The debate about MOND often degenerates into something that falls well short of the sober, objective discussion that is suppose to characterize scientific debates. One can tell when voices are raised and baseless ad hominem accusations made. I have, with disturbing frequency, found myself accused of partisanship and intellectual dishonesty (usually by people who are as fair and balanced as Fox News).
Let me state with absolute clarity that intellectual honesty is a bedrock principle of mine. It is a nerdy and dated referncee, but I think my attitude is summed up well by the character Merlin in the movie Excalibur: "When a man lies, he murders some part of the world." [As best I can tell, this is originally attributable to the 17th century cleric Paul Gerhardt.] Indeed, I would extend this to ignoring facts. One should not only be truthful, but also as complete as possible. It does not suffice to be truthful while leaving unpleasant or unpopular facts unsaid.
I "grew up" believing in dark matter. Specifically, Cold Dark Matter, presumably a WIMP. I didn't think MOND was wrong so much as I didn't think about it at all. Barely heard of it; not worth the bother. So I was shocked - and angered - when it its predictions came true in my data for low surface brightness galaxies. So I understand when my colleagues have the same reaction.
Nevertheless, Milgrom got the prediction right. I had a prediction, it was wrong. There were other conventional predictions, they were also wrong. Indeed, dark matter based theories generically have a very hard time explaining these data. In a Bayesian sense, given the prior that we live in a ΛCDM universe, the probability that MONDian phenomenology would be observed is practically zero. Yet this phenomenology is observed. (This is very well established, and has been for some time.)
So - confronted with an unpopular theory that nevertheless had some important predicitons come true, I reported that fact. I could have ignored it, pretended it didn't happen, covered my eyes and shouted LA LA LA NOT LISTENING. With the benefit of hindsight, that certainly would have been the savvy career move. But it would also be ignoring a fact, and tantamount to a lie.
In short, though it was painful and protracted, I changed my mind. Isn't that what the scientific method says we're suppose to do when confronted with experimental evidence?
That is not to say I believe MOND has to be correct. But I realized I had been wrong to believe so fervently that CDM had to exist. After all, we only infer the existence of non-baryonic dark matter through our absolute faith in our theory of gravity - a theory that is only tested on the scales of galaxies by the data that tend to support MOND. My concerns on this point would be greatly allayed by actual detection of the dark matter. If we're so sure it exists, show me a piece! That's the great thing about experimental evidence - it provokes one to change one's mind: I am open to changing mine again. I am disturbed by how many scientists are not so open, even as the evidence for supersymmetry (a prerequisite for WIMP dark matter) crumbles. I undestand discomfort with MOND as a theory. I am unimpressed with the presumptive superiority of a theory that invokes the existence of invisible particles that have no place in the standard model of particle physics. I'm open to the possibility. I'm just not willing to grant the presumption of their existence in the absence of laboratory evidence, much less the presumed superiority of this world view. One may truthfully say that ΛCDM provides a coherent description for the large scale structure of the universe. One cannot truthfully say that it must be correct - especially as those who do so pointedly ignore the observational fact that MOND is the effective force-law in galaxies. Maybe that is a consequence of dark matter. But that has yet to be demonstrated. It is a fact more conveniently ignored. Sadly, most cosmologists at the dawn of the 21st century have not even progressed as far as the attitude encapsulated by this quote from J.K. Galbraith: "Faced with the choice between changing one's mind and proving that there is no need to do so, almost everybody gets busy on the proof."
For those of you who are not partisan, here is a simple trick for identifying dark matter partisans. They inevitably say some variation on "MOND works in galaxies, but not on larger scales." This is a gross over-simplification of the real situation. It is also a dead give away that the speaker hasn't bothered to think seriously on the subject, prefering instead to follow this more direct line of reasoning. (Obviously I made this flow chart as a joke, but sadly it is not as unfair a description of how many cosmologists behave as it should be.)
Nevertheless, I can relate to the negative attitude many harbor towards MOND. I was once guilty of the same attitude myself. MOND had to bite me in my own data to get my attention. That was a necessary but not sufficient condition. I also had to be honest enough to consider the possibility that I might have been wrong to be so sure about dark matter.
To many cosmologists, even considering MOND is a crime. (For example, see this email which I received some years after writing this web page.) I do not find persuasive arguments to the effect that I must be dishonest since I consider MOND. (As a colleague put it upon returning from a dark matter conference, the prevailing attitude was "People should just stop talking about MOND." Because that is a mature and well considered scientific perspective.) It is certainly possible that MOND is wrong. There have of course been [exceedingly rare] occasions on which I personally have been wrong. But is it dishonest simply to consider MOND? Or not to?
I am simply bemused by the presumption of those who don't know me who seem to imagine that they know what I think or how I'll behave. Various assertions to this effect occasionally reach my attention. My favorite examples involve the presumption that MOND must somehow pollute my teaching. One prominent astronomer once asked a student how I had "indoctrinated" him into MOND. That's sort of like asking "have you stopped beating your wife lately?" I find this presumption especially amusing (if offensive) because I know what it is to be indoctrinated (remember when Ωm had to be unity?) and I take care to avoid this in all subjects. Just as in publication, where I always write the conventional paper first, I am careful to teach the standard model before discussing any alternatives (if they even come up). To be sure, I do sometimes use dark matter and MOND in class as a living example of the scientific method in action. Is that indoctrination? That must be why the last time I did this the tally of anonymous student votes came out 7 for dark matter, 3 for MOND, with 3 abstentions. Obvious bias!
I ask those who presume that I'm biased to ask themselves why I might be so misguided. I have no loyalty to Milgrom, who I didn't know from Adam when I first encountered his theory. It certainly isn't for profit. MOND has brought me ample helpings of grief, career-wise. In that respect at least, it is the idea that keeps on giving.
So why do I do it? Because intellectual honesty demands it. Not to do so would be ignoring a salient fact, and I refuse to be party to a lie.