In the Dark

I haven’t had time to write much about astrophysics and cosmology recently, so this morning I back a few days through the arXiv – where every research paper worth reading in these fields can be found – and found a fascinating paper by Kelly et al. about the gravitational lensing of a supernova known as SN Refsdal after the pioneer of gravitational lensing, Sfur Refsdal. When first observed in 2014 this supernova was observed as four images produced by the gravitational lensing of light from the supernova through a massive cluster of galaxies, a phenomenon known as an Einstein Cross.

Lens modellers quickly got to work on this system and concluded that two further images should exist. Given the difference in light travel times for lensed paths and the relatively short lifetime of a supernova, neither would occur at the same time as the four original ones. One image would have been observed at some point in the 1990s, had anyone been looking, but that wasn’t the case. However, another was predicted to occur in 2015 and that was observed. I call it Refsdal’s Ghost because of the French word revenant, which means a ghost but also someone who returns after a long absence. Anyway, the revenant is marked SX in the image below (obtained from here). The unobserved image is SY and the four originals S1-S4.

Multiple images of a point source a in a system such as this, with measured time delays, provide theorists with a great deal of information they can use to model the distribution of mass inside the cluster, including its physical size. That in turn allows one to measure its distance. With a measurement of redshift, this allows one to determine the Hubble Constant.

That’s the background. Now here’s the abstract of the paper by Kelly et al.:

The gravitationally lensed Supernova Refsdal appeared in multiple images, produced through gravitational lensing by a massive foreground galaxy cluster. After the supernova appeared in 2014, lens models of the galaxy cluster predicted an additional image of the supernova would appear in 2015, which was subsequently observed. We use the time delays between the images to perform a blinded measurement of the expansion rate of the Universe, quantified by the Hubble constant (H0). Using eight cluster lens models, we infer H₀ = 64.8 +4.4-4.3 km / s / Mpc, where Mpc is the megaparsec. Using the two models most consistent with the observations, we find H₀ = 66.6 +4.1-3.3 km / s / Mpc. The observations are best reproduced by models that assign dark-matter halos to individual galaxies and the overall cluster.

Anyone who has been following developments in cosmology knows that there is currently some “tension” over different measurements of the Hubble constant, as illustrated in this figure (which is slightly dated but which makes the point):

There is some uncertainty of course, but it is interesting that the Kelly et al. measurement aligns with most of what are called the “early” measurements in this plot. As I have mentioned before, though, there is another common factor in the “early” measurements, which is that they are based on geometrical distances obtained from angular distances whereas most of the others are based on luminosity. If the Hubble tension were to resolve into a differences between these two types of measure then it would be of fundamental importance to cosmology. At present, however, there is nowhere near enough evidence to be sure one way or the other.