A Quantum Call from the Infant Universe
In the first moments after the Big Bang, the Universe expanded faster than light could cross it. This burst, which physicists call inflation, stretched tiny quantum jitters to the size of galaxies. It is usually assumed that after such stretching the jitters lose their quantum character and become ordinary, like ripples on water. However, a new paper proposes a way to test whether a quantum link called entanglement has survived in them.
Entanglement is like a pair of coins that always land on opposite sides, even if tossed in different cities. If one comes up heads, the other instantly turns up tails, and this is not a coincidence but a deep connection. In the early Universe, such pairs could be born from gravitational waves, or more precisely, from their smallest quanta, gravitons. A graviton is like a photon, but it carries not light but the force of gravity. Every graviton has a polarization, that is, a direction of oscillation, like a stretched string.
The authors show that such an entangled pair leaves a trace in the distribution of matter when the gravitons interact with ordinary density perturbations. This trace can be sought in how galaxies align along invisible filaments of dark matter. To distinguish the quantum trace from a classical one, Bell's inequality, the famous test for entanglement, is used. If the measured quantity exceeds a certain limit, then the link was quantum.
The horizon of the Universe during inflation played the role of a measuring device. It collected information about the path each fluctuation took, and thereby destroyed the quantum link. This is similar to how observing a particle in an interferometer fixes its trajectory and destroys the interference. But even after such destruction, the correlation between polarizations persists, and it can be calculated from the shape of the four-point function, that is, from the statistics of the positions of four galaxies.
Today this trace can be sought in data from sky surveys such as Euclid or DESI. If the signal is found, it will be the first direct proof that quantum mechanics operated on the scales of the entire Universe at the moment of its birth. So far there is no such proof, but now we have a concrete recipe for how to obtain it. Perhaps the answer is already hidden in the distribution of galaxies, and all we have left to do is read it correctly.
The cosmological horizon during inflation acts as a measuring device: it gathers information about which path a fluctuation took, and in doing so destroys quantum coherence. It is similar to how observing a particle in an interferometer pins down its trajectory.
The idea that the large-scale structure of the Universe stores quantum correlations echoes a motif from Greg Egan's novel Schild's Ladder, where the fundamental laws of physics turn out to be encoded in the geometry of spacetime.