors: neither the community manager nor the publishing director has an (astro)physical background, it was the first time that SciLogs.com had experienced an attack against one of its blogs.

So, the result was that four days after the tweets about The Dark Matter Crisis were posted, our blog was discontinued. Interestingly, only a few hours later the blogger who complained about our blog tweeted: “Shout out to the @SciLogscom  team, esp. @notscientific  and @laurawheelers, for stepping up & vetting their #science blogs for quality!”. (@laurawheelers was not involved in the decision to discontinue our blog. She only referred @StartsWithABang to SciLogs.com’s community manager.) @StartsWithABang added “They are storing the archives, but the blog is inactive and will not be continued”. While until then this situation was only an example of one blogger attacking our blog and our research with contorted accusations, the reactions of a few other Twitter users  were disheartening.  Some of them, science communicators and even an active astronomer, welcomed the blog’s discontinuation. One would have hoped that they would see the value of our science blog, regardless of their own opinions on the controversial topic we blog about.

Some slightly earlier attacks

The incident seems to be related to a recently published paper by us: Kroupa, Pawlowski & Milgrom (2012). When the paper appeared on the preprint server arXiv on January 18, this lead to a short discussion on Twitter, during which the same blogger who would later led to the short-timed discontinuation of our blog, made some pretty harsh accusations against “the MOND zealots”, whom he seems to call a mix of skeptics and liars and deniers who trot out misinformation and undermine confidence in science. In reaction to our paper, he published a blog post in which he claims to rule out MOND with one graph. Unfortunately, his blog post does not address any of the issues discussed in our recent paper, nor does it address those discussed in many other papers over the recent years.

In reaction to the accusations and contorted depiction of our research, I submitted a comment to the blog post. It asks for a clarification of the accusations and tries to start an objective discussion. There was no reason to censor it. Nevertheless, the comment was not published the first time, so I submitted it again the following day. Again, it was not published. I then decided to ignore the issue and the blogger in the future, as a factual debate seemed to be undesired and emotion-laden quarreling on the web is a waste of time. However, as our blog was actively attacked only a few days later by that very same blogger, the comment is being published here for transparency:

“When I understand your Twitter tweets from yesterday correctly, you think that “Kroupa and some of the other MOND zealots” are, at least to a certain extend, liars and deniers who “trot out misinformation & undermine confidence in science”. Is this what you were saying or did I misunderstand something? My honest opinion is that this would be unnecessarily aggressive, insulting, unprofessional and unscientific as it does not help to establish a well-founded discussion of the scientific issues.

The fact that you do not address the numerous problems of LCDM, many of which are mentioned in the recent paper, does not help shaping a discussion. In your blog post, you base your argumentation on only one problem of MOND: the the strong oscillations in the matter power spectrum. However, according to e.g. Famaey & McGaugh (2012), this problem is not as clear-cut as you claim. They write: “the non-linearity of MOND can lead to mode mixing that washes out the initially strong signal by z = 0”, and even suggests a more robust test.

More fundamentally, basic logic tells us that falsifying one hypothesis does not provide information about the validity of an opposing one. Just to give an example: Disproving that the world is a disk does not prove that the guy who is claiming that the earth is donut-shaped is right. As it turns out, the earth is neither a disk nor a donut, but essentially a sphere. Nevertheless, you jump from this graph to a conclusion about “MOND, MOG, TeVeS, or any other dark-matter-free alternative”. In addition, if you would consider the numerous failures of the LCDM model in a similar way like those of MOND, according to your argumentation we would have to give up on both, modified gravity theories and dark matter.

As a last note, I’d like to point out that in our recent paper we do not present MOND as the final answer. The fact that there is not a single “MOND”, but many different attempts to construct a full theory of modified gravity (see Sect. 6) already demonstrates that more work needs to be done. But in order to search for a solution of the many problems LCDM has on scales of many Mpc and below (where MOND is very successful), scientists should be encouraged to investigate this possibility. That is what a paradigm shift is, in my opinion: acknowledging that there are problems and being open-minded for new or alternative explanations, without hiding the problems that these alternatives may themselves face. As we acknowledge in the paper, mass discrepancies in galaxy clusters and building a consistent cosmology are real challenges for MOND, but there exist more or less convincing answers to these problems in the various effective covariant theories that have been proposed to date (see e.g. the list of theories in Famaey & McGaugh 2012 and their Section 9.2). Even if most of these tentative new explanations will turn out to be unsuccessful, I am sure there still is much to learn about the Universe. We have made this clear in the final sentences of our paper, too: “Understanding the deeper physical meaning of MOND remains a challenging aim. It involves the realistic likelihood that a major new insight into gravitation will emerge, which would have significant implications for our understanding of space, time and matter.”

