25 years a heretic

People seem to like to do retrospectives at year’s end. I take a longer view, but the end of 2020 seems like a fitting time to do that. Below is the text of a paper I wrote in 1995 with collaborators at the Kapteyn Institute of the University of Groningen. The last edit date is from December of that year, so this text (in plain TeX, not LaTeX!) is now a quarter century old. I am just going to cut & paste it as-was; I even managed to recover the original figures and translate them into something web-friendly (postscript to jpeg). This is exactly how it was.

This was my first attempt to express in the scientific literature my concerns for the viability of the dark matter paradigm, and my puzzlement that the only theory to get any genuine predictions right was MOND. It was the hardest admission in my career that this could be even a remote possibility. Nevertheless, intellectual honesty demanded that I report it. To fail to do so would be an act of reality denial antithetical to the foundational principles of science.

It was never published. There were three referees. Initially, one was positive, one was negative, and one insisted that rotation curves weren’t flat. There was one iteration; this is the resubmitted version in which the concerns of the second referee were addressed to his apparent satisfaction by making the third figure a lot more complicated. The third referee persisted that none of this was valid because rotation curves weren’t flat. Seems like he had a problem with something beyond the scope of this paper, but the net result was rejection.

One valid concern that ran through the refereeing process from all sides was “what about everything else?” This is a good question that couldn’t fit into a short letter like this. Thanks to the support of Vera Rubin and a Carnegie Fellowship, I spent the next couple of years looking into everything else. The results were published in 1998 in a series of three long papers: one on dark matter, one on MOND, and one making detailed fits.

This had started from a very different place intellectually with my efforts to write a paper on galaxy formation that would have been similar to contemporaneous papers like Dalcanton, Spergel, & Summers and Mo, Mao, & White. This would have followed from my thesis and from work with Houjun Mo, who was an office mate when we were postdocs at the IoA in Cambridge. (The ideas discussed in Mo, McGaugh, & Bothun have been reborn recently in the galaxy formation literature under the moniker of “assembly bias.”) But I had realized by then that my ideas – and those in the papers cited – were wrong. So I didn’t write a paper that I knew to be wrong. I wrote this one instead.

Nothing substantive has changed since. Reading it afresh, I’m amazed how many of the arguments over the past quarter century were anticipated here. As a scientific community, we are stuck in a rut, and seem to prefer to spin the wheels to dig ourselves in deeper than consider the plain if difficult path out.

Testing hypotheses of dark matter and alternative gravity with low surface density galaxies

The missing mass problem remains one of the most vexing in astrophysics. Observations clearly indicate either the presence of a tremendous amount of as yet unidentified dark matter^1,2, or the need to modify the law of gravity^3-7. These hypotheses make vastly different predictions as a function of density. Observations of the rotation curves of galaxies of much lower surface brightness than previously studied therefore provide a powerful test for discriminating between them. The dark matter hypothesis requires a surprisingly strong relation between the surface brightness and mass to light ratio⁸, placing stringent constraints on theories of galaxy formation and evolution. Alternatively, the observed behaviour is predicted⁴ by one of the hypothesised alterations of gravity known as modified Newtonian dynamics^3,5 (MOND).

Spiral galaxies are observed to have asymptotically flat [i.e., V(R) ~ constant for large R] rotation curves that extend well beyond their optical edges. This trend continues for as far (many, sometimes > 10 galaxy scale lengths) as can be probed by gaseous tracers^1,2 or by the orbits of satellite galaxies⁹. Outside a galaxy’s optical radius, the gravitational acceleration is a_N = GM/R² = V²/R so one expects V(R) ~ R^-1/2. This Keplerian behaviour is not observed in galaxies.

One approach to this problem is to increase M in the outer parts of galaxies in order to provide the extra gravitational acceleration necessary to keep the rotation curves flat. Indeed, this is the only option within the framework of Newtonian gravity since both V and R are directly measured. The additional mass must be invisible, dominant, and extend well beyond the optical edge of the galaxies.

Postulating the existence of this large amount of dark matter which reveals itself only by its gravitational effects is a radical hypothesis. Yet the kinematic data force it upon us, so much so that the existence of dark matter is generally accepted. Enormous effort has gone into attempting to theoretically predict its nature and experimentally verify its existence, but to date there exists no convincing detection of any hypothesised dark matter candidate, and many plausible candidates have been ruled out¹⁰.

Another possible solution is to alter the fundamental equation a_N = GM/R². Our faith in this simple equation is very well founded on extensive experimental tests of Newtonian gravity. Since it is so fundamental, altering it is an even more radical hypothesis than invoking the existence of large amounts of dark matter of completely unknown constituent components. However, a radical solution is required either way, so both possibilities must be considered and tested.

A phenomenological theory specifically introduced to address the problem of the flat rotation curves is MOND³. It has no other motivation and so far there is no firm physical basis for the theory. It provides no satisfactory cosmology, having yet to be reconciled with General Relativity. However, with the introduction of one new fundamental constant (an acceleration a₀), it is empirically quite successful in fitting galaxy rotation curves^11-14. It hypothesises that for accelerations a < a₀ = 1.2 x 10^-10 m s^-2, the effective acceleration is given by a_eff = (a_N a₀)^1/2. This simple prescription works well with essentially only one free parameter per galaxy, the stellar mass to light ratio, which is subject to independent constraint by stellar evolution theory. More importantly, MOND makes predictions which are distinct and testable. One specific prediction⁴ is that the asymptotic (flat) value of the rotation velocity, V_a, is V_a = (GMa₀)^1/4. Note that V_a does not depend on R, but only on M in the regime of small accelerations (a < a₀).

In contrast, Newtonian gravity depends on both M and R. Replacing R with a mass surface density variable S = M(R)/R², the Newtonian prediction becomes M S ~ V_a⁴ which contrasts with the MOND prediction M ~ V_a⁴. These relations are the theoretical basis in each case for the observed luminosity-linewidth relation L ~ V_a⁴ (better known as the Tully-Fisher¹⁵ relation. Note that the observed value of the exponent is bandpass dependent, but does obtain the theoretical value of 4 in the near infrared¹⁶ which is considered the best indicator of the stellar mass. The systematic variation with bandpass is a very small effect compared to the difference between the two gravitational theories, and must be attributed to dust or stars under either theory.) To transform from theory to observation one requires the mass to light ratio Y: Y = M/L = S/s, where s is the surface brightness. Note that in the purely Newtonian case, M and L are very different functions of R, so Y is itself a strong function of R. We define Y to be the mass to light ratio within the optical radius R_*, as this is the only radius which can be measured by observation. The global mass to light ratio would be very different (since M ~ R for R > R_*, the total masses of dark haloes are not measurable), but the particular choice of definition does not affect the relevant functional dependences is all that matters. The predictions become Y²sL ~ V_a⁴ for Newtonian gravity^8,16 and YL ~ V_a⁴ for MOND⁴.

