The Dark Matter of Astronomy Hype

Posted in Astrohype, Bad Statistics, The Universe and Stuff with tags , , , , on April 16, 2018 by telescoper

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.

 

 

 

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Sports Round-up

Posted in Cricket, Football with tags , , , , on April 15, 2018 by telescoper

Feeling a bit under the weather – and mindful that I have to get the early flight back to Cardiff tomorrow morning – I’ve been taking it easy today. I did, however, venture forth to the local pub in Maynooth (which is approximately fifty yards from my flat) after lunch to have a pint and watch the second half of Newcastle Utd against Arsenal in the Premiership League. I’m glad I did, as Newcastle won 2-1, which cheered me up considerably.

After four consecutive wins Newcastle have now got 41 points from 33 games and are in tenth place. Though relegation is still mathematically possible, that would require a very improbable sequence of results.

Even the bottom club, West Bromwich Albion, still have a chance of avoiding relegation. They even managed to beat Manchester Utd today, but it still looks most probable that they, Stoke City and Southampton will be in the Championship next year.

Manchester United’s defeat later this afternoon hands the Premiership title to their Midlands rivals Manchester City. Congratulations to City. They’ve clearly been the best team all season, a little end-of-term wobble notwithstanding.

Wolverhampton Wanderers gained promotion yesterday, but the second automatic spot is yet to be decided. Cardiff City looked to have blown it in recent matches, but had a good win yesterday against Midlands side Norwich City while a last-gasp goal from Brentford meant Fulham only drew. With a game in hand, the Bluebirds are in a strong position but you never know.

At the other end of the Championship lies Sunderland, but even they have a (remote) chance of avoiding relegation.

Anyway, as always, the last handful of matches of the football season coincides with the start of the cricket season. The County Championship started on Friday but many games were badly affected by the weather. Where play was possible, conditions favoured bowling rather than batting: Kent, for example, were bowled out for 64 in their first innings against Gloucestershire..

Glamorgan have been playing a three-day match against a Cardiff MCCU team. The students had Glamorgan in trouble at 28-3 chasing their first innings 249 but the professionals recovered to reach 217-5 at yesterday’s close. No play was possible today because of rain so the match ended in a draw.

Hopefully, when teaching and exam marking are finished in both Maynooth and Cardiff, I’ll be able to make use of my season ticket for some of Glamorgan’s matches at the SSE SWALEC Stadium in Sophia Gardens. The Welsh weather may, of course, have something to say about that!

Pictures from a Mediaeval Bestiary, No. 69 – The Turtle

Posted in Uncategorized on April 14, 2018 by telescoper

The Day’s Events

Posted in Uncategorized with tags , , , , on April 13, 2018 by telescoper

Today has been a very strange day. The strangeness started last night when, without any notice being given to us mere residents, a crew arrived at 8pm to do roadworks (resurfacing). There then followed an excruciating racket right outside my window well into the early hours of the morning.

So, having not had much sleep at all, I wasn’t in a very good mood when I got up. Things went from bad to worse when, just after 8am, all the electricity in my flat went off. I checked the trip switches and saw no problem. I then went outside and saw the traffic lights were off. It turned out that the power supply to all of Maynooth was off (including the entire University), as well as quite a bit of the rest of County Kildare.

With no electricity I couldn’t have a shower or make any coffee or have the bacon sandwich I’d planned to have for breakfast. The prospect of sitting in a cold flat all morning with nothing to do and not even the radio to listen to didn’t appeal so I got dressed and went to the office (which is only 15 minutes walk away). No shops were open on the way. There was no electricity anywhere on campus, so no internet connection, and quite a lot of students sitting around wondering what to do. At least the office was fairly warm and I had plenty of things I could do without a computer.

News eventually started coming through that power was returning gradually to the campus buildings. Ours came back at about 10.30. At that point I finally got a cup of coffee. I still don’t know what caused the fault.

The other major event of the day was that the result came through from the Universities and Colleges Union ballot on whether to accept the Employers’ offer on pensions. A majority of the members voted `yes’, so strike action – which had been planned to resume at Cardiff on Monday 16th April – is now suspended. I wouldn’t bet against a resumption later this year, as the major issues seem to me unresolved. However, I will be leaving Cardiff in July so that’s the end of the matter for me.

Anyway, this now means that I’ll be resuming my teaching in Cardiff on Tuesday next week (17th April). I’d already decided to spend this weekend in Ireland so I’ll be going back on Monday morning, Flybe willing…

Fun with the Airy Equation

Posted in Education, mathematics with tags , , , , , , on April 12, 2018 by telescoper

Today being a Maynooth Thursday, it has, as usual, has been dominated by computational physics teaching. We’re currently doing methods for solving ordinary differential equations. At the last minute before this afternoon’s lab session I decided to include an exercise that involved solving the following harmless-looking equation: y'' = xy.

