How fast is the universe expanding? New data keeps mystery open Premium

Cosmologists investigate the Hubble tension, exploring the universe's expansion and the accuracy of current models.

A big open problem in cosmology is the Hubble tension. There are two equally valid ways to measure how fast the universe is expanding, but they have yielded two very different estimates. No amount of rechecking and refining calculations has made this tension go away.

In a study published recently in Monthly Notices of The Royal Astronomical Society (MNRAS), scientists from Germany and the U.K. led with a radical explanation for the tension: our model used to understand the universe is wrong.

This model is called Λ cold dark matter, or “lambda CDM”. It’s currently the simplest model that explains various features of the universe, including radiation leftover from the Big Bang, the arrangement of galaxies in the universe, and the fact that the universe is expanding.

But cosmologists are also looking for a new, better model that can explain some things the Λ CDM model can’t, such as the Hubble tension. Repeated measurements and computations have ascertained the Hubble tension exists and that it’s not some aberration in the data.

In a paper published after the MNRAS one in The Astrophysical Journal Letters, a different group disproved a flaw some scientists had suspected in one of the two ways to measure the universe’s expansion – meaning the tension is real.

For now, the model does seem to be the problem.

These results also come against the backdrop of a meeting this week in London, where cosmologists will gather to discuss whether this model has become outdated for other reasons as well.