The universe's most transformative changes occurred in just the first few minutes
In short: Before galaxies existed, the cosmos was a searing, opaque soup of particles. In a cosmic blink of an eye, a period of rapid expansion smoothed the universe, setting the stage for the first atoms, stars, and the very light we observe today.
Cosmic inflation marked the universe's start, occurring approximately 13.8 billion years ago as space underwent superluminal expansion for a brief moment. While scientists are unsure what powered this expansion or what preceded it, inflation played a crucial role in smoothing out the universe's large-scale curvature and magnifying tiny quantum density differences. These fluctuations eventually became the seeds for all large-scale structures, including galaxies.
As inflation ceased, its energy transferred into matter and light, marking the Big Bang. One second after this event, the universe was a primordial soup of particles at temperatures reaching 10 billion degrees Celsius (18 billion degrees Fahrenheit). During the subsequent minutes, a process called nucleosynthesis occurred. The first elements, including hydrogen, helium, and small amounts of lithium and beryllium, were created through proton and neutron impacts. By the five-minute mark, most of the universe's helium had formed, though the cosmos remained too hot for stable atoms to exist.
For hundreds of thousands of years, the universe remained an opaque fog of free electrons that scattered light. It was not until the epoch of recombination, approximately 380,000 years after the Big Bang, that the universe cooled enough for nuclei to capture electrons. This cleared the cosmic fog, making the universe transparent and releasing the cosmic microwave background—the oldest light detectable today. This transition led into the 'Dark Ages,' a 200-million-year period of darkness before gravity pulled dense gas clouds together to ignite the first massive, brilliant stars.
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Sources
- Summary by Intellect · source: A Brief History of the Universe: Crash Course Astronomy #44
- Also: Overview - NASA Science
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