The Big Bang
The Big Bang theory describes how the universe evolved from an extremely hot, dense state about 13.8 billion years ago. Several independent lines of evidence support it.
What the Big Bang is
The Big Bang theory says the universe was once extremely hot and dense and has been expanding and cooling ever since. It was not an explosion into empty space; it happened everywhere at once, as space itself began expanding. The theory describes what happened after the earliest moments, not what (if anything) came before.
The first minutes
As the universe cooled, fundamental particles formed. Within about three minutes, protons and neutrons fused into light nuclei: Big Bang nucleosynthesis produced about 75% hydrogen and 25% helium by mass, with traces of deuterium and lithium. These predicted abundances match what astronomers observe in the oldest stars and gas clouds.
The cosmic microwave background
About 380,000 years after the Big Bang, the universe cooled enough (about 3,000 K) for electrons and nuclei to combine into atoms, making space transparent. The light released then has been stretched by expansion into microwaves, now at 2.725 K. Arno Penzias and Robert Wilson discovered this cosmic microwave background (CMB) by accident in 1965.
Mapping the CMB
Satellites such as COBE (1989), WMAP (2001), and Planck (2009) mapped tiny temperature variations in the CMB, about one part in 100,000. These ripples reveal the seeds of later galaxies and clusters, and their patterns let cosmologists measure the universe's age, composition, and geometry with high precision.
Evidence for the Big Bang
Four main lines of evidence support the Big Bang: the expansion of the universe (Hubble's law); the cosmic microwave background; the observed abundances of light elements; and the evolution of galaxies over time, with distant (earlier) galaxies looking younger. The rival steady state theory could not explain the CMB.
Cosmic inflation
Inflation theory, proposed by Alan Guth in 1980, suggests that a tiny fraction of a second after the beginning, the universe expanded extremely rapidly. Inflation explains why the universe is so uniform and nearly flat, and how quantum fluctuations grew into the CMB ripples. It is widely accepted but still being tested.
The first stars and galaxies
After the CMB was released, the universe entered the 'cosmic dark ages', with no stars. The first stars formed perhaps 100-200 million years after the Big Bang, and their light reionised hydrogen gas. JWST is now observing galaxies from this era.
Worked example
The CMB was released at about 3,000 K and is now 2.725 K, so its wavelengths have stretched by a factor of about 3,000/2.725, or about 1,100. This means the universe has expanded about 1,100 times in each direction since the CMB was emitted, a redshift of about z = 1,100.
Key takeaways
The Big Bang theory describes a universe expanding and cooling from a hot, dense state about 13.8 billion years ago, happening everywhere. Big Bang nucleosynthesis made about 75% hydrogen and 25% helium. The CMB, released about 380,000 years after the Big Bang, was discovered in 1965 and is now 2.725 K. COBE, WMAP, and Planck mapped its ripples. Expansion, the CMB, light-element abundances, and galaxy evolution support the theory. Inflation explains uniformity. The first stars formed after the dark ages.
Flashcard
Universe expanding from a hot, dense state
Check your understanding
What is the cosmic microwave background?