WoU-MMA: Multi-Messenger Gravitational Lensing with Asymmetric Lenses and Sources

University Of Texas At Dallas, Richardson TX

Investigators

Abstract

This award is concerned with three spectacular predictions of Einstein’s theory of general relativity: black holes (BHs), gravitational waves (GWs) and gravitational lensing. When massive compact objects such as BHs rapidly orbit each other and merge due to the huge gravitational force between them, they create ripples in space-time – GWs. These GWs are detected on earth as tiny changes in the lengths of the arms of a detector such as LIGO. The masses, spins, and orbital parameters of the BHs determine the observed frequency and amplitude of the GW signal as a function of time. Since the first detection by the LIGO collaboration in 2015, more than 90 binary compact-object mergers have been observed. This has opened an exciting new window on the Universe that complements what is seen using telescopes that capture electromagnetic (EM) waves such as optical light. When GWs or EM waves pass through the Universe they can be deflected and focused by intervening massive objects very close to their path. Although this strong gravitational lensing phenomenon has not yet been identified for GW sources, it is expected to split GW signals into multiple magnified copies, each taking a slightly different path to the observer. An EM analog of this strong lensing process, as seen on HST or JWST images, are the distant galaxies stretched into eye-catching arcs by foreground galaxy cluster lenses. Lensing signatures depend on the gravity of the intervening lens, making it an essential tool to weigh and study dark matter. With the increasing sensitivity of ground-based GW detectors and the advent of space-based detectors like LISA, GW sources will be seen to vast distances where the chance of an intervening lens is considerable. This award will increase our understanding of the properties and gravity of BHs across cosmic history, and the distribution of dark matter in intervening lenses. It will provide new theoretical models of strong lensing of GWs. The award will also facilitate several educational and outreach activities of benefit to society, such as an extension of the interactive VIGOR (Virtual Interaction with Gravitational Waves to Observe Relativity) simulation of binary black holes, development of curriculum materials for local high schools and undergraduate classes, and engagement with museums. Graduate and undergraduate students will be immersed in the research, inspiring and training the next generation of scientists for academia and industry. Increasing numbers of high-redshift GW events will be observed in the future, and a significant fraction of these sources will experience strong gravitational lensing. Most current research assumes axisymmetry in both the binary black hole (BBH) sources about their orbital angular momentum and in the lens models about the optical axis passing from the observer through the lens. These assumptions will be relaxed in three interrelated projects. (1) Lensing of extreme-mass-ratio inspirals (EMRIs) in which the orbital angular momentum is generically misaligned with the supermassive black hole (SMBH) spin, and where lensing could be misinterpreted as deviations from general relativity. (2) Lensing amplification factors for more realistic lens models that include embedded SMBHs at galactic centers, cored density profiles, and asymmetry in either the lens galaxy or an external shear. (3) Synergies between EM observations of source galaxies or transients, and GW sources. Simultaneously characterizing precession and lensing in GW sources is critical for their use as probes of both astrophysics and gravity. More realistic asymmetric lens models are also essential for real data analysis, as axisymmetric models qualitatively fail to account for additional images in the geometric-optics limit and employing more complicated wave-optics is essential over much of the source plane. Most BBH mergers will probably lack transient EM counterparts, so multi-messenger studies correlating GW and EM surveys will be required to identify host galaxies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.