|0 second||Time t=0 is not really part of the Big Bang timeline. This is because the two theories of physics which are at the heart of the Big Bang theory—general relativity and the standard model of particle physics— are mutually incompatible. The predicting power of physics breaks down at time 0 and traditional Big Bang cosmology predicts a gravitational singularity.|
|10–43 second||The Planck Era: This era is the closest that current physics makes meaningful predictions. The universe is incredibly hot, dense and turbulent. All the fundamental forces—gravitation, electromagnetism, strong and weak forces—are unified. Physicists hope that newly proposed theories of quantum gravitation such as string theory will eventually provide a better understanding of this epoch.|
|10–36 second||The Grand Unification Era: Cosmic inflation is triggered causing an exponential expansion of the universe from far smaller than a subatomic particle to what became our observable universe, flat (no curvature) and incredibly smooth (with only tiny variations in density due to quantum effects). All the forces of nature except gravity (the electromagnetic, strong and weak forces) might have been unified into a single force—an idea called grand unified theory (GUT).|
|10–32 second||Inflation ends. Particles of matter are created from energy via Einstein’s equation E=mc^2. Initially a mix of matter and antimatter, most of the particles annihilate each other in a spurt of radiation, leaving behind randomly scattered pockets of matter.|
|10–11 second||The Electroweak Era: Electromagnetic and weak force became distinct.|
|100 seconds||Big Bang nucleosynthesis starts when nuclear reactions produced light nuclides, such as deuterium and helium-3 (earlier the universe was too hot for light nuclides to form). Strong and weak forces are dominant.|
|380,000 years||The Recombination Era: More and more electrons and protons (and other nuclei) combined to form neutral hydrogen atoms. The universe became increasingly transparent. This is the earliest visible signature of the universe that we can still see today. CMB is released.
Gravity takes over as the dominant force.
|Dark ages: While electrons in neutral hydrogen can absorb photons of some wavelengths by going to an excited state, a universe full of neutral hydrogen is relatively opaque only at those absorbed wavelengths, but transparent throughout most of the spectrum. The Dark Ages start at that point, because there were no light sources other than the gradually darkening CMB.|
|600 million years||The Reionization Era: The universe went through a phase called reionization when the energetic ultraviolet light from these first stars began splitting the hydrogen atoms back into electrons and protons (or ionizing them)|
|7 billion years||Expansion of universe begins to accelerate|
|13.7 billion years||Today|
|20 billion years||Milky Way and Andromeda galaxies collide|
|100 billion years||All other galaxies are invisible|
|1 trillion years||Primordial isotopes are lost or diluted|
|100 trillion years||Last star burns out|