LISA sources and science

TL;DR

The article surveys LISA sources organized by spectral character, highlighting stochastic cosmological backgrounds, galactic binary stars, massive black hole mergers, and extreme mass ratio inspirals as key regimes in the (0.03-0.1) mHz to 0.1 Hz band. It emphasizes how LISA can perform high-precision parameter estimation, measuring black hole masses and spins to percent-level accuracy and tracking their cosmic growth, while also offering deep probes of strong-field gravity through EMRIs and potential multi-messenger opportunities with MBH mergers. The discussion underlines the discovery potential of cosmological backgrounds (e.g., phase transitions and cosmic strings) and the practical prospects for identifying electromagnetic counterparts to MBH coalescences. Overall, LISA promises a transformative view of black hole growth, galaxy evolution, and spacetime dynamics in the distant, dynamical universe.

Abstract

LISA is a planned space-based gravitational-wave (GW) detector that would be sensitive to waves from low-frequency sources, in the band of roughly (0.03 - 0.1) mHz < f < 0.1 Hz. This is expected to be an extremely rich chunk of the GW spectrum -- observing these waves will provide a unique view of dynamical processes in astrophysics. Here we give a quick survey of some key LISA sources and what GWs can uniquely teach us about these sources. Particularly noteworthy science which is highlighted here is the potential for LISA to track the moderate to high redshift evolution of black hole masses and spins through the measurement of GWs generated from massive black hole binaries (which in turn form by the merger of galaxies and protogalaxies). Measurement of these binary black hole waves has the potential to determine the masses and spins of the constituent black holes with percent-level accuracy or better, providing a unique high-precision probe of an aspect of early structure growth. This article is based on the "Astrophysics and Relativity using LISA" talk given by the author at the Seventh Edoardo Amaldi Conference on Gravitational Waves; it is largely an updating of the author's writeup of a talk given at the Sixth International LISA Symposium.

Figures (1)

• Figure 1: Left panel: Distribution of measured mass errors from a Monte-Carlo simulation of $10^4$ binaries, randomly distributed on the sky, randomly oriented, and with random spins and spin orientations. All binaries are placed at redshift $z = 1$, and have masses $m_1 = 10^6\,M_\odot$, $m_2 = 3\times10^5\,M_\odot$. The solid line gives the distribution of errors in $m_1$, the dotted line gives the distribution in $m_2$. Both distributions peak at a relative error of about $0.1\%$, and are almost entirely confined to errors smaller than $1\%$. Right panel: Errors in the Kerr spin parameter $\chi = |{\bf S}|/M^2$. For most events in the distribution, the spins are determined to within $\delta\chi = 0.01$. The spin of the larger black hole (solid line) is typically better measured than that of the smaller hole (dashed line), since the magnitude of its spin vector $|{\bf S}|$ is larger and has a more important impact on the waveform.