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

Astronomers and cosmologists are not known for their amazing ability to pronounce adjectives. Take the Very Large Telescope, the European Extremely Large Telescope, or even the Big Bang. But they were right about the 1920 event now called the Great Debate.

It was spring at the US National Academy of Sciences in Washington, and two great astronomers began one of the most controversial issues in the field by expressing opposing opinions about what they called Scale of the Universe. You see, the Universe is expanding. At each moment of time, more and more space appears between the stars, and everything is getting further and further apart. And, as we now know, this is happening faster and faster.

This expansion is taken into account in astronomical calculations by a number called Hubble constantintroduced by astronomer Edwin Hubble in 1929. But debate about what that number is—how fast the universe is expanding—began long before this year. In the early 1900s, many scientists thought that the Milky Way galaxy was the entire universe. After all, we didn't yet have the technology to see beyond our own galaxy. A few strange spots changed everything. At first these spots were called spiral nebulae, and cosmologists around the world were consumed with debate about whether they were located inside our galaxy or whether they were actually galaxies themselves.

In 1920, all these disputes culminated in the Great Debates. Two renowned researchers, Harlow Shapley and Heber Curtis, gave prepared talks to the general public on whether spiral nebulae, including what we now call Andromeda, were small clouds at the edge of our galaxy (which would mean that our galaxy was a single object), or whether the nebulae were actually galaxies beyond our own, implying a much larger and wilder universe.

Shapley's argument was based on measurements of distances to stars known as Cepheid variables, which led him to believe that we live in a vast galaxy about 300,000 light-years wide. This is 10 times larger than previously thought, and according to Shapley, spiral nebulae could not be further than this.

Curtis, on the other hand, argued that these strange nebulae were so-called island universes—essentially other galaxies. He observed explosions of stars called novae and found that Andromeda had more of them than other stars. Milky Way. He reasoned: if this was only a small part of our galaxy, why would there be so many explosions there than in any other part? Additionally, the spiral nebulae seemed to move very quickly throughout the galaxy. If they were actually moving that fast, there is no way they could be gravitationally bound to our galaxy and still fit the prevailing astrophysics models at the time.

The two presented their arguments in a pair of lectures followed by a series of articles, but no conclusion was reached and no transcripts of the lectures survive. In my opinion, this was not just the Great Debate, but the First Great Debate. Although Curtis was ultimately proven right, the debate about the Hubble constant—and therefore the size and age of our Universe—continues. And while today's arguments are based on newer and better data than we had in 1920, they are built on the foundation laid by Shapley and Curtis.

The Hubble constant is measured in units called kilometers per second per megaparsec. A megaparsec is just over 3.25 million light years, making it a unit of measurement that astronomers use to measure particularly vast distances. A Hubble constant of 1 means that for every megaparsec we move away from our position on Earth, objects move away from us 1 kilometer per second faster. Think of it this way: if every meter of space becomes 1 centimeter longer, then what was once 1 meter away moves away quite a bit, and what used to be 1,700 kilometers away moves away a lot. The original value of the Hubble constant, calculated by Hubble himself in 1929, was about 500 kilometers per second per megaparsec, so he believed that for every megaparsec we move away from Earth, galaxies are moving 500 kilometers per second faster.

This figure immediately caused controversy. First, if we assume that the Universe has been expanding at the same rate since its origin – something we generally assumed then, although it is no longer believed to be true – this would mean age of the universe was about 2 billion years. And thanks to radioactive dating of rocks, we already knew in the 1920s that the Earth was at least 2 billion years old, if not older. So, if the Hubble constant was 500, it could mean that our planet is older than the universe, which cannot be true.

By about the 1980s, things had become such that most astronomers held one of two views on the Hubble constant. It was again like the Great Debate in slow motion, this time between the French astronomer Gerard de Vaucouleurs and the American Allan Sandage. De Vaucouleurs thought the Hubble constant was about 100, while Sandage thought it was lower, about 50. They used similar methods, but each disagreed with the other's assumptions and measurements. They've been writing articles on this issue for over ten years, and nothing has budged.

This began to change again in the 90s, when telescopes again improved greatly with the launch of the Hubble Space Telescope and the arrival of a young cosmologist named Wendy Friedman. She led a project called the Hubble Key, which measured all kinds of objects—including Cepheid variables, supernovae and other so-called standard candles, whose predictable luminosity makes them so important to understanding the Hubble constant—with much greater precision than we had access to before. These efforts eventually resulted in a value for the Hubble constant of about 72. Over time, all other methods using standard candles to measure distances slowly converged to the same value. Even the most recent standard measurements of candles indicate that the Hubble constant is about 73 kilometers per second per megaparsec.

It means the Hubble constant controversy has been resolvedright? Not really. In the early 2000s, astronomers began using the cosmic microwave background (CMB), leftover light from the Big Bang, to measure the expansion of the Universe. While all other direct measurements are called the local distance ladder, this method is based on measuring the state of the early Universe and extrapolating it forward using our best models of the Universe. The CMB method gives a Hubble constant of about 67.

Now we have another great debate – the third one, if you count. This time it's not 50 versus 100; It's 67 to 73, so we're getting close. But both sides are equally adamant that there is no problem with their measurements.

Along with the debate about which Hubble constant is correct, another, broader debate arises: can both Are the measurements correct? Known as Hubble voltageThis suggests that as distance ladder measurements become more precise, and as we rule out more and more possible sources of error, the argument that they are both right becomes stronger and stronger – which may mean that we need entirely new physics that we haven't thought of yet.

This modern version of the Great Debate is more complex than ever, making it even more difficult to resolve, even though both sides are closer to each other than ever. To reach a final conclusion, we will need more independent methods of measuring the Hubble constant. Friedman is working on several of them, using different types of stars, and other astronomers are using more exotic methods that involve analyzing the propagation of gravity throughout the universe to achieve completely independent measurements of cosmic expansion. For now, the debate continues.

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