In the first instants after the Big Bang, the universe expanded faster than light could cross it. Like an inflating balloon, but instead of rubber, it was space itself. On this balloon, ripples inevitably formed — tiny compressions and rarefactions from which all galaxies and stars later grew. Physicists usually describe these ripples separately, as if they appear on a smooth, calm background of inflation. But is that really the case?
The authors asked: what if the ripples and the inflation process are not independent? In the quantum world, particles can be entangled — linked so that the state of one instantly determines the state of the other, even at a distance. It is like two pendulums swinging in sync: nudge one, and the other responds, even though nothing connects them. The researchers proposed that the wave of the universe describing inflation and the wave of the ripples could be entangled in exactly the same way. Then the ripples we see in the sky as faint temperature fluctuations in the cosmic microwave background should carry the imprint of this connection.
The scientists built a simplified model where inflation leads to a future resembling today's accelerating universe. They calculated what the ripple spectrum looks like if entanglement is honestly accounted for, rather than discarded as was done before. It turned out that deviations from standard predictions do exist. They are tiny, but importantly, noticeably larger than the corrections usually associated with quantum gravity. It is like searching not for a grain of sand on a beach, but for a small pebble — still difficult, but no longer hopeless.
For now, the effect is too small for today's telescopes. But future maps of the cosmic microwave background might be able to detect it. Then we would see, for the first time, direct evidence that the birth of structure in the cosmos was a quantum-entangled process. And that would change our understanding of the very beginning, as if we finally heard not just the melody, but the full orchestration of the music of the Big Bang.
Temperature fluctuations in the cosmic microwave background are only about 0.001% of the average: that is like noticing a bump of one hundredth of a millimeter on a perfectly polished table a meter long.
In Carl Sagan's novel Contact, the protagonists find a message from the creators of the universe in the cosmic microwave background — perhaps the real imprint of quantum entanglement will turn out to be just as mysterious.