Titile: Dark Matter under the Gravitational Lens

Speaker: Jeremy Lim

Institute: University of Hong Kong

Host：方陶陶

Time：2026年10月13日周二 14:30

Location：物理楼552

Abstract：

The existence of Dark Matter (DM) is the clearest sign that the Standard Model of particle physics is incomplete.  Hypothetical particles invoked by different theoretical extensions to the Standard Model constitute the leading candidates for DM; the identification of the DM particle will therefore point the correct path towards New Physics.  In lieu of a laboratory detection, the most powerful tool for testing the nature of the DM particle is provided by gravitational lensing.  In this talk, I present two tests of whether the DM particle comprises ultra-massive particles such as WIMPs or ultra-light particles such as Axions, the two leading contenders for DM.  First, we show that lensing anomalies (an over 2-decade-old problem in astrophysics) can be resolved if quantum interference among ultra-light DM particles generate pervasive density modulations in the DM halos of galaxies.   Second, in a very clean test, we show that the spatial distribution of lensed star transients across the critical curves of galaxy clusters demand pervasive density modulations in the DM halos of galaxy clusters.  The inferred density modulations in the DM halos of galaxies are required to have characteristic scales of ~100 pc, whereas those in the DM halos of galaxy clusters are required to have characteristic scales of ~10 pc: both consistent with the de-Broglie wavelengths of ultra-light particles having masses of ~10-22 eV. These tests – along with others involving observations of baryonic light – tilt the balance toward New Physics invoking ultra-light particles such as (ultra-light) Axions.

Bio：

Jeremy Lim is a Professor in the Physics Department at the University of Hong Kong.  Born in Malaysia, he obtained his PhD in Australia (a naturalised citizen of this country), before holding postdoc positions at the University of Maryland and CalTech in the USA.  He served as a staff member at the Academia Sinica Institute of Astronomy and Astrophysics for over a decade, before joining the University of Hong Kong in 2009.  He leads a large team at the University of Hong Kong working on the astrophysical applications of gravitational lensing – with a particular focus on elucidating the nature of the Dark Matter particle.  In his spare time, he works on outstanding problems in galaxy clusters.  Apart from research, he is passionate about teaching both undergraduate courses as well as high-school programs, and fostering research in astrophysics in South-East Asia.

Related papers:

https://iopscience.iop.org/article/10.3847/2041-8213/ad9aa8/pdf

https://www.nature.com/articles/s41550-023-01943-9