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A picture from the James Webb Space Telescope of galaxies.

Scientists can’t agree on how fast the universe is expanding – why this matters so much for our understanding of the cosmos

It’s one of the biggest puzzles in cosmology. Why two different methods used to calculate the rate at which the universe is expanding don’t produce the same result. Known as the Hubble tension, the enigma suggests that there could be something wrong with the standard model of cosmology used to explain the forces in the universe.

Now, recent observations using the new James Webb Space Telescope (JWST) are shaking up the debate on how close the mystery is to being resolved.

In this episode of The Conversation Weekly podcast, two professors of astronomy explain why the Hubble tension matters so much for our understanding of the universe.

In February, the Nobel prize-winning physicist Adam Reiss, published a new paper. It said that new observations of far-away stars using the JWST matched those obtained by the Hubble Space Telescope.

These stars, called Cepheids, are commonly used in one method of calculating the rate at which the universe is expanding. Known as the local distance ladder, or cosmic distance ladder, this method has been around since observations first made by Edwin Hubble himself in 1929. And it generally produces a rate of expansion of around 73km per second per mega parsec.

But a second method, using predictions of the cosmic microwave background radiation left over by the Big Bang, has constantly arrived at a different number for the rate of expansion of the universe: 67km per second per mega parsec.

Reiss said that when the new data confirmed the earlier observations from the Hubble Space Telescope, the gap between the numbers remains unresolved. “What remains is the real and exciting possibility that we have misunderstood the universe,” he said.

A few months later, however, more data from the JWST, presented by Wendy Freedman, a physicist at the University of Chicago, using observations from a different set of stars, arrived at 69km per second per mega parsec, a number closer to the cosmic microwave background figure of 67. Freedman is excited that the numbers seem to be converging.


Listen to The Conversation’s podcast series Great Mysteries of Physics for more about the greatest mysteries facing physicists today – and the radical proposals for solving them. Hosted by Miriam Frankel it features interviews with some of the worlds leading scientists including Sean Carroll, Sabine Hossenfelder and Jim Al-Khalili.


Vicent Martínez and Bernard Jones are fascinated by the Hubble tension. Jones is an emeritus professor of astronomy at the University of Groningen in the Netherlands. Martínez, his former student, is now a professor of astronomy and astrophysics at the University of València in Spain.

“The fundamental basis of science, what distinguishes science from science fiction, is our ability to verify the information we are getting,” explains Jones.

That’s why Martinez says the mystery of the Hubble tension is still driving people to:

Research and imagine experiments and organise huge projects with the complicated observation of the cosmos in order to understand what’s going on. At the end, this will affect your idea of the whole universe and probably you will need to change some fundamental ingredient of your cosmological model.

Martinez and Jones have just written a book, along with their co-author Virginia Trimble, about moments in history when scientists realised they’d got something very wrong, and had to readjust their way of thinking. Martínez thinks this could happen again with the Hubble tension:

It could happen that, for example, a new theory of gravity could solve the problem of dark energy or dark matter. We have to be open to those ideas.

Listen to Bernard Jones and Vicent Martínez talk more about the Hubble tension, and how it fits in the wider history of science, on The Conversation Weekly podcast. The episode also features an introduction from Lorena Sánchez, science editor at The Conversation in Spain.

A transcript of this episode will be available shortly.

This episode of The Conversation Weekly was written and produced by Katie Flood, with assistance from Mend Mariwany. Gemma Ware was the executive producer. Sound design was by Eloise Stevens, and our theme music is by Neeta Sarl. Stephen Khan is our global executive editor and Soraya Nandy helps with our transcripts.

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