The First Three Minutes

Part 3: The Timeline of the Universe

Step back and put the whole history in order. Each epoch ends when a different component takes over or a key transition occurs — and the sequence is exactly the budget from the last reading, played out in time.

• Inflation ($\sim 10^{-36}~\mathrm{s}$): accelerating expansion stretches quantum fluctuations into structure seeds; it ends by reheating into the hot Big Bang.
• Radiation domination: the hot plasma, with radiation the densest component ($\rho_r \propto a^{-4}$). Big Bang nucleosynthesis happens here, at $\sim 1~\mathrm{s}$ to $3~\mathrm{min}$ — inside the radiation era, not before it.
• Matter–radiation equality ($z \approx 3400$, $\sim 50{,}000~\mathrm{yr}$): matter’s slower dilution lets it overtake radiation; matter domination begins.
• Recombination ($z \approx 1100$, $\sim 380{,}000~\mathrm{yr}$): the universe cools enough for atoms to form and becomes transparent, releasing the cosmic microwave background. Structure now grows in earnest.
• Dark-energy domination (the last few billion years): the expansion begins to accelerate as dark energy’s repulsion overcomes matter’s pull (around $z \approx 0.6$), and dark energy grows to dominate the density budget by $z \approx 0.3$ — where the previous reading left off.