Most people have heard of the Hubble Space Telescope (HST), but relatively few people are familiar with the astronomer after whom it was named. Edwin Hubble (1889-1954) was a key figure in 20th-century astronomy, so much so that not only was a space telescope named for him but a constant of physics, as well. This article will explore the highlights of Hubble’s work and their significance for modern-day astronomers and cosmologists.
Hubble began his career as an astronomer in the early 20th century. At the time, three questions dominated the field, with no consensus in sight. First, is Earth’s solar system located at the center of the Milky Way Galaxy or somewhere in the periphery? Second, what is the size of the Milky Way in light-years? Finally, and arguably most importantly, is Earth’s galaxy the only one in the universe, or do other galaxies exist outside of the Milky Way?
In 1920, two leading astronomers, Shapley and Curtis, debated these questions at a famous forum held in Washington, DC. Predictably, the debate generated more heat than light. Unbeknownst to all concerned, Hubble would tackle these questions over the next two decades. In the process, he forged a permanent link between the sciences of astronomy and cosmology.
Hubble’s first breakthrough was the detection of stars called Cepheid variables in the Andromeda “Nebula.” These stars fluctuate in brightness in a predictable cycle of months to years. Compared to their counterparts in the Milky Way, the Cepheids in Andromeda cycled far more slowly. This confirmed that Andromeda was not a gas cloud within the Milky Way Galaxy but rather a spiral galaxy located far beyond the Milky Way – some 2.5 million light years away. As larger telescopes became available during the 1920s, astronomers discovered hundreds, then thousands, of galaxies separate from the Milky Way.
Discovering that our galaxy is one of a multitude of galaxies was a tremendous feat in and of itself. Hubble’s next discovery was truly monumental. By 1929, he noticed that the spectral emission lines of nearly all observable galaxies are shifted or stretched toward the red end of the electromagnetic spectrum. This meant that, aside from gravitationally bound clusters of galaxies, most galaxies are moving away from one another. Then Hubble discovered a trend that changed our view of the universe forever. The more distant a galaxy was from the Milky Way, the larger its red shift and the faster it was receding. Essentially, galactic red shifts were the first solid evidence of an expanding universe.
While astronomers (including Hubble) struggled to make sense of these observations, it was not long before cosmologists took Hubble’s discovery to its logical conclusion: if galaxies throughout the universe are rushing away from each other, at some point in the distant past they must have shared a common origin – in an unimaginably hot, dense ball of matter, energy, space and time that scientists call the Big Bang.
So, how long ago did the expansion of the universe begin? In other words, when was the universe born? That all depends upon how fast the universe has expanded from the moment of the Big Bang up to the present, captured by the deceptively simple equation Velocity = Distance X Rate constant. Appropriately enough, the rate term was named the Hubble Constant, denoted Ho.
The Hubble Constant is technically not a constant, but rather a rough estimate of the expansion rate of the universe. The units of Ho are kilometers per second per megaparsec. One parsec is equal to just over three light years; a megaparsec is equal to over 3 million light years. Most astronomers think the value of Ho falls between 50 and 100 km/s/Mpc. This means that with each passing second, between 50 and 100 kilometers of intergalactic space opens up over the vast distance of 3 million light years.
The reciprocal of Ho gives the approximate time elapsed since the expansion of the universe began. If the current estimate of Ho (68 km/sec/Mpc) stands correct, this corresponds to a universe between 12 and 15 billion years old.
According to astronomers, the expansion of the universe is most noticeable in the gigantic voids between galaxies and galactic clusters. Within individual galaxies such as the Milky Way, gravitational attraction between stars and other matter counteracts the force of expansion; otherwise galaxies would be ripped apart (or would never have formed in the first place).
Interestingly, astronomers cannot account for the majority of matter responsible for gravitational attraction within the Milky Way or other galaxies. This led them to propose the existence of “dark matter,” a heretofore uncharacterized substance that comprises most of the matter in the universe. Some astronomers think that particles called neutrinos account for the majority of dark matter. Others invoke the existence of weakly interacting massive particles, or so called WIMPS. Still others think the presence of supermassive black holes at the center of most galaxies may account for dark matter and its gravitational effects. The debate continues.
The reason Ho is technically not a “constant” is that in the very early universe, the expansion rate may have been higher before the formation of the first stars and galaxies. Once the universe evolved from a hot, high energy state into a cooler one where matter could aggregate, gravity took hold over large distances, and the expansion rate of the universe slowed down.
In recent years, however, some astronomers think they have discovered a strange trend – the expansion of the universe may actually be accelerating. They attribute this acceleration to the existence of “dark energy,” the mysterious counterpart of so-called dark matter. Others are not convinced, pointing out that if a presumably tangible entity like dark matter remains so elusive, how can scientists ever expect to confirm the existence of dark energy? As with all debates in modern astronomy, this one is likely to continue for quite some time.