Cosmology Conundrum

Some colleagues have suggested that my posting on Planck Day was overly frivolous, given the huge importance of what Planck has achieved. (Nicely written up by Andrew Jaffe.) Other colleagues have suggested that Planck Day was bad press, as it was such a huge public non-event, with a big fuss about mild parameter adjustment. I find both these things true, leaving a weird sense of tension and excitement. I’d better explain myself.

First, lets be clear about the technical achievement. Planck is an absolute triumph of technology, engineering, management, and organisation. An amazing machine that has worked beautifully. It also represents a stunning scientific achievement. The lamda-CDM model, and its beautifully articulated engine of prediction, is a conceptual and analytic triumph. Of course this triumph belongs not just to the Planck team, but represents the accumulated achievement of many scientists all over the world over a number of years.

Lets just look at that power spectrum fit. It is not just a question of the theoretical curve going vaguely up and down, in more or less the same way as the data. The detailed agreement is gob-smacking – multiple peaks, their positions, their sizes, their widths, their second and third order curvature. Any scientist will look at this and think “no way is this a fluke”. Don’t let doubters trot out that coffee-time stuff about being able to fit anything with enough parameters. Firstly, that old chestnut is largely nonsense, and secondly, the fit quality is way beyond that.

Parfait. Everybody love parfait

And yet – outside the world of the CMB, the CDM paradigm has problems, as we were reminded here at ROE the day after Planck Day, in a nice wee coffee talk by Jorge Penarrubia.The best known problems are that CDM predicts far too many dwarf galaxies, and galaxy profiles that are much cuspier than observed,  but there are other claims, such as the existence of a very unlikely polar structure of dwarfs surrounding the Milky Way,  and of suspicious uniformities in galaxy rotation curves. All these problems may still get patched up by astrophysical fixes to do with feedback, IMF games, etc… but we don’t know yet.

Its common to hear people say that Particle Physics and Cosmology are in a similar situation – a model rather than a theory – a perfect fit but with no explanation. Why do all those parameters have those particular values? It seems so arbitrary. And what are dark matter and dark energy anyway? In this view, even the boringly successful fit is tantalising because it tells us there must be a deeper theory in waiting that will explain the perfect fit.

That may describe particle physics, but it doesn’t describe cosmology. Rather, what we have is perfect but fishy. How can the model be so perfect in some places and so poor in others? Have we missed something simple? It may well be that the astrophysical fixes do the trick, and then cosmology does look indeed like particle physics – successful but unsatisfying. Or it could be that some conceptual change is needed, and a revolution is waiting.

I’d give the revolution 2:1 against. But thats good enough odds that your eyes are glued to the table… Mesdames et Messieurs, faites vos jeux.

14 Responses to Cosmology Conundrum

  1. “All these problems may still get patched up by astrophysical fixes to do with feedback, IMF games, etc… but we don’t know yet.”

    My bet is on gastrophysics. The Planck results don’t depend on the details of the behaviour of CDM, while galaxy formation etc obviously does.

    What if those dwarf galaxies are there, but we can’t see them because they don’t shine? OK, perhaps one expects stars in them, but this involves the interaction between CDM and star formation, which goes far beyond the CDM knowledge needed for the CMB.

    The interesting question is whether lambda+Omega=1.0056, which should bring a Nobel Prize to Barrow and Shaw. However, the Planck data aren’t precise enough to decide.

  2. andyxl says:

    I thought you were a radical in the cosmology world Phillip!

    • In the sense of “…and this proves it”? If so, are you referring to gastrophysics, or to my mention of Barrow and Shaw?

      Or in the sense of “…but apparently you’re not”?

      Inquiring minds want to know.

      • andyxl says:

        In the sense of “but apparently you’re not”

      • I am relieved! My reputation is saved!

        The important thing is not whether one is radical, but whether one is right. (This applies not only to cosmology, but to everything in life. For example, one could argue that running a VMS cluster at home is both radical and right.)

      • For the record, I direct the Gentle Readers to where the following refereed-journal publications argue in favour of a conventional explanation, debunking a non-orthodox hypothesis put forward by someone else: 3, 4, 14. (10 demonstrates the “trust the observations, rather than the theory” approach, though in this case the observations weren’t conclusive.) With the wisdom of hindsight, one might add 1 as well. The jury is still out on 16.

      • And, blowing my own horn for the last time today, all of the others are quite conventional, actually. Bread-and-butter stuff. But proper German bread and Swedish salted butter. 🙂

  3. skyweek says:

    My prediction is that the multi-sigma discrepancy between the Hubble constants coming out of the Planck fit and from numerous direct methods – that have lately converged on a higher value (near 74 km/s/Mpc) – will keep folks busy for years. And will *perhaps* lead to (some) “new physics” in the end. The bold view of the Planck folks in their paper XVI that basically all other astrophysicists have screwed up at one point in their Hubble constant determinations is untenable …

    • Albert says:

      I expect that the ‘direct’ methods suffer from a bandwagon effect, and that the standard candles can be re-calibrated to get the lower Hubble constant. The megamaser determinations are more ‘direct’ but are consistent with the lower value. No new physics needed..phew.

      • skyweek says:

        The Megamaser Cosmology Project – in progress – is indeed currently moving into Planck’s direction, but analysis of the classical megamaser case, NGC 4258, does not and rather confirms the high value of the Carnegie Hubble Program (i.e. former Ho Key Project). So then: “re-calibrate” their work in – widely hailed last summer as a breakthrough in Ho determination, by the way – and make it fit with Planck …

      • Albert says:

        NGC 4258 is a bit nearby to get an accurate value for H_0. I’ll leave the recalibration of the Cepheids to others – certainly lots of people will try. But I find their derived uncertainties optimistic: the calibration hangs on only 10 galactic stars, not really enough to define a meaningful sigma, and they have to fit two parameters. So my guess would be that the uncertainties are underestimated. But now I should probably go in hiding rather than fight in the Hubble wars.

  4. andyxl says:

    Your prediction is noted, and yes that discrepancy is interesting, as are the mild discrepancies at small l. It just seemed to me that the dwarf and cusp problems are much more glaring.

  5. I’ll take the 2 to 1 odds. I bet on dollar to your two that this is the right model.

  6. There are much more glaring problems with the present Cosmological Model.
    Where is the evidence of time dilation in the energy variations from Quasars?
    Where are the high red shift quasars?
    I describe even more fundamental issues, (which are resolved) in my paper.
    Please give me a chance. Do not make a rush to judgment decision without consideration.
    John Kulick

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