The only sensible¹⁷ null hypothesis that can be constructed is that the mass to light ratio be roughly constant from galaxy to galaxy. Clearly distinct predictions thus emerge if galaxies of different surface brightnesses s are examined. In the Newtonian case there should be a family of parallel Tully-Fisher relations for each surface brightness. In the case of MOND, all galaxies should follow the same Tully-Fisher relation irrespective of surface brightness.

Recently it has been shown that extreme objects such as low surface brightness galaxies^8,18 (those with central surface brightnesses fainter than s₀ = 23 B mag./[] corresponding 40 L_☉ pc^-2) obey the same Tully-Fisher relation as do the high surface brightness galaxies (typically with s₀ = 21.65 B mag./[] or 140 L_☉ pc^-2) which originally¹⁵ defined it. Fig. 1 shows the luminosity-linewidth plane for galaxies ranging over a factor of 40 in surface brightness. Regardless of surface brightness, galaxies fall on the same Tully-Fisher relation.

The luminosity-linewidth (Tully-Fisher) relation for spiral galaxies over a large range in surface brightness. The B-band relation is shown; the same result is obtained in all bands^8,18. Absolute magnitudes are measured from apparent magnitudes assuming H₀ = 75 km/s/Mpc. Rotation velocities V_a are directly proportional to observed 21 cm linewidths (measured as the full width at 20% of maximum) W₂₀ corrected for inclination [sin^-1(i)]. Open symbols are an independent sample which defines⁴² the Tully-Fisher relation (solid line). The dotted lines show the expected shift of the Tully-Fisher relation for each step in surface brightness away from the canonical value s₀ = 21.5 if the mass to light ratio remains constant. Low surface brightness galaxies are plotted as solid symbols, binned by surface brightness: red triangles: 22 < s₀ < 23; green squares: 23 < s₀ < 24; blue circles: s₀ > 24. One galaxy with two independent measurements is connected by a line. This gives an indication of the typical uncertainty which is sufficient to explain nearly all the scatter. Contrary to the clear expectation of a readily detectable shift as indicated by the dotted lines, galaxies fall on the same Tully-Fisher relation regardless of surface brightness, as predicted by MOND.

MOND predicts this behaviour in spite of the very different surface densities of low surface brightness galaxies. In order to understand this observational fact in the framework of standard Newtonian gravity requires a subtle relation⁸ between surface brightness and the mass to light ratio to keep the product sY² constant. If we retain normal gravity and the dark matter hypothesis, this result is unavoidable, and the null hypothesis of similar mass to light ratios (which, together with an assumed constancy of surface brightness, is usually invoked to explain the Tully-Fisher relation) is strongly rejected. Instead, the current epoch surface brightness is tightly correlated with the properties of the dark matter halo, placing strict constraints on models of galaxy formation and evolution.

The mass to light ratios computed for both cases are shown as a function of surface brightness in Fig. 2. Fig. 2 is based solely on galaxies with full rotation curves^19,20 and surface photometry, so V_a and R_* are directly measured. The correlation in the Newtonian case is very clear (Fig. 2a), confirming our inference⁸ from the Tully-Fisher relation. Such tight correlations are very rare in extragalactic astronomy, and the Y-s relation is probably the real cause of an inferred Y-L relation. The latter is much weaker because surface brightness and luminosity are only weakly correlated^21-24.

The mass to light ratio Y (in M_☉/L_☉) determined with (a) Newtonian dynamics and (b) MOND, plotted as a function of central surface brightness. The mass determination for Newtonian dynamics is M = V² R_*/G and for MOND is M = V⁴/(G a₀). We have adopted as a consistent definition of the optical radius R_* four scale lengths of the exponential optical disc. This is where discs tend to have edges, and contains essentially all the light^21,22. The definition of R_* makes a tremendous difference to the absolute value of the mass to light ratio in the Newtonian case, but makes no difference at all to the functional relation will be present regardless of the precise definition. These mass measurements are more sensitive to the inclination corrections than is the Tully-Fisher relation since there is a sin^-2(i) term in the Newtonian case and one of sin^-4(i) for MOND. It is thus very important that the inclination be accurately measured, and we have retained only galaxies which have adequate inclination determinations — error bars are plotted for a nominal uncertainty of 6 degrees. The sensitivity to inclination manifests itself as an increase in the scatter from (a) to (b). The derived mass is also very sensitive to the measured value of the asymptotic velocity itself, so we have used only those galaxies for which this can be taken directly from a full rotation curve^19,20,42. We do not employ profile widths; the velocity measurements here are independent of those in Fig. 1. In both cases, we have subtracted off the known atomic gas mass^19,20,42, so what remains is essentially only the stars and any dark matter that may exist. A very strong correlation (regression coefficient = 0.85) is apparent in (a): this is the mass to light ratio — surface brightness conspiracy. The slope is consistent (within the errors) with the theoretical expectation s ~ Y^-2 derived from the Tully-Fisher relation⁸. At the highest surface brightnesses, the mass to light ratio is similar to that expected for the stellar population. At the faintest surface brightnesses, it has increased by a factor of nearly ten, indicating increasing dark matter domination within the optical disc as surface brightness decreases or a very systematic change in the stellar population, or both. In (b), the mass to light ratio scatters about a constant value of 2. This mean value, and the lack of a trend, is what is expected for stellar populations^17,21-24.

The Y-s relation is not predicted by any dark matter theory^25,26. It can not be purely an effect of the stellar mass to light ratio, since no other stellar population indicator such as color^21-24 or metallicity^27,28 is so tightly correlated with surface brightness. In principle it could be an effect of the stellar mass fraction, as the gas mass to light ratio follows a relation very similar to that of total mass to light ratio²⁰. We correct for this in Fig. 2 by subtracting the known atomic gas mass so that Y refers only to the stars and any dark matter. We do not correct for molecular gas, as this has never been detected in low surface brightness galaxies to rather sensitive limits³⁰ so the total mass of such gas is unimportant if current estimates³¹ of the variation of the CO to H₂ conversion factor with metallicity are correct. These corrections have no discernible effect at all in Fig. 2 because the dark mass is totally dominant. It is thus very hard to see how any evolutionary effect in the luminous matter can be relevant.

In the case of MOND, the mass to light ratio directly reflects that of the stellar population once the correction for gas mass fraction is made. There is no trend of Y* with surface brightness (Fig. 2b), a more natural result and one which is consistent with our studies of the stellar populations of low surface brightness galaxies^21-23. These suggest that Y* should be roughly constant or slightly declining as surface brightness decreases, with much scatter. The mean value Y* = 2 is also expected from stellar evolutionary theory¹⁷, which always gives a number 0 < Y* < 10 and usually gives 0.5 < Y* < 3 for disk galaxies. This is particularly striking since Y* is the only free parameter allowed to MOND, and the observed mean is very close to that directly observed²⁹ in the Milky Way (1.7 ± 0.5 M_☉/L_☉).

