The field of cosmology has been in crisis for a while. One of the most important parameters in cosmology, known as the Hubble constant (H0), seems to have different values depending on how you're measuring it. H0 characterises how quickly the universe is expanding and tells us how fast something is moving away from us given its distance (with an unusual unit of kilometres per second per megaparsec). 

One method to calculate H0 uses the Cosmic Microwave Background (CMB), which gives a value of 67.4 +/- 0.6. Another method uses the cosmic distance ladder to look at Cepheid variables in nearby galaxies and gives a value of 73.04 +/- 1.04. We can clearly see that these two values differ, and more importantly, their error bars aren't close to overlapping. This discrepancy is known as the Hubble tension.

The two approaches above are the current primary methods which yield the most precise values, but there are also other ways to calculate H0. For example, a recent release by Pascale et al. uses 'time delay cosmography' – a technique which utilises gravitational lensing to observe an astronomical transient at various stages of its life.

Pascale et al. were processing JWST data back in March 2023 when they noticed something weird. They were looking at a massive galaxy cluster, dubbed PLCK G165.7+67.0, which is gravitationally lensing several background galaxies. They found one particular lensed galaxy that showed up three times with a supernova explosion within it.

JWST made two more observations of the cluster over the next couple months, including spectroscopic data. They were able to determine that the supernova was of Type Ia, which occur when a white dwarf becomes too big. When a gravitationally lensed object shows up multiple times (like in this case), it means that its light has taken multiple different paths around the massive object doing the lensing (the galaxy cluster). Sometimes these routes have different lengths, meaning that each copy would represent a different time relative to one another. If the object being lensed is something which varies in brightness over time (i.e., a supernova), then we can get a good idea as to how much of a delay there is between images. We thus have 9 snapshots of the galaxy in total – 3 JWST images containing 3 different copies of the galaxy.

Using the light curve, we're able to calculate the delay between the three lensed images. In addition, you could also calculate the expected delay by generating a model of the lens itself. Comparing these delay parameters then allows us to obtain an estimate for H0! 

The most precise value determined was 75.4 +8.1/-5.4 – the error bars are quite large, so it's hard to conclusively say how it compares to other H0 values. The team behind this paper will continue looking at the supernova till it has completely faded to get even more accurate brightness measurements, and they're also scanning the sky for more multiply imaged Type Ia supernovae. With just a few more, they'll be able to bring their error bars to be comparable to the CMB and Cepheid measurements. Super exciting !!

published: 20/04/24 by kaan evcimen