False Convergence and the Bandwagon Effect

In idle moments, such as can be found during sunny sunday summer afternoons in the garden, it’s  interesting to reminisce about things you worked on in the past. Sometimes such trips down memory lane turn up some quite interesting lessons for the present, especially when you look back at old papers which were published when the prevailing paradigms were different. In this spirit I was lazily looking through some old manuscripts on an ancient laptop I bought in 1993. I thought it was bust, but it turns out to be perfectly functional; they clearly made things to last in those days! I found a paper by Plionis et al. which I co-wrote in 1992; the abstract is here

We have reanalyzed the QDOT survey in order to investigate the convergence properties of the estimated dipole and the consequent reliability of the derived value of \Omega^{0.6}/b. We find that there is no compelling evidence that the QDOT dipole has converged within the limits of reliable determination and completeness. The value of  \Omega_0 derived by Rowan-Robinson et al. (1990) should therefore be considered only as an upper limit. We find strong evidence that the shell between 140 and 160/h Mpc does contribute significantly to the total dipole anisotropy, and therefore to the motion of the Local Group with respect to the cosmic microwave background. This shell contains the Shapley concentration, but we argue that this concentration itself cannot explain all the gravitational acceleration produced by it; there must exist a coherent anisotropy which includes this structure, but extends greatly beyond it. With the QDOT data alone, we cannot determine precisely the magnitude of any such anisotropy.

(I’ve added a link to the Rowan-Robinson et al. paper for reference). This was  a time long before the establishment of the current standard model of cosmology (“ΛCDM”) and in those days the favoured theoretical paradigm was a flat universe, but one without a cosmological constant but with a critical density of matter, corresponding to a value of the density parameter \Omega_0 =1.

In the late eighties and early nineties, a large number of observational papers emerged claiming to provide evidence for the (then) standard model, the Rowan-Robinson et al. paper being just one. The idea behind this analysis is very neat. When we observe the cosmic microwave background we find it has a significant variation in temperature across the sky on a scale of 180°, i.e. it has a strong dipole component

There is also some contamination from Galactic emission in the middle, but you can see the dipole in the above map from COBE. The interpretation of this is that the Earth is not at rest. The  temperature variation causes by our motion with respect to a frame in which the cosmic microwave background (CMB) would be isotropic (i.e. be the same temperature everywhere on the sky) is just \Delta T/T \sim v/c. However, the Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. We know these velocities pretty well, but they don’t account for the size of the observed dipole anisotropy. The extra bit must be due the gravitational pull of larger scale structures.

If one can map the distribution of galaxies over the whole sky, as was first done with the QDOT galaxy redshift survey, then one can compare the dipole expected from the distribution of galaxies with that measured using the CMB. We can only count the galaxies – we don’t know how much mass is associated with each one but if we find that the CMB and the galaxy dipole line up in direction we can estimate the total amount of mass needed to give the right magnitude. I refer you to the papers for details.

Rowan-Robinson et al. argued that the QDOT galaxy dipole reaches convergence with the CMB dipole (i.e. they line up with one another) within a relatively small volume – small by cosmological standards, I mean, i.e. 100 Mpc or so- which means that  there has to be quite a lot of mass in that small volume to generate the relatively large velocity indicated by the CMB dipole. Hence the result is taken to indicate a high density universe.

In our paper we questioned whether convergence had actually been reached within the QDOT sample. This is crucial because if there is significant structure beyond the scale encompassed by the survey a lower overall density of matter may be indicated. We looked at a deeper survey (of galaxy clusters) and found evidence of a large-scale structure (up to 200 Mpc) that was lined up with the smaller scale anisotropy found by the earlier paper. Our best estimate was \Omega_0\sim 0.3, with a  large uncertainty. Now, 20 years later, we have a  different standard cosmology which does indeed have \Omega_0 \simeq 0.3. We were right.

Now I’m not saying that there was anything actually wrong with the Rowan-Robinson et al. paper – the uncertainties in their analysis are clearly stated, in the body of the paper as well as in the abstract. However, that result was widely touted as evidence for a high-density universe which was an incorrect interpretation. Many other papers published at the time involved similar misinterpretations. It’s good to have a standard model, but it can lead to a publication bandwagon – papers that agree with the paradigm get published easily, while those that challenge it (and are consequently much more interesting) struggle to make it past referees. The accumulated weight of evidence in cosmology is much stronger now than it was in 1990, of course, so the standard model is a more robust entity than the corresponding version of twenty years ago. Nevertheless, there’s still a danger that by treating ΛCDM as if it were the absolute truth, we might be closing our eyes to precisely those clues that will lead us to an even better understanding.  The perils of false convergence  are real even now.

As a grumpy postscript, let me just add that Plionis et al. has attracted a meagre 18 citations whereas Rowan-Robinson et al. has 178. Being right doesn’t always get you cited.

