The Big Bang theory is the standard theory of the modern universe. The universe began about 13.8 billion years ago in an extremely hot and dense state and reached its present form while expanding and cooling.
Basic Knowledge
Major milestones in the history of the universe:
① t=0: Big Bang: The universe begins from an extremely hot, dense point (a singularity).
② ~10⁻³⁶ seconds later: Inflation: The universe expands rapidly and exponentially. It is used to explain the uniformity of the universe observed today.
③ ~3 minutes later: Big Bang nucleosynthesis: Protons and neutrons combine to form the nuclei of H (hydrogen), D (deuterium), He (helium), and Li (lithium).
④ ~380,000 years later: Recombination: As the universe cools (to about 3000K), electrons combine with protons to form neutral hydrogen. Light becomes able to travel freely. The remnant of this light is the cosmic microwave background radiation (CMB), now observed as radio waves at about 2.7K.
⑤ ~100 million years later and afterward: Formation of the first stars and galaxies.
⑥ ~4.6 billion years ago: Formation of the Solar System.
⑦ Present (about 13.8 billion years later): The universe continues to expand at an accelerating rate.
Big Bang (The beginning of the universe about 13.8 billion years ago. Expansion from an extremely hot and dense state)
Inflation (The exponential rapid expansion immediately after the Big Bang. Explains the uniformity of the universe)
Recombination (The period about 380,000 years later when light became able to propagate freely)
Cosmic microwave background radiation (CMB) (The afterglow of light from recombination. Observed across the entire sky at about 2.7K)
Big Bang nucleosynthesis (The process by which the nuclei of H, He, and Li were synthesized several minutes after the Big Bang)
Age of the universe (About 13.8 billion years. Calculated from the CMB and the Hubble constant)
Further Exploration (Background and Significance)
The three major pieces of evidence supporting the Big Bang theory:
① Hubble's law (galaxy redshift = observational evidence of the universe's expansion).
② Cosmic microwave background radiation (CMB) (discovered in 1965 by Penzias and Wilson; Nobel Prize). The CMB is evidence that the universe was once hot.
③ Abundance of light elements (the universe's helium content is about 25%, matching the prediction of Big Bang nucleosynthesis).
The present composition of the universe is about 5% ordinary matter, 27% dark matter, and 68% dark energy. Matter made of the atoms we know makes up only 5%.
The inflationary theory was proposed in the 1980s by Katsuhiko Sato, Alan Guth, Andrei Linde, and others. It provided answers to questions such as "Why is the universe so uniform?" and "Why is space flat?" However, direct observational evidence has not yet been established.
- Age of the universe = about 13.8 billion years
- Three major pieces of Big Bang evidence: Hubble's law, CMB, and light-element abundances
- CMB = remnant of light from recombination (about 380,000 years later), about 2.7K
- Inflation = rapid expansion immediately after the Big Bang
- Composition of the universe: 5% ordinary matter, 27% dark matter, 68% dark energy
- Big Bang = not an "explosion" but the beginning of the rapid expansion of space
- The present universe is undergoing accelerated expansion
Points of Caution (Easy to Confuse)
① The Big Bang was not an "explosion" but the beginning of the expansion of space. It was not that "something exploded somewhere"; the entire universe expanded. ② The CMB is light from recombination, not light from the Big Bang itself. ③ Dark matter (source of gravity) and dark energy (source of accelerated expansion) are completely different. ④ The age of the universe, 13.8 billion years, is measured in "years," not "light-years."
Practice
- Briefly explain what cosmic microwave background radiation (CMB) is.
- Give two pieces of observational evidence supporting the Big Bang theory.
- Give the approximate proportions of ordinary matter, dark matter, and dark energy in the present universe.