How quickly is the universe expanding? Astronomers and theorists can't agree

There's breaking news from the astronomy world. Astronomers have confirmed that we really don't understand something we only suspected we didn't understand, in yet another study after years of their observations clashing with theorists' predictions. And we can hope their new measurements helps us ultimately understand it because, at least, now we know what we were wrong about.

What am I talking about? Well, it's something called the "Hubble tension," and it's a dispute between observational astronomers and theoretical cosmologists about who's been wrong about how fast the universe is expanding.

Now new observations from the James Webb Space telescope, published this week in The Astrophysical Journal, back up previous measurements that peg the speed of the expansion as faster than what the theoreticians believe it should be.

That's made the score 2-1 for the astronomers and the theoreticians have some explaining to do.

Let's take this one step by step. 

In 1929, American astronomer Edwin Hubble announced his discovery that galaxies scattered throughout the universe are moving away from each other — and that the farther away they are, the faster they are moving. This suggests that the universe is expanding as the space between the distant galaxies stretches outwards.

Hubble's landmark discovery has challenged scientists ever since, as they try to determine just how fast that expansion is with a value known as the Hubble constant.

Fast forward to 1998 when two independent teams of astronomers used images from the Hubble Space Telescope to peer deeper and farther back into the universe than had ever been possible before. They discovered, shockingly, that something very weird was happening.

They found that the rate at which the universe was expanding was increasing over time. This was like throwing a baseball up in the air and finding it not falling back to Earth, but accelerating into outer space.

Thus was born the notion of dark energy, the mysterious force that somehow causes the universe's expansion to accelerate, possibly by causing space itself to expand.

These paradoxical discoveries threw a wrench into the standard model of cosmology, a mathematical model of the history and evolution of the universe that told the story of its expansion from the moments after the Big Bang. 

Theorists happily integrated the dark energy into their math, as well as another mysterious factor: dark matter, an invisible and undetectable cosmological phenomenon whose immense gravitational influence pervades the universe.

It was a beautiful new theory. But the math and observations from the universe wouldn't cooperate. 

The standard model predicts that the modern universe should be expanding at a rate of 67.4 kilometres per second per megaparsec (km/s/Mpc), where a megaparsec covers a huge distance of 3.26 million light-years.

But models are only as good as the blackboards they are written on.

The Hubble telescope precisely measured the rate at which the universe is expanding by looking at relatively nearby galaxies, and in 2018 found a higher rate of 72.8 km/s/Mpc.

This, I can only imagine, led to awkward encounters in physics department common rooms in universities around the world, as the theoreticians and the astronomers desperately pretended that those few megaparsecs weren't that big a deal. As I said — "Hubble tension."

In 2023, observations from the James Webb Space Telescope seemed to back up the Hubble Space Telescope findings. Then in February 2024, the same team updated their research with even more data backing up their initial findings. The tension, in other words, continued.

The latest study using the largest sample of Webb observations yet, and three different methods to cross-check the Hubble telescope data, arrive at a very similar value of 72.6 km/s/Mpc.

The two space telescopes' findings bolster the case there is something, other than measurement error, influencing the universe's expansion rate — and that our theoretical understanding of the universe is incomplete. 

This, for both the theorists and the astronomers, is good news. The astronomers get to be right, which is nice. The theorists get to look for one of the holy grails of science: new physics, which roughly translates as "clean the blackboards — it's game on!" They know their old math was wrong, which means there's something new out there to discover.

This is often how basic science works. The theoreticians and experimentalists are volleying back and forth to find the ultimate truth.

This may sound far removed from everyday life — and while it won't influence how much tax you pay this year, it is attempting to answer the basic question of how we got here. And when we probe the fundamental forces that shape the universe as a whole, sometimes new discoveries are made that could have amazing implications in the future.