On the kitchen counter, there's a flour scale and a measuring cup for water. Individually, they're precise, but when you bake bread, the recipe calls for the perfect ratio. Astronomers have a similar dispute about the universe's expansion rate. Direct measurements from stars give one number, while calculations from the cosmic microwave background (the light left over from the Big Bang) give another, slightly smaller one. The difference is small but stubbornly persistent.
The authors of the new study added two ingredients to the 'recipe.' First: a tiny fraction of dark matter, the invisible substance that holds galaxies together with its gravity, is not eternal. It slowly decays into dark radiation (particles that don't interact with ordinary matter but carry energy). This happens when the universe was about 50,000 years old, just as radiation was ceasing to be the dominant force and matter was beginning to clump. Second: dark energy itself, which pushes space apart, slightly changes its push over time rather than remaining strictly constant.
The scientists fed this model to three independent datasets: maps of the cosmic microwave background, galaxy distributions, and lensing (how massive objects bend light rays). The resulting expansion rate was 69.8 km/s per megaparsec. This is closer to direct measurements (about 73 km/s/Mpc) than the standard answer (67.4). The gap shrank to about two sigma — in statistical terms, a chance coincidence is still possible but unlikely. Moreover, when direct supernova data were added to the analysis, the rate rose to 70.2, nearly matching the Cepheid measurements.
So what's the takeaway? Dark matter isn't just invisible glue; it's also the quiet conductor of the cosmic orchestra. Its slow decay in the distant past could shift the expansion rate today. Perhaps the mystery of the universe's speed isn't a measurement error but a hint that the dark sector is more complex than we thought.
If all the dark matter in our galaxy were spread evenly throughout its volume, each cubic centimeter would contain only about 0.3 times the mass of a proton — nearly a perfect vacuum, but on galactic scales it's enough to keep stars from flying apart.
In Peter Watts's novel 'Blindsight,' alien ships use dark matter decay as an energy source — much like in this model, but as an engine rather than a cosmic clock.