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This date in science: Edwin Hubble and the expanding universe

Hubble helped change our cosmology: our idea of the universe as a whole.

November 20, 1889. Happy birthday, Edwin Hubble! The Hubble Space Telescope is named for this astronomer. How did this honor come to be? Hubble’s work was pivotal in changing our entire cosmology: our idea of the universe as a whole.

A hundred years ago, most astronomers believed that the whole universe consisted of just one galaxy, our own Milky Way. In the 1920s, Hubble was among the first to recognize that there is a universe of galaxies located beyond the boundaries of our Milky Way.

He also showed that our universe of galaxies is expanding.

During the 1920s, Edwin Hubble observed stars that vary in brightness in a patch of light known at the time as the Andromeda nebula. He knew that these stars changed in brightness in a way that depended on their true brightness. He then saw how bright they looked to find the distance to the Andromeda nebula.

At the time, many astronomers believed that the Andromeda nebula was a forming solar system, located within the Milky Way’s boundaries. Hubble showed that this patch of light was really a separate galaxy – what we know today as the Andromeda Galaxy – the nearest large spiral galaxy beyond our Milky Way.

This image is the Hubble eXtreme Deep Field, released in 2012.  Read more about this image here.

This image is the Hubble eXtreme Deep Field, released in 2012. Nearly every speck of light here is a separate galaxy, beyond our Milky Way. Read more about this image here.

As soon as other nebulae were revealed as separate galaxies, the known universe got much bigger!

But was this huge universe stationary? Or was it expanding, or contracting?

The answer involved the light of galaxies as a whole. Astronomers observed that the light of distant galaxies was shifted toward the red end of the light spectrum. This red shift was interpreted as a sign that the galaxies are moving away from us. Hubble and his colleagues compared the distance estimates to galaxies with their red shifts. And – on March 15, 1929 – Hubble published his observation that the farthest galaxies are moving away faster than the closest ones.

This insight became known as Hubble’s Law. It was the first recognition that the galaxies are moving away from each other – that our universe is expanding.

It’s said that Albert Einstein was elated to hear of Hubble’s work. Einstein’s Theory of Relativity implied that the universe must either be expanding or contracting. But Einstein himself rejected this notion in favor of the accepted idea that the universe was stationary and had always existed. When Hubble presented his evidence of the expansion of the universe, Einstein embraced the idea. He called his adherence to the old idea “my greatest blunder.”

In recent years, the expansion of the universe has been seen to be undergoing a mysterious speeding-up. Read about the Texas telescope poised to explore dark energy, the unknown force proposed to explain the expansion.

The Andromeda galaxy and two satellite galaxies as seen through a powerful telescope.  In Hubble's time, astronomers believed this object resided within our own Milky Way galaxy.  Hubble's work revealed that it is an island of stars in space, external to our Milky Way.  Image via NOAO

The Andromeda galaxy and two satellite galaxies as seen through a powerful telescope. In Hubble’s time, astronomers believed this object resided within our own Milky Way galaxy. Hubble used a class of variable stars called Cepheid variables to show that the Andromeda galaxy is an island of stars in space, external to our Milky Way. Image via NOAO

Hubble was a multi-talented man. Although he majored in science as an undergraduate at the University of Chicago, a promise to his dying father caused him to take up a study of the law. He was also an amateur heavyweight boxer, and reportedly turned down the chance to fight professionally. He returned to science as a graduate student at Yerkes Observatory in Wisconsin. In 1919, he accepted a position at the prestigious Mount Wilson Observatory in California.

But the story of Hubble’s great insights begins earlier. In 1908, an astronomer named Henrietta Leavitt had discovered a relationship between the period and luminosity of a class of pulsating stars called Cepheid variables. By timing its period, astronomers could work out the true luminosity of a Cepheid – and by comparing the true luminosity with the observed brightness, they could work out its distance.

This worked fine for judging distances inside the Milky Way, but it wasn’t until the 1920s that telescopes existed that were powerful enough to observe Cepheids in other galaxies. Hubble spotted his first Cepheid in the Andromeda ‘spiral nebula’ in 1924.

It was Vesto Slipher of Lowell Observatory whose study of spiral nebulae showed that these objects exhibit red shifts. Afterwards, Hubble and another astronomer, Milton Humason, observed the Cepheids in 18 of Slipher’s objects, using the new 100-inch Hooker Telescope at Mount Wilson.

The pulsation of the Cepheid variables let them estimate true distances to these objects. That’s how they showed that the objects are really separate galaxies, located extremely far away.

The nearest galaxy, the Andromeda galaxy, is 2.2 million light-years beyond our Milky Way. But other galaxies extend around us in space for many billions of light-years.

Bottom line: Edwin Hubble’s birth date was November 20, 1889. Hubble showed there are galaxies beyond our Milky Way. Then he showed that the universe of galaxies is expanding. The Hubble Space Telescope is named for him.

Deepest view we have yet, into our universe: the Hubble eXtreme Deep Field

Deborah Byrd

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