The Dark Matter of Astronomy Hype

Just before Easter (and, perhaps more significantly, just before April Fool’s Day) a paper by van Dokkum et al. was published in Nature with the title A Galaxy Lacking Dark Matter. As is often the case with scientific publications presented in Nature, the press machine kicked into action and stories about this mysterious galaxy appeared in print and online all round the world.

So what was the result? Here’s the abstract of the Nature paper:


Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052–DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052–DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.


I had a quick look at the paper at the time and wasn’t very impressed by the quality of the data. To see why look at the main plot, a histogram formed from just ten observations (of globular clusters used as velocity tracers):

I didn’t have time to read the paper thoroughly before the Easter weekend,  but did draft a sceptical blog on the paper only to decide not to publish it as I thought it might be too inflammatory even by my standards! Suffice to say that I was unconvinced.

Anyway, it turns out I was far from the only astrophysicist to have doubts about this result; you can find a nice summary of the discussion on social media here and here. Fortunately, people more expert than me have found the time to look in more detail at the Dokkum et al. claim. There’s now a paper on the arXiv by Martin et al.

It was recently proposed that the globular cluster system of the very low surface-brightness galaxy NGC1052-DF2 is dynamically very cold, leading to the conclusion that this dwarf galaxy has little or no dark matter. Here, we show that a robust statistical measure of the velocity dispersion of the tracer globular clusters implies a mundane velocity dispersion and a poorly constrained mass-to-light ratio. Models that include the possibility that some of the tracers are field contaminants do not yield a more constraining inference. We derive only a weak constraint on the mass-to-light ratio of the system within the half-light radius or within the radius of the furthest tracer (M/L_V<8.1 at the 90-percent confidence level). Typical mass-to-light ratios measured for dwarf galaxies of the same stellar mass as NGC1052-DF2 are well within this limit. With this study, we emphasize the need to properly account for measurement uncertainties and to stay as close as possible to the data when determining dynamical masses from very small data sets of tracers.

More information about this system has been posted by Pieter van Dokkum on his website here.

Whatever turns out in the final analysis of NGC1052-DF2 it is undoubtedly an interesting system. It may indeed turn out to  have less dark matter than expected though I don’t think the evidence available right now warrants such an inference with such confidence. What worries me most however, is the way this result was presented in the media, with virtually no regard for the manifest statistical uncertainty inherent in the analysis. This kind of hype can be extremely damaging to science in general, and to explain why I’ll go off on a rant that I’ve indulged in a few times before on this blog.

A few years ago there was an interesting paper  (in Nature of all places), the opening paragraph of which reads:

The past few years have seen a slew of announcements of major discoveries in particle astrophysics and cosmology. The list includes faster-than-light neutrinos; dark-matter particles producing γ-rays; X-rays scattering off nuclei underground; and even evidence in the cosmic microwave background for gravitational waves caused by the rapid inflation of the early Universe. Most of these turned out to be false alarms; and in my view, that is the probable fate of the rest.

The piece went on to berate physicists for being too trigger-happy in claiming discoveries, the BICEP2 fiasco being a prime example. I agree that this is a problem, but it goes far beyond physics. In fact its endemic throughout science. A major cause of it is abuse of statistical reasoning.

Anyway, I thought I’d take the opportunity to re-iterate why I statistics and statistical reasoning are so important to science. In fact, I think they lie at the very core of the scientific method, although I am still surprised how few practising scientists are comfortable with even basic statistical language. A more important problem is the popular impression that science is about facts and absolute truths. It isn’t. It’s a <em>process</em>. In order to advance it has to question itself. Getting this message wrong – whether by error or on purpose -is immensely dangerous.

Statistical reasoning also applies to many facets of everyday life, including business, commerce, transport, the media, and politics. Probability even plays a role in personal relationships, though mostly at a subconscious level. It is a feature of everyday life that science and technology are deeply embedded in every aspect of what we do each day. Science has given us greater levels of comfort, better health care, and a plethora of labour-saving devices. It has also given us unprecedented ability to destroy the environment and each other, whether through accident or design.

