The DESI spectrograph gazed billions of light-years into the cosmos and saw a strange picture. Light from distant galaxies reaches us with a redshift — spectral lines shifted toward the red, like the siren of a receding vehicle. But the shift turned out to be not quite what the laws predict. This suggests that something is off with the expansion of the universe.

In 1998, Adam Riess, Saul Perlmutter, and Brian Schmidt used light from supernovae to discover that the expansion is accelerating. The culprit was dubbed dark energy. But DESI data hint that the strength of this energy may not be uniform everywhere. It's as if the cosmic sea of dark energy has become rippled.

One idea is that we've stumbled into a local bubble. Inside it, the density of dark energy is 10% lower than outside. Such a bubble could have arisen during a phase transition — a vacuum jump, akin to a bubble forming in boiling water. If we're not at the center, light from distant objects passes through its wall, where spacetime curvature changes, acting like a funhouse mirror: distances measured using the large-scale structure (frozen sound waves from the early universe) get distorted.

But the universe has a perfect 'polygraph' — the cosmic microwave background. This faint glow, leftover from the fireball of the early universe, is nearly uniform. But any large bubble would necessarily distort this picture. Electrons in galaxy clusters scatter the warped light, producing a noticeable signal. Calculations show it should be hundreds of times brighter than what we see in the sky.

So the sea of dark energy remains smooth for now. The bubble idea has been discarded, but it's not a failure — now we know that any inhomogeneities in dark energy must reveal themselves in the ancient light. Upcoming surveys like Euclid will hunt for exactly these traces.