When a car on cruise control hits a bump, the speed jumps for a moment, but the engine runs smoothly. Something similar was recently noticed with dark energy — the force that makes the Universe expand faster and faster. Scientists describe it with the parameter w: if it drops below -1, the Universe would accelerate toward the 'Big Rip.' DESI project data showed that w twice briefly dipped into this dangerous zone, but without disaster. Usually, such behavior requires weird physics, but a new model gives a simple explanation.
In it, dark matter particles interact with dark energy like a bump in the road: their coupling gently nudges the system, and the parameter w temporarily falls below -1, while dark energy itself remains stable. The model was checked against three sources: distances to galaxies (DESI), properties of the cosmic microwave background (Planck), and the brightness of distant supernovae. Everything matched.
The most unexpected conclusion: dark matter particles must be incredibly light — around 0.002 electronvolts. The electron's mass is almost half a million times larger. Such 'fluffs' might be part of an unknown family of particles yet to be discovered.
Dark energy behaves like water that momentarily boils without forming bubbles — pressure spikes, but the system quickly settles down.
The idea of unstable dark energy spawned many stories about the 'Big Rip,' such as in Paul McAuley's novel 'The War with the Remotes'.