It’s official – we don’t know how fast the universe is expanding

The Hubble constant describes how fast the universe is expanding, but our measurements won’t line up, which may mean our standard model of the universe is wrong

The expansion of the universe is accelerating, but we don’t know how quickly. With new observations, this issue has only got more severe, and now some astronomers are saying that it is officially a real problem – not one caused by uncertainties in the measurements.

There are two main ways we measure the Hubble constant, which describes the expansion of the universe. The first is to examine the cosmic microwave background – a relic of the first light to shine through the universe after the big bang – and use our standard model of cosmology to calculate what the expansion rate should be like today. This puts the rate of acceleration at about 67 kilometres per second per megaparsec.

The other method, called the local method or the distance ladder, involves measuring the distances to stars called cepheids and then using those distances to extrapolate to supernovae in other galaxies. These distances allow us to calculate the Hubble constant, which the latest measurements from Adam Riess at Johns Hopkins University in Maryland and his colleagues have put at about 73 kilometres per second per megaparsec.

For decades, it has been plausible that these two methods would eventually converge on a single true value of the Hubble constant. Now, Riess and his team say that is extraordinarily unlikely – which would mean that something is wrong with our standard model of the universe.

Even after analysing the data in many different ways and including results from other teams, “it’s really hard for us to get below about 72.5 or above about 73.5,” says Riess. The disagreement between the two calculations is known as the Hubble tension.

By his team’s calculations, the two methods of measurement disagree with one another at a statistical level referred to as “5 sigma”, generally considered a gold standard in physics for demonstrating that measurements are a true discovery and not a statistical fluke. This means there is only about a 1 in 3.5 million chance that the Hubble tension is just a fluke.

However, other astronomers have pointed out that even a 5-sigma discrepancy doesn’t rule out the possibility of errors or systematic uncertainty in our measurements of stars. “It doesn’t matter how many sigma away you are, it’s whether you have determined all of the potential errors out there that had led to that place,” says Barry Madore at the Carnegie Institution for Science in California.

While the measurements may point towards the Hubble tension being a real problem, we cannot know for certain until it is confirmed by several methods of measurement, says Madore. Thankfully, the James Webb Space Telescope should be able to help with that and researchers are also working on other methods, such as using gravitational waves.

If the tension is shown to exist, the specifics of the new physics we would need to explain it are still up in the air. “We’re very clear that we don’t know what the source of the Hubble tension is,” says Riess. “There have been some intriguing ideas, but none of them are a perfect fit yet.”

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