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# The Big Bang Theory The prevailing model for the universe is known as the Big Bang theory. The name comes from the idea that the universe was once in an extremely hot and dense state that expanded rapidly. This expansion caused the universe to cool and resulted in its present size and state. **E...

# The Big Bang Theory The prevailing model for the universe is known as the Big Bang theory. The name comes from the idea that the universe was once in an extremely hot and dense state that expanded rapidly. This expansion caused the universe to cool and resulted in its present size and state. **Evidence for the Big Bang** * **Expansion of the Universe:** * In the 1920's, Edwin Hubble discovered that the distance to other galaxies was generally proportional to their redshift. * Redshift occurs when light seen coming from an object that is moving away from you is proportionally decreased in frequency, or redshifted. * Thus, more distant galaxies have greater redshift and are moving away faster. * **Cosmic Microwave Background Radiation:** * In the 1960's, Arno Penzias and Robert Wilson discovered the cosmic microwave background (CMB). * The CMB is thermal radiation left over from the time of recombination in the early universe. * Recombination occurred when the universe had cooled sufficiently to allow protons and electrons to combine and form neutral hydrogen atoms. * **Abundance of Light Elements:** * The Big Bang theory predicts the abundance of light elements such as hydrogen, helium, and lithium in the universe. * This is because in the early universe, the temperature was high enough for nuclear fusion to occur. * The predicted abundance of light elements agrees with observations. **Timeline of the Big Bang** | Time | Event | | :------------------ | :------------------------------------------------------------------------------------------------------------------------ | | $t=0$ | Big Bang | | $t=10^{-43} s$ | Planck Era: All four fundamental forces are unified. | | $t=10^{-36} s$ | Inflation: The universe expands exponentially. | | $t=10^{-12} s$ | Electroweak Era: Electromagnetism and the weak force separate. | | $t=10^{-6} s$ | Quark Era: Quarks and gluons exist in a plasma. | | $t=1 s$ | Hadron Era: Quarks combine to form hadrons such as protons and neutrons. | | $t=10 s$ | Lepton Era: Leptons such as electrons and neutrinos dominate the universe. | | $t=3 min$ | Nucleosynthesis: Protons and neutrons combine to form light nuclei such as helium and lithium. | | $t=380,000 years$ | Recombination: Electrons and nuclei combine to form neutral atoms. The universe becomes transparent to radiation. | | $t=1 billion years$ | Galaxy Formation: Gravity causes matter to collapse and form galaxies. | | $t=13.8 billion years$ | Present: The universe continues to expand and evolve. | **Open Questions** * **What caused the Big Bang?** * **What is the nature of dark matter and dark energy?** * **What is the ultimate fate of the universe?**