Cosmology’s Great Debate began a century ago – and is still going

Astronomers and cosmologists aren’t known for being incredible at adjectives. Take the Very Large Telescope, or the European Extremely Large Telescope, or even the big bang. But they weren’t wrong about the 1920 event now referred to as the Great Debate.

It was spring at the US National Academy of Sciences in Washington DC, and two great astronomers kicked off one of the most contentious issues in the field with opposing opinions on what they referred to as The Scale of the Universe. See, the universe is expanding. At every single moment in time, more space is appearing between the stars, and everything is getting further and further apart. And, as we now know, it’s happening faster and faster.

That expansion is accounted for in astronomical calculations by a number called the Hubble constant, introduced by astronomer Edwin Hubble in 1929. But the argument over just what that number is – how fast the universe is making more universe – began well before that year. In the early 1900s, many scientists thought that the Milky Way galaxy was the entire universe – after all, we didn’t have the technology yet to see beyond our own galaxy. A few strange smudges changed everything. At first, these smudges were called spiral nebulae, and cosmologists around the world were consumed with an argument over whether they were within our own galaxy or if they were indeed galaxies themselves.

In 1920, all that arguing culminated in the Great Debate. Two renowned researchers, Harlow Shapley and Heber Curtis, gave prepared talks to the general public on whether the spiral nebulae, including what we now call Andromeda, were small clouds on the edge of our galaxy – which would mean our galaxy was the only thing out there – or if the nebulae were actually galaxies beyond our own, implying a much bigger and wilder universe.

Shapley’s argument was based on measurements of the distance to stars known as Cepheid variables, which led him to believe that we lived in a vast galaxy about 300,000 light years wide. That’s 10 times bigger than anyone had previously thought and, according to Shapley, there was no way the spiral nebulae were further than that.

Curtis, on the other hand, argued that these strange nebulae were so-called island universes – in essence, other galaxies. He had looked at stellar explosions called novae and found that Andromeda had more of them than the rest of the Milky Way. He reasoned, if it was just a small part of our galaxy, why would it have so many more explosions than any other part? Plus, the spiral nebulae seemed to be moving extremely quickly around the galaxy. If they were really moving so quickly, there’s no way they could be gravitationally bound to our galaxy and still fit within the prevailing models of astrophysics at the time.

The two presented their arguments in a pair of lectures, followed by a series of papers, but no conclusion was reached, and no transcript of the lectures remains. In my view, it wasn’t simply the Great Debate, but the First Great Debate. While Curtis was eventually proved correct, the argument over the Hubble constant – and therefore, the size and age of our universe – rages on. And while the arguments today are based on newer and better data than we had in 1920, they are built on the foundations laid by Shapley and Curtis.

The Hubble constant is measured in units called kilometres per second per megaparsec. A megaparsec is a little over 3.25 million light years, making it a unit astronomers use for particularly huge distances. A Hubble constant of 1 would mean that for every megaparsec we move away from our position on Earth, objects are moving away from us 1 kilometre per second faster. Think about it this way: if every meter of space gets 1 centimeter longer, then something that was previously 1 meter away moves away just a little bit, but something that was previously 1700 kilometers away moves away a whole lot. The original value for the Hubble constant calculated by Hubble himself in 1929 was about 500 kilometres per second per megaparsec, so he thought that for every megaparsec we move away from Earth, the galaxies were hurtling away 500 kilometres per second faster.

That number was immediately controversial. For one thing, if we assume the universe has been expanding at a uniform rate since its inception – something that we did commonly assume then, although now that is no longer thought to be true – that would mean the age of the universe was about 2 billion years. And from radioactive dating of rocks, we already knew in the 1920s that Earth was at least 2 billion years old, if not older. So, if the Hubble constant was 500, that might mean that our planet was older than the universe, which couldn’t possibly be true.

By about the 1980s, things had crystallised such that most astronomers held one of two views on the Hubble constant. It was like a slow-motion Great Debate all over again, this time between the French astronomer Gérard de Vaucouleurs and the American Allan Sandage. De Vaucouleurs thought the Hubble constant was about 100, and Sandage thought it was lower, around 50. They were using similar methods, but each took issue with the other’s assumptions and measurements. They wrote papers back and forth on this for more than a decade, and neither one would budge.

Things started to move again in the 90s, when once again telescopes vastly improved with the launch of the Hubble Space Telescope, and the arrival of a young cosmologist named Wendy Freedman. She led what came to be called the Hubble Key Project, which measured all sorts of objects – including the Cepheid variables, supernovae and other so-called standard candles, whose predictable luminosities make them so important to understanding the Hubble constant – with much more precision than we’d had access to before. This effort eventually led to a value for the Hubble constant of about 72. Over time, all the other methods using standard candles to measure distances slowly converged on the same value. Even the most recent standard candle measurements are holding at a Hubble constant of about 73 kilometres per second per megaparsec.

That means the argument over the Hubble constant was resolved, right? Very much no. In the early 2000s, astronomers started using the cosmic microwave background (CMB), the remnants of light from the big bang, to measure the expansion of the universe. Whereas all the other direct measurements are referred to as the local distance ladder, this method relies on measuring the state of the early universe and extrapolating it forward using our best models of the universe. The CMB method yields a Hubble constant of about 67.

Now, once again, we have another Great Debate – the third one, if you’re counting. This time, it’s not 50 versus 100; it’s 67 versus 73, so we’re getting closer. But each side is similarly adamant that there are no problems with their measurements.

What comes along with the debate over which Hubble constant is the correct one is another, wider debate: could both measurements be right? Known as the Hubble tension, this suggests that as the distance ladder measurements get more and more precise and as we rule out more and more possible sources of error, the argument that they are both right gets stronger and stronger – which could mean we need entirely new physics that we haven’t thought of yet.

This modern version of the Great Debate is more complex than ever, so it will be even harder to resolve, even though the two sides are closer together than they’ve ever been. To reach a final conclusion, we will need more independent methods of measuring the Hubble constant. Freedman is working on a couple of those using different types of stars, and other astronomers are using more exotic methods that involve analysing the propagation of gravity through the universe to reach completely independent measurements of cosmic expansion. For now, the debates rage on.

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