Edwin Hubble's groundbreaking discovery of the expanding universe a century ago, while working at Carnegie Observatories, revolutionized the understanding of the cosmos, revealing the Milky Way as just one among countless galaxies. This finding led to the realization that the universe is expanding, forming the foundation of modern cosmology, including the concepts of redshift and the Hubble Constant.
In commemoration of Edwin Hubble's discovery of a Cepheid variable class star, called V1, in the neighboring Andromeda galaxy 100 years ago, astronomers partnered with the American Association of Variable Star Observers (AAVSO) to study the star. AAVSO observers followed V1 for six months, producing a plot, or light curve, of the rhythmic rise and fall of the star's light. Based on this data, the Hubble Space Telescope was scheduled to capture the star at its dimmest and brightest light. Edwin Hubble's observations of V1 became the critical first step in uncovering a larger, grander universe than some astronomers imagined at the time. Once dismissed as a nearby "spiral nebula" measurements of Andromeda with its embedded Cepheid star served as a stellar milepost marker. It definitively showed that Andromeda was far outside of our Milky Way. Edwin Hubble went on to measure the distances to many galaxies beyond the Milky Way by finding Cepheid variables within those levels. In turn, the velocities of those galaxies allowed him to determine that the universe was expanding. Image Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA); Acknowledgment: R. Gendler
This momentous discovery ushered in a new era of human curiosity, enabling humans to scientifically contemplate the origins through the cosmic message encoded in starlight. To commemorate this centennial, Carnegie Science and NASA are hosting a special event at the 245th meeting of the American Astronomical Society in Washington, D.C.
The Sun is the most significant star in the universe, and the Andromeda galaxy contains the second-most significant star. The flickering star is located 2.2 million light-years away and is 1/100,000th as bright as the faintest star visible to the naked eye.
The ostensibly unlucky star, known as V1, opened a treasure trove of time and space mysteries that continue to confound astronomers today. Hubble discovered the modest star in 1923 using the world's largest telescope at the time, the 100-inch Hooker Telescope, which was funded by the Carnegie Foundation and located at Mount Wilson Observatory in California. Milepost markers for far-off celestial objects are made from this uncommon pulsating star called a Cepheid variable.
Although there are no tape measures in space, Henrietta Swan Leavitt found, by the early 1900s, that the luminosity of Cepheid variables is directly related to their pulsation period.
Hubble believed there was a bigger universe out there, but this was the concrete evidence. He was taken aback when he wrote an exclamation point on the Andromeda photographic plate, which identified the variable star.
Cosmology as a science consequently took off almost immediately. When Hubble informed the renowned Harvard astronomer Harlow Shapley of the discovery, Shapley was devastated.
Three years prior, Shapley had spent an evening at the Smithsonian Museum of Natural History in Washington arguing his observational interpretation of a much smaller universe. Shapley insisted that the Milky Way galaxy must contain the entire universe because it is so massive. Shapley maintained that the enigmatic, hazy "spiral nebulae," like Andromeda, were trivial, just stars forming on the edge of the Milky Way.
Hubble could not have predicted that the legacy would be carried on 70 years later by an incredible telescope bearing his name, placed hundreds of miles above the ground. "Hubble" became a household name, synonymous with amazing astronomy, using the amazing telescope.
Edwin Hubble could never have imagined the extent to which NASA's Hubble Space Telescope is pushing the boundaries of knowledge today. Among the celestial fireworks of the interaction between matter and energy, the space telescope has revealed a compulsive universe full of active stars, colliding galaxies, and runaway black holes.
Edwin Hubble, the first astronomer to take the steps that would eventually lead to the Hubble Space Telescope, revealed a seemingly endless ocean of galaxies. Despite their abundance, he believed galaxies were only found in a few distinct shapes: oddball irregular galaxies, football-shaped ellipticals, and pinwheel spirals.
He believed that these could be hints of galaxy evolution, but the answer would have to wait until 1994 when the renowned Hubble Deep Field was installed on the Hubble Space Telescope.
Edwin Hubble's analysis revealed the most significant result: galaxies appear to be moving away from Earth at a faster rate the farther they are from Earth. It appeared as though the universe was expanding like a balloon. The premise behind this was that Hubble linked the distances of galaxies to the reddening of light, or redshift, which grew proportionately farther apart.
Ten years before Hubble, astronomer Vesto Slipher of Lowell Observatory conducted a spectroscopic study of the "spiral nebulae" and gathered the first redshift data. Hubble made the connection, but Slipher was unaware that they were extragalactic. Slipher initially interpreted his redshift data as a Doppler effect example. This phenomenon occurs when light is stretched to longer, redder wavelengths as a source moves away. Slipher found it intriguing that all the spiral nebulae seemed to be receding from Earth.
The Belgian physicist and Jesuit priest Georges Lemaître first concluded that the universe was expanding two years before Hubble published his findings after he examined the Hubble and Slifer observations. Today, this relationship between redshifts and galaxy distances is known as Hubble-Lemaître's law.
Lemaître also discovered that the expansion rate could be rolled back in time, much like rewinding a film, because the universe seemed to be expanding uniformly. This would make the universe unthinkably dense, hot, and small. The term "big bang" did not become popular until 1949.
Albert Einstein, a contemporary of Edwin Hubble, was relieved because he had concluded that the universe could not stay motionless without collapsing under the force of gravity. Today, the Hubble Constant is used to describe the rate of cosmic expansion.
Ironically, Hubble was never entirely convinced that the runaway universe was a plausible explanation for the redshift data. Hubble had a suspicion that the galaxies were appearing to be flying apart due to an unidentified physics phenomenon. Hubble was partially correct in explaining redshift as a function of time-dilation proportional to the stretching of expanding space in Einstein's theory of special relativity. All the galaxies seem to be doing is speeding through the cosmos. Instead, space is growing.
The Hubble telescope arrived to precisely determine the expansion rate after decades of meticulous measurements, granting the universe an age of 13.8 billion years. To create a yardstick for distant galaxies, the first rung of what astronomers refer to as the "cosmic distance ladder" had to be established. The Hubble telescope can detect Cepheid variable stars, which are cousins to V1, more than 100 times farther away from Earth than the star Edwin Hubble discovered initially.
In 1998, the Hubble telescope and other observatories found that the universe was expanding at an ever-increasing rate, a phenomenon known as "dark energy." This discovery completely upended astrophysics once more. This concept of a repulsive type of gravity in space was initially explored by Einstein, who dubbed it the cosmological constant.
Theorists are further perplexed by the fact that the current expansion rate seems to deviate from what contemporary cosmological models of the expanding universe would predict. Astronomers are currently grappling with the possibility that whatever is speeding up the universe is also changing over time. With its capacity to conduct extensive cosmic surveys, NASA's Roman Space Telescope should provide fresh perspectives on the behavior of dark matter and dark energy. Roman will probably use lensed supernovae to measure the Hubble constant.
At the Mount Wilson Observatory, which is located high above Los Angeles, Hubble took a photograph of the Andromeda galaxy, a massive smudge of light, which marked the beginning of this epic century-long journey into the unknown.
Edwin Hubble is, in essence, the man who destroyed the old universe and found a new one that would make humanity seem like a tiny dot in the vast cosmos.