According to the widely accepted Big Bang theory, the Universe began at some point in space and time around 13,700,000,000 years ago, as evidenced by the Doppler effect observed in all stars and a 3 degree Kelvin background radiation observed elsewhere.
- In about 10E-43 seconds, physics became defined and gravity separated from other forces. The Universe was 10E32 K in temperature.
- At 10E-35 seconds, the Universe expanded to the size of a softball and the strong nuclear force separated. Energy turned into quarks and electrons and their anti-matter counterparts. The temperature was 1027 K.
- At 10E-6 seconds, quarks began to bind into protons and neutrons; matter and antimatter destroyed each other and the balance turned out to be in favor of matter. The Universe was about the size of our solar system and 1013 K hot.
- 1 second after its inception, the electromagnetic and weak nuclear forces appeared as the Universe cooled to 109 K.
- 3 minutes later, protons and neutrons fused into nuclei.
- After 100,000 years, the electrons and the nuclei came together to form atoms; photons separated from matter and there was light.
- In the next 1,000,000,000 years the Universe became clumpy as galaxies began to take shape.
- Ever since then, the Universe has cooled down to 3 K, galaxies have developed, generations of stars have passed, creating heavier elements, and life has appeared. Our galaxy, the Milky Way, contains about 200,000,000,000 stars and lies 1 million light years from its nearest neighbor.
- The solar system was formed around 4,600,000,000 years ago as dust collapsed into the protostar that was to become the Sun, and into planetesimals orbiting it. The inner planets, from Mercury to Mars, are solid because the solar winds have driven gases away and have a core; the outer planets are larger and consist of gas.