The universe is expanding, and galaxies are flying apart like raisins in rising dough. The rate of this expansion, the Hubble constant, is a key number in cosmology. To measure it, you usually need the 'sound horizon': a trace of sound waves that rippled through the hot plasma in the first few hundred thousand years after the Big Bang. This trace shows up in the distribution of galaxies and in the cosmic microwave background. The problem is that the horizon itself depends on the composition of the universe, and if we get the composition wrong, we'll get the expansion rate wrong too.
Astrophysicists from the BOSS collaboration took a different route. They used a map of 1.5 million galaxies and built a power spectrum — a plot of how often clumps of different sizes occur. Then they pulled a clever trick: they recalculated that plot so that the sound horizon played no role at all. Instead, they used the scale of the clumps themselves. Next, they added data on weak gravitational lensing (when light from distant galaxies is slightly bent on its way), on the relative positions of galaxies, and on dark matter, which is visible through its gravitational pull. They combined all of this into a single analysis.
In the end, they got a Hubble constant of 0.702 with 3–4 percent precision. When they added supernova data (stellar explosions that serve as 'standard candles'), the precision improved to 2.6 percent, and the value became 0.686. This is still noticeably lower than the direct measurement from nearby galaxies (about 0.73), but the gap is no longer as dramatic. The authors also noticed a slight deviation in the parameter describing the position of the sound horizon — at 1.8 sigma. This could just be a statistical fluke, but it might also point to something new.
If this is confirmed, our picture of the universe will need a rethink. For now, the new result is an important step toward measuring the expansion rate without relying on assumptions about its past.
The Hubble constant is named after Edwin Hubble, who in 1929 discovered that distant galaxies recede from us faster the farther away they are. He had no idea about dark energy: it wasn't discovered until 1998.
In Liu Cixin's novel 'The Three-Body Problem,' an alien civilization tries to slow the expansion of the universe to survive in the far future. Knowing the Hubble constant precisely is the first step to understanding whether that's possible.