The universe remembers: astronomy with non-linear memory of gravitational waves, Shubhanshu Tiwari, University of Zurich

Non-linear memory is one of the most intriguing predictions of general relativity which is generated by the passage of gravitational waves (GWs) leaving the spacetime permanently deformed. For example a GW signal from binary black hole (BBH) will have two parts the oscillatory part which is known as the “chirp” and a much fainter non-oscillatory (DC like) part which is non-linear memory. GW interferometer will perceive this signal not as the oscillations of the mirrors but as a persistent change in the relative position of the mirrors. Due to non-linear memory the change in the position of the mirrors are persistent, but the signature of non-linear memory on the GWs detectors can only be captured while the wave is passing through. This makes non-linear memory a transient signal peaking at the lowest possible frequency a detector can measure. A non-linear memory is produced by all the sources of GWs and  has the peculiarity that even if the oscillatory part of the source lies at high frequency the non-linear memory will be available at low frequency. This property of non-linear memory makes it a valuable resource for GW data analysis.

In this talk I will provide and introduction to how we can use gravitational waves memory as a resource for the current and future ground based detectors. To do this I will recap some of the noteworthy/interesting detections recently made by the LIGO-Virgo-KAGRA collaboration where the nature of the compact object is not fully known. I will also motivate how we can creatively use the non-linear memory to probe seemingly inaccessible sources of GWs like ultra low mass compact binary mergers where the oscillatory part lies at outside the reach of any current detectors and only non-linear memory could be detected if these sources exist.