StochRewind · Stochastic Rewind and fast-forward: calibrating LISA with LIGO's black holes and stochastic background

Stochastic Rewind and fast-forward: calibrating LISA with LIGO's black holes and stochastic background

Since the first direct measurement of a gravitational wave (GW) from a merging binary system of stellar-mass black holes (BH) in 2015 by the Laser Interferometer GW Observatory (LIGO), the field of GW astronomy has made giant leaps, counting now over 100 events. The catalogue of individually detected events grows with each new data run, and is used to infer the properties of the underlying population of merging binary BHs (BBHs) in the Universe. Furthermore, ground-based GW detectors such as LIGO, Virgo, and KAGRA (LVK) can potentially see much more than the individual signals; the resolved events counted in the catalogues make up a tiny fraction of all the GWs which reach our detectors, while most lie below the confusion limit and go undetected. Like voices in a crowded room, the collection of unresolved events gives rise to a background signal, referred to as the stochastic GW background (SGWB). While we witness the explosion of ground-based detector science, the GW community is actively preparing for the upcoming revolution: the Laser Interferometer Space Antenna (LISA). Led by the European Space Agency, the LISA constellation will provide a deep coverage of the low-frequency GW sky, which is populated not only by unique sources (e.g. supermassive BHs, extreme mass ratio inspirals) but also by the same stellar-mass BHs observed by LIGO; crucially, these are observed at a much earlier phase of their lives. The overlapping of many sources, both in time and in frequency, will make LISA data analysis an extremely challenging task, which is why the so-called “LISA global fit” is now the holy grail of GW astronomy. It is necessary to fully exploit the data from LVK today to dig deeper in the LISA data of tomorrow: Calibrating LISA with LIGO’s black holes and stochastic background is exactly the bold idea behind the StochRewind project. We (i) deliver crucial improvements to current measurements of the stochastic GW background, (ii) combine it with LIGO’s individual detections, and (iii) exploit it to predict the stochastic background of stellar-mass BHs in LISA. This is a novel handle to anchor one of the many moving parts of the LISA global fit using products from the current ground-based detector network. Why now: The LISA ground segment is under active development and many of its foundational choices are still being made. Completing StochRewind now is vital to fully realise the promise of GW astronomy. Combining both individual and stochastic BBH signals in LVK and LISA, StochRewind will pioneer an holistic view of GW astronomy, bound to have repercussions for decades to come.

Data: CORDIS, © European Union