ONE of the most basic facts about the universe is that it is expanding. This observation, made by Edwin Hubble (pictured) in 1929, leads to all sorts of mind-stretching ideas. That the universe is growing implies it was smaller in the past—possibly a lot smaller. Which leads to the thought that a “Big Bang” kicked everything off. It also opens the question of whether the universe will expand for ever, or will eventually see its expansion halted and reversed by gravity, thus ending in a Big Crunch.

Things got stranger in 1998, when a group of astrophysicists discovered that the rate of expansion is increasing, for this finding raised another question in turn. The acceleration of the expansion was so great that it seemed something was actively pushing the universe apart. Thus was born the notion of “dark energy”—a new component of the cosmos, invoked to balance the equations.

Trying to work out what dark energy really is (or if it even exists) requires accurate measurements, particularly of the rate at which the universe is expanding. This rate is known as the Hubble constant, and there are two ways of measuring it. Unfortunately, the answers these methods come up with disagree. That is not necessarily a problem. Previous observational conflicts (for example, that the oldest stars in the universe were older than the universe itself) have gone away as measurements improved. But in this case a new set of measurements has confirmed the discrepancy. And that has got those who study astrophysics flummoxed.

The new measurements were made by a team led by Adam Riess, one of the researchers who discovered the accelerating expansion. Dr Riess works at the Space Telescope Science Institute in Baltimore. In a study just uploaded to the arXiv, a repository of papers awaiting formal approval for publication, he and his colleagues offer a new set of measurements of 50 stars of a type known as Cepheid variables.

Cepheids are important to astronomers because they are a rung on what is known as the cosmic distance ladder. A Cepheid pulsates at a frequency related to its intrinsic brightness. This makes it possible, by comparing the intrinsic brightness of such a star with its apparent brightness, as seen from Earth, to work out how far away it is relative to other Cepheids. Then, if the actual distances to some nearby Cepheids can be measured directly, those relative distances can be turned into absolute ones.

This is done by observing their parallax. As Earth orbits the sun, the positions of nearby stars will seem to shift relative to those farther away, in the same way that, to a passenger on a train, trees in the middle distance appear to move with respect to far-off mountains. This means their distances can be worked out by triangulation. The result is a method that has been used since Hubble’s day to work out the distances to nearby galaxies in which individual Cepheids, which are extremely bright stars, can be detected. Then, with this rung in place, other objects, such as certain sorts of supernovae that have predictable energy outputs, can be observed in galaxies of known distance and used to extend the ladder.

The accuracy of the ladder, though, depends on the measurement of each rung. With this in mind, Dr Riess and his colleagues combined data from two space telescopes—Hubble, which has been in orbit since 1990, and Gaia, launched in 2013—to measure with unprecedented accuracy the distances to nearby Cepheids in the hope that this might make the cosmic-ladder-based estimate of the Hubble constant converge with one derived from observations of the Cosmic Microwave Background (CMB), a thin soup of radiation suffusing the universe that is left over from its earliest moments. It did not. Rather, it confirmed the previous estimate.

According to the cosmic ladder, the universe is expanding at a rate of 73.24km per second per megaparsec. (In English, this means that for each additional megaparsec of distance—about 3.3m light years—the speed at which galaxies are moving away from each other rises by 73km per second.) According to the CMB method the rate is 67km per second. That suggests there really is something wrong with current understanding of the universe. Perhaps this is no more than a mismeasurement of one of the other steps on the cosmic distance ladder. But it could be quite profound. Which is good news for the employment prospects of astrophysicists.