Civilized societies face rigorous challenges in this century. We must confront the threat of climate change and forthcoming energy crises. We must find better ways of resolving conflicts peacefully lest nuclear or chemical or even conventional weapons lead us to global catastrophe. We must stop large-scale pollution or systematic destruction of the biosphere that nurtures us. And we must do all of these things without abandoning the many positive things that science has brought us. Abandoning science and rationality by retreating into religious or political fundamentalism would be a catastrophe for humanity.

Unfortunately, recent decades have seen a wholesale breakdown of trust between scientists and the public at large. This is due partly to the deliberate abuse of science for immoral purposes, and partly to the sheer carelessness with which various agencies have exploited scientific discoveries without proper evaluation of the risks involved. The abuse of statistical arguments have undoubtedly contributed to the suspicion with which many individuals view science.

There is an increasing alienation between scientists and the general public. Many fewer students enrol for courses in physics and chemistry than a a few decades ago. Fewer graduates mean fewer qualified science teachers in schools. This is a vicious cycle that threatens our future. It must be broken.

The danger is that the decreasing level of understanding of science in society means that knowledge (as well as its consequent power) becomes concentrated in the minds of a few individuals. This could have dire consequences for the future of our democracy. Even as things stand now, very few Members of Parliament are scientifically literate. How can we expect to control the application of science when the necessary understanding rests with an unelected “priesthood” that is hardly understood by, or represented in, our democratic institutions?

Very few journalists or television producers know enough about science to report sensibly on the latest discoveries or controversies. As a result, important matters that the public needs to know about do not appear at all in the media, or if they do it is in such a garbled fashion that they do more harm than good.

Years ago I used to listen to radio interviews with scientists on the Today programme on BBC Radio 4. I even did such an interview once. It is a deeply frustrating experience. The scientist usually starts by explaining what the discovery is about in the way a scientist should, with careful statements of what is assumed, how the data is interpreted, and what other possible interpretations might be and the likely sources of error. The interviewer then loses patience and asks for a yes or no answer. The scientist tries to continue, but is badgered. Either the interview ends as a row, or the scientist ends up stating a grossly oversimplified version of the story.

Some scientists offer the oversimplified version at the outset, of course, and these are the ones that contribute to the image of scientists as priests. Such individuals often believe in their theories in exactly the same way that some people believe religiously. Not with the conditional and possibly temporary belief that characterizes the scientific method, but with the unquestioning fervour of an unthinking zealot. This approach may pay off for the individual in the short term, in popular esteem and media recognition – but when it goes wrong it is science as a whole that suffers. When a result that has been proclaimed certain is later shown to be false, the result is widespread disillusionment.

The worst example of this tendency that I can think of is the constant use of the phrase “Mind of God” by theoretical physicists to describe fundamental theories. This is not only meaningless but also damaging. As scientists we should know better than to use it. Our theories do not represent absolute truths: they are just the best we can do with the available data and the limited powers of the human mind. We believe in our theories, but only to the extent that we need to accept working hypotheses in order to make progress. Our approach is pragmatic rather than idealistic. We should be humble and avoid making extravagant claims that can’t be justified either theoretically or experimentally.

The more that people get used to the image of “scientist as priest” the more dissatisfied they are with real science. Most of the questions asked of scientists simply can’t be answered with “yes” or “no”. This leaves many with the impression that science is very vague and subjective. The public also tend to lose faith in science when it is unable to come up with quick answers. Science is a process, a way of looking at problems not a list of ready-made answers to impossible problems. Of course it is sometimes vague, but I think it is vague in a rational way and that’s what makes it worthwhile. It is also the reason why science has led to so many objectively measurable advances in our understanding of the World.

I don’t have any easy answers to the question of how to cure this malaise, but do have a few suggestions. It would be easy for a scientist such as myself to blame everything on the media and the education system, but in fact I think the responsibility lies mainly with ourselves. We are usually so obsessed with our own research, and the need to publish specialist papers by the lorry-load in order to advance our own careers that we usually spend very little time explaining what we do to the public or why.

