Tension in the Hubble constant

A few months ago I blogged about the apparent “tension” between different measurements of the Hubble constant. Here is an alternative view of the situation, with some recent updates. The plot has thickened a bit, but it’s still unclear to me whether there’s really a significant discrepancy.

Anyway, here’s a totally unscientific poll on the issue! Do feel free to register your vote.

Triton Station

There has been some hand-wringing of late about the tension between the value of the expansion rate of the universe – the famous Hubble constant, H, measured directly from observed redshifts and distances, and that obtained by multi-parameter fits to the cosmic microwave background. Direct determinations consistently give values in the low to mid-70s, like Riess et al. (2016): H = 73.24 ± 1.74 km/s/Mpc while the latest CMB fit from Planck gives H = 67.8 ± 0.9 km/s/Mpc. These are formally discrepant at a modest level: enough to be annoying, but not enough to be conclusive.

The widespread presumption is that there is a subtle systematic error somewhere. Who is to blame depends on what you work on. People who work on the CMB and appreciate its phenomenal sensitivity to cosmic geometry generally presume the problem is with galaxy measurements. To people who work on local galaxies, the CMB value is…

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30 Responses to “Tension in the Hubble constant”

  1. Sesh Nadathur Says:

    That post is full of such nonsense! I’m afraid it is motivated reasoning from someone who doesn’t like the CMB because it conclusively rules out his favourite theory (MOND with no dark matter).

    For instance, he says “The CMB data now allow only a narrow trench. I worry that it may wink out entirely. Were that to happen, it would falsify our current model of cosmology.”

    The “narrow trench” referred to here is simply a consequence of the fact that the extremely well-measured location of the CMB acoustic peaks translates to an extremely well-measured value of the combination of parameters Omega_m*h^3. This looks like a “trench” in a plot of Omega_m vs h. So what? That’s just reflecting a natural degeneracy in parameter space and has zero relevance to falsifying the current model of cosmology.

    • telescoper Says:

      My own view, for what it’s worth, is that galaxies are inherently unreliable….

    • Sesh Nadathur Says:

      I agree the H0 tension is probably not a show-stopper. I disagree with the speculation that the tension can be resolved by allowing non-zero curvature to shift the Planck value of H0, among various other silly things in that blog post. I tried to explain some of the problems in a comment on that page.

      It’s possible there are some strange unnoticed systematics still affecting Planck CMB data, which happen to lead to a low value of H0 that is compatible with WMAP, ACT, lensing, BAO and SNe data. I think it is more likely that there are systematics affecting the distance ladder measurements. Time will tell.

      As for good criticisms of MOND-without-dark-matter, the clincher as far as I am concerned is that the relative heights of odd and even acoustic peaks in the CMB necessarily require that there must be a source for the gravitational potential that is not coincident with the position of the baryons. Unless the theory of gravity becomes non-local (which MOND is not), this means some form of dark matter must necessarily exist. One could similarly point to the subdominant BAO wiggles in the matter power spectrum, as done by Dodelson.

      I also agree that galaxies are inherently unreliable – another way of saying we understand linear perturbation theory better than all the stuff going on in galaxies.

    • Sesh Nadathur Says:

      My experience of arguing with Stacy McGaugh is that he will admit the CMB is evidence for CDM one minute and then turn around and argue that dark matter doesn’t exist the next minute. If you point out his statements about the CMB are wrong, he’ll say it’s so boring that you’re telling him stuff he knows already … and then he’ll make the same incorrect statements again a few weeks later.

      Disclaimer: other MOND supporters are available.

    • Shantanu Says:

      Sesh, Peter all : Mike Turner (a strong dark matter proponent) and who coined the name “dark energy” has argued that MOND phenomenology is a consequence of CDM. https://arxiv.org/abs/astro-ph/0107284. Do you people agree with this claim? I asked this question to N of NFW a few years ago and he disagrees. Would be interesting to hear your take on this

      • Shantanu Says:

        Philip. I am not supporting this result or claim and I agree this paper is an order of magnitude calculation. However what is interesting from a sociological point of view is that one of the leading proponent of the dark matter camp admits that the empirical observations which lead Milgrom et al to propose MOND are *correct* and the only thing they differ about is interpretation.
        Whereas from most other dark matter proponents, I never even receive a straight answer as to what they think of their empirical fits to the data are correct or not.

      • Shantanu Says:

        Philip, I sort of disagree with your statement that “I am not aware of anyone who disputes these observational data”.
        I once heard a seminar by Dan Hooper who said that T-F is a complete fluke. I once asked Rocky Kolb about T-F relation His reply was a facetious one ” FIshy-Tully relation” which sort of means he doesn’t care about it.
        Most others use the magic word “feedback” as a panacea for all these problems.
        That said Stacy’s latest paper on MADR has made multiple CDM simulators trying to explain the results through their simulations.

  2. Sesh Nadathur Says:

    In terms of the H0 value, no, none whatsoever.

  3. Shantanu Says:

    Peter : If galaxies are so complicated , how come they obey simple empirical scaling laws (Renzo’s rule, Tully-Fisher relation, fundamental plane, constant asymptotic acceleration over a wide range of scales, mass acceleration discrepancy relation)? They shouldn’t obey these simple deterministic laws.