The essence of the problem is illustrated by Fig. 3, which shows the rotation curves of two galaxies of essentially the same luminosity but vastly different surface brightnesses. Though the asymptotic velocities are the same (as required by the Tully-Fisher relation), the rotation curve of the low surface brightness galaxy rises less quickly than that of the high surface brightness galaxy as expected if the mass is distributed like the light. Indeed, the ratio of surface brightnesses is correct to explain the ratio of velocities at small radii if both galaxies have similar mass to light ratios. However, if this continues to be the case as R increases, the low surface brightness galaxy should reach a lower asymptotic velocity simply because R_* must be larger for the same L. That this does not occur is the problem, and poses very significant systematic constraints on the dark matter distribution.

The rotation curves of two galaxies, one of high surface brightness¹¹ (NGC 2403; open circles) and one of low surface brightness¹⁹ (UGC 128; filled circles). The two galaxies have very nearly the same asymptotic velocity, and hence luminosity, as required by the Tully-Fisher relation. However, they have central surface brightnesses which differ by a factor of 13. The lines give the contributions to the rotation curves of the various components. Green: luminous disk. Blue: dark matter halo. Red: luminous disk (stars and gas) with MOND. Solid lines refer to NGC 2403 and dotted lines to UGC 128. The fits for NGC 2403 are taken from ref. 11, for which the stars have Y* = 1.5 *M_☉/L_☉*. For UGC 128, no specific fit is made: the blue and green dotted lines are simply the NGC 2403 fits scaled by the ratio of disk scale lengths h. This provides a remarkably good description of the UGC 128 rotation curve and illustrates one possible manifestation of the fine tuning problem: if disks have similar Y, the halo parameters p₀ and R₀ must scale with the disk parameters s₀ and h while conspiring to keep the product *p₀ R₀*² fixed at any given luminosity. Note also that the halo of NGC 2403 gives an adequate fit to the rotation curve of UGC 128. This is another possible manifestation of the fine tuning problem: all galaxies of the same luminosity have the same halo, with Y systematically varying with s₀ so that Y* goes to zero as s₀ goes to zero. Neither of these is exactly correct because the contribution of the gas can not be set to zero as is mathematically possible with the stars. This causes the resulting fin tuning problems to be even more complex, involving more parameters. Alternatively, the green dotted line is the rotation curve expected by MOND for a galaxy with the observed luminous mass distribution of UGC 128.

Satisfying the Tully-Fisher relation has led to some expectation that haloes all have the same density structure. This simplest possibility is immediately ruled out. In order to obtain L ~ V_a⁴ ~ MS, one might suppose that the mass surface density S is constant from galaxy to galaxy, irrespective of the luminous surface density s. This achieves the correct asymptotic velocity V_a, but requires that the mass distribution, and hence the complete rotation curve, be essentially identical for all galaxies of the same luminosity. This is obviously not the case (Fig. 3), as the rotation curves of lower surface brightness galaxies rise much more gradually than those of higher surface brightness galaxies (also a prediction⁴ of MOND). It might be possible to have approximately constant density haloes if the highest surface brightness disks are maximal and the lowest minimal in their contribution to the inner parts of the rotation curves, but this then requires fine tuning of Y* with this systematically decreasing with surface brightness.

The expected form of the halo mass distribution depends on the dominant form of dark matter. This could exist in three general categories: baryonic (e.g., MACHOs), hot (e.g., neutrinos), and cold exotic particles (e.g., WIMPs). The first two make no specific predictions. Baryonic dark matter candidates are most subject to direct detection, and most plausible candidates have been ruled out¹⁰ with remaining suggestions of necessity sounding increasingly contrived³². Hot dark matter is not relevant to the present problem. Even if neutrinos have a small mass, their velocities considerably exceed the escape velocities of the haloes of low mass galaxies where the problem is most severe. Cosmological simulations involving exotic cold dark matter^33,34 have advanced to the point where predictions are being made about the density structure of haloes. These take the form^33,34 p(R) = p_H/[R(R+R_H)^b] where p_H characterises the halo density and R_H its radius, with b ~ 2 to 3. The characteristic density depends on the mean density of the universe at the collapse epoch, and is generally expected to be greater for lower mass galaxies since these collapse first in such scenarios. This goes in the opposite sense of the observations, which show that low mass and low surface brightness galaxies are less, not more, dense. The observed behaviour is actually expected in scenarios which do not smooth on a particular mass scale and hence allow galaxies of the same mass to collapse at a variety of epochs²⁵, but in this case the Tully-Fisher relation should not be universal. Worse, note that at small R < R_H, p(R) ~ R^-1. It has already been noted^32,35 that such a steep interior density distribution is completely inconsistent with the few (4) analysed observations of dwarf galaxies. Our data^19,20 confirm and considerably extend this conclusion for 24 low surface brightness galaxies over a wide range in luminosity.

The failure of the predicted exotic cold dark matter density distribution either rules out this form of dark matter, indicates some failing in the simulations (in spite of wide-spread consensus), or requires some mechanism to redistribute the mass. Feedback from star formation is usually invoked for the last of these, but this can not work for two reasons. First, an objection in principle: a small mass of stars and gas must have a dramatic impact on the distribution of the dominant dark mass, with which they can only interact gravitationally. More mass redistribution is required in less luminous galaxies since they start out denser but end up more diffuse; of course progressively less baryonic material is available to bring this about as luminosity declines. Second, an empirical objection: in this scenario, galaxies explode and gas is lost. However, progressively fainter and lower surface brightness galaxies, which need to suffer more severe explosions, are actually very gas rich.

Observationally, dark matter haloes are inferred to have density distributions^1,2,11 with constant density cores, p(R) = p₀/[1 + (R/R₀)^g]. Here, p₀ is the core density and R₀ is the core size with g ~ 2 being required to produce flat rotation curves. For g = 2, the rotation curve resulting from this mass distribution is V(R) = V_a [1-(R₀/R) tan^-1({R/R₀)]^1/2 where the asymptotic velocity is V_a = (4πG p₀ R₀²)^1/2. To satisfy the Tully-Fisher relation, V_a, and hence the product p₀ R₀², must be the same for all galaxies of the same luminosity. To decrease the rate of rise of the rotation curves as surface brightness decreases, R₀ must increase. Together, these two require a fine tuning conspiracy to keep the product p₀ R₀² constant while R₀ must vary with the surface brightness at a given luminosity. Luminosity and surface brightness themselves are only weakly correlated, so there exists a wide range in one parameter at any fixed value of the other. Thus the structural properties of the invisible dark matter halo dictate those of the luminous disk, or vice versa. So, s and L give the essential information about the mass distribution without recourse to kinematic information.