I think every working scientist in the country should be required to spend at least 10% of their time working in schools or with the general media on “outreach”, including writing blogs like this. People in my field – astronomers and cosmologists – do this quite a lot, but these are areas where the public has some empathy with what we do. If only biologists, chemists, nuclear physicists and the rest were viewed in such a friendly light. Doing this sort of thing is not easy, especially when it comes to saying something on the radio that the interviewer does not want to hear. Media training for scientists has been a welcome recent innovation for some branches of science, but most of my colleagues have never had any help at all in this direction.

The second thing that must be done is to improve the dire state of science education in schools. Over the last two decades the national curriculum for British schools has been dumbed down to the point of absurdity. Pupils that leave school at 18 having taken “Advanced Level” physics do so with no useful knowledge of physics at all, even if they have obtained the highest grade. I do not at all blame the students for this; they can only do what they are asked to do. It’s all the fault of the educationalists, who have done the best they can for a long time to convince our young people that science is too hard for them. Science can be difficult, of course, and not everyone will be able to make a career out of it. But that doesn’t mean that it should not be taught properly to those that can take it in. If some students find it is not for them, then so be it. We don’t everyone to be a scientist, but we do need many more people to understand how science really works.

I realise I must sound very gloomy about this, but I do think there are good prospects that the gap between science and society may gradually be healed. The fact that the public distrust scientists leads many of them to question us, which is a very good thing. They should question us and we should be prepared to answer them. If they ask us why, we should be prepared to give reasons. If enough scientists engage in this process then what will emerge is and understanding of the enduring value of science. I don’t just mean through the DVD players and computer games science has given us, but through its cultural impact. It is part of human nature to question our place in the Universe, so science is part of what we are. It gives us purpose. But it also shows us a way of living our lives. Except for a few individuals, the scientific community is tolerant, open, internationally-minded, and imbued with a philosophy of cooperation. It values reason and looks to the future rather than the past. Like anyone else, scientists will always make mistakes, but we can always learn from them. The logic of science may not be infallible, but it’s probably the best logic there is in a world so filled with uncertainty.




33 Responses to “The Dark Matter of Astronomy Hype”

  1. Someguywhoyouprobablythinkiscomplelywrong Says:

    I don’t think mandating scientists spend time on outreach is likely to improve scientific understanding, nor do I think that science, in general, can be taught to a majority of people without entirely changing our entire society.

    If you reject Cartesian dualism, and accept that the mind has a physical basis and is shaped by experience, within boundaries and limits imposed by nature (eg. genetics, along with some random fluctuations of course), then it seems obvious that the typical person will not be able to think scientifically.
    Nor do I ever expect them to be able to, no matter how much time you spend on outreach, or how much you encourage people to study physics and chemistry.

    The day to day lives of people seem to, on average, entirely preclude the chance for the development of a rational, statistics and evidence based reasoning process.

    I would actually expect relying on heuristics and emotionally weighted judgements, along with conforming to tribal (eg. political party etc) group values to be stronger in general at dominating over scientific reasoning.

    There is then the, often ideologically rejected, claims about heritability (correlation with parents etc) of academic achievement ,and other metrics associated with an ability to use and understand scientific reasoning.
    Add to that the fact that scientifically educated people have a much lower fertility rate, and it becomes apparent that our society acts as a meat grinder for scientific potential, and the ability, background, and importantly inclination or desire to want to understand science, in the general population.

    I think you would do more to increase public awareness of science by encouraging scientists to have as many, or more, children as some extreme religious sects do, than by trying to educate the general public.

    If people’s environment doesn’t reward rational, evidence based reasoning, but instead punishes it (or at the very least makes the propagation of the very properties that facilitate it less than those that don’t), why would any sane person expect things to be different?

    This is at an individual, not a society wide level, of course our society reaps great rewards from science, but I don’t think individuals do in general, and the exact veracity of the meme aside, “selfish” genes don’t seem to gain from it either.