This is usually known as Airy’s equation and it comes up quite frequently in problems connected with optics. It was first investigated by a former Astronomer Royal George Airy, after whom the function is named; incidentally, he was born in Alnwick (Northumberland, i.e. not the Midlands).

Despite its apparent simplicity, the Airy equation describes some very interesting phenomena. Indeed it is the simplest ODE (that I know of) that has the property that there’s a point at which the behaviour of the solution turns from oscillatory to exponential. Here’s a result of a numerical integration of the equation: obtained using a simple Python script:

(I stopped the integration at x=5 as the magnitude of the solution grows very quickly beyond that value for the particular initial conditions chosen).

One of the reasons for including this example (apart from the fact that Airy was a Geordie) is that the students were so surprised by the behaviour of the solution and most of them assumed that there was some problem with the numerical stability of their results. Some integration methods do struggle with such simple equations as the simple harmonic oscillator, but just sometimes weird numerical results are not mere numerical artifacts!

Anyway, my point is not about this particular equation or even about computational physics, but a general pedagogical one: finding interesting results for yourself is much more likely to motivate you to think about what they mean than if they’re just described to you by someone else. I think that goes for numerical experiments in a computer lab just as much as it does for any other kind of practical experiment in a science laboratory.

Mathematical operations with the Normal distribution

Posted in Uncategorized on April 12, 2018 by telescoper

Interesting post about the USS pension `deficit’ and why it is strongly dependent on the valuation method.

corp.ling.stats

This post is a little off-topic, as the exercise I am about to illustrate is not one that most corpus linguists will have to engage in.

However, I think it is a good example of why a mathematical approach to statistics (instead of the usual rote-learning of tests) is extremely valuable.

Case study: The declared ‘deficit’ in the USS pension scheme

At the time of writing nearly two hundred thousand university staff in the UK are active members of a pension scheme called USS. This scheme draws in income from these members and pays out to pensioners. Every three years the pension is valued, which is not a simple process. The valuation consists of two aspects, both uncertain:

  • to value the liabilities of the pension fund, which means the obligations to current pensioners and future pensioners (current active members), and
  • to estimate the future asset value of the pension fund…

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Cardiff to Dublin via Belfast

Posted in Biographical, Cardiff with tags , , , on April 11, 2018 by telescoper

I thought I write a brief post to arising from today’s travel difficulties, really just to make a record of the episode for posterity.

As last Wednesday I got up early this morning (4.30am) to get the 7am FlyBe flight from Cardiff to Dublin, due into Dublin at 8.05am. Having only hand luggage, as usual, I proceeded straight upstairs to the departure area only to find that the screen said the flight was cancelled, and directed passengers intending to travel on it back downstairs to the `Disruption Desk’ .

A long queue had already formed by the time I got there, but I got to the desk fairly quickly. The assistant explained that the cancellation was due to `staff sickness’ (i.e. they were short of a pilot) and the only option on offer was to fly to Belfast whence a bus would be provided to Dublin. The Belfast flight should have left at 6.15am but was being held for passengers to Dublin. I was also given a £5 refreshments voucher.

I thought a moment and then decided to accept this offer. I didn’t have any morning appointments today, but definitely had to get to Maynooth somehow by tomorrow morning as I have a lecture to deliver.

The plane left Cardiff about 7.15am and arrived in Belfast around 8.10am but when I emerged from the arrivals area there was no bus. In fact it took about 45 minutes to arrive, and we didn’t get going until about 8.55am. Many of the passengers were clearly nervous about missing connecting flights in Dublin, including a group of women planning onward travel to the USA, but the trip was fairly uneventful apart from the fact that the toilet was out of commission necessitating a stop so that people could use the facilities in a service station.

I can confirm that there is no visible border between Northern Ireland and the Republic on the road between Belfast and Dublin, although it does change name from A1 to M1 on the way. The distance between Belfast and Dublin by road is about 100 miles and it took just over two hours. I arrived at Terminal 1 of Dublin Airport at 11.05, almost exactly three hours late. I thought that wasn’t too bad a result given the chaos in Cardiff when I left but it was still a frustrating morning. I had to wait until 11.50 for the bus to Maynooth, where I finally arrived about 12.40.

I will of course be submitting a request for the compensation to which I am entitled for the delay, but above all I hope that those whose arrangements were even more seriously disrupted than mine managed to get to where they needed to go in the end.