380 000 years after the Big Bang, the universe was almost smooth — density variations were no more than one hundred-thousandth. Today it's a grand ripple: in some places dark matter and galaxies crowd together, in others voids yawn. Like ripples on water, these waves stretch across billions of light-years. They were shaped by gravity, which pulled matter into clumps. For a long time, we thought the ripples were passive, just stretching along with the expansion of the universe, while the expansion itself was driven by a mysterious repulsive force.
The new study overturns this picture. It turns out that the curvature of space, its spacetime curvature, inside clumps and voids behaves not as a passive background. Its square doesn't vanish when averaged over a large volume and creates extra energy. Imagine an echo in a cave: the main sound fades, but the reflections still hum for a long time. It's the same here: when we average the density, we remove the direct peaks, but the afterglow of curvature remains. It nudges dark energy (the field that pushes space apart), forcing it to change over time.
To test this, scientists used two approaches: perturbation theory and gradient expansion (the method of smooth variations). Numerical simulations confirm: the contribution doesn't disappear. They computed corrections to the properties of dark energy: both turned out to have the same sign and be close in magnitude. This means dark energy can't suddenly change its state — for example, in the future it will either stay ordinary or always have been 'phantom' (with growing density). By measuring the redshift of distant galaxies, telescopes like Euclid and Roman will be able to refine the expansion history. But without accounting for the ripples, they might see a phantom transition where there is none. It was Edwin Hubble who discovered that the universe is expanding, and Saul Perlmutter and Adam Riess who found that the expansion is accelerating. Now a new layer is added to their discoveries: the acceleration is driven not only by dark energy but also by the very structure of the ripples.
So the ripples, once seen as just a pattern, have turned out to be the conductor of expansion. The circles on the cosmic pond don't just rock the galaxies: they change the depth of the pond itself.
The effect of curvature doesn't vanish even if the average spatial curvature is zero — the contribution comes from the unevenness itself, not the overall shape.
In Alan Lightman's book 'Einstein's Dreams,' time flows differently in different neighborhoods. Here the cosmic ripples do something similar: they determine how fast each region expands, and that changes the overall picture of the world.