So, I don’t think there is any lying, denying or misinformation involved on part of us as active scientists. It is just that the Universe is a hard nut to crack. Having the strength to admit that none of the current models are the final answer should in fact increase our confidence in science.”

It is ironic that in a comment on this very blog post, the blogger suggests to a critical reader that if he does not like his way of blogging, the reader could get his own blog. Only a few days later the blogger seems to have worked towards the discontinuation of our blog …

The aftermath and an upcoming guest post

After being informed about the discontinuation and after having discovered the background story on Twitter, we got in touch with the staff responsible for SciLogs.com. As mentioned before, they quickly realized that the discontinuation of The Dark Matter Crisis was a mistake. After discussing the issue with Richard Zinken, the publishing director of Spektrum der Wissenschaft (who is also responsible for the SciLogs.com blog network), he and the community manager apologized for the incident. We have accepted the apology and understand that mistakes can happen. During the last weeks, we worked together with the SciLogs.com team, thinking about what would be the best way to re-open the blog and how to handle the recent events in a constructive way. Together with Richard and the community manager we developed this blog post on the difficulties faced when communicating controversial research.

Together, we also decided to invite a guest blogger to The Dark Matter Crisis, preferably a cosmologist who is skeptical about our views. We hope that this helps to shape the debate and keep it at a scientific level, in contrast to the seemingly emotionally driven attacks which misshape the public’s view of how science handles controversial research. We have asked a few colleagues for such posts, and are content that one experienced scientist has agreed to act as our guest blogger. We know that he is well-respected in the field. His guest post will go online tomorrow.

UPDATE (March 09 2013): In a recent blog post, supposedly trying to shut off people working on dark matter alternatives forever, the blogger attacking us wrote: “Courtesy of Scott Dodelson, I present to you the one graph that incontrovertibly settles the matter.” We now rather offer you a guest blog post on that matter by … Scott Dodelson.

In the meantime, Jim Peebles, Albert Einstein Professor Emeritus of Science at Princeton University, gave us his explicit permission to publish the full, unedited email in which he explains that he would not like to be our guest blogger. We would like to thank him for this and, given our recent experience, fully understand that he prefers to not start blogging:

“Hello Pavel

Sorry for the delay. I have been thinking about your email, and have decided that I will not contribute a commentary on your situation.

I agree with many of your points. The behavior of [SciLogs.com] is silly; this is not the way of science. As you indicate, the community is remarkably optimistic about galaxy formation within the standard LCDM cosmology. I consider this an example of the human herd instinct. With you I distrust talk of precision cosmology; we are still seeking an accurate cosmology. But I think we differ on the weight of evidence for LCDM. I am deeply impressed by the variety of independent lines of evidence that point to LCDM, and conclude that the case for LCDM as a useful approximation to reality on the scale of the Hubble length is about a good as one gets in physical science. No one can prove that there is not another cosmology without dark matter that fits the data as well as LCDM, and no one can prove that there is not another theory that works as well as quantum mechanics. I expect we both put the odds on the latter as too low to matter. I feel close to the same about the former.

You are entirely entitled to take the approach I see in your blog, but I do not want to state my opinion on your blog. I don’t want to take up [blogging] anywhere!

Regards, Jim”

In addition, you can have a look at a recent article in New Scientist: “Dark matter rival boosted by dwarf galaxies”. The article mentions James Binney, from the University of Oxford, who says that he “believes that some sort of MOND-like behaviour may manifest itself on small scales”, while Avi Loeb, of Harvard University, being skeptical about MOND, nevertheless states that: “The theory deserves a lot of respect.”

We believe that all astronomers, whether skeptical or not of our controversial research, are able to agree with Loeb’s statement, and it is in this spirit that we would like to continue our endeavours in online science communication.

By Marcel S. Pawlowski and Pavel Kroupa  (08.03.2013): “The Dark Matter Crisis continues: on the difficulties of communicating controversial science” on SciLogs. See the overview of topics in The Dark Matter Crisis.

