The early universe was an extremely hot and dense medium. In this medium, like bubbles of steam in boiling water, bubbles of a new state of space spontaneously emerged — regions where physical properties changed abruptly. Usually, such bubbles are considered relatively simple structures, but some of them carried a colossal number of hidden 'patterns'—microscopic states that physicists call entropy. Imagine a soap bubble: its thin film shimmers with a rainbow, but here the bubble's surface glows not with seven colors but with billions of different shades, each corresponding to a separate quantum variant.
When such bubbles collide and merge, their quantum entanglement does not vanish without a trace but significantly affects the dynamics of the merger. This influence leaves a characteristic imprint on gravitational waves — ripples in the fabric of spacetime that travel at the speed of light. Unlike the merger of black holes, where information about the internal structure, according to classical ideas, seems to be lost to an outside observer, quantum bubbles generously pass on their 'memories' through the gravitational-wave signal. In this signal, a clear quantum trace is preserved — an echo of their multiplicity, manifesting in subtle features of the waveform.
Future detectors, whose sensitivity continues to improve following the example of instruments built by Rainer Weiss (a pioneer of laser interferometry for detecting gravitational waves), could pick up these characteristic signals. Detecting such a quantum echo would allow us to glimpse the quantum 'personality' of space itself, revealing its fundamental nature at a level inaccessible to any other experiment.
Each bubble was born in all possible states at once, as if a tossed coin landed both heads and tails simultaneously. The bubble's complexity was astronomical: the number of its microstates exceeds the number of atoms in the observable universe.
Science fiction often plays with the theme of multiple universes, but here the 'bubble universes' are also quantum-entangled, as if they share a common memory of the moment of creation.