The space antennas LISA and Taiji each consist of three satellites flying around the Sun behind Earth, forming a triangle with sides millions of kilometers long. They measure how the distance between them changes when a gravitational wave passes, like ripples on a pond from a thrown stone. But the satellites themselves don't stay still: their triangle breathes and rotates, and with it, their ability to pick up signals from different parts of the sky changes.
The authors built a precise model of this motion over a full year. It turns out that at low frequencies, the sensitivity resembles a four-lobed map — in some directions the detector hears better, in others worse. The difference from the usual 'averaged' approach reaches 20%. And when they recalculated how many sources could be spotted, the spread was already 70%.
This means that for a reliable count of binary stars and black holes in our Galaxy and beyond, a single sensitivity number is not enough. A full celestial map that updates every moment is needed. Without it, weak signals could be missed or source properties misjudged.
as written by the authors arXiv:2606.28071
Space-based gravitational-wave (GW) laser interferometers, including LISA and Taiji, are designed to observe gravitational waves in the millihertz band and are expected to open up a frequency range that is otherwise inaccessible. The sensitivity and response of these instruments are central to their scientific goals, mission design and parameter estimation capabilities. However, they are commonly modeled as static, equilateral triangular constellations, an approximation that neglects both orbital motion and directional dependence. In this work, we systematically examine the direction-dependent response and sensitivity of dynamic LISA-like detectors over an entire year of heliocentric orbit. Based on an analytical, time-dependent heliocentric orbital model and an adiabatic unequal-arm interferometer configuration, we construct direction-dependent sensitivity curves in the Michelson interferometric channel for dynamic LISA and Taiji. We obtain analytic expressions for the angular-dependent sensitivity and demonstrate the emergence of a quadrant-like pattern in sky maps at low frequencies. We show that, relative to the static approximation, the low-frequency sensitivity varies by roughly $$20\%$$, which in turn produces about a $$70\%$$ variation in the directional dependence of the number of detectable GW sources, with even larger discrepancies at higher frequencies. Therefore, for accurate predictions of the total GW source counts and reliable parameter inference for binary systems, it is necessary to employ fully dynamic, direction-dependent sensitivity curves.
The frequency of gravitational waves from double white dwarfs is comparable to the rhythm of a calm heartbeat — one beat every few tens of seconds.
In Carl Sagan's novel 'Contact', a signal from space is received by a network of radio telescopes — LISA and Taiji could be the first to hear the 'gravitational whisper' of the Universe.