WE THOUGHT THE UNIVERSE WAS ALMOST JUST ONE GALAXY.

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A CENTURY AGO, WE THOUGHT THE UNIVERSE WAS ALMOST JUST ONE GALAXY.

 It is difficult to imagine today, but just over a century ago, astronomers were still trying to determine whether the Milky Way was the entire universe or merely one part of something vastly larger.

 Strange objects known as "spiral nebulae" were observed in telescopes at the beginning of the 20th century, but their true nature was unknown. Were they clouds of gas and stars located somewhere inside the Milky Way—or were they enormous, distant systems comparable to our own galaxy?

 During the 1920 Shapley–Curtis Great Debate, this question gained notoriety. Heber Curtis argued that the spiral nebulae could be independent galaxies far beyond the Milky Way, while Harlow Shapley proposed that the spiral nebulae were not separate galaxies in a very large Milky Way. Then came one of the most transformative developments in the history of astronomy.  Edwin Hubble's observations in the 1920s provided convincing proof that some of these enigmatic spiral nebulae were in fact distant galaxies. The Milky Way was not the entire universe; it was just one galaxy among many others, and the implication was huge. Today, NASA estimates that the Milky Way’s stellar disk spans more than 100,000 light-years, while the observable universe extends roughly 93 billion light-years across.

 And the numbers become almost impossible to comprehend.  NASA says the observable universe contains at least 100 billion galaxies, with each galaxy potentially containing millions, billions, or even trillions of stars.

 In barely a century, humanity’s cosmic perspective changed dramatically—from imagining the Milky Way as the great island universe to understanding that it is just one member of a vast cosmic web of galaxies, clusters, filaments and enormous voids.

The fact that the observable universe is not necessarily the entire universe is perhaps the most humbling aspect. We can only observe the region from which light has had time to reach us.  What exists beyond that cosmic horizon remains unknown.

Our understanding of the cosmos has expanded from one galaxy to a universe filled with at least hundreds of billions of galaxies—and possibly something far larger beyond what we can ever observe.

 The universe didn’t suddenly become bigger.

 Our understanding of it did.

 Source: NASA Science



   Writer: Binod Kumar Simkhada
 binodkumarsimkhada315@gmail.com

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