Why should it be that galaxies stand still where the greatest part of the universe can be overlooked? The latest reconstruction of the dark matter by the James Webb Space Telescope turns that question into a geometry problem that is solvable: observe enough distant galaxies, measure the distortion of their shapes, and deduce the mass that is doing the bending. In a COSMOS-field study by Jacqueline McCleary and others, the researchers took into account the apparent shapes of some 250,000 background galaxies and constructed the most detailed dark matter map to date, over the area of the sky equal to about the size of the full Moon. The conclusion solves the cosmic web almost 2 times sharper than the previous Hubble based maps and exposes structures that have no counterpart that is bright.
It is weak gravitational lensing, a regime in which the distortions are extremely small and disappear to the eye. They are, statistically, cumulative. “Those galaxies are basically the cosmic wallpaper,” or the reason is not the inherent diversity of the galaxies but their diversity as a whole in response to the foreground gravity having acted. Practically, scientists analyze each galaxy as a noisy shape measurement; only when the samples are very large are the “shear” shapes patterns visible, following the mass distribution that is not visible to light itself.
That framing is one way of understanding the reason that Webb transforms the game. The number of usable background galaxies and their clarity limits the number of weak lensing maps rather than the telescope time. The infrared sensitivity of Webb increases the number of faint and high-redshift sources and enhances the measurements of shapes in the rich deep fields. The COSMOS area has long been analyzed, but the imaging by Webb takes the lensing signal back to earlier periods, such as 10-11 billion years ago, when stars were actively formed in galaxies. The map is not just a picture in this sense: it is a piece of information, which links the distribution of matter to the growth of galaxies over much of the length of cosmic time.
Among the most striking elements, by far, are the most disturbing ones: filaments, clumps, which are visible in the lensing reconstruction, without a visible concentration of the visible galaxies. Those discrepancies are significant since they challenge one of the fundamental premises of cosmology, which is that ordinary matter can tend to settle into the same gravitational wells that the dark matter has bored out. In places where such a correspondence is lost, we are left with measurement limits or the complexity of astrophysics or an incomplete account of the manner in which matter collects.
To date, the Webb map generally fits the lambda-cold dark matter perspective, the standard model in which a dark matter is the long-lived scaffolding of the universe and a dark energy is the cause of accelerated expansion. McCleary’s caution remains intact: “Although at a glance it’s a match for lambda-CDM, I’m not giving up yet – I’m withholding judgment until our analysis is finished.” The more valuable is that sharper maps turn the qualitative agreement into quantitative stress tests and narrowing the range within which alternatives may conceal.
These stress tests are also coming in a new direction. This was alongside an independent, large-scale project, the Dark Energy Survey, which compared weak lensing and galaxy clustering over 6 years of observations to find expansion history constraints that were said to be over twice as strong as previous DES analyses, on a catalog of 669 million galaxies in 758 nights. A single parameter of the way matter clumps at late times continues to exhibit the ongoing tension with the early-universe expectations, despite the overall results being widely in line with the standard cosmology.
Collectively all these approaches are complementary in nature. Webb gives a slender, yet incredibly detailed map of lensing – an anatomical scan of the cosmic web, whereas surveys like DES give a context of the population at scale on huge portions of sky. The next procedure with the Webb field is to scale the map to depth and make use of the data on the distances to create a three-dimensional reconstruction and to tie specific populations of galaxies to the dark matters they inhabit. In weak lensing, to map the universe, the universe does not have to shine. It merely requires to bend light, by per cent. or more, in the same direction sufficiently frequently, to cause mathematics to be able to see what the eyes cannot.
