Introduction: The Discovery of an Expanding Universe
The journey to understanding the cosmos began with a fundamental mystery about how the universe evolves. In 1916, Albert Einstein completed his general theory of relativity, which redefined gravity as a property of space and time rather than just a force between objects. Unlike previous models, this theory could describe the evolution of the entire universe. However, his equations suggested a dynamic universe, which contradicted the 1917 scientific consensus that the cosmos was static, eternal, and consisted solely of the Milky Way galaxy.
The tension between theory and observation was eventually broken by the development of better tools for measuring the heavens. Henrietta Swan Leavitt discovered a relationship between the brightness of specific pulsing stars and the timing of their pulses, providing a reliable cosmic yardstick. By measuring how long a star took to pulse, astronomers could determine its true brightness and its exact distance from Earth. Using this method in 1925, Edwin Hubble proved that distant fuzzy patches in the sky were actually separate galaxies located far beyond our own Milky Way.
Hubble soon noticed a startling pattern when comparing the distances of these newly discovered galaxies to the light they emitted. Light waves from distant objects stretch as they move away, making them appear redder in a phenomenon known as redshift. Hubble found that almost all galaxies are moving away from us, and the farther away they are, the faster they recede. This linear relationship demonstrated conclusively that the universe is actively expanding.
Georges Lemaître, a priest and physicist, used Einstein’s equations to propose that this expanding universe must have begun as a single, incredibly dense point. This Big Bang model suggested that everything in the observable universe was once concentrated in an intensely hot state. Modern astronomy confirms this hot beginning by observing the abundance of light elements like hydrogen and helium, which perfectly matches theoretical predictions of the early universe. By observing distant stellar explosions, scientists have determined that this continuously growing cosmos is approximately 13.7 billion years old.
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