There is a new article in Science on the expansion rate of the universe, very much along the lines of my recent post. It is a good read that I recommend. It includes some of the human elements that influence the science.

When I started this blog, I recalled my experience in the ’80s moving from a theory-infused institution to a more observationally and empirically oriented one. At that time, the theory-infused cosmologists assured us that Sandage had to be correct: H0 = 50. As a young student, I bought into this. Big time. I had no reason not to; I was very certain of the transmitted lore. The reasons to believe it then seemed every bit as convincing a the reasons to believe ΛCDM today. When I encountered people actually making the measurement, like Greg Bothun, they said “looks to be about 80.”

This caused me a lot of cognitive dissonance. This couldn’t be true. The universe would be too young (at most ∼12 Gyr) to contain the oldest stars (thought to be ∼18 Gyr at that time). Worse, there was no way to reconcile this with Inflation, which demanded Ωm = 1. The large deceleration of the expansion caused by high Ωm greatly exacerbated the age problem (only ∼8 Gyr accounting for deceleration). Reconciling the age problem with Ωm = 1 was hard enough without raising the Hubble constant.

Presented with this dissonant information, I did what most of us humans do: I ignored it. Some of my first work involved computing the luminosity function of quasars. With the huge distance scale of H0 = 50, I remember noticing how more distant quasars got progressively brighter. By a lot. Yes, they’re the most luminous things in the early universe. But they weren’t just outshining a galaxy’s worth of stars; they were outshining a galaxy of galaxies.

That was a clue that the metric I was assuming was very wrong. And indeed, since that time, every number of cosmological significance that I was assured in confident tones by Great Men that I Had to Believe has changed by far more than its formal uncertainty. In struggling with this, I’ve learned not to be so presumptuous in my beliefs. The universe is there for us to explore and discover. We inevitably err when we try to dictate how it Must Be.

The amplitude of the discrepancy in the Hubble constant is smaller now, but the same attitudes are playing out. Individual attitudes vary, of course, but there are many in the cosmological community who take the attitude that the Planck data give H0 = 67.8 so that is the right number. All other data are irrelevant; or at best flawed until brought into concordance with the right number.

It is Known, Khaleesi. 

Often these are the same people who assured us we had to believe Ωm = 1 and H0 = 50 back in the day. This continues the tradition of arrogance about how things must be. This attitude remains rampant in cosmology, and is subsumed by new generations of students just as it was by me. They’re very certain of the transmitted lore. I’ve even been trolled by some who seem particularly eager to repeat the mistakes of the past.

From hard experience, I would advocate a little humility. Yes, Virginia, there is a real tension in the Hubble constant. And yes, it remains quite possible that essential elements of our cosmology may prove to be wrong. I personally have no doubt about the empirical pillars of the Big Bang – cosmic expansion, Big Bang Nucleosynthesis, and the primordial nature of the Cosmic Microwave Background. But Dark Matter and Dark Energy may well turn out to be mere proxies for some deeper cosmic truth. IF that is so, we will never recognize it if we proceed with the attitude that LCDM is Known, Khaleesi.

Stacy McGaugh is an astrophysicist and cosmologist who studies galaxies, dark matter, and theories of modified gravity. He is an expert on low surface brightness galaxies, a class of objects in which the stars are spread thin compared to bright galaxies like our own Milky Way. He demonstrated that these dim galaxies appear to be dark matter dominated, providing unique tests of theories of galaxy formation and modified gravity. Professor McGaugh is currently the chair of the Department of Astronomy at Case Western Reserve University in Cleveland, Ohio, and director of the Warner and Swasey Observatory. Previously he was a member of the faculty at the University of Maryland, having also held research fellowships at Rutgers, the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, and the Institute of Astronomy at the University of Cambridge after earning his Ph.D. from the University of Michigan.

9 thoughts on “Hubble constant redux”

  1. One sentence from the Science article had me laughing loudly:
    -But critics like Hiranya Peiris, a Planck astrophysicist based at University College London, says variable dark energy seems “ad hoc and contrived.”-
    Variable Dark Energy seems contrived, but Dark Energy doesn’t? One cannot deny that something is going on, but Dark Energy is just a place holder label, it is not an actual, specific, model. Such place holders are, virtually by definition, ad hoc and contrived.

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  2. Indeed – which was exactly Merritt’s point. There is a remarkable sociology here… once we name something, after a while it becomes familiar and then becomes normal. We’ve already contrived dark energy, so it is normal. Adjusting it is the new contrived.

    A related observation: there are many theorists seeking modified theories of gravity as an explanation of dark energy. Precious few of them seem to have thought to also address the dark matter problem. That is often treated as separate and immutably established. To me, dark energy is an additional anomaly, on top of the dark matter problem. Having two anomalies would seem to beg for a fundamental approach to both.

    As I put it at an Aspen meeting in 1997: Lambda is a big stop sign in the sky. It is telling us we’re on the wrong road.

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  7. Re: “But Dark Matter and Dark Energy may well turn out to be mere proxies for some deeper cosmic truth.”
    Stacy, while you and your colleagues are shedding observational light on Dark Matter, you may find solace in the news that a few theoretical cosmologists are providing a more plausible explanation for the data that provoked the concept of Dark Energy, within the framework of General Relativity. David L. Wiltshire and colleagues have published a series of papers over the last decade addressing what he considers to be neglected implications of GR that bear directly on the apparent accelerating expansion. For example, in arXiv:0809.1183 (doi= 10.1103/PhysRevD.78.084032), he proposes that “Dark energy is a misidentification of cosmological gravitational energy gradients in an inhomogeneous void-dominated universe.” As I understand it, he finds that clocks run slower in the gravitational field of (clusters of) galaxies than in the voids, so that voids grow faster than our clocks lead us to expect, leading to a 38% “age difference” accumulated over the age of the universe. He estimates the effect to be much larger than the well-studied backreaction, which also relates to accelerating evolution of voids. (Nevertheless, he joins several others in savaging recent attempts to “prove” that backreaction of inhomogeneities is irrelevant in cosmology — see arXiv:1505.07800 doi= 10.1088/0264-9381/32/21/215021.)

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