The origin of supermassive black holes at galactic centers remains a major mystery in astronomy. These black holes may have formed in the early Universe or grown over time by consuming matter and other black holes. When a supermassive black hole consumes another large black hole, it emits gravitational waves, which are ripples in spacetime.

The Challenge of Detecting Massive Black Holes
Gravitational waves have been detected from small stellar black holes, the remnants of stars. However, detecting signals from pairs of supermassive black holes is currently impossible due to the low frequencies of their gravitational waves. Future detectors like the space-based ESA-led mission LISA will improve sensitivity but still may not detect the most massive black hole pairs.

High Frequencies to Measure Lower Frequencies
The research team, led by former University of Zurich students, proposes using gravitational waves from small stellar black hole binaries to detect larger black hole pairs. This method, requiring a deci-Hz gravitational-wave detector, would uncover supermassive black hole binaries that are otherwise undetectable.

"Our idea basically works like listening to a radio channel. We propose to use the signal from pairs of small black holes similar to how radio waves carry the signal. The supermassive black holes are the music that is encoded in the frequency modulation (FM) of the detected signal," said Jakob Stegmann, lead author of the study who started this work at the University of Zurich as a visiting student and since then moved to the Max Planck Institute for Astrophysics as a postdoctoral research fellow. "The novel aspect of this idea is to utilize high frequencies that are easy to detect to probe lower frequencies that we are not sensitive to yet."

A Beacon for Larger Black Holes
Recent findings from pulsar timing arrays suggest the existence of merging supermassive black hole binaries, but this evidence is indirect and based on collective signals from many distant binaries.

The proposed method leverages changes caused by supermassive black holes in the gravitational waves emitted by nearby small stellar-mass black hole binaries. This technique acts like a beacon, revealing larger black holes. Detecting subtle modulations in signals from small black hole binaries could identify hidden supermassive black hole binaries with masses from 10 million to 100 million times that of our Sun, even at vast distances.

Lucio Mayer, a co-author and black hole theorist at the University of Zurich, added, "As the path for the Laser Interferometer Space Antenna (LISA) is now set, after adoption by ESA last January, the community needs to evaluate the best strategy for the following generation of gravitational wave detectors, in particular which frequency range they should target - studies like this bring a strong motivation to prioritize a deci-Hz detector design."

Research Report:Imprints of massive black-hole binaries on neighbouring decihertz gravitational-wave sources

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