Some synths give you a filter and a couple of oscillators and call it a day. CosmicSynth hands you two colliding black holes and says go make a record with that. It is a free VST3 built by Kael Barba that turns real gravitational wave physics into playable instruments, and I am still wrapping my head around how much fun it is.
There are five presets and each one is a different cosmic event. Binary Pad holds a chord that wobbles with the strain of two black holes spiraling into each other, then the merger sweeps through the chord every few seconds. Inspiral Pluck starts every note below pitch and chirps it up along the merger curve until it lands in tune 140 milliseconds later, which sounds exactly as weird and wonderful as it reads. Ringdown Bell plays the ringing of the black hole left behind after the merger, tuned by the remnant's measured spin, and the overtones sit at non-integer ratios so it rings like a proper metallic bell.
Then it gets stranger in the best way. Kozai-Lidov Triple pumps the orbit's eccentricity so the tone gets buzzier and pulses once per orbit, and Gravitational Lens splits the signal into two arrivals so a heavy lens gives you an echo up to a second late while a light one flanges it. There is a live spectrogram running the whole time so you can watch the physics happen while you play, plus depth controls for amp, pitch, filter and timbre, and a GW LIVE switch that turns the physics off if you just want the plain synth underneath.
You can even load your own waveform data as a CSV and the plugin measures the merger time, ringdown frequency and remnant spin from it. It ships with a built-in simulated merger of two 11.5 solar mass black holes so it works straight out of the box, and the whole thing is open source.
The honest bits: it is Windows only and VST3 only, so Mac folks are out of luck for now, and this is version 0.1.0 from a first-time developer, so expect rough edges rather than a polished flagship. But for industrial and dark ambient work, those chirps and ringdowns are the kind of sounds you cannot get anywhere else. I fed a slow pad through it and got something that sounded like the universe clearing its throat.
Price: free and open source. VST3 on Windows.
Grab it here: https://github.com/barkael327-a11y/cosmicsynth