High School Earth Science: Stars, Galaxies, and the Universe - The Universe — Flashcards

Formation of the Universe    
The Big Bang theory is the most widely accepted cosmological explanation for how the universe formed. According to the Big Bang theory, the universe began about 13.7 billion years ago, before which everything in the universe was squeezed into a very small volume. This means that at the beginning, the entire known universe was a single, hot, chaotic mass. Then, an enormous explosion—a big bang—caused the universe to start expanding rapidly. According to the Big Bang theory, all of the matter and energy in the universe, and even space itself, came out of this explosion.    
In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons began to form.    
After a few minutes, protons and neutrons came together to create hydrogen nuclei. Energy in the universe was great enough to initiate nuclear fusion, and hydrogen nuclei fused together to become helium nuclei. However, the first neutral atoms (atoms that included electrons) did not form until about 380,000 years later. Scientists think that matter in the early universe was not evenly distributed across space. Instead, there existed dense clumps of matter held together by gravity. Eventually, these clumps became the countless trillions of stars, billions of galaxies, and other structures we now know to make up the visible mass of the universe.    

When it was first proposed, the Big Bang theory was just a hypothesis. Many astronomers did not accept it and thought that the universe was static. However, nearly all astronomers came to accept the hypothesis when an important line of evidence for the Big Bang was discovered in 1964. After that, the Big Bang hypothesis achieved the status of a scientific theory. Two researchers at Bell Laboratories, using a microwave receiver, learned that the background radiation of the universe gave space a temperature of 3 Kelvin, not 0 Kelvin, which would be expected in a static universe. Although this is a small amount of heat, it was enough for most scientists to agree that it must be left over from the Big Bang.