How did the universe begin? Cosmologists aren't the only ones plagued by this question. When we look up at a starry sky on a clear night, we can't help but ask ourselves: Where did it all come from? How did it all begin? Modern cosmologists attempt to answer these questions with the Big Bang Theory.
Before the Big Bang, there was no time or space. The Big Bang marked the beginning of the universe's expansion from a singularity (or something close to a singularity) — a single point that was infinitely small, infinitely hot, and infinitely dense.
Since the Big Bang, the universe has gone through several eras distinguished by the behavior of the universe's fundamental forces and particles.
Though physicists have a decent understanding of the early stages of the universe, the immediate fractions of a second following the Big Bang, known as the Planck Era, are not well understood. From the moment of initial expansion to 10-43 seconds afterwards, cosmologists suspect that the four fundamental forces at work in the universe today (strong, weak, electromagnetism, and gravity) were combined into a single unified force.
Grand Unification Era
The Grand Unification Era followed the Planck Era, taking place between 10-43 seconds and 10-35 seconds. The era began with gravity’s separation from the other three forces and ended with the separation of the strong force from the electroweak force.
At the beginning of the Electroweak Era (10-35 to 10-10 seconds), the strong force decoupled from the electroweak force, releasing a tremendous amount of energy and triggering a sudden rapid expansion known as inflation. As space expanded more rapidly than the speed of light, extremely energetic interactions created elementary particles such as photons, gluons, and quarks. The era ended with the separation of electromagnetism from the weak force.
Elementary Particle Era
Between 10-10 seconds and 0.001 second, the Elementary Particle Era, a “particle soup” filled the universe. Quarks and antiquarks, electrons and positrons, and other particles and antiparticles continually swapped mass for energy via matter-antimatter collisions. As the universe cooled, the temperature dipped too low to re-create pairs of particles from photons and the particles continued to annihilate without being replaced. A slight asymmetry between the amount (or possibly the behavior) of matter and antimatter enabled matter to dominate and become the universe’s primary ingredient. The cooler temperature also enabled the strong nuclear force to draw quarks together to form protons and neutrons.
Era of Nucleosynthesis
Fusion continued in the Era of Nucleosynthesis (0.001 seconds – 3 minutes), when protons and neutrons combined into the first atomic nuclei, hydrogen, some of which fused further into helium and lithium. Cooling continued and soon temperatures dipped too low for fusion to continue in the Era of Nuclei (3 minutes – 380,000 years). Big Bang nucleosynthesis had left the universe with roughly 75% hydrogen nuclei, 25% helium nuclei, and trace amounts of lithium and deuterium nuclei. The plasma of positively charged nuclei and negatively charged free electrons filled the universe, trapping photons in its midst.
Era of Atoms
The Era of Atoms (380,000 years – 1 billion years or so) began as the universe finally cooled and expanded enough for the nuclei to capture free electrons, forming fully-fledged, neutral atoms. Previously trapped photons were finally free to move through space, and the universe became transparent for the first time. These photons have been passing through space ever since, forming the cosmic microwave background. The universe’s expansion has redshifted the initially energetic photons to microwave wavelengths. The CMB also marks the furthest point back in time we can observe — the time before is sometimes referred to as the dark age.
The differences in density seen in the CMB provided the seeds for galaxy formation. The first galaxies formed when the universe was roughly 1 billion years old and heralded the current Era of Galaxies.