Talks

Primordial black hole (PBH) dark matter can be probed by "picolensing". Widely spatially separated gamma-ray detectors near Earth would observe parallax of an intervening PBH lens with respect to a cosmologically distant gamma-ray burst (GRB). This parallax can be of order the Einstein angle of the lens, resulting in differential magnification of the source as viewed from the two detectors. Simultaneous brightness measurements of the same GRB made by two detectors is sensitive to this effect. Two recent studies in the literature have shown this approach could be a promising way to search for PBH dark matter in part of the "asteroid mass window", roughly (few) * 1e-15 < M_{PBH} / M_{Sun} < (few) * 1e-11. In this talk, I will discuss some ongoing work to explore the robustness of this signal to various uncertainties not previously carefully accounted for: e.g., uncertainties in the transverse extent of the GRB emission region, its intensity profile, detector background rates, sensitivity of the projection to outlier GRB events, etc. I'll show that, while the large GRB source size uncertainties do degrade previous projections somewhat, it is still possible to probe most of the PBH DM asteroid mass window with a mission that employs two Swift/BAT-class detectors separated by a distance on the order of an AU. Depending on the total number of GRBs that such a mission ultimately observes, it may even be possible to robustly probe new subcomponent dark-matter parameter space at PBH masses above the window, potentially as high as (few) * 1e-6 M_{Sun}.

Pulsar timing arrays (PTAs) consist of a set of regularly monitored millisecond pulsars with extremely stable rotational periods. The arrival time of pulses can be altered by the passage of gravitational waves (GWs) between them and the Earth, thus serving as a galaxy-wide GW detector. Evidence for the first detection of low-frequency (~nHz) gravitational waves has recently been reported across multiple PTA collaborations, opening a new observational window into the Universe. Although the origin of the GW signal is yet to be determined, the dominant sources are expected to be inpiralling supermassive black holes (SMBHs). I will discuss a recent work in which we compare the GW detections by PTAs with the expected signal implied by existing electromagnetic observations in a simple but robust manner. This study suggests that the currently measured GW amplitude is larger than expected by a significant amount. I will then show that additional information regarding the typical number of sources contributing to the background can already be inferred from current PTA data.