The early universe wasn't silent. Long before stars and galaxies, in the era of inflation, quantum fluctuations stretched to cosmic scales. Amplified clumps of spacetime, upon compression, gave birth to primordial black holes — objects that could become the seeds of supermassive monsters or the dark matter from which the universe is woven. But, as it turns out, these same processes set off a dual gravitational-wave concert.

Imagine striking a bell. The low hum is the scalar-induced background of gravitational waves, born as inhomogeneities enter the horizon. Pulsar timing arrays listen to it at nanohertz frequencies, and LISA will catch it at millihertz. The high overtones are bursts from the mergers of the black holes themselves, sounding at hundreds and thousands of hertz, accessible to LIGO and the future Einstein Telescope. Both voices arise from the same primordial curvature spectrum — one hammer strike on space created an entire chord.

This approach turns gravitational-wave astronomy truly multi-wavelength: from nanohertz to kilohertz. Constraints on the abundance of primordial black holes from the cosmic microwave background and galactic dynamics can now be translated into predictions for ground-based detectors. Future projects like the Einstein Telescope or the DECIGO space observatory will be able to test inflation models on scales previously inaccessible to direct observation.

We see a picture of the early universe as an orchestra, with the score written before matter even existed. We just need to add a third part — gravitational waves from the evaporation of light black holes via Stephen Hawking's mechanism. Then the chord becomes a full triad, and physics gets a unique calibrated tool to study what happened just before expansion. The most surprising part: this connection works for any inflation model, turning gravitational waves into a universal tuning fork, tuned to the frequency of the Big Bang itself.