The History and Future of the Cosmos
The Big Bang Theory is our best model for the origin and evolution of the universe.
A Brief History of the Universe
t=0: The Big Bang. The universe began in an incredibly hot, dense state.
First few minutes: The universe was a hot soup of elementary particles. As it cooled, protons and neutrons formed, and then Big Bang Nucleosynthesis occurred, fusing them into hydrogen, helium, and a tiny amount of lithium.
~380,000 years: The universe cooled enough for electrons to combine with nuclei to form the first neutral atoms. This event is called recombination. The universe became transparent to light for the first time, and the light released at this moment is what we now observe as the Cosmic Microwave Background (CMB).
~400 million years: The first stars and galaxies began to form from the gravitational collapse of dense clouds of gas.
~9 billion years: Our solar system formed.
13.8 billion years: The present day.
The 'Dark Side' of the Universe
Observations of the universe's large-scale structure and expansion reveal that the ordinary matter we know makes up only about 5% of the universe. The rest is composed of two mysterious components:
Dark Matter (~27%): An unknown type of matter that does not emit or interact with light, but whose gravitational effects are observable. It is responsible for holding galaxies and galaxy clusters together. Without it, galaxies would fly apart.
Dark Energy (~68%): An unknown form of energy that is causing the expansion of the universe to accelerate. It is a repulsive force that is overcoming gravity on the largest scales.
The Fate of the Universe
The ultimate fate of the universe depends on the battle between the inward pull of gravity (from matter and dark matter) and the outward push of dark energy.
The Big Crunch (Closed Universe): If the density of the universe were high enough, gravity would eventually overcome the expansion, causing the universe to collapse back in on itself. (Now considered unlikely).
The Big Freeze / Heat Death (Open or Flat Universe): If dark energy continues to dominate, the universe will continue to expand at an accelerating rate forever. Galaxies will become increasingly distant, stars will eventually burn out, and the universe will end as a cold, dark, and empty place. This is the currently favored model.