A strict s-p₀-R₀ relation is rigorously obeyed only if the haloes are spherical and dominate throughout. This is probably a good approximation for low surface brightness galaxies but may not be for the those of the highest surface brightness. However, a significant non-halo contribution can at best replace one fine tuning problem with another (e.g., surface brightness being strongly correlated with the stellar population mass to light ratio instead of halo core density) and generally causes additional conspiracies.

There are two perspectives for interpreting these relations, with the preferred perspective depending strongly on the philosophical attitude one has towards empirical and theoretical knowledge. One view is that these are real relations which galaxies and their haloes obey. As such, they provide a positive link between models of galaxy formation and evolution and reality.

The other view is that this list of fine tuning requirements makes it rather unattractive to maintain the dark matter hypothesis. MOND provides an empirically more natural explanation for these observations. In addition to the Tully-Fisher relation, MOND correctly predicts the systematics of the shapes of the rotation curves of low surface brightness galaxies^19,20 and fits the specific case of UGC 128 (Fig. 3). Low surface brightness galaxies were stipulated⁴ to be a stringent test of the theory because they should be well into the regime a < a₀. This is now observed to be true, and to the limit of observational accuracy the predictions of MOND are confirmed. The critical acceleration scale a₀ is apparently universal, so there is a single force law acting in galactic disks for which MOND provides the correct description. The cause of this could be either a particular dark matter distribution³⁶ or a real modification of gravity. The former is difficult to arrange, and a single force law strongly supports the latter hypothesis since in principle the dark matter could have any number of distributions which would give rise to a variety of effective force laws. Even if MOND is not correct, it is essential to understand why it so closely describe the observations. Though the data can not exclude Newtonian dynamics, with a working empirical alternative (really an extension) at hand, we would not hesitate to reject as incomplete any less venerable hypothesis.

Nevertheless, MOND itself remains incomplete as a theory, being more of a Kepler’s Law for galaxies. It provides only an empirical description of kinematic data. While successful for disk galaxies, it was thought to fail in clusters of galaxies³⁷. Recently it has been recognized that there exist two missing mass problems in galaxy clusters, one of which is now solved³⁸: most of the luminous matter is in X-ray gas, not galaxies. This vastly improves the consistency of MOND with with cluster dynamics³⁹. The problem with the theory remains a reconciliation with Relativity and thereby standard cosmology (which is itself in considerable difficulty^38,40), and a lack of any prediction about gravitational lensing⁴¹. These are theoretical problems which need to be more widely addressed in light of MOND’s empirical success.

ACKNOWLEDGEMENTS. We thank R. Sanders and M. Milgrom for clarifying aspects of a theory with which we were previously unfamiliar. SSM is grateful to the Kapteyn Astronomical Institute for enormous hospitality during visits when much of this work was done. [Note added in 2020: this work was supported by a cooperative grant funded by the EU and would no longer be possible thanks to Brexit.]

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Oh… you don’t want to look in there

This post is a recent conversation with David Garofalo for his blog.

Today we talk to Dr. Stacy McGaugh, Chair of the Astronomy Department at Case Western Reserve University.

David: Hi Stacy. You had set out to disprove MOND and instead found evidence to support it. That sounds like the poster child for how science works. Was praise forthcoming?

Stacy: In the late 1980s and into the 1990s, I set out to try to understand low surface brightness galaxies. These are diffuse systems of stars and gas that rotate like the familiar bright spirals, but whose stars are much more spread out. Why? How did these things come to be? Why were they different from brighter galaxies? How could we explain their properties? These were the problems I started out working on that inadvertently set me on a collision course with MOND.

I did not set out to prove or disprove either MOND or dark matter. I was not really even aware of MOND at that time. I had head of it only on a couple of occasions, but I hadn’t payed any attention, and didn’t really know anything about it. Why would I bother? It was already well established that there had to be dark matter.

I worked to develop our understanding of low surface brightness galaxies in the context of dark matter. Their blue colors, low metallicities, high gas fractions, and overall diffuse nature could be explained if they had formed in dark matter halos that are themselves lower than average density: they occupy the low concentration side of the distribution of dark matter halos at a given mass. I found this interpretation quite satisfactory, so gave me no cause to doubt dark matter to that point.

This picture made two genuine predictions that had yet to be tested. First, low surface brightness galaxies should be less strongly clustered than brighter galaxies. Second, having their mass spread over a larger area, they should shift off of the Tully-Fisher relation defined by denser galaxies. The first prediction came true, and for a period I was jubilant that we had made an important new contribution to out understanding of both galaxies and dark matter. The second prediction failed badly: low surface brightness galaxies adhere to the same Tully-Fisher relation that other galaxies follow.

I tried desperately to understand the failure of the second prediction in terms of dark matter. I tried what seemed like a thousand ways to explain this, but ultimately they were all tautological: I could only explain it if I assumed the answer from the start. The adherence of low surface brightness galaxies to the Tully-Fisher relation poses a serious fine-tuning problem: the distribution of dark matter must be adjusted to exactly counterbalance that of the visible matter so as not to leave any residuals. This makes no sense, and anyone who claims it does is not thinking clearly.

It was in this crisis of comprehension in which I became aware that MOND predicted exactly what I was seeing. No fine-tuning was required. Low surface brightness galaxies followed the same Tully-Fisher relation as other galaxies because the modified force law stipulates that they must. It was only at this point (in the mid-’90s) at which I started to take MOND seriously. If it had got this prediction right, what else did it predict?

I was still convinced that the right answer had to be dark matter. There was, after all, so much evidence for it. So this one prediction must be a fluke; surely it would fail the next test. That was not what happened: MOND passed test after test after test, successfully predicting observations both basic and detailed that dark matter theory got wrong or did not even address. It was only after this experience that I realized that what I thought was evidence for dark matter was really just evidence that something was wrong: the data cannot be explained with ordinary gravity without invisible mass. The data – and here I mean ALL the data – were mostly ambiguous: they did not clearly distinguish whether the problem was with mass we couldn’t see or with the underlying equations from which we inferred the need for dark matter.

So to get back to your original question, yes – this is how science should work. I hadn’t set out to test MOND, but I had inadvertently performed exactly the right experiment for that purpose. MOND had its predictions come true where the predictions of other theories did not: both my own theory and those of others who were working in the context of dark matter. We got it wrong while MOND got it right. That led me to change my mind: I had been wrong to be sure the answer had to be dark matter, and to be so quick to dismiss MOND. Admitting this was the most difficult struggle I ever faced in my career.

David: From the perspective of dark matter, how does one understand MOND’s success?

Stacy: One does not.

That the predictions of MOND should come true in a universe dominated by dark matter makes no sense.

