Abstract
Unveiling the true nature of Dark Matter (DM), which manifests itself only through gravity, is one of the principal quests in physics. Leading candidates for DM are weakly interacting massive particles (WIMPs) or ultralight bosons (axions), at opposite extremes in mass scales, that have been postulated by competing theories to solve deficiencies in the Standard Model of particle physics. Whereas DM WIMPs behave like discrete particles (ρDM), quantum interference between DM axions is manifested as waves (ψDM). In my talk, I present our research (Amruth et al. Nature Astronomy (2023)) which shows that gravitational lensing leaves signatures in multiply-lensed images of background galaxies that reveal whether the foreground lensing galaxy inhabits a ρDM or ψDM halo. Whereas ρDM lens models leave well documented anomalies between the predicted and observed brightnesses and positions of multiply-lensed images, ψDM lens models correctly predict the statistical level of anomalies left over by ρDM lens models.
More challengingly, when subjected to a battery of tests for reproducing the quadruply-lensed triplet images in the system HS 0810+2554, ψDM is able to reproduce all aspects of this system whereas ρDM often fails. In a second test of ψDM, we look at the observations of the first gravitationally lensed type Ia supernova, iPTF16geu, which exhibits anomalous brightnesses when ρDM lens models are used. I show that not only can no smooth ρDM lens model reproduce the brightnesses, but that iPTF16geu also exhibits anomalous positions in addition to anomalous brightnesses. I then show that we can resolve these anomalies with lens models constructed using ψDM.
This work paves the way for future tests of ψDM in the context of gravitational lensing. The most immediate applications are to other systems involving a single lensing galaxy, in particular to other multiply lensed SN Ia that have recently been found. Future work should also address lensing anomalies found in galaxy clusters, for which I show how density fluctuations can be imposed on ϱDM lens models to mimic ψDM lens models. The ability of ψDM to resolve lensing anomalies even in demanding cases like HS 0810+2554, together with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axions.
Anyone interested is welcome to attend.