Sunghoon Jung, Chang Sub Shin

Utilizing gravitational-wave (GW) lensing opens a new way to understand the history and structure of the universe. In spite of coarse angular resolution and short duration of observation, we show that LIGO can detect the GW lensing induced by small structures, in particular by compact dark matter (DM) of $10 - 10^5 M_{\rm sun}$, which remains an interesting DM candidate. The lensing is detected through GW frequency chirping, creating the natural and rapid change of lensing patterns: frequency-dependent amplification and modulation of waveforms. As a highest-frequency GW detector, LIGO is a unique GW lab to probe such light compact DM. With design sensitivity of Advanced LIGO, one-year observation can detect as many as 1000 lensed GWs and constrain compact DM fraction as small as $f_{\rm DM} \gtrsim 10^{-3}$.

José Luis Bernal, Alvise Raccanelli, Licia Verde, Joseph Silk

It is broadly accepted that Supermassive Black Holes (SMBHs) are located in the centers of most massive galaxies, although there is still no convincing scenario for the origin of their massive seeds. It has been suggested that primordial black holes (PBHs) of masses $\gtrsim 10^{2} M_\odot$ may provide such seeds, which would grow to become SMBHs. We suggest an observational test to constrain this hypothesis: gas accretion around PBHs during the cosmic dark ages powers the emission of high energy photons which would modify the spin temperature as measured by 21cm Intensity Mapping (IM) observations. We model and compute their contribution to the standard sky-averaged signal and power spectrum of 21cm IM, accounting for its substructure and angular dependence for the first time. If PBHs exist, the sky-averaged 21cm IM signal in absorption would be higher, while we expect an increase in the power spectrum for $\ell~\gtrsim 10^2-10^3$. We also forecast PBH detectability and measurement errors in the abundance and Eddington ratios for different fiducial parameter configurations for various future experiments, ranging from SKA to a futuristic radio array on the dark side of the Moon. While the SKA could provide a detection, only a more ambitious experiment would provide accurate measurements.