The universe did not always speed up. For its first few billion years, it was slamming the brakes — gravity dragging the expansion to a crawl. Then, around 6 billion years ago, something took over. Enter a cosmic age and find out when the brakes let go.
Here is the story nobody expects: the universe spent its first few billion years slowing down. Right after the Big Bang, the cosmos was expanding furiously — but gravity was pulling back. Radiation and matter filled space, and their mutual attraction acted like a cosmic brake pedal. For roughly the first 7 billion years, the expansion rate was falling. Galaxies were being dragged apart slower and slower.
Then, around 6 billion years ago — when the universe was about 7.6 billion years old — the pedal flipped. Dark energy, a pressure that does not thin out as space grows, overtook matter. The deceleration stopped. The expansion began to speed up, and it has been flooring it ever since. Cosmologists call this moment the "cosmic jerk": the instant the universe stopped braking and started accelerating.
The reason for the switch is a density race. Matter and radiation dilute as the universe expands — the same stuff spread over more space. Dark energy does not. Its density stays constant, so its share of the cosmic budget only grows. Today it is 68% of everything and climbing. The brakes are gone for good.
This is not just trivia. The discovery that the expansion is accelerating won the 2011 Nobel Prize, and it remains the biggest unexplained number in physics: why is dark energy this strong? In 2026, new analyses of 1,700+ supernovae and the DESI survey are questioning whether the acceleration is even real or just an illusion in the data. Either way, the story of the brakes letting go is the hinge of modern cosmology.
No. The first ~7 billion years were decelerating: radiation and matter dominated, and gravity dragged the expansion down. The switch to acceleration happened when the universe was roughly 7.6 billion years old, about 6 billion years ago.
Because matter dilutes as space grows while dark energy does not. Early on, matter was dense enough to win the tug-of-war. As the universe expanded, matter thinned out and dark energy's constant density gained the upper hand — like a heavier car that only wins once the road is long enough.
In the standard ΛCDM model, yes — dark energy acts like a constant, so acceleration strengthens as matter keeps diluting. But if dark energy evolves (a live question in 2026 DESI analyses), the future could change: it might weaken, or even start braking again.
Much faster in absolute terms, but falling. At 1 billion years old, the universe was expanding about 11 times faster than today — and slowing down hard. It took the switch at ~7.6 billion years for the rate to bottom out and begin climbing again.