    (If you are a transhumanist fanatic we can pray to the gods of germline engineering and hope that everyone can be bootstrapped to scientifically interested genius asap, and therefore all our problems don’t need to be addressed at all…..but that seems very unlikely to me).

    Tldr; I think the problem is the behaviours and traits incentivized to propagate the most are not compatible with a rational, statistics and evidence based, scientific reasoning process.
    I also believe that emotional heuristics and tribal in group signalling are likely to grind an ever increasing gap between those can think scientifically and those who can’t.

    Outreach and education are bandages to a wounded society, but not a cure.

    • “If you reject Cartesian dualism, and accept that the mind has a physical basis and is shaped by experience, within boundaries and limits imposed by nature (eg. genetics, along with some random fluctuations of course), then it seems obvious that the typical person will not be able to think scientifically.”

      I don’t think this is ‘obvious’ at all. The ability to think scientifically is a distinct evolutionary and cultural advantage.

  2. ‘Scientists Thought All Galaxies Had Dark Matter, but They Just Found One Without It’

    Dark matter fills ’empty’ space, strongly interacts with visible matter and is displaced by visible matter.

    The reason for the mistaken notion the galaxy is missing dark matter is that the galaxy is so diffuse that it doesn’t displace the dark matter outward and away from the galaxy to the degree that the dark matter is able to push back and cause the stars far away from the galactic center to speed up.

    It’s not that there is no dark matter connected to and neighboring the visible matter. It’s that the galaxy has not coalesced enough to displace the dark matter to such an extent that it forms a ‘halo’ around the galaxy.

    A galaxy’s halo is not a clump of dark matter traveling with the galaxy. A galaxy’s halo is the state of displacement of the dark matter.

  3. Anton Garrett Says:

    There is an increasing alienation between scientists and the general public. Many fewer students enrol for courses in physics and chemistry than a a few decades ago. Fewer graduates mean fewer qualified science teachers in schools. This is a vicious cycle that threatens our future. It must be broken.

    The reason for this is very deep. It is the outworking of the cultural shift that saw around 1980 the science-fiction shelves in our bookshops be replaced wholesale by sword-and-sorcery. What was going on? People had twigged that secular materialism did not bring personal happiness and fulfilment and were looking elsewhere. The tragic irony is that real scientists, rather than social commentators, never said it would.

    • Michel C. Says:

      People can make the difference between sorcery and science, but how about Super Hero tv series disguised in science-fiction? Is it a symptom or is it a cause? Or both?

    • If I weren’t sure that you’ve never inhaled, I would have asked what you have been smoking. 🙂

      Yes, as a fan of (some) old-school hard science fiction, I was surprised at the move to swords and sorcery. (Interestingly, Asimov stopped writing science fiction for a long time primarily because he saw a need to write popular-science books after the “Sputnik shock”.) However, I’m sure that it is unrelated to secular materialism. Note that many (most?) writers of old-school hard science fiction are (were) secular materialists (Asimov is a prime example).

      Yes, the swords-and-sorcery stuff is an example of people looking elsewhere, perhaps dissatisfied by conventional religion (probably more so than being dissatisfied by secular materialism). However, only a small fraction of people read swords-and-sorcery stuff, so I can’t see this as being related to the alienization between science and society. Moreover, many swords-and-sorcery people actually work in science and technical jobs (it’s almost a cliché).

      I think a much bigger problem is fake news and self-styled scientists who call into question conclusions which, by any sensible definition, are well established: evolution, anthropogenic global warming, etc. Alex Jones and his info wars and the people who fall for it are the real enemies of science, not fans of Conan the Barbarian.

      • Anton Garrett Says:

        Well, Phillip, it is possible to be both sure and wrong…

        Do you reckon that the proportion of people who read sword-and-sorcery is much different from the proportion that read science fiction?

        I take it that by “conventional religion” you mean institutional forms of Christianity. As a nonconformist I don’t think much of those either.

