Carnegie Science celebrates Edwin Hubble's discovery of the universe

For humans, the most important star in the universe is the Sun. The second most important star is nestled inside in the Andromeda galaxy. Don’t go looking for it. The flickering star is 2.2 million light-years away and is 1/100,000th the brightness of  the faintest star visible to the human eye.

Yet, a century ago, its discovery by Carnegie Science astronomer Edwin Hubble ushered in the coming-of-age for humans as a curious species that could scientifically ponder our own creation as encoded in the message of starlight. Carnegie Science is celebrating this centennial this week at the 245th meeting of the American Astronomical Society in Washington, D.C.

The seemingly inauspicious star, simply named V1, flung open a Pandora’s box full of mysteries about time and space that are still challenging astronomers today.  The demure star was discovered in 1923 by Carnegie astronomer Edwin Hubble in a relentless needle-in-haystack search for this rare type of pulsating star, called a Cepheid variable. Such stars are used as a milepost markers. There are no tape-measures in space. But his special type of star had already been proven to be a reliable distance marker to targets within our galaxy thanks to the work of Harvard University “computer” Henrietta Swan Leavitt. 

The discovery of this lighthouse beacon led Hubble to the jaw-dropping realization that the universe extended far beyond our own Milky Way galaxy. In fact, Hubble had suspected there was a larger universe out there, but here was the proof in the pudding. He was so amazed he scribbled an exclamation mark on the photographic plate of Andromeda that pinpointed the variable star.

As a result, the science of cosmology exploded almost overnight. V1 was direct evidence that there was a vastly bigger cosmos that once only existed in the imaginations of ancient world philosophers. Hubble’s contemporary, the distinguished Harvard astronomer Harlow Shapley, upon Hubble notifying him of the discovery, was devastated. “Here is the letter that destroyed my universe,” he lamented to fellow astronomer Cecilia Payne, who was in his office when he opened Hubble’s message.

Just three years earlier, Shapley had presented his observational interpretation of a much smaller universe in a debate one evening at the Smithsonian Museum of Natural History in Washington, D.C., He maintained that the Milky Way galaxy was so huge it must encompass is the entirety of the universe. Shapley insisted that the mysteriously fuzzy "spiral nebulae," such as Andromeda, were simply stars forming on the periphery of our Milky Way, and inconsequential.

Little could Hubble have imagined that 70 years later, an extraordinary telescope named after him, lofted hundreds of miles above the Earth, would continue his legacy. The marvelous telescope made “Hubble” a household word,  synonymous with wonderous astronomy.

Today, NASA’s Hubble Space Telescope is pushing the frontiers of knowledge over ten times farther than Edwin Hubble could ever see. The space telescope lifted the curtain on a compulsive universe full of discordant stars, colliding galaxies, and runaway black holes,  among the celestial fireworks of the interplay between matter and energy.

Edwin Hubble was the first astronomer to take this initial steps, which would ultimately lead to the Hubble Space Telescope revealing a seemingly infinite ocean of galaxies. Edwin Hubble pondered that, despite their abundance, galaxies came in just a few specific shapes: pinwheel  spirals, football shaped eipticals, and oddball irregular galaxies. He thought these might be clues to galaxy evolution. But the answer had to wait for the Hubble Space Telescope’s far vision in the legendary Hubble Deep Field in 1994.

The most impactful finding is that Edwin Hubble's analysis showed that the farther the galaxy is, the faster it appears to be receding from Earth. The universe looked like it was expanding like a balloon. This was based on Hubble tying galaxy distances to the reddening of light, the so-called redshift, which proportionally increased the father away the galaxies are.

The redshift data was collected by Lowell observatory astronomer Vesto Slifer who spectroscopically studied the “spiral nebula ” a decade before  Hubble. Slifer did not know they were extragalactic. Hubble made the connection. But Slifer first interpreted his redshift data an example of the Doppler effect, which describes how light is stretched to longer, redder wavelengths if a source is moving away from us. To Slifer, it was curious that all the spiral nebulae appeared to be moving away from us.

Two years prior to Hubble publishing his findings, the Belgian physicist and Jesuit priest  Georges Lemaître analyzed the Hubble and Slifer observations and first came to the conclusion of an expanding universe.   This proportionality between galaxies' distances and redshifts is today termed Hubble–Lemaître's law.

because the universe appeared to be uniformly expanding Lemaître further realized that the expansion rate could be run back into time, like rewinding a movie, until the universe was unimaginably small, hot and dense.

The term for a compact origin to the universe  was later dubbed the Big Bang in a 1949 radio show interview with antagonist Fred Hoyle, who favored an eternal universe. The nickname stuck all these years.  For the first time in human consciousness, we could assign an age to the universe, like counting the number of candles in a birthday cake.

This was a relief to Edwin Hubble’s contemporary, Albert Einstein, who deduced the universe could not remain stationary without imploding under gravity’s pull. The rate of cosmic expansion is now known as the Hubble Constant.  

Ironically, Hubble himself never fully accepted the runaway universe as an interpretation of the redshift data. He suspected  that some unknown physics phenomenon was giving the illusion that the galaxies were flying away from each other. He was partly right in that Einstein’s theory of special relativity explained redshift as an effect of time-dilation that is proportional to the stretching of expanding space.  The galaxies only appear to be zooming through the universe. Space is expanding instead.

After decades of precise measurements, the Hubble telescope came along to nail down the expansion rate precisely, thanks to work spearheaded by former Carnegie Science Observatories Director Wendy Freedman, giving the universe an age of 13.8 billion years. This required establishing the first rung of what astronomers call the “cosmic distance ladder” needed to build a yardstick to far-flung galaxies. They  are cousins to V1, Cepheid variable stars that the Hubble telescope can detect out to over 100 times farther from Earth that the star Edwin Hubble first found. 

Astrophysics was again turned on its head again in 1996 when the Hubble telescope and other observatories discovered that the universe was expanding at an ever-faster rate. Einstein first toyed with this idea of a repulsive form of gravity in space calling it the cosmological constant.

Even more mysteriously, that expansion rate appears to be different in the early universe, further confounding theoreticians and suggesting new physics are needed.  Today astronomers are wrestling with the idea that whatever is accelerating the universe, dubbed dark energy, may be changing over time.

This grand century-long adventure, plumbing depths of the unknown, began with Edwin Hubble photographing a large smudge of light, the Andromeda galaxy, at the Mount Wilson Observatory high above Los Angeles. 

In short, Edwin Hubble is the man who wiped away the ancient universe and discovered a new universe that would shrink humanity’s self-perception into being an insignificant speck in the cosmos.

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This text was adapted from the Space Telescope Science Institute.