This lesson surveys the standard Big Bang theory of modern cosmology, the observational evidence supporting it, and the still-unexplained dark matter, dark energy, and inflation.
Basic Knowledge
The universe is thought to have begun about 13.8 billion years ago from an extremely hot and dense state (the Big Bang). Major events in chronological order:
・10⁻³⁶〜10⁻³² seconds: inflation (exponential expansion; the universe expanded by a factor of 10²⁶)
・10⁻⁶ seconds: protons and neutrons formed (quark confinement)
・3 minutes: Big Bang nucleosynthesis (nuclei of H, He, and Li formed; He accounts for about 25% of the mass of the universe)
・380,000 years: recombination (electrons and protons combined to form hydrogen atoms, allowing light to travel freely)
・Present (13.8 billion years): radius of the observable universe about 46.5 billion light-years
The light from the time of “recombination” cooled and is now observed from the entire sky as electromagnetic radiation at about 2.7K, called the cosmic microwave background radiation (CMB). Tiny temperature fluctuations in the CMB (differences of 10⁻⁵) were the seeds of today’s large-scale structure.
Inflation (exponential expansion immediately after the Big Bang; solves the horizon problem and flatness problem)
Big Bang nucleosynthesis (formation of H, He, and Li nuclei during the first 3 minutes; He has a mass fraction of about 25%)
Recombination (the turning point 380,000 years after the beginning when light became able to travel freely)
Cosmic microwave background radiation(CMB) (the remnant of light from recombination; currently about 2.7K)
Dark matter (matter observed gravitationally but emitting no light; about 27% of the universe)
Dark energy (unknown energy that accelerates cosmic expansion; about 68% of the universe)
Further Study (Background and Significance)
Inflation theory (independently proposed in 1981 by Katsuhiko Sato and Alan Guth) solves the horizon problem (the CMB has the same temperature in distant directions; why is it uniform even though there should have been no causal connection?) and the flatness problem (why is the curvature of the universe nearly zero?). Quantum fluctuations during inflation were stretched and became the seeds of cosmic structure (primordial density fluctuations).
Dark matter candidates include WIMPs (weakly interacting massive particles), axions, and sterile neutrinos. However, as of 2025, they have not yet been directly detected.
Dark energy is formulated as the cosmological constant Λ (a constant that Einstein once withdrew). The fact that 95% of the universe consists of unknown components is one of the greatest mysteries in modern physics.
- Beginning of the universe: about 13.8 billion years ago, the Big Bang
- Inflation: 10⁻³⁶〜10⁻³² seconds, exponential expansion
- Big Bang nucleosynthesis (3 minutes): He mass fraction about 25%
- Recombination (380,000 years later) → CMB began to travel freely
- CMB: currently 2.7K, observed across the entire sky
- Composition of the universe: ordinary matter 5%, DM 27%, DE 68%
- ΛCDM is the standard model of cosmology
Points of Caution (Easily Confused)
① Keep the order of inflation (10⁻³⁶s), Big Bang nucleosynthesis (3 minutes), and recombination (380,000 years) accurate. ② The CMB is not “light from the Big Bang” but “the remnant of light from recombination” (light immediately after the Big Bang could not reach us because the universe was opaque). ③ Do not confuse dark matter (gravity and attraction) with dark energy (accelerated expansion and a repulsive effect). ④ The radius of the observable universe, 46.5 billion light-years, is not equal to the age of the universe × the speed of light because of cosmic expansion.
Practice
- What is the current temperature of the CMB, approximately in K?
- Name one problem that inflation theory attempts to solve.
- State the composition of the universe in the ΛCDM model (the percentages of ordinary matter, dark matter, and dark energy).