4th Wall

The Big Bang Theory

1. Overview

The Big Bang theory is the leading cosmological model for the observable universe from its earliest known periods through its subsequent large-scale evolution. It describes how the universe expanded from an initial state of extremely high density and temperature.

2. Key Concepts
  • Singularity: The universe began from an infinitely dense and hot point, a singularity, approximately 13.8 billion years ago.
  • Inflation: A period of exponential expansion immediately after the Big Bang, lasting for a tiny fraction of a second (10^{-32} seconds), which smoothed out irregularities and stretched the universe to immense sizes.
  • Cooling and Formation: As the universe expanded, it cooled, allowing fundamental particles to combine:
    • Quark-gluon plasma: In the first few microseconds, the universe was a superheated plasma of quarks and gluons.
    • Nucleosynthesis: Within the first few minutes, protons and neutrons formed, and then light nuclei (primarily hydrogen and helium) were created through nuclear fusion.
    • Recombination: Approximately 380\,000 years after the Big Bang, the universe cooled enough for electrons to combine with atomic nuclei, forming neutral atoms. This event made the universe transparent, allowing photons to travel freely, which we observe today as the Cosmic Microwave Background (CMB).
3. Evidence Supporting the Big Bang
  • Hubble's Law (Expansion of the Universe):
    • In 1929, Edwin Hubble observed that galaxies are moving away from Earth, and the velocity of recession is proportional to their distance (Hubble's Law: v = H0 d where v is velocity, H0 is the Hubble constant, and d is distance).
    • This observation implies that the universe is expanding, consistent with starting from a compressed state.
  • Cosmic Microwave Background (CMB) Radiation:
    • Discovered in 1964 by Penzias and Wilson, the CMB is a faint, uniform glow of microwave radiation coming from all directions in space.
    • It is interpreted as the residual heat or "afterglow" from the Big Bang's recombination period, providing direct evidence of a hot, dense early universe.
  • Abundance of Light Elements:
    • The Big Bang model accurately predicts the observed relative abundance of light elements (hydrogen, helium, and lithium) in the universe.
    • These elements were formed during the brief period of Big Bang Nucleosynthesis when the universe was hot enough for nuclear fusion.
  • Large-Scale Structure of the Universe:
    • The distribution of galaxies and galaxy clusters on large scales, forming a cosmic web of filaments and voids, is consistent with predictions made by the Big Bang model regarding the gravitational growth of initial density fluctuations.