Most cosmologists agree that the universe begins as a hot, dense point. It’s just a fraction of the size of an atom, yet so dense it contains everything that exists in the universe: matter, energy and space. In the beginning, the fundamental forces of physics— gravity, strong force, weak force, and electromagnetism—are all united into one force.
Gravity separates from the other fundamental forces. The first subatomic particles and antiparticles appear.
10−36 seconds to 10−32 seconds:
Strong nuclear force separates from the remaining fundamental forces, causing the universe to scale up at break-neck speed—from the fraction of the size of an atom to about the size of a softball.
10−12 seconds to 10−6 seconds:
The fundamental forces of physics are now distinct from each other, behaving just as they do today. At this point, subatomic particle and antiparticle pairs are constantly being created and destroyed. But not everything is destroyed because there are slightly more particles than antiparticles.
10−6 seconds to 3 minutes:
Some of the surviving particles become protons and neutrons. The other surviving particles are leptons, which collide with antiparticles and release photon energy.
3 minutes to 20 minutes:
The expanding universe cools just enough for the nuclei of the basic elements hydrogen and helium to form.
Hydrogen and helium atoms form. High-energy photons begin to travel freely through space and are detectable today as cosmic background radiation.
100 million years:
Gravity begins to pull hydrogen and helium atoms together to form stars.
1 billion years:
Matter clusters into billions of galaxies where heat and pressure create new chemical elements. When stars explode, the elements inside them become the building blocks of planets.
8.5 – 9 billion years:
The sun forms by pulling the debris of exploded stars toward itself.
9.2 billion years:
Earth takes shape.
10.2 billion years:
The first living organisms form.
Present day, 13.7 billion years