Before I became aware of MOND, I spent lots of time trying to come up with dark matter-based explanations for what I was seeing. It didn’t work. Since then, I have continued to search for a viable explanation with dark matter. I have not been successful. Others have claimed such success, but whenever I look at their work, it always seems that what they assert to be a great success is just a specific elaboration of a model I had already considered and rejected as obviously unworkable. The difference boils down to Occam’s razor. If you give dark matter theory enough free parameters, it can be adjusted to “predict” pretty much anything. But the best we can hope to do with dark matter theory is to retroactively explain what MOND successfully predicted in advance. Why should we be impressed by that?

David: Does MOND fail in clusters?

Stacy: Yes and no: there are multiple tests in clusters. MOND passes some and flunks others – as does dark matter.

The most famous test is the baryon fraction. This should be one in MOND – all the mass is normal baryonic matter. With dark matter, it should be the cosmic ratio of normal to dark matter (about 1:5).

MOND fails this test: it explains most of the discrepancy in clusters, but not all of it. The dark matter picture does somewhat better here, as the baryon fraction is close to the cosmic expectation — at least for the richest clusters of galaxies. In smaller clusters and groups of galaxies, the normal matter content falls short of the cosmic value. So both theories suffer a “missing baryon” problem: MOND in rich clusters; dark matter in everything smaller.

Another test is the mass-temperature relation. Both theories predict a relation between the mass of a cluster and the temperature of the gas it contains, but they predict different slopes for this relation. MOND gets the slope right but the amplitude wrong, leading to the missing baryon problem above. Dark matter gets the amplitude right for the most massive clusters, but gets the slope wrong – which leads to it having a missing baryon problem for systems smaller than the largest clusters.

There are other tests. Clusters continue to merge; the collision velocity of merging clusters is predicted to be higher in MOND than with dark matter. For example, the famous bullet cluster, which is often cited as a contradiction to MOND, has a collision speed that is practically impossible with dark matter: there just isn’t enough time for the two components of the bullet to accelerate up to the observed relative speed if they fall together under the influence of normal gravity and the required amount of dark mass. People have argued over the severity of this perplexing problem, but the high collision speed happens quite naturally in MOND as a consequence of its greater effective force of attraction. So, taken at face value, the bullet cluster both confirms and refutes both theories!

I could go on… one expects clusters to form earlier and become more massive in MOND than in dark matter. There are some indications that this is the case – the highest redshift clusters came as a surprise to conventional structure formation theory – but the relative numbers of clusters as a function of mass seems to agree well with current expectations with dark matter. So clusters are a mixed bag.

More generally, there is a widespread myth that MOND fits rotation curves, but gets nothing else right. This is what I expected to find when I started fact checking, but the opposite is true. MOND explains a huge variety of data well. The presumptive superiority of dark matter is just that – a presumption.

David: At a physics colloquium two decades ago, Vera Rubin described how theorists were willing and eager to explain her data to her. At an astronomy colloquium a few years later, you echoed that sentiment in relation to your data on velocity curves. One concludes that theorists are uniquely insightful and generous people. Is there anyone you would like to thank for putting you straight?

Stacy: So they perceive themselves to be.

MOND has made many successful a priori predictions. This is the golden standard of the scientific method. If there is another explanation for it, I’d like to know what it is.

As your questions supposes, many theorists have offered such explanations. At most one of them can be correct. I have yet to hear a satisfactory explanation.

David: What are MOND people working on these days?

Stacy: Any problem that is interesting in extragalactic astronomy is interesting in the context of MOND. Outstanding questions include planes of satellite dwarf galaxies, clusters of galaxies, the formation of large scale structure, and the microwave background. MOND-specific topics include the precise value of the MOND acceleration constant, predicting the velocity dispersions of dwarf galaxies, and the search for the predicted external field effect, which is a unique signature of MOND.

The phrasing of this question raises a sociological issue. I don’t know what a “MOND person” is. Before now, I have only heard it used as a pejorative.

I am a scientist who has worked on many topics. MOND is just one of them. Does that make me a “MOND person”? I have also worked on dark matter, so am I also a “dark matter person”? Are these mutually exclusive?

I have attended conferences where I have heard people say ‘“MOND people” do this’ or ‘“MOND people” fail to do that.’ Never does the speaker of these words specify who they’re talking about: “MOND people” are a nameless Other. In all cases, I am more familiar with the people and the research they pretend to describe, but in no way do I recognize what they’re talking about. It is just a way of saying “Those People” are Bad.

There are many experts on dark matter in the world. I am one of them. There are rather fewer experts on MOND. I am also one of them. Every one of these “MOND people” is also an expert on dark matter. This situation is not reciprocated: many experts on dark matter are shockingly ignorant about MOND. I was once guilty of that myself, but realized that ignorance is not a sound basis on which to base a scientific judgement.

David: Are you tired of getting these types of questions?

Stacy: Yes and no.

No, in that these are interesting questions about fundamental science. That is always fun to talk about.

Yes, in that I find myself having the same arguments over and over again, usually with scientists who remain trapped in the misconceptions I suffered myself a quarter century ago, but whose minds are closed to ideas that threaten their sacred cows. If dark matter is a real, physical substance, then show me a piece already.

Cosmology, then and now

I have been busy teaching cosmology this semester. When I started on the faculty of the University of Maryland in 1998, there was no advanced course on the subject. This seemed like an obvious hole to fill, so I developed one. I remember with fond bemusement the senior faculty, many of them planetary scientists, sending Mike A’Hearn as a stately ambassador to politely inquire if cosmology had evolved beyond a dodgy subject and was now rigorous enough to be worthy of a 3 credit graduate course.

Back then, we used transparencies or wrote on the board. It was novel to have a course web page. I still have those notes, and marvel at the breadth and depth of work performed by my younger self. Now that I’m teaching it for the first time in a decade, I find it challenging to keep up. Everything has to be adapted to an electronic format, and be delivered remotely during this damnable pandemic. It is a less satisfactory experience, and it has precluded posting much here.

Another thing I notice is that attitudes have evolved along with the subject. The baseline cosmology, LCDM, has not changed much. We’ve tilted the power spectrum and spiked it with extra baryons, but the basic picture is that which emerged from the application of classical observational cosmology – measurements of the Hubble constant, the mass density, the ages of the oldest stars, the abundances of the light elements, number counts of faint galaxies, and a wealth of other observational constraints built up over decades of effort. Here is an example of combining such constraints, and exercise I have students do every time I teach the course:

Observational constraints in the mass density-Hubble constant plane assembled by students in my cosmology course in 2002. The gray area is excluded. The open window is the only space allowed; this is LCDM. The box represents the first WMAP estimate in 2003. CMB estimates have subsequently migrated out of the allowed region to lower H0 and higher mass density, but the other constraints have not changed much, most famously H0, which remains entrenched in the low to mid-70s.

These things were known by the mid-90s. Nowadays, people seem to think Type Ia SN discovered Lambda, when really they were just icing on a cake that was already baked. The location of the first peak in the acoustic power spectrum of the microwave background was corroborative of the flat geometry required by the picture that had developed, but trailed the development of LCDM rather than informing its construction. But students entering the field now seem to have been given the impression that these were the only observations that mattered.

