A Century After the Dawn of the Galactic Era

One hundred years ago, we were able to prove for the first time that the Milky Way galaxy is not the entirety of our universe, but merely one of the scattered star islands in the cosmos. Over the past century, we have not only expanded our understanding of the boundaries of the existing universe but are also striving to comprehend the ultimate limits of space and time. It may be hard for many of us to believe today, but there are still people alive on our planet who were born at a time when the existence of other galaxies was a theoretical discussion; it was only in 1924 that we found definitive evidence of these star islands. This astonishing speed of knowledge development may seem even more remarkable when we recall how long previous models of the universe lasted. Since ancient times, the Earth-centered theory of the universe lasted for several millennia, asserting that our Earth is at the center of the universe with everything revolving around it. It was in the mid-16th century that mathematical tools and a plethora of accumulated observations finally disproved the Ptolemaic theory and the Earth-centered model, showing that the sun is at the center of our universe, with planets and Earth revolving around it. The heliocentric era did not last as long as its predecessor, and we soon realized that other stars in the sky are other suns, and we, our solar system, and our parent star are nestled in a corner of a starry island called the galaxy. However, the lifespan of this era also came to an end quickly. The first close-up image of a star outside the Milky Way was recorded. Although in the 18th century thinkers like Immanuel Kant and William Herschel proposed the existence of islands of other stars, which we now call galaxies, the tools of that day could not confirm this hypothesis. Finally, the 1920s arrived, and a great debate began in the astronomical community. Perhaps the peak of this debate occurred between Harlow Shapley and Heber Curtis, two prominent astronomers, on April 26, 1920, at the Smithsonian National Museum of Natural History. This debate, known as the 'Great Debate,' was about the size of the universe and the nature of nebula-like objects such as the Andromeda galaxy. While Shapley argued that these nebulae were peripheral parts of the Milky Way (which was thought to be the entire universe), Curtis proposed that these objects were actually collections of very distant stars. This debate concluded in 1924 when Edwin Hubble, using the 100-inch telescope at Mount Wilson on the outskirts of Los Angeles, successfully discovered a star from the Cepheid variable family within the Andromeda galaxy. Henrietta Leavitt had established a direct relationship between their periods and the brightness of these stars; therefore, if we observe them anywhere in the universe and determine their periods, we can understand how bright the star actually is, and by examining its apparent and intrinsic brightness, we can estimate its distance. This was what Edwin Hubble did to usher us into the galactic era. Just five years later, Hubble formulated the 'Hubble Law' by studying the galaxies of the universe, concluding that not only is our universe filled with galaxies, but these galaxies are moving away from each other due to the expansion of space. Observational tools across various wavelengths, as well as mathematical models and theories, helped scientists quickly overturn all our old beliefs. Within a few decades, we not only realized the vastness of a universe filled with galaxies and galaxy clusters but also discovered enormous walls of superclusters. More importantly, we have realized that all the visible matter we see in the universe, existing in the form of stars and galaxies, constitutes less than five percent of the universe's total matter. About 26 percent of our universe is made up of dark matter, and around 70 percent consists of a mysterious and currently unknown entity called 'dark energy,' which causes the continued expansion of our universe. This is a long journey we have traveled in less than a hundred years. A journey that, aided by modern science and emerging from debates and mathematical models, has led us from a world we thought was composed of a few planets revolving around the sun to a vast and breathtaking expanse. How will our understanding of the universe we live in evolve in the next hundred years?