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Abstract:
At the present time, the two main contenders for Dark Matter (DM) are either ultra-massive particles (WIMPs) or ultra-light particles (Axions or ultra-light Axion-like particles). Astronomical evidence that DM may comprise ultra-light particles is building, but all suffer from uncertainties or ambiguities (just like for astronomical evidence that DM comprises ultra-massive particles). Here, we present arguably the cleanest test to distinguish between ultra-massive versus ultra-light DM particles: transients comprising individual background stars lensed by a foreground galaxy cluster augmented by lensing generated by individual stars in the cluster. Such transients are seen close to the cluster critical curve and skewed inwards where negative-parity images form. We show that an abundance of low-mass DM halos, predicted by cosmological simulations employing ultra-massive DM particles, predict transients that are skewed outwards of the cluster critical curve (positive parity). On the other hand, pervasive density modulations owing to quantum interference of ultra-light particles predict transients that are skewed inwards (negative parity) and have the same spatial distribution as is observed. This work further tilts the balance towards new physics invoking ultra-light Axions-like particles having masses of ~10-22 eV.
Biography:
Prof. Lim has been at HKU since 2009 and currently holds the position of Professor."
He leads a research group at the University of Hong Kong working on the astrophysical applications of gravitational lensing, with a particular focus on elucidating the nature of Dark Matter. Together with overseas collaborators, they have shown that different astronomical observations point towards the likelihood that Dark Matter comprises ultra-light particles (e.g., ultra-light Axion-like particles) rather ultra-massive particles (e.g., WIMPs) as has long been thought.
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