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How did the universe change as it expanded and cooled after the big bang?
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Universe
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The big-bang theory describes the universe as beginning approximately 13.8 billion years ago in an extremely hot, dense state. As the universe expanded, it underwent a significant decrease in temperature and density, triggering a series of physical transitions that shaped the cosmos.123
Key Stages of Evolution
- Early Expansion and Cooling: In the initial moments, the universe was dominated by radiation. As it cooled, subatomic particles formed. Within the first few minutes, nuclear reactions produced the first light elements, primarily hydrogen and helium.31
- Formation of Atoms: After several hundred thousand years, the temperature dropped sufficiently for electrons to attach to nuclei, forming neutral atoms. This allowed electromagnetic radiation to travel freely through space, creating the cosmic microwave background observed today.342
- Structure Formation: Over millions of years, gravity caused matter to clump together, eventually giving rise to the first stars and galaxies.13
This ongoing expansion continues to shape the universe, which remains filled with the remnants of these early processes.1