Big Bang Theory (Cosmological Model)

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The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe.[1] According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, the Big Bang occurred approximately 13.75 billion years ago,[2][3] which is thus considered the age of the Universe.[4][5] After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons. While protons and neutrons combined to form the first atomic nuclei only a few minutes after the Big Bang, it would take thousands of years for electrons to combine with them and create electrically neutral atoms. The first element produced was hydrogen, along with traces of helium and lithium. Giant clouds of these primordial elements would coalesce through gravity to form stars and galaxies, and the heavier elements would be synthesized either within stars or during supernovae. The Big Bang is a well-tested scientific theory and is widely accepted within the scientific community. It offers a comprehensive explanation for a broad range of observed phenomena. Since its conception, abundant evidence has been uncovered in support of the model.[6] The core ideas of the Big Bang—the expansion, the early hot state, the formation of helium, and the formation of galaxies—are derived from many observations that are independent from any cosmological model; these include the abundance of light elements, the cosmic microwave background, large scale structure, and the Hubble diagram for Type Ia supernovae. As the distance between galaxy clusters is increasing today, it can be inferred that everything was closer together in the past. This idea has been considered in detail back...
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