38. Are there two types of dwarf galaxies in the universe?

Dwarf galaxies, that is galaxies less massive than a few billion solar masses, are expected to be formed through two processes. They might either be the luminous components of small dark matter halos, formed early in the universe when gas fell into the potential well of those halos. These dwarf galaxies are called primordial dwarf galaxies (PDGs) and are expected to be dominated by their dark matter content.

The other formation mechanism is a process observed even in the present-day universe. When two major disk galaxies collide, the gas and the stars in the disks are expelled by tidal forces induced by the encounter to large distances. An example for a very prominent structure that has been created through tidal interactions between disk galaxies is the ‘tail’ that extends to the upper right corner in the figure below. Within this tidal debris, new objects of dwarf galaxy mass form. This is why dwarf galaxies of this second type are called tidal dwarf galaxies, or TDGs.

Thus, TDGs form from the baryonic material in the galactic disks of the progenitor galaxies, but can they also contain dark matter? Even in a disk galaxy with a massive dark matter halo, the vast majority of the dark matter would be located outside the galaxy’s disks. Of the small amount of dark matter within the disk, only a tiny fraction would furthermore be moving in the same direction and would have the same velocity as the stars and the gas in the disks. The vast majority of the dark matter would therefore have different initial conditions regarding its location and motion than the gas and the stars. But during a galaxy collision, only material with similar initial conditions is thrown on similar trajectories by the tidal forces and has a chance of becoming bound to the gravitational field of a forming TDG. The vast majority of the dark matter, having different initial conditions, will therefore be thrown onto different trajectories. While the dark matter on such different trajectories may be able to cross the shallow gravitational field of a TDG, it would do so at a high relative velocity. Therefore, this dark matter cannot become bound to the TDG. As an analogy for an encounter between a TDG and a chunk of dark matter, consider two spaceships orbiting a planet. Even if they orbit the planet at the same altitude, they can only rendezvous if they follow each other on the same orbit. For all other possible choices of orbits (say one is flying to the south and the other is flying to the west), the spaceships would fly past each other quickly if they do not crash.

In summary, it is one of the major characteristics of TDGs that they cannot contain much dark matter, even if their progenitor galaxies did (e.g Bournaud 2010).


Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

If the standard model of cold dark matter is correct, there should be a co-existence of these two types of dwarf galaxies in the universe: dark-matter dominated PDGs and TDGs without significant dark matter content. This is the Dual Dwarf Galaxy Theorem (Kroupa 2012).As they would have very different compositions, the two types should fall into two easily distinguishable groups. The natural question to ask in order to test this prediction is:

Are there really two distinct populations of dwarf galaxies in the universe?

This is investigated in the article “Dwarf elliptical galaxies as ancient tidal dwarf galaxies” by Dabringhausen & Kroupa (2013). The principle of their study is simple: they just had to compare the observed properties of old dwarf galaxies with known tidal dwarf galaxies. For the comparison, they use two properties, which are easy to determine observationally. These properties are:

  • The stellar mass, i.e. only the mass in stars, without the mass in gas, dust or dark matter. It can be determined from the luminosity of the system (more stars = brighter object).
  • The projected half-light radius, which is a measure of how extended the system is.

There are extensive catalogs listing these two properties for so-called pressure-supported systems, i.e. systems of stars in which the stars move on chaotic orbits (in contrast to the ordered rotation of  disc galaxies). The following plot shows these data points.

Screenshot from 2020-10-26 11-27-10

                                           Credit: Dabringhausen & Kroupa (2013)

These objects include globular clusters (GCs), ultra-compact dwarf galaxies (UCDs), massive elliptical galaxies (nEs), and dwarf elliptical galaxies (dEs). The first two types of objects (green points) appear to be free of dark matter, while the second two (red points) are generally assumed to sit in dark matter halos. The study of Dabringhausen & Kroupa is particularly interested in the dEs, as these are in the mass- and size-range of observed TDGs, but are generally assumed to be PDGs.

Adding Tidal Dwarf Galaxies

For a meaningful comparison, the properties of these dEs have to be compared with those of known TDGs. To be confident that an object is a TDG, it has to be associated with interacting galaxies (another possibility is to look at numerical simulations of galaxy collisions and extract the properties of TDGs formed in those models). However, this gives rise to a complication: TDGs associated with a pair of interacting galaxies are young, many of them are still forming some stars and such young TDGs can contain a lot of gas. The dEs, in contrast, are old systems without gas. So the observed properties of the young TDGs have to be aged before they can be compared to the dEs. As the TDGs age, they will loose their gas. The paper lists three possible processes:

  1. The gas is converted into stars.
  2. The gas is removed because the feedback of massive stars in the TDG heat it.
  3. The gas can be removed through ram-pressure stripping as the TDG moves through the intergalactic medium.

