Chapter 22

The Early Universe

The universe began extremely hot and dense and has been cooling as it expands. In the early universe, photons could become particle-antiparticle pairs, and vice versa. This made the early universe full of particles and radiation

Fundamental Forces

Strong force

Electromagnetism

Weak force

Gravity

At very high temperatures, these forces may unify.

How the Early Universe Changed with Time

Planck Era: This is the earliest period, and we don't have a good theory for it yet. GUT Era: During this time, the strong, weak, and electromagnetic forces may have been unified. Electroweak Era: The weak and electromagnetic forces separated. Particle Era: There was almost an equal amount of matter and antimatter. Era of Nucleosynthesis: Matter and antimatter annihilated, and nuclei began to fuse. Era of Nuclei: Helium nuclei formed. Era of Atoms: Atoms formed, and background radiation was released. Era of Galaxies: Galaxies began to form.

Evidence for the Big Bang

We can detect leftover radiation from the Big Bang. The Big Bang theory correctly predicts the amount of helium and other light elements in the universe.

Cosmic Microwave Background

This is the radiation left over from the Big Bang, discovered in 1965. It has been traveling across the universe since atoms formed. It has a temperature of about 2.73 K. The expansion of the universe has stretched its wavelength to the microwave part of the spectrum.

Helium Formation

The Big Bang theory predicts that about 25% of the universe's mass should be helium, which matches what we observe.

Mysteries of the Universe

Where does the structure of the universe come from? Why is matter distributed so evenly? Why is the universe's density so close to the critical density?

Inflation Theory

A period of rapid expansion in the early universe can explain these mysteries.

Density Enhancements

Inflation can stretch tiny quantum ripples to a huge size, creating the seeds for structures.

Uniformity

Areas that are now far apart were once close together, explaining why they have similar temperatures.

Geometry

Inflation flattens the universe's geometry, making its density very close to the critical density.

Evidence for Inflation

The universe's geometry is flat. The total mass and energy have a critical density. Ordinary matter makes up about 5% of the total. Total matter is about 32% (including dark matter). Dark energy is about 68% of the total.

Why is the Night Sky Dark?

Olbers' Paradox: If the universe were infinite, unchanging, and the same everywhere, the night sky would be full of stars and bright. The Big Bang Explanation: The night sky is dark because the universe changes over time. When we look far out into space, we're looking back to a time before stars existed.