Cosmologists staged a showdown between two kinds of cosmic explosions. Type Ia supernovae are stars that flare up with nearly identical brightness, making them reliable beacons for measuring distances across the universe. Gamma-ray bursts are far more powerful but also more finicky: they can be seen from enormous distances, but the strength of each flash varies. To turn them into a cosmic ruler, scientists spend a long time calibrating the relationship between explosion energy and decay time — much like adjusting a scale before weighing goods.
The authors of the new study took 15 years of gamma-ray burst observations from the Fermi space telescope and the Pantheon+ supernova catalog. They tested four models of the universe: from the simplest (ΛCDM, where dark energy is constant) to trickier ones where dark energy can flow into dark matter. Such models are sometimes proposed to smooth over the long-standing debate about the universe's expansion rate — the so-called Hubble tension. But even accounting for all the uncertainties and errors in gamma-ray bursts, the data showed no advantage for the complex models. The simple ΛCDM performed just as well, and by a statistical criterion that penalizes extra details, it even came out preferred.
The outcome is reminiscent of inspecting an old bridge: engineers suspected hidden cracks, but a thorough examination showed the structure is sound without extra supports. Gamma-ray bursts, for now, don't require us to revise our basic ideas about dark energy. In the future, as measurement precision improves, new nuances may emerge, but today, the cosmic ruler says: all is well.
The most distant known gamma-ray burst occurred when the universe was just 400 million years old, like peering into its infancy.
In Carl Sagan's novel Contact, a signal from an extraterrestrial civilization was initially mistaken for a gamma-ray burst, so powerful it seemed.