Two clocks tell different times. One sits on our desk, the other lies far in the past, in the glow left over from the Big Bang. The first, looking at nearby galaxies, ticks off the expansion rate of the universe — about 73 kilometers per second per megaparsec of distance. The second, peering into ancient light, gives a noticeably lower value — roughly 67. The difference exceeds the error bars, and cosmologists have dubbed it the “Hubble tension.” It’s as if the thermometer outside your window read +25, while a satellite sensor above the city read +18, and both were working perfectly.
New work suggests not fixing the clocks but rethinking what they measure. In the standard picture of the cosmos, dark matter and dark energy are two separate ingredients that never mix. The former, like an invisible scaffold, pulls galaxies into clusters. The latter, by contrast, pushes space itself apart, making the universe expand ever faster. The authors allowed for an exchange between them: a tiny fraction of dark matter gradually turns into dark energy. The flow is barely noticeable, roughly one ten-thousandth of the total reservoir over the entire history, but it’s enough to give expansion a slight nudge over the past few billion years. Then the “nearby” clock, measuring today’s rate, yields a higher value, while the “distant” one, averaging over the early epoch, stays almost unchanged.
Interestingly, the flow goes in an unusual direction: from matter to energy. In everyday life, we’re used to the opposite — energy condensing into matter (for instance, steam condensing into water droplets). Here, dark matter seems to evaporate into an even more rarefied substance that accelerates the cosmos. A side effect is the slowed growth of large structures: galaxy clusters turn out slightly less massive than the standard model predicts. This brings theory closer to observations of weak lensing, where light from distant galaxies is bent by invisible clumps of mass.
So the two clocks gradually bring their readings together. Not because one is broken, but because a hidden river runs between them, slowly carrying dark matter over to the side of expansion. It remains to be seen whether future sky surveys will spot this river — for example, through a slightly different pattern of galaxy clusters.
To spot the discrepancy in the expansion rate, astronomers compare two “echoes”: one from nearby supernovae, the other from a hot clump of plasma frozen 380,000 years after the Big Bang. The difference is like a mountain echo returning slightly faster than the physics of sound would predict.
In Peter Watts’s novel Blindsight, invisible dark matter turns out to be linked to consciousness, and here it engages in a silent chemical reaction with dark energy — almost like an unseen ocean changing its density.