Worse, they seem to think these things are Known, as if there’s never been a time that we cosmologists have been sure about something only to find later that we had it quite wrong. This attitude is deleterious to the progress of science, as it precludes us from seeing important clues when they fail to conform to our preconceptions. To give one recent example, everyone seems to have decided that the EDGES observation of 21 cm absorption during the dark ages is wrong. The reason? Because it is impossible in LCDM. There are technical reasons why it might be wrong, but these are subsidiary to Attitude: we can’t believe it’s true, so we don’t. But that’s what makes a result important: something that makes us reexamine how we perceive the universe. If we’re unwilling to do that, we’re no longer doing science.

The Other

I am a white American male. As such, I realize that there is no way for me to grasp and viscerally appreciate all the ways in which racism afflicts black Americans. Or, for that matter, all the ways in which sexism afflicts women. But I can acknowledge that these things exist. I can recognize when it happens. I’ve seen it happen to others, both friends and strangers. I can try not to be part of the problem.

It isn’t just black and white or male and female. There are so many other ways in which we classify and mistreat each other. Black Americans were enslaved; Native Americans were largely eradicated. It is easy to think of still more examples – religious heretics, colonized peoples, members of the LGBT community – anything that sets one apart as the Other. Being the Other makes one less than human and more akin to vermin that should be controlled or exterminated: clearly the attitude taken by Nazis towards Jews in occupied Europe.

When I was a child, my family moved around a lot. [It doesn’t matter why; there was no good reason.] We moved every other year. I was born in Oklahoma, but my only memory of it is from visiting relatives later: we moved to central Illinois when I was still a baby. We lived in a series of small towns – Decatur, Sullivan, made a brief detour to Escondido, California, then back to Shelbyville. My earliest memories are of the rich smell of the fertile Illinois landscape coming to life in springtime as my consciousness dawned in a beautiful wooded landscape about which I was infinitely curious. The shady forests and little creeks were as much my classrooms as the brick schoolhouses inhabited by teachers, friends, and bullies.

I was painfully, cripplingly shy as a child. It took a year to start to make new friends, and another to establish them. Then we would move away.

Bullies came more quickly than friends. Every bully wants to pick on others, but especially if they are different – the Other. I was different in so many ways. I was from somewhere else, an alien immigrant to each parochial little town. I was small for my age and young for my grade, having skipped first grade. I was an egghead, a nerd in a time where the only thing society seemed to value was size and strength. Worst of all, I did not attend the same little church that they did, so I was going to hell, and many illiterate bible-thumping bullies seemed to take it as their religious duty to speed me on my way.

When I was 13, we moved to Flint, Michigan. We went from a tiny farm town to an urban industrial area the epitomizes “rust belt.” I could no longer see the stars at night because the sky was pink – a lurid, poisonous pink – from the lights of the nearby AC Spark Plugs factory (then an active facility in which I briefly worked; now a vast empty slab of concrete). I still wandered in the limited little woods wedged between the freeway and a golf course, but the creek there ran thick with the sheen of petrochemical runoff.

I became a part of the 1970s effort at desegregation. The white religious bigot bullies were replaced with black ghetto bullies. Some seemed to think it to be their duty to return the shit white people had given them by being shitty to white people whenever they could. I didn’t really get that at the time. To me, they were just bullies. Same old, same old. Their hatred for the Other was palpably the same.

But you know what? Most people aren’t bullies. Bullies are just the first in line to greet Others onto whom they hope to unload their own self-loathing. Given time, I met better people in each and every place I lived. And what I found, over and over again, is that people are people. There are craven, nasty people and their are extraordinary, wonderful people, and everything else you can imagine in between. I’ve lived in all-white neighborhoods and mostly black neighborhoods and pretty well integrated neighborhoods. I’ve seen differences in culture but zero evidence that one race is better or worse or even meaningfully different from the other. Both have a tendency to mistrust the Other that seems deeply ingrained in human nature. We aren’t quite human to each other until we’re personally known. Once you meet the Other, they cease to be the Other and become an individual with a name and a personality. I suspect that’s what people mean when they claim not to see color – it’s not that they cease to see it, but for the people they’ve actually met, it ceases to be their defining characteristic.

And yet we persist in making implicitly racist assumptions. To give just one tiny example, a few years back a friend was helping to organize the Larchmere Porch Fest, and asked my wife and I to help. This is a wonderful event in which people in the Larchmere neighborhood offer their porches as stages for musical performances. One can wander up and down and hear all manner of music. On this occasion, I wound up helping to set up one porch for a performance by Obnox. I realized that some electricity would be needed, so knocked on the homeowner’s door. A woman appeared, and after a brief discussion, she provided an extension cord with a pink, Barbie-themed power strip that we threaded through an open window. Lamont Thomas and his drummer arrived, and set up went fairly smoothly, but he thought of something else, so also knocked on the door. I don’t remember what he was looking for, but I remember the reaction of the woman upon opening the door. Lamont is a tall, imposing black man. Her eyes got as big as saucers. She closed the door without a word. We heard the -snick- of the lock and her retreating footsteps. Lamont looked at the door that had been shut in his face, then looked at me and spoke softly: “My lyrics are kinda… raw. Is that going to be a problem?” I could only shrug. “She signed up for this,” I replied.

I don’t know what went through her mind. I would guess that like a lot of white people in the U.S., she had conveniently forgotten that black people exist – or at least, weren’t a presence in her regular circle of life. So when she chose to participate in a positive civic activity, in this case porch fest, it simply hadn’t occurred to her that black people might be involved. Who would have guessed that some musicians might be black!

That episode is but one tiny example of the pervasive, reflexive fear of the Other that still pervades American culture. More generally, I marvel at the human potential that we must have wasted in this way. The persecution of minorities, both ethnic and religious, the suppression of novel thought outside the mainstream, the utter disregard for women in far too many societies… For every Newton, for every Einstein, for every brilliant person who became famous for making a positive impact on the world, how many comparably brilliant people found themselves in circumstances that prevented them from making the contributions that they might otherwise have made? Einstein happened to be visiting the U.S. when Hitler came to power, and wisely declined to return home to Germany. He was already famous, so it was possible to financially arrange to keep him on. How might it have gone if the timing were otherwise? How many were less fortunate? What have we lost? Why do we continue to throw away so much human potential?

Obnox_larchmere2014 — Obnox performs at the Larchmere Porch Fest in 2014.

Black Lives Matter

I started this blog as a place to discuss science, and have refrained from discussing overtly political matters. This is no longer possible. Today is June 10, 2020 – the date set to strike for black lives. I want to contribute in a tiny way by writing here. If that seems inappropriate to you or otherwise makes you uncomfortable, then that probably means that you need to read it and reflect on the reasons for your discomfort.