18 Responses to “False Convergence and the Bandwagon Effect”

  1. The bandwagoning already exists in physics – the alternative ideas are censored from there faster, than for example racism or sexually explicit posts. Apparently, too many people fears of their jobs, social status and informational monopoly. We shouldn’t tolerate it, because everybody of us could become victim of this religious approach to physics tomorrow.

  2. >> Being right doesn’t always get you cited.

    Or being first… I’m sure we have all got a publication injustice story to tell.

  3. > Perhaps, but all that really matters is that someone eventually finds the truth.

    For the goodness of science, yes, but for furthering your career, possibly not.

  4. John Peacock Says:

    Hi Peter. Ah, the Good Old Days. I wrote a paper on just the same subject in 1992. The build-up of the total peculiar velocity of the Local Group has an interesting property, in that it converges much faster in direction than it does in magnitude. So with data on density fluctuations to (less than)100 Mpc depth you can nail the dipole direction to just a few degrees, and yet still have the magnitude wrong by a factor (greater than)2.

    This issue of unprobed large local fluctuations is still with us, of course, in the minds of those who seeks to dispense with Lambda by invoking a colossal local hole. Can’t remember if you’ve put the boot into that in the blog, but if not then I hope you’ll consider it worth a rant.

    Strangely, my 1992 paper also has 18 citations, just like yours: but one of those is from you, so belated thanks!

    • telescoper Says:

      John,

      I edited this comment to include the later amendment. I hope that’s OK.

      I think the real outcome of all this was that people decided that the CMB dipole isn’t really a very good way of measuring Omega…

      Peter

  5. Bryn Jones Says:

    I sometimes wonder how today’s state of astronomical and cosmological knowledge will look in 20 to 30 years’ time. Will people look back and wonder why we didn’t see the answers to some of our biggest problems. Is the evidence to solve some of the biggest problems already available, requiring only a clear mind to see through the confusion?

    • telescoper Says:

      I was once asked that question in a job interview. My reply was that, although I might have a plastic knee, I don’t have a crystal ball. I didn’t get the job.

  6. John Peacock Says:

    Bryn: truth (as currently perceived) was often staring us in the face in the past, but had to wait until the climate of ideas was right. e.g. the 1985 DEFW paper that recently won the Gruber Prize was lauded at the time for high-bias galaxy formation, which allowed a matter-only Omega=1 model to match the data of the day. But they also said “or you could have a low-density universe with a cosmological constant”, even though this suggestion was largely ignored. So it may well be that all the ideas we need to understand the universe have already been written down, but are just obscured by current fashion. To be clear: I’m not suggesting that Lambda or CDM are passing fashions, but elements of galaxy formation may well be.

    • telescoper Says:

      I think it’s quite likely that in the future, when people have figured out what Lambda really is, they’ll look back at us with amusement. What I mean is that although I believe the observational results (or most of them at any rate), the theoretical interpretation is probably missing something vital which in hindsight is also obvious.

    • Bryn Jones Says:

      I suspect one other important factor that acts to obscure the truth is confusion by some spurious results, be they observational or theoretical. With hindsight we can see what the “correct” results were indicating, but tend to overlook the confusion imposed by other studies.

      Twenty years ago I worked on stars and the Galaxy. Perhaps therefore my own perspective then on cosmology, as something of an outsider looking on, attached particular importance to the ages of globular clusters and to constraints from primordial nucleosynthesis. In those days, globular clusters were widely believed to be 15 to 18 Gyr old, imposing particular constraints on cosmological parameters. Improvements in stellar evolutionary models have now lowered the ages of globular clusters to 12 to 13 Gyr. In contrast the nucleosynthesis results stand, and indicated a low baryonic density.

      People have difficulty in seeing through the confusion of contradictory information. The contradictions vanish as new research tightens constraints to converge on the truth.

  7. >> Dennis Overbye, in “Lonely Hearts in the Cosmos”

    Doesn’t Dennis describe Simon as a “sandy haired irish man” ?

    • Bryn Jones Says:

      My copy of Overbye’s Lonely Hearts of the Cosmos states in page 308:

      Rather than rail about about it, some astronomers were trying to figure out how to use dark matter to solve cosmological problems. One of the first was Simon White, a graduate student at Cambridge University who was to become an aficionado of dark matter astrophysics. White, a gangly Irishman with unruly blond hair, had done his Ph.D. thesis on numerical simulations of galaxy clusters under Rees, the fast-talking, faster-thinking ex-student of Sciama’s.

      (Despite that, Lonely Hearts of the Cosmos is really worth reading.)

  8. telescoper Says:

    I won’t say it was but he isn’t particularly big and – to the best of my knowledge – doesn’t wear a wig.

  9. > White, a gangly Irishman with unruly blond hair,

    Yeah – that’s it. I always wondered if Dennis had actually spoken to Richard McMahon 🙂

    • Bryn Jones Says:

      Yes, a confusion between Simon White and Richard McMahon would explain it, at least in terms of confusing what the two look like!

    • telescoper Says:

      The only problem was that a certain person forgot to turn off italics…

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