Because those gas-removal processes happen slowly, their major effect on the TDG properties is an increase of the system’s half-light radius: as (gas) mass is lost, the TDG will be less bound and the distribution of stars will expand. This allowed Dabringhausen & Kroupa (2013) to estimate where aged TDGs would show up in the figure:

Screenshot from 2020-10-26 11-30-25

                                             Credit: Dabringhausen & Kroupa 2013

The TDGs (blue symbols) fit in quite nicely with the dEs. The lower points on the error bars represent the TDG properties as observed, i.e. still young. Their radii are a lower limit: the TDGs cannot shrink as they slowly loose their gas. The upper end of the error bars assumes that most of the TDG’s mass, 75% to be precise, has been lost. This coincides nicely with the upper end of the dE distribution, too. There is in principle no reason why a TDG couldn’t loose even more of its initial mass, but such TDGs are likely to be destroyed very easily (see further below).

So, the TDGs and the dEs populate the same region in the figure. What does this tell us?

Due to their different composition (PDGs being dark matter dominated, TDGs being dark matter free), one would expect to observe two distinguishable groups of dwarf galaxies. The opposite is found: dEs populate only one region in the plot, and the same region is covered by (aged) TDGs. Consequently, this suggests that the observed dEs are in fact old TDGs. But then there is no room for primordial, dark matter-dominated dwarf galaxies.

This finding is also consistent with the expected numbers of TDGs in the universe. Numerical simulations of close encounters between possible progenitor galaxies show that on average one or two long-lived, massive TDGs are created per such encounter (see Bournaud & Duc 2006). By considering the total number of encounters between possible progenitor galaxies until the present day, Okazaki & Taniguchi (2000) found that such a rate of TDG-production would already be enough to account for all dEs in the Universe.

The black lines in the second plot give another hint at a connection between dEs and TDGs. Because TDGs are formed by colliding galaxies, many of the TDGs will end up as satellite galaxies. When such satellites orbit around a much more massive host galaxy, they will be affected by tidal forces. If the satellite is too extended, its own gravity is not strong enough to keep it bound against the tidal forces of the host. The exact radius depends on the masses of the host and the satellite, as well as the satellite’s orbit. The black lines in the plot give an impression of the tidal radius of satellite galaxies, assuming they orbit at a typical satellite distance of 100 kpc around different host galaxies. For the lowermost line, the host is assumed to be heavy, while the uppermost line corresponds to a rather light host. Above a given line, a satellite of a galaxy with the corresponding mass is not stable anymore, but will be disrupted by tidal forces. So if a TDG loses so much mass that it expands above this line, it will be destroyed and vanish from the plot. Thus, if the dEs are indeed TDGs, the position and slope of the cutoff at large half-light radii is easily explained.


The results of Dabringhausen & Kroupa (2013), if confirmed by future studies, suggest that there is only one type of dwarf galaxies in the Universe. Virtually every galaxy that is classified as an old dwarf galaxy, i.e. a dE, would be an aged TDG which originated from the debris of interacting galaxies. We emphasize also that TDGs have been shown to lie on the baryonic Tully-Fisher Relation (Gentile et al. 2007), which they cannot if this relation is defined by dark matter. These results are very problematic for cold dark-matter based models, which predict that in addition to TDGs a plethora of primordial dwarf galaxies with a completely different composition exists as a second group of dwarf galaxies.  However, the result of Dabringhausen & Kroupa (2013) fits in nicely with the peculiarities of the Milky Way (e.g. Pawlowski et al. 2012) and Andromeda (Ibata et al. 2013) satellite galaxies: they co-orbit within thin planes, which is expected for a population of TDGs. But again this distribution is at odds with the predicted distributions of primordial galaxies.

When it comes to their properties and distribution, tidal dwarf galaxies seem to develop a lead over dark-matter dominated, primordial dwarf galaxies.

By Marcel S. Pawlowski and Pavel Kroupa  (07.03.2013): “Are there two types of dwarf galaxies in the universe?” on SciLogs. See the overview of topics in The Dark Matter Crisis.