    • telescoper Says:

      They’re not exact deterministic relations, they are correlations. The fundamental plane, for example is neither fundamental nor a plane.

      • Shantanu Says:

        Stacy has shown that the scatter in all these relations is very tiny. How about an explanation of Figure 3 of arXiv:1401.1146?

      • telescoper Says:

        It’s not “very tiny”!

        Especially not when you remember that most of the plots are logarithmic.

        Here is Figure 3 of that paper:

        Tiny Scatter

        If you think that’s “tiny scatter” you really need to get out more.

      • Shantanu Says:

        I meant tiny scatter in Baryonic T-F relation and also in the recently proposed mass acceleration discrepancy relation.

        regarding figure 3 of arXiv:1401.1146, the key plot is the right panel for DM dominated systems, where the asymptotic acceleration shows a value of about 1 km^2/sec^{-2}/pc.
        I find this pretty intriguing and have asked a few simulators about this, but have not get a good explanation for this.
        But maybe you are right and this should not be taken seriously and will go away with better data.

      • Shantanu Says:

        More specifically do you agree with the spirit of the 2001 paper by Turner and Kaplinghat which argues that all these (quasi)-deterministic relations can be explained within CDM and no need to modify gravity?

      • telescoper Says:

        I can safely say that I don’t know..this needs to be answered with simulations that include realistic star formation and hydrodynamics. I don’t think we’re close to that yet, despite what the simulators say. A huge amount of important physics happens way below the resolution scale of even the best simulations.

        So I’m an agnostic. I’m not a fervent CDM supporter because of small-scale clumpiness issues which I take reasonably seriously. I’m actually quite interested in “fuzzy” DM theories, etc. I can perfectly well accept the idea that GR might not be the full story, but the evidence isn’t strong enough for me to accept that it absolutely needs modifying.

  4. Shantanu Says:

    Hi, Peter thanks for explaining your perspective. I agree with what you say regarding simulations. However as a physicist, its not obvious to me how by including realistic star formation, hydrodynamic feedback etc (which are all dissipative non-linear processes) you will magically get simple deterministic scaling laws for a diverse range of galaxes with just one free parameter , which is what Milgrom first found in 1983 and as Dave Merritt mentioned on this blog ( a few years back) all predictions made by Milgrom have turned out to be true.

    • telescoper Says:

      They’re not “simple deterministic scaling laws”, they’re empirical correlations.

      And of course it’s “not obvious” – that’s why we need simulations. What you seem to be assuming, however, is that it is obvious that it can’t work and that I think is unjustified.

    • telescoper Says:

      By the way, here’s a paper from last year which says that it all works fine in LCDM…


      • Shantanu Says:

        See my comments on March 2.
        “That said Stacy’s latest paper on MADR has made multiple CDM simulators trying to explain the results through their simulations.”
        The above paper is one of those.
        So its good that people are now trying to explain this using conventional Lambda CDM. So now at least everyone agrees the empirical correlations are correct.

  5. Shantanu Says:

    Peter and others: Check out this insightful article by Dave Merritt
    in a philosophy journal


    Table 1 is really disturbing in that almost no Cosmology textbook/monograph mentions about the critical acceleration scale
    or Mass acceleration discrepancy relation. Peter’s book is not in this table (although I think this is restriction o books after 2005)

  6. Shantanu Says:

    Philip or anyone else: Do you know how many books mention black hole M-sigma relation (which also is another empirical observed correlation, which we don’t understand). I haven’t done a similar survey, but am pretty sure it is more than 0. Similarly I think at least since no dispute observations 2, 3 its strange that not a single textbook mentions them.

  7. The Hubble constant is a thumb rule. It does not explain the process concerned. It also does not explain why the moon is accelerating away from the earth or why the earth is accelerating away from the sun or why Andromeda is accelerating towards Milky Way. I think that Hubble’s formula is incomplete.

  8. Telescoper says two out of three things, not two out of four, the first of which being the process of galaxies accelerating away from us. But the thing about the taste of some kinds of cheese is absolutely true 🙂 Anyway, the accelerating away of the moon is tue (now about 4 m per year and increasing); the earth now about 15 cm per year and increasing (recent Japanese measurements).

    • telescoper Says:

      The moon is not accelerating away from the Earth, it is accelerating towards the Earth, which is why it is in a near circular orbit. The radius of this orbit is increasing slowly with time due to tidal dissipation but that does not mean that the Moon is accelerating away from the Earth.

      • Thank you for clearing up my my misunderstanding. So is it the increase of the radius that might be responsible in the far future for the earth becoming unstable? (this is what I read some time ago)

      • telescoper Says:

        Estimated timescale is 50 billion years, far longer than the lifetime of the Sun. I’m not worried. I’ll be retired by then.

  9. M. E. de Souza has published a new model for the Universe that incorporates the good results of the Big Bang Model and solves the problems. Besides, the model shows that the Universe is eternal in time with infinite cycles of time. The paper is calle The Multi-Bang Universe: the Never-Ending REalm of Galaxies. Another important paper by the same author is Dark Matter does not Exist at all. Both articles can be found in the web page http://www.primons.com

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