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.