From a Static Universe to the Big Bang
Modern cosmology began when gravity stopped being treated as a simple force and started being understood as part of space and time themselves. Einstein’s general relativity showed that matter bends space, and that this bending guides motion. Once this new theory was applied to the whole cosmos, it suggested something unsettling to scientists of the time: the universe should not sit still. It should either expand or contract.
That result clashed with the early 1900s picture of a static, eternal universe made up only of the Milky Way. To avoid collapse, Einstein added a term to his equations so the universe could remain motionless. He later regretted that move, but it reflected how strongly scientists were shaped by the observations available then. Better evidence, not argument alone, would soon overturn the old picture.
The first major step came from measuring the distances to stars and nebulae. Henrietta Swan Leavitt discovered that certain stars, called Cepheid variables, brighten and dim in a regular way that reveals their true brightness. That gave astronomers a reliable distance marker. Using this method, Edwin Hubble showed that Andromeda was not a cloud inside the Milky Way but a separate galaxy far beyond it.
Hubble then found something even more important. The light from distant galaxies was shifted toward the red end of the spectrum, which showed that those galaxies were moving away from us. More than that, the farther away a galaxy was, the faster it was receding. The simplest explanation was that space itself is expanding, carrying galaxies apart.
Once expansion was established, the idea of a hot, dense beginning became hard to avoid. Georges Lemaître saw that if the universe is growing now, then in the past everything must have been compressed into a much smaller state. That early universe would have been incredibly hot, and as it cooled, it could form only the lightest elements at first. The observed amounts of hydrogen, helium, and a little lithium match that picture remarkably well.
Other evidence strengthened the case. Stars make heavier elements such as carbon and oxygen, and supernova explosions spread them into space. Type Ia supernovae, which explode with nearly predictable brightness, also help measure cosmic distances. Together these findings built a coherent history: the universe is old, expanding, and has evolved from an earlier hot and dense state.
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