To start, I quote the statement made by my colleagues and myself:

The CWRU Department of Astronomy stands in solidarity with our Black colleagues and fellow citizens across the United States in expressing what should be a clear moral absolute: that people of color should enjoy the same freedoms as other Americans to life, liberty, and the pursuit of happiness. We condemn the de facto system of racial oppression that leads to pervasive police brutality up to and including the extrajudicial murders of Black Americans like George Floyd and far too many others.

We strive to build an academic community that welcomes, encourages, and supports students and scientists of color. To achieve this goal, we recognize that we must continually reflect on the injustices faced by under-represented and marginalized people, and repair the institutional structures that place them at a disadvantage. We encourage our colleagues in astronomy, throughout academia, and more broadly across society to do the same.

We will participate in the Strike for Black Lives this Wednesday, June 10, and encourage others to join us.

As the current chairperson of the CWRU Department of Astronomy, I was initially reluctant to post something about the Black Lives Matter movement on the department website. It is a different thing to make a statement on behalf of an organization of many people than it is to do so for oneself. Moreover, we are a science entity, not a political one. But we are also people, and cannot separate our humanity from our vocation. There comes a point when way too much is ever so much more than more than enough. We have reached such a point. So when I contacted my colleagues about doing this, there was unanimous agreement and eager consent to do so among all the faculty and scientific staff.

I value the freedom of speech enshrined in the first amendment of the constitution of the United States of America. I think it is worth reproducing here:

Amendment I

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.

Freedom of speech is often construed to mean the right to espouse whatever opinion one might hold, and I think that is indeed an essential personal freedom that Americans take for granted in a way that is rather special in the history of humankind. Note also that the first amendment explicitly includes “the right of the people peaceably to assemble” – a right that Americans sometimes exercise but also frequently attempt to deny to each other.

Why does this come up now? Well, if you haven’t been keeping up with current events, George Floyd died in custody after being arrested in Minneapolis, sparking protests – peaceable assemblages – across the country and around the world.

In the last sentence, I intentionally use a misleading structure common in both the press and in police reports: “George Floyd died…”, as if it were something that just happened, like a butterfly happening to pass by. Indeed, the initial police report on the incident stated that Floyd “seemed to be in medical distress” while omitting mention of any causal factor for that distress. Similarly, the medical examiner’s report exonerated the police, attributing Floyd’s death to “underlying medical conditions.”

That is some major league bullshit.

The cause of Floyd’s death is not mysterious. Officer Derek Chauvin crushed Floyd’s windpipe by kneeling on his neck for eight minutes and forty six seconds. That is considerably longer than the longest TV commercial break you have ever been modestly annoyed by. Who among us has never raged WILL THESE COMMERCIALS NEVER END? Now imagine feeling the life being crushed out of you for a considerably longer period while lying flat on your belly with your hands already cuffed behind your back. That’s right – George Floyd was already handcuffed and on the ground while being pinned by the neck. In no way can this be construed as resisting arrest. He was already under police control and in no position to resist anything, up to and including being murdered.

A more accurate statement using the active voice would be “Police arrested George Floyd, then brutally murdered him as he lay helplessly handcuffed on the ground.” There was an obvious cause for his “medical distress:” Derek Chauvin’s knee and body weight. “Underlying conditions” played no role. Before being pinned and crushed, Floyd was alive. After, he was dead. It didn’t matter if he had been suffering from terminal cancer: that’s not what killed him. Officer Chauvin did. There is no alleged about it: we can all personally witness this heinous act through now-ubiquitous video recordings.

The more puritanical grammarians might object that I am not merely using the active voice that the police and coroner’s report (and some press accounts) take care to avoid. I am also using pejorative adverbs: brutally and helplessly. Yes. Yes I am. Because those words apply. If you want an illustration to go along with the dictionary definition of these words, then go watch all 8:46 of the execution of George Floyd.

As egregious as this case is, it is not an isolated incident. That both the police and coroner’s reports whitewash the incident with intentionally vague and passive language is a dead give away that this is standard operating procedure. They’ve done it before. Many times. So many times that there is a well-rehearsed language of obfuscation to subvert the plain facts of the matter.

This event has sparked protests around the country because it illustrates an all too familiar pattern of police behavior in black communities. I’ve heard various people say things like “It can’t be that bad.” Yet this systematic police brutality is what protesters are saying is their life experience of being black in America. Are you in a position to know better than they?

I’ve heard people say worse things. Like blaming the victim. Floyd was a career criminal, so he deserved what he got. This is such a common sentiment, apparently, that it affected a Google search I did the other day. I was trying to look up a geology term, and got as far as typing “geo” when Google auto-suggested

Really? This is such a common conceit that the mere three letters g e o leads Google to think I’m searching on George Floyd’s criminal past? I can think of a lot of more likely things to follow from g e o. Given the timing, I can see how his name would come up quickly. Just his name. Why add on “criminal past”? How many people must be doing that search for this to be Google’s top hit?

News flash: people are supposed to be innocent until proven guilty. It is the purpose of police to apprehend suspects and that of the courts and a jury of citizens to decide guilt or innocence. Whatever the alleged crime, the punishment is not summary execution by the police on the spot. As much as some few of them seem to want to be, the police are not and should not be Judge Dredd.

The same victim-blaming is going on with the protests. People have assembled in communities all over the country to protest – a right guaranteed by the first amendment. As near as I can tell, most of these assemblies have been peaceable. Given the righteous, raw anger over the arbitrary state-abetted murder of American citizens, it is hardly surprising that some of these assemblies devolve into riots. The odds of this happening are seen time and again to be greatly enhanced when the police show up to “keep order.” All too often we have seen the police act as the aggressors and instigators of violence. If you haven’t seen that, then you are not paying attention – or not following a credible news source. Fox, OANN, Breitbart, the Sinclair broadcasting network – these are not credible new sources. They are propaganda machines that are keen on focusing attention on the bad behavior of a minority of protesters in the hopes that you’ll be distracted from the police brutality that sparked the demonstrations in the first place.

Victim-blaming is an excuse closet racists use to dodge engagement with the real issue of police misconduct. “He was a career criminal! He deserved it!” and “Riots are bad! Police must keep order and protect property!” These are distractions from the real issue. Property is not as important as life, liberty, and the pursuit of happiness. Black Americans are not assured of any of those. When they peacefully assemble to petition the government for a redress of grievances, they are met with masses of police in riot gear hurling flash-bangs and teargas. Even if a few of these assemblages lead to riots and some looting, so what? That is nothing in comparison with existential threat to life and liberty suffered by all too many Americans because of the color of their skin.

