The Big Bang model

What the Big Bang means

The Big Bang model says that the observable universe has expanded and cooled from an earlier hot, dense state. It does not describe an explosion of matter into preexisting empty space. Instead, it describes the expansion of space itself.

Running cosmic expansion backward implies that galaxies were closer together, radiation was hotter, and matter was denser.

Main evidence

Three major pillars support the Big Bang model. First, galaxies show cosmological redshifts consistent with expansion. Second, the cosmic microwave background is relic radiation from a hot early phase. Third, the observed abundances of light elements match predictions from early-universe nuclear reactions.

Together, these form a coherent picture of cosmic history.

Early hot universe

As the scale factor $a(t)$ was smaller in the past, radiation temperature was higher. For freely expanding radiation,

Tpropto rac{1}{a}

High temperature means high particle energies. Early on, matter existed as a hot plasma of particles, antiparticles, and radiation interacting frequently.

Timeline overview

At very early times, physics reaches energies where known theories are incomplete. After the earliest unknown era, the universe passed through stages involving particle interactions, possible inflation, quark confinement, nucleosynthesis, recombination, first stars, galaxies, and large-scale structure.

Recombination occurred about 380,000 years after the Big Bang, allowing the CMB photons to travel freely. The first stars formed much later, after gravity amplified density fluctuations.

Expansion equations

In homogeneous and isotropic cosmology, expansion is described by the Friedmann equation. A simplified form is

ight)^2=rac{8pi G}{3} ho-rac{kc^2}{a^2}+rac{Lambda c^2}{3}$$ Here $ ho$ is energy density, $k$ describes spatial curvature, and $Lambda$ is the cosmological constant. This equation connects expansion to the universe's contents. ## Misconceptions The Big Bang did not happen at one point in space. Every region of today's observable universe was once hotter and denser. There is no center of expansion within the universe in the standard homogeneous model. Also, the Big Bang model does not necessarily describe the absolute beginning of existence. It describes the universe's evolution from an early hot dense state. What happened at the earliest moments may require quantum gravity. ## Observable universe We can observe only the region from which light has had time to reach us since the early universe became transparent. This is the observable universe. The whole universe may be much larger, possibly infinite. ## The big idea The Big Bang model is the framework that the universe has expanded and cooled from a hot dense past. Redshifts, the CMB, and light-element abundances strongly support it. It is not an explosion into space, but a history of expanding space, cooling radiation, and evolving structure.