According to the Big Bang theory, the following sequence of events is believed to have occurred. The starting point for this timeline, 13.7 ± 0.2 billion years ago, is the time at which in general relativity there exists a gravitational singularity. At this time, general relativity is unable to make statements about what the Universe is like because the theory gives infinite values for the temperature and density of the universe.
It is believed that general relativity is insufficient to make predictions about the very beginning of the universe
and that a theory of quantum gravity will be needed to do so. Nevertheless the time at which general relativity predicts a singularity makes a convenient starting point to begin the timeline, despite the fact that this singularity may or may not actually have existed.
One concept which is important to understand this table is the concept of decoupling or freezeout. Imagine a block of ice and an aluminum Coca-Cola can. If you increase the temperature to an extremely high value, then both objects will vaporize and one will have a mixture of water and aluminum vapor which can be considered a single entity. Now if one decreases the temperature, then below a certain value the aluminium will condense and freeze and stop interacting with the water vapor. The exact temperature at which this occurs can be estimated.
A similar process occurs during the course of the Big Bang
as entities freeze out and decouple from the rest of the soup that makes up the universe. The temperature at which freezeout occurs can be estimated and the temperature corresponds to the time after the Big Bang.
One final note is that this timeline will refer to the diameter of the universe. This is not the total size of the universe, which may be infinite. Rather one starts with the current size of the observable universe which is about thirteen billion light years because thirteen billion years is the estimated length of time since the beginning of the universe and anything outside that sphere cannot be observed. One then calculates how large that sphere is at a particular time.
Stephen Hawking has theorized that the events of the Big Bang (the expansion of a singularity into the current space time continuum) can be seen as a reversal of the events that occur in a black hole, where space-time condenses into a singularity.
Science tells us nothing about what happened from the time of the Big Bang until 10-43 seconds, a concept known as Planck time. After this, the time is grouped into epochs.
The Planck Epoch covers the time from 10-43 to 10-35 seconds after the Big Bang. The temperature during this epoch is estimated to decrease from 1032 K to 1027 K.
- A length of 10-43 seconds is known as Planck time. At this point, the force of gravity separated from the other three forces, collectively known as the electronuclear force. This is important because it is currently unknown how gravity combines with the other forces. The diameter of the currently observable universe is theorized as 10-33 cm.
- Separation of the strong force from the electronuclear force, leaving three forces: gravity, strong, and electroweak forces. The particles which are involved in the strong force are considerably more massive than the particles which are involved with the other forces and so are believed to "condense" out earlier.
Grand Unification Epoch
The Grand Unification Epoch covers the time from 10-35 to 10-12 seconds after the Big Bang. The temperature during this epoch is estimated to decrease from 1027 K to 1015 K.
- For the period of time between 10-35 seconds and 10-33 seconds, it is believed that the size of the universe expands by a factor of approximately 1020 to 1030 cm. Postulating the existence of inflation solves a number of problems which are described in cosmic inflation.
- This period is also very important for the existence of matter in the universe. Individually, the strong and the electroweak forces behave exactly the same way toward matter and antimatter. Which means that there is no opportunity after this time for more matter to be created than antimatter. The strong and the electroweak forces are mixed and act as a single force. Grand unification theories suggest that when this is the case, it may be possible to have particle reactions which create more matter than antimatter.
- The temperature of the Universe is approximately 1025 Kelvin. The Quark-Antiquark Freezeout begins and lasts until 10-5 seconds. At these temperatures, quarks are able to condense out but the temperatures are still too hot for protons and neutrons to exist.
- Birth of quarks, which appear in particle-antiparticle pairs. Quarks and anti-quarks annihilate each other to create photons, but quarks are created at a ratio of approximately 109 (1 billion) anti-quarks to 109+1 (1,000,000,001) quarks, resulting in one quark per billion matter-antimatter interactions. Free quarks multiply rapidly.
The Electroweak Epoch covers the time from 10-12 to 10-6 seconds after the Big Bang. The temperature during this epoch is estimated to decrease from 1015 K to 1013 K.
- The diameter of the currently observable universe increases to approximately 1013 meters. The weak force, which involves massive particles, condenses and separates from the electromagnetic force, which involves a massless particle, leaving us with the four separate forces we know today.
The Hadron Epoch covers the time from 10-6 seconds to 1 second after the Big Bang. The temperature during this epoch is estimated to decrease from 1013 K to 1010 K.
- Electrons and positrons annihilate each other during the hadron epoch.
- The temperature of the Universe is approximately 1013 K. Quarks combine to form protons and neutrons. The lowering temperature allows quark/anti-quark pairs to combine into mesons. After this period quarks and anti-quarks can no longer exist as free particles.
- The temperature of the Universe is approximately 1010 (10 million) Kelvin. The existence of antimatter is cancelled out, as lepton/anti-lepton pairs are annihilated by existing photons. Neutrinos break free and exist on their own.
The Lepton Epoch covers the time from 1 second to 3 minutes after the Big Bang. The temperature during this epoch is estimated to decrease from 1010 K to 109 K.
1 second after the Big Bang
- Formation of hydrogen nucleus, the first atomic nuclei. Nuclear fusion begins to occur as the universe is now cool enough for atomic nuclei to form and still hot enough for them to collide to form heavier elements.
Epoch of Nucelosynthesis
The Epoch of Nucleosynthesis covers the time from 3 minutes to 300,000 years after the Big Bang. The temperature during this epoch is estimated to decrease from 109 K to 3000 K.
3 minutes after the Big Bang
300,000 years after the Big Bang
- Three minutes after the Big Bang, the universe is too cool for nuclear activity to occur, and these reactions stop. At this point the universe consists of about 75% hydrogen, