The Big Bang: 13.8 billion years, step by step

The Three Biggest Pieces of Evidence for the Big Bang

The Expanding Universe

Distant galaxies all exhibit redshift, showing that they are moving away from each other.

Cosmic Microwave Background Radiation

This is the leftover light from the early universe, currently observed as thermal radiation at around 2.7 K.

Proportions of Light Elements

The Big Bang model accurately predicts that the early universe contained approximately:

• 75% hydrogen
• 25% helium

along with tiny amounts of deuterium and lithium.

Timeline of the Universe

Planck Epoch (First 10⁻⁴³ Seconds)

Current physics cannot yet describe this phase.

Cosmic Inflation

In a tiny fraction of the first second, space expanded extremely rapidly, transforming a microscopic region into cosmic scale.

Formation of Protons and Neutrons

As the universe cooled, quarks combined to form protons and neutrons.

The First Three Minutes

Nuclei of hydrogen, helium, and small amounts of lithium appeared.

380,000 Years After the Big Bang

Electrons combined with nuclei to form neutral atoms.

For the first time, light could travel freely through space, creating the cosmic microwave background radiation we observe today.

Around 200 Million Years

The first stars began to form.

Around 1 Billion Years

The first galaxies appeared.

Present Day

The universe is approximately 13.8 billion years old and continues to expand.

What Changed in 1998?

Previously, many scientists believed that gravity would slow down the expansion over time.

However, observations of Type Ia supernovae in 1998 revealed the opposite:

The universe is expanding at an accelerating rate.

This discovery led to the hypothesis of dark energy, a component believed to account for approximately:

• 68% of the energy of the universe
• 27% dark matter
• 5% ordinary matter

The Future of the Universe

If dark energy continues to behave as it does now, the universe will continue to expand for an extremely long time.

Some notable milestones:

• In about 8 billion years: The Sun becomes a white dwarf.
• In about 100 trillion years: The last star burns out.
• In about 10¹⁰⁰ years: The last black holes evaporate.

The most widely supported scenario today is the heat death, as the universe becomes increasingly cold, diffuse, and devoid of usable energy to sustain large-scale physical processes.

🌌 The greatest strength of the Big Bang model is not that it tells a beautiful story about the origin of the universe, but that it predicted phenomena such as the microwave background radiation and the proportions of light elements in advance. When scientists went looking for them, they actually observed them.

Further Reading

• Multiverse: four levels and a difficult question
• Dark matter and dark energy: the Universe's great "Tug of War" battle
• Dark stars may have left echoes in the form of gravitational waves across the universe