On the connection between PTAs and LISA

Massive black hole (MBH) binaries are powerful sources of low-frequency gravitational waves (GWs), and these objects should form as a natural consequence of galaxy mergers. The strong evidence for a stochastic, low-frequency GW background presented last year by NANOGrav and other pulsar timing arrays (PTAs) is consistent with theoretical predictions for a cosmic population of MBH binaries. If indeed the PTA signal is dominated by MBH binaries, this constitutes the first robust indication that MBH binaries can overcome the final parsec problem and merge in less than a Hubble time. This exciting development raises promising prospects for constraining MBH evolution not only in the supermassive, low-redshift regime, but also in the earliest epochs of their existence. The Laser Interferometer Space Antenna (LISA), planned for launch in the late 2030's, will be able to detect MBH mergers out to z ~ 20. LISA will be sensitive to lower-mass MBH mergers, making it an ideal probe of the poorly understood origins and early evolution of MBHs. JWST is also opening a new window into this regime as it uncovers high-redshift accreting MBH candidates at a rapid pace. I will describe a suite of recently developed models for MBH formation in cosmological hydrodynamics simulations, including a novel prescription for representing the descendants of low-mass MBH seeds in large-volume simulations. Predictions of GW source population characteristics from such models will be crucial for disentangling MBH seeding and growth channels using LISA data, as well as combining multi-messenger data from PTAs, LISA, and JWST to build a more comprehensive picture of MBH evolution across cosmic time.