Talks

The 21-cm line from neutral hydrogen holds immense potential as a probe of early astrophysics and cosmic evolution. Realizing the potential of this observational probe, however, is limited by our ability to control systematic effects in the data and model the cosmological 21-cm signal. In this talk, I will provide an overview of the observational prospects made possible through the cosmic 21-cm signal and discuss the challenges confronting interferometric experiments that are targeting a high-redshift detection. I will focus on recent developments from the Hydrogen Epoch of Reionization Array (HERA), covering our latest upper limits on the 21-cm power spectrum and their implications for early X-ray heating of the intergalactic medium. I will additionally discuss our latest efforts to understand and mitigate mutual coupling, a complex systematic effect in the data that threatens to thwart our efforts to detect the cosmological 21-cm signal. I will conclude by discussing the path forward with HERA, with a preliminary view of what to expect from forthcoming analyses.

According to general relativity, the remnant of a binary black hole merger is a perturbed Kerr black hole. Perturbed Kerr black holes emit "ringdown" radiation which is well described by a superposition of damped exponentials ("quasinormal modes”), with frequencies and damping times that depend only on the mass and spin of the remnant. The observation of gravitational radiation emitted by black hole mergers might finally provide direct evidence of black holes, just like the 21 cm line identifies interstellar hydrogen. I will review the current status of this "black hole spectroscopy" program. I will focus on: (1) the role of nonlinearities in ringdown modeling, (2) the current observational status of black hole spectroscopy, and (3) future prospects for the observability of modified gravity effects and nonlinear modes.