Scientists Still Don’t Know How Fast The Universe Is Expanding
“The uncertainties on these two methods are both pretty low, but are also mutually incompatible. If the Universe has less matter and more dark energy than we presently think, the numbers on the ‘leftover relic’ method could increase to line up with the higher values. If there are errors at any stage in our distance measurements, whether from parallax, calibrations, supernova evolution, or Cepheid distances, the ‘distance ladder’ method could be artificially high. There’s also the possibility, favored by many, that the true value lies somewhere in between.”
How quickly is the fabric of space expanding? That depends on how we ask the Universe. If we look at things like the leftover glow from the Big Bang or the large-scale clustering of galaxies, we get a consistent value of 67 km/s/Mpc. But if we look at individual galaxies through a variety of methods, we get a different consistent value: 74 km/s/Mpc. The uncertainties on each method are small and do not overlap, and a potential third way of measuring this (merging neutron stars) have problems and biases all their own. A generation ago, we argued whether the Hubble constant was 50 or 100; the answer turned out to be 70. Now, we argue over whether it’s 67 or 74… or, as a few people propose, that it’s again some value in between the two.