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.
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.