Science Matters

How the "universe" got bigger in 1925

Boyce Rensberger

(3/2025) The universe as we know it is 100 years old. Stick with me here.

Until 1925 most scientists thought that our Milky Way galaxy was the entire universe. Indeed, most astronomers thought that everything they saw in their telescopes existed within our galaxy and that nothing lay beyond. As some put it, we live in an "island universe," a lone galaxy in a sea of nothingness. Other astronomers suggested that the universe surely contained other galaxies, but they had no proof. This was one of the great debates within astronomy in those days.

Then, exactly one century ago the American astronomer Edwin Hubble delivered a scientific paper presenting strong evidence that some of the fuzzy patches of light they could see in their telescopes (so nebulous that they called them nebulae) were actually entire galaxies of their own -- galaxies far beyond the edge of our beloved Milky Way. That was the year when the universe as we know it was born.

Actually, our knowledge of the universe has improved since then, and you are free to argue that the universe "as we know it" is younger than I said. But 100 is a nice, round number to celebrate. So that’s where I’m going to start. I may devote a future column to the newer changes.

For now, I should explain the evidence that Hubble presented, but to do that I must go back a few years earlier, back to 1908 and the work of a deaf astronomer named Henrietta Swan Leavitt. She’s another of the female scientists whose work paved the way for a male scientist who became more famous. She was one of the giants on whose shoulders Hubble stood.

Working at the Harvard College Observatory, Leavitt examined hundreds of photographs taken through a telescope and measured the position and brightness of each of many thousands of stars. The pictures were of specific patches of sky, each photographed repeatedly every few days.

Leavitt noticed that a few dots of light fluctuated in brightness over time. They were stars that somehow got brighter and then dimmer in regular cycles that ranged from days to months. And she went a step further. She discovered a pattern. She saw that the bright stars cycled slowly while the dim ones were faster. It was a pattern she recognized in hundreds of stars. Because she found these in the constellation Cepheus, they were called "Cepheid variables."

So what? Well, Leavitt had uncovered a key relationship. The cycling rate corresponded directly to its intrinsic brightness. For example, let’s say a certain Cepheid variable in our galaxy takes eight-days to cycle, and it has a certain brightness. Now we look somewhere else in the sky and see another variable star that takes eight days. But if that star looks much dimmer, we can infer that it really is as bright as an eight-day Cepheid but must be farther away to look so dim to us.

As scientists usually put it, the intrinsic or actual brightness of a star can be deduced from its apparent brightness depending on how fast it cycles. Put another way, they realized that there was a direct relationship between apparent brightness and distance. An eight-day star that was half as bright as the eight-day Cepheid was twice as far away.

What Hubble described in his 1925 presentation was that he had found numerous variable stars cycling at rates measured by Leavitt. But they were so much dimmer than the original Cepheids that they had to be in galaxies far beyond the Milky Way.

So, one century ago, Earthlings discovered that we were not a lonely island in a sea of nothingness; we were one island in a sea of many other islands. One of the great debates in astronomy was settled by evidence that everybody agreed on. The universe has many galaxies, trillions as we know today.

This discovery dealt another blow to the long-held belief that Earth is at the center of all that exists. That view dates back to Aristotle, Ptolemy and beyond. It began to crumble in 1543 when Nicolaus Copernicus published evidence that Earth is not the center of the universe but that it orbited a different center, the sun. In 1610 Galileo Galilei published further evidence that confirmed Copernicus. The Roman Catholic Church condemned these findings as heresy because they contradicted a literal reading of the Bible. Under threat of torture, the Roman Inquisition forced Galileo to recant and sentenced him to house arrest, where he remained for nine years until his death.

Hubble would, of course, go on to change the known universe again in 1929. He discovered that galaxies are moving away from each other and that the farther away each galaxy is, the faster it is receding from us. The evidence came from studying the color spectrum of light coming from distant galaxies. He noticed that the colors were stretched farther apart than in the spectrum we know on Earth, all of them appearing to have been shifted to the red end of the spectrum.

Hubble interpreted this as similar to the Doppler effect on sound. To repeat a familiar example, if a siren is coming toward us, we hear a certain pitch. But once the siren goes past, the pitch lowers. This happens because the oncoming vehicle’s speed compresses the sound waves, making them seem higher pitched. Once the siren goes past, the vehicle’s speed stretches the sound waves, shifting them to a lower pitch.

It turns out that all the other galaxies are red-shifted. So they must be speeding away from us. The universe, Hubble concluded, is expanding. Moreover, if you traced the movements backward in time, there once was a beginning when everything was compressed into a point that had undergone a Big Bang.

Henrietta Swan Leavitt, for her part, was nominated for the Nobel Prize but, sadly, she had died four years earlier, in 1921. Nobels are awarded only to the living. She succumbed to stomach cancer at the age of 53.

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FUN FACT: The name "Milky Way" goes back at least 2500 years. The ancient Romans, for example, called it "via lactea." Our word "galaxy" comes from the ancient Greek "galaxias," meaning "milky." Until modern times, nobody knew that the Milky Way was our edge-on view of a giant wheel of at least 100 billion stars.

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