The early universe resembled an overcrowded hall where heavy particles collided and decayed at a deafening pace. Among them could have been scalar leptoquarks — particles that simultaneously latch onto a quark (a building block of matter) and a lepton (such as an electron). Within the framework of grand unified theory, where all forces of nature merge into one, these leptoquarks must be extremely massive. Otherwise, the proton would decay too quickly, and experiments do not see that.
When such a behemoth decays, quantum gravity forces it to emit gravitons — quanta of gravitational waves. This is akin to bremsstrahlung: the particle abruptly changes its state and 'shudders' with a ripple in spacetime. The heavier the leptoquark, the more pronounced this process. The authors traced how the density of these particles changed in the expanding universe by solving the Boltzmann equation — the primary tool for describing the evolution of particle number. It turned out that the gravitational waves thus generated should fill the cosmos with a high-frequency background.
Such a signal cannot be caught by conventional laser interferometers like LIGO. Instead, detectors based on resonant cavities are needed — essentially tiny metal chambers where a gravitational wave makes the walls vibrate at a strictly defined frequency. If the signal is detected, it would be the first direct evidence of the quantum nature of gravity and a window into an era when all interactions were unified.
Gravitational waves from leptoquarks could have frequencies on the order of gigahertz — roughly the same as your Wi-Fi router emits, but instead of carrying internet cat videos, they carry the imprint of the universe's birth.
In Carl Sagan's novel 'Contact', an alien signal contains blueprints for a machine capable of generating gravitational waves for travel through spacetime — much like leptoquarks naturally produce them during decay.