An old friend tried to make the case to me that, basically, “mobs are bad.” I reacted poorly to his clueless but apparently sincere buy-in to the misdirection of victim-blaming, and felt bad about it afterwards. But he was wrong, in an absolute moral sense, and I have no patience left for blaming the victim. Yes. Mobs are bad. Duh. But going straight to that willfully misses the point. This didn’t start with mob violence out nowhere. It started with the systematic oppression of an entire group of American citizens defined in literally the most superficial way possible – the pigmentation of their skin. The police have many roles in our society, some for the good, some not. One of the bad roles has been as enforcers of a de facto system of white supremacy – a system so deeply ingrained that most white people aren’t even aware that it exists.

I would like to believe, as many white folk apparently do, that white supremacy is a thing of the past. An ugly chapter in our past now relegated to the dustbin of history. Yet I look around and see that it is alive and well all around us.

We – all of us who are American citizens – have an obligation to make things better for our fellow citizens. At a very minimum, that means listening to their concerns, not denying their experience. Just because it is horrible doesn’t make it untrue. So don’t try to tell me about the evils of riots and mobs until you first engage with the underlying causes therefore. These are mere symptoms of the societal cancer that is white supremacy. They are natural, inevitable reactions to decades upon decades of degradation and disenfranchisement heaped on top of centuries of dehumanization through slavery and lynchings. Until you acknowledge and engage meaningfully with these brutal aspects of history and modern-day reality, you have zero credibility to complain about any of their toxic offspring. Doing so is a clear sign that you are part of the problem.

Fuzzy Thing!

I was contacted today by a colleague at NASA’s Goddard Space Flight Center who was seeking to return some photographic plates of Halley’s comet that had been obtained with the Burrell Schmidt telescope. I at first misread the email – I get so many requests for data, I initially assumed that he was looking for said plates. That sent me into a frenzy of where the heck are they? about data obtained by others well before my time as the director of the Warner & Swasey Observatory. Comet Halley last came by in 1986.

Fortunately, reading comprehension kicked in, and I realized that all I really needed to figure out was where they should go. The lower pressure version of where the heck are they? That would be the Pisgah Astronomical Research Institute, which has had the good sense to archive the vast treasury of astronomical plates that many observatories obtained in the pre-digital era but don’t always have the ability to preserve. But this post isn’t about that; it is just a spark to the memory.

In 1986, I was a first-year graduate student in the Princeton physics department. As such, I had at that time little more competence in observing the sky than any other physicist (practically none). Nevertheless, I traipsed out into an open field at the dark edge of town on a clear night with a pair of binoculars and a vague knowledge of what part of the sky Comet Halley should be in. How hard could it be to spot the most famous comet in history?

Impossibly hard. There was nothing to see, so far as I could find. The apparition of 1986 was a bust. This informed in me a bad attitude towards comets. There had never been a good apparition in my lifetime (all of 22 years at that point), and Halley certainly wasn’t one. I accepted that decent comets must be a rare occurrence.

Flash forward a decade to 1996, by which time I was an accomplished observer with a good working knowledge of the celestial sphere. A new comet was discovered – Hyakutake – and with it came much hype. Yeah, yeah, I’d heard it all before. Boring. Comets were always a flop.

Comet Hyakutake made a close approach to Earth in March of 1996. Its wikipedia page is pretty good, with a nice illustration of its orbit and its path on the sky as perceived from the Earth. I was working at DTM at the time, where there were lots of planetary scientists as well as a few astronomers. Someone posted an ephemeris, so despite my distrust of comets I found myself peeking at what its trajectory would be. Nevertheless, we had a long period of cloudy weather, so there was nothing to see even if there was something to see, which I expected there wasn’t.

At this time, my elder daughter Caitlyn was two years old. I made a habit of taking her out and pointing things out in the sky. We watched the sunset, the moon set after it near new moon, and the moon rise near full moon. She seemed content to listen to her old man babble about the lights in the sky. Apparently more of that sank in than I realized.

My wife Anne was teaching at Loyola, and her department chair had invited us over for a party around the vernal equinox. We enjoyed the adult company and Caitlyn put up well with it – up to a point. It got dark and we bid our farewells and headed out. We had parked across the street, and on the way out Betsy (our hostess) said “Stacy – you’re an astronomer. Where’s the comet?”

I got this pained expression. Stupid comets. But it had cleared up for the first time in nearly a week, and looking up from the front door, I could quickly orient myself on the sky. Doing so, I realize that the comet was behind the house. So I pointed up and over, towards the back yard and through the roof: “Over there.” I continued across the street to the car with the toddler cradled in my left arm, fiddling with the keys with my right hand.

We did not have a nice car: one had to insert the key manually into the door to unlock it. As I went around the car to get to the driver’s side, I was focused on this mundane task. It did not occur to me to look up in the direction I had just pointed. I felt Caitlyn stretch her arm to point at the sky, exclaiming “Fuzzy thing!”

I looked up. There is was: a big, bright, fuzzy ball. A brilliant cometary apparition, the coma easily visible even in Baltimore. My two-year old daughter spotted it and accurately classified it before I even looked up.

Comet Hyakutake was an amazing event. Not only spectacular to look at, but it drove home celestial mechanics in a visceral way. It was at this time very close to Earth (by the scale of such things). That meant it made noticeable progress in its orbit from night to night. You couldn’t see it moving just staring at it, but one night is was here, the next night it was there, the following night over there. It was skipping through the constellations at a dizzying speed for an object that takes c. 70,000 years to complete one orbit. But we were close enough that one could easily see the progress it made across the sky from night to night, if not minute to minute. If you wanted to take a picture with a telescope, you had to track the telescope to account for this – hence the star trails in the image above: the stars appear as streaks because the telescope is moving with the comet, not with the sky.

The path of Comet Huyakutake across the sky.

This figure (credit: Tom Ruen) shows the orbital path of Comet Huyakutake projected on the sky (constellations outlined in blue). Most of the time, the comet is far away near the aphelion of its orbit. As it fell in towards the sun, its path made annual ellipses due to the reflex motion of the Earth’s own orbit – the parallax. These grew in size until the comet came sweeping by in the month of March, 1996. Think about it: it spent tens of thousands of years spiraling down towards us, only to shoot by, transitioning well across the sky in only a couple of weeks. Celestial mechanics made visible.

Not long after Hyakutake started to fade, Comet Hale-Bopp became visible. Hale-Bopp did not pass as close to the Earth as Hyakutake, so it didn’t leap across the sky like Tom Bombadil. But Hale-Bopp was a physically larger comet. As such, it got bright and stayed bright for a long time, remaining visible to the naked eye for a record year and half. In the months after Hyakutake’s apparition, we could see Hale-Bopp chasing the sunset from the balcony of our apartment. Caitlyn and I would sit there and watch it as the twilight faded into dark. Her experience of comets had been the opposite of mine: where in my thirty years (before that point) they had been rare and disappointing, in her (by then) three years they had been common and spectacular.

The sky is full of marvels. You never know when you might get to see one.

Triton Station

A Blog About the Science and Sociology of Cosmology and Dark Matter

Category: Personal Experience