For decades the universe has refused to balance its books. Measurements of the young cosmos indicate that ordinary matter – the baryons that make up protons, neutrons and everything built from them – accounted for roughly 17 percent of all matter shortly after the Big Bang. Add up every star, galaxy and glowing cloud visible today and you reach only about a tenth of that. The rest had to be out there somewhere, spread so thinly – on the order of a single proton per cubic metre – that no telescope could photograph it.
A team led from MIT has now mapped where much of it sits, and the answer is stranger than the models allowed for. Writing in Physical Review Letters, members of the CHIME/FRB Collaboration describe vast, wispy envelopes of gas reaching millions of light-years out from groups of galaxies – far beyond the neighbourhoods simulations had assigned them.
Cosmic flashes as a measuring stick
The measurement rests on fast radio bursts, millisecond flares of radio waves from distant galaxies first detected in 2007. Their origins are still argued over, but their behaviour in transit is well understood: as a burst crosses intervening material, the signal is stretched out in time, and the degree of stretching scales with how much matter stood in the way. "We can measure that smearing very precisely," says lead author Haochen Wang, a graduate student at MIT's Kavli Institute for Astrophysics and Space Research.
Earlier work had used that effect to confirm that thin gas exists between galaxies at all. The new step was geometric. The team cross-correlated thousands of burst measurements from CHIME, a radio telescope in British Columbia that sweeps the entire northern sky, with the positions of millions of galaxies catalogued by the Dark Energy Spectroscopic Instrument survey. Where galaxies cluster, the bursts smear more – and the pattern of that excess traces the shape of the invisible gas, not just its presence.
That shape came out unexpectedly broad. A galaxy spans a few hundred thousand light-years; the team found associated matter extending to roughly four million. "That's further than the simulations predict, by quite a bit," says co-author Kiyoshi Masui, an associate professor of physics at MIT.
The implication points back at the galaxies themselves. To fling gas that far, the jets of supermassive black holes and the shockwaves of exploding stars must be considerably more energetic than models assume – galaxies behaving less like tidy reservoirs and more like fountains. Rather than breaking the standard picture of cosmology, then, the missing baryons appear to have been hiding exactly where an under-modelled process would have put them. And the method should sharpen on its own: every new burst CHIME records adds another probe of the space in between.