The Big Bang theory is the prevailing cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution.
Origin and Expansion
The universe began as an extremely hot, dense point called a singularity. About 13.8 billion years ago, this singularity began to expand rapidly—an event known as the Big Bang.
|
Time Period |
Event |
Description |
|---|---|---|
|
Inflationary Epoch |
Rapid Expansion |
The universe expanded exponentially in a fraction of a second. |
|
Quark Epoch |
Formation of Subatomic Particles |
Quarks and leptons form in a hot, dense plasma. |
|
Hadron Epoch |
Formation of Protons and Neutrons |
Quarks combine to form protons and neutrons. |
|
Nucleosynthesis |
Formation of Light Elements |
Protons and neutrons combine to form the nuclei of hydrogen, helium, and trace lithium. |
|
Recombination |
Formation of Atoms |
Electrons combine with nuclei to form the first neutral atoms, making the universe transparent. |
Evidence Supporting the Theory
Several lines of evidence strongly support the Big Bang model.
1. Cosmic Microwave Background (CMB)
The CMB is faint radiation filling all space, almost uniform in all directions. This radiation is a relic heat from the universe's early, hot, dense state.
2. Redshift of Galaxies and Hubble's Law
Observations show that almost all galaxies are moving away from us, and the farther away a galaxy is, the faster it is receding. This is known as Hubble's Law and indicates that the universe is currently expanding.
3. Abundance of Light Elements
The measured abundance of light elements (Hydrogen, Helium, and Lithium) in the universe closely matches the theoretical predictions made by Big Bang Nucleosynthesis.
Key Concepts
- Dark Matter and Dark Energy: While the Big Bang describes the origin, a significant portion of the universe's composition remains unknown. Dark matter accounts for about 27% of the universe's mass/energy, and dark energy accounts for about 68%. Only about 5% is composed of the ordinary matter we can observe.
- The Horizon Problem: This problem asks why the CMB is so uniform in temperature across the sky, despite regions being too far apart to have ever interacted. The theory of Inflation is proposed to resolve this.