      • “Well, Phillip, it is possible to be both sure and wrong…”

        According to Landau’s theorem, that proves that I am a cosmologist. 🙂

        “Do you reckon that the proportion of people who read sword-and-sorcery is much different from the proportion that read science fiction?”

        I don’t know, but it’s probably about the same; a small fraction in both cases.

        “I take it that by “conventional religion” you mean institutional forms of Christianity. As a nonconformist I don’t think much of those either.”

        Right. I’ve often heard of people becoming dissatisfied with traditional religion. I have rarely if ever heard of someone who was disappointed with secular humanism. And, if so, I doubt that he would turn to swords and sorcery.

      • Anton Garrett Says:

        If people are disappointed in their faith system (of which secular humanism is one, just not theistic) then they seldom speak out about it until or unless they find another. It means admitting something unpalatable to yourself, and probably losing some friends.

      • “Well, Phillip, it is possible to be both sure and wrong…”

        OK, for the record it is now official that you’ve smoked some illicit substances.

        By the way, Monica Lewinsky missed the chance for the best one-liner in history: But I didn’t inhale. 🙂

    • Perhaps the main difference is that Conan fans know that they are reading fiction. 😐

  4. telescoper Says:

    Yet again I remind potential commenters that I require a name and valid email address before your comment can be published.

  5. Alan Heavens Says:

    I agree with you about statistics being at the core of scientific reasoning. Sadly, it is often seen as an afterthought in the scientific method, and is not valued as it should be. It is just as important as good experimental technique and good theoretical modelling, but there is often a lack of understanding (even in very high level grants panels) and even any recognition that it really matters. Arguably, the BICEP2 situation arose because of statistical failure – in this case to marginalise correctly over uncertainties in foreground emission. It is curious that standards vary wildly from sub-field to sub-field – in our own area, microwave background and gravitational wave analysis are generally very good examples, but in other fields the typical analysis methods often lag behind and really need to raise their game.

  6. I think these are two very different things. There is certainly pressure on scientists to jump to conclusions, ranging from career prospects to journals wanting impact. Uncertainties are not always well known but if you play a cautious game, the rash adventurers may get there first.

    The respect for science in the general public has nothing to do with this, I think. In my experience, people have no problem with scientists being wrong at times. It is when they attack the public that problems occur. We don’t attack people for misunderstanding quantum physics, so why do we go on the attack when it is about religion? Does it hurt people in any way if some assign an age of 6000 years to the world? You mention religious and political fundamentalism. Why leave out the fundamentalists of science? All suffer from a lack of respect for others. I remember someone saying that religious people can’t do science – and that was a member of a grants panel. Damaging intolerance happens on all sides. Yes, it would be very helpful to get a better understanding of science. But that is not achieved by attacking what people feel strongly about.

    There are things that do need strong action, where behaviour damages other people directly or indirectly, in churches, in politics, in universities. But we often instead seem to fight very unnecessary battles.

    • “Does it hurt people in any way if some assign an age of 6000 years to the world?”

      Focussing on just this one example, believing this implies a rejection of the way science works. It is no accident that people who believe in a young Earth also often doubt other scientific conclusions, which can indeed hurt people. For example, global warming is a serious problem and it is probably man-made for the most part. But that knowledge is based in part on studies of how temperatures and atmospheric composition have changed in the past, going back much longer than 6000 years.

      • I disagree: the rejection of science isn’t because of their belief, it is because they are attacked by scientists. There is no reason why those people should not accept climate change, dangers of pollution, engineering, etc, other than us making reasons for them. What the heck: the story of Noah involves people ignoring climate change, and the consequences..

      • Really? Visit a fundamentalist church in the American midwest. Many of the people have heard about scientists from one source only: the pulpit. They don’t even know any scientists, much less have they been attacked. “God said it; I believe it; that settles it.” This really is their philosophy. (This is not a small, irrelevant minority. They are the people who elected Trump.)

