The universe was once thought to be static, but it is actually expanding.
In short: For three thousand years, scientists assumed the universe was unchanging. It took a massive paradigm shift in the twentieth century to realize that space itself is stretching, driving galaxies away from us. This discovery fundamentally altered our understanding of cosmic origins and the very nature of space.
Before the 1920s, the scientific consensus was that the universe was static. This idea was even enshrined as a solution to Albert Einstein's equations of general relativity. However, the first cracks in this edifice appeared through the work of Willem de Sitter, who noted redshifts in galaxies measured by astronomer Vesto de Slipher. A redshift indicates a velocity moving away from us, whereas a blueshift indicates movement towards us.
In 1929, Edwin Hubble announced that galaxy redshifts increased systematically with their distance from Earth, suggesting an expanding universe. While Hubble's name is often associated with this law, Belgian cosmologist Georges Lemaitre had independently discovered it two years earlier. Additionally, in 1922, Russian mathematician Alexander Friedmann published solutions to Einstein's equations that proved a non-static universe was mathematically possible.
The expansion is not galaxies moving through space, but the space in which they are embedded that is expanding. This concept leads to profound questions about the geometry of the cosmos. While a 'flat' universe is a leading model, it remains unknown if the universe is finite or infinite. A flat universe could be infinite like a plane, or finite like a torus—a doughnut-shaped surface that is spatially flat but has a finite volume.
This shift in understanding was not without controversy. During the 1930s, the large measured value of Hubble's constant sparked debates, as some scientists questioned whether redshifts truly meant space was expanding. This was particularly contentious because the calculated age of the universe appeared to be less than the age of the Earth.
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Sources
- Summary by Intellect · source: The Expanding Universe - Professor Joseph Silk
- Also: ESA Science & Technology - Joseph Silk, astronomer
- Also: The Expanding Universe
- Also: royalsociety.org — Professor Joseph Silk FRS | Royal Society Fellow
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