The Universe began 13.8 billion years ago in an event called the Big Bang. The Big Bang was not an explosion of matter in space, but the sudden appearance and expansion of space itself. The expansion has continued ever since, creating a cosmos of unimaginable vastness.
Although light travels extremely quickly, it still takes it billions of years to cross the Universe. This means that peering into deep space allows us to look back in time and study the Universe’s early years.
The Big Bang
The Universe materializes out of nothing. It is smaller than an atom but has all the energy and mass it will ever have. In the first trillionth of a trillionth of a trillionth of a second, it expands to the size of a football - a process known as inflation.
Within a second, the incredible energy of the expanding Universe produces tiny particles of matter. Most of these collide, destroy each other, and vanish, but a tiny fraction remain. These leftovers build up to form larger particles called protons and neutrons - the building blocks of atoms.
It takes 300,000 years for the Universe to cool sufficiently for protons and neutrons to form the first atoms: hydrogen and helium. These gases form a thin cloud that fills the Universe. Light can now travel freely, making space transparent. This ancient light can still be captured by astronomers today.
Stars and galaxies
Gravity pulls thicker areas of gas into clumps that get tighter and tighter. This heats their cores, triggering nuclear reactions, and so giving birth to stars. The newborn stars cluster by the billion in vast whirlpools - galaxies.
The Solar System (4.6 billion years ago)
Our local star, the Sun, forms from a cloud of gas and dust left by dying stars. Not all the material is absorbed by the new star though - a gigantic disk of dust and gas is left in orbit around it. Over time, the particles of matter in this disk stick together to form the planets, moons, asteroids, and comets of our Solar System.
Farther from the Sun than scalding Venus but not as far as freezing Mars, planet Earth is just the right temperature for liquid water to settle on its surface. A random chemical reaction between carbon-based chemicals in the water produces a molecule that can make copies of itself, as DNA can today. It is the first form of life.
After oceans first formed on Earth, it didn’t take long for life to appear on the planet. How life began remains one of the great mysteries of science, but most scientists believe the first living things developed from carbon-based chemicals in water. No trace of these remains, but the animals and plants that evolved from them left numerous fossils behind. The fossil record shows that the story of life on Earth has had twists and turns, with occasional mass extinctions wiping out the dominant species and allowing new forms of life to emerge.
Future: The Sun dies
About 5 billion years in the future, the Sun will turn into a red giant star as its supply of fuel begins to run out. It will swell in size, its outer layers engulfing the planets Mercury, Venus, and probably Earth. The heat will vaporize any water left on Earth, and possibly our planet’s crust, too, making life impossible.
Future: The Big Freeze
The Universe may continue expanding forever. Matter and energy will become ever more thinly dispersed, preventing new stars from forming. After the last star burns out, the Universe will be permanently dark and freezing cold - an endless void with no activity.