        Religion has attacked (and in some cases burned at the stake) many more scientists than scientists have attacked religious people. They might attack religion, but that’s OK. Science is about finding the truth, or at least a better approximation to it. If science and religion disagree (and they often do; they are not non-overlapping magisteria), religion has been wrong, every time. Saying that the Earth is 6000 years old is like saying vaccines cause autism: it’s bullshit, pure and simple. Unless one takes the stance than every statement is equally true, there is nothing wrong with calling a spade a spade.

      • Entirely different things. Believing in a 6000-year old earth doesn’t hurt others. Arguing that vaccines cause autism does cause damage and needs to be addressed. But in my experience, the latter comes from a different section of society, some fringe groups and some groups who get their opinions from the sensationalist press. Accusing people with one opinion of holding the other one is neither helpful nor, in many cases, correct.

        And people have been capable of amazing atrocities. You mention the religious prosecutions, but not the atheistic french revolution, the communist prosecutions, the biochemists of the nazi era. Are scientists better people? Ask the students and postdocs and you will hear stories of abuse by senior people there as well. My point is that accusing people because they belong to a group you disagree with, will turn them into anti-science.

      • Anton Garrett Says:

        You were asked: Does it hurt people in any way if some assign an age of 6000 years to the world?

        You replied: Focussing on just this one example, believing this implies a rejection of the way science works. It is no accident that people who believe in a young Earth also often doubt other scientific conclusions, which can indeed hurt people.

        That’s a pretty long-winded way of saying No.

      • But I was saying “yes”.

      • Anton Garrett Says:

        Not very well !

        You are saying that this absurd belief (and I write that as a Christian) correlates with some other beliefs that are harmful. That is not the same thing. As a matter of fact I agree with this statement, although I think it also correlates with some that are good.

      • I would add to Phillip’s comment that if someone believes that the age of the Earth is only 6000 years, then that in and of itself doesn’t hurt anyone. So in that very limited sense, Anton is correct. However, I would interpret Phillip’s comments more generally, in that people rarely keep their beliefs to themselves, but tend to want to impose them on others. This is especially true of those who adhere to belief systems that have a very long history of wanting to control others. So here in the good ol’ US of A we have many cases of people who hold a belief in a 6000 year old Earth who want to remove from schools books that take a more….um….reasoned, rational, and evidenced based approach to the age of the Earth. Such people get themselves elected to school boards and are thus able to affect book purchases etc. (especially true of the Lone Star State). This does harm to children who now get conflicting information from their teachers, books, and parents.

        So yes, such beliefs generally do do harm to others.

        I would also second Phillip’s remark that those who tend to hold such beliefs tend to have little attachment to reality are easily swayed by the propaganda of politically savvy individuals who are willing to manipulate others, as we are continuing to see around the world. This who accept things blindly on faith tend to be easily persuaded.

      • Anton Garrett Says:

        What matters when differing belief systems meet is not which is true (if any) but how they instruct their adherents to treat nonbelievers.

  7. Michel C. Says:

    The reason sciences are attacked is simply because the most powerful people are generally stupid in regards to science. And the problem is science is the prove of their stupidity but they have the power to silence…

  8. Insulting people is not the best way of making a case for science.

    Philip mentioned the vaccine and autism problem. That actually came out of a scientific paper that suffered from all the problems Peter identifies in his blog: publicity over accuracy, and a journal looking for impact. Science also has a case to answer.

    • In this case, it did answer. The paper was withdrawn, Wakefield lost his licence, etc. The scientific community has done a huge amount of work to debunk this. Still, many people believe it. This belief is dangerous. The way forward is for people to understand how science works. This is incompatible with believing in a young Earth.

    • Michel C. Says:

      The insulted is not insulted unless he or she reads the insults. And is the truth insulting?

  9. David Whitehouse Says:

    A very interesting article, much food for thought.

  10. […] hier, hier und hier, eine Replik des Original-Autors, Zusammenfassungen der Debatte hier, hier und hier und nachfolgende Papers mit Kritik hier, hier und hier. Und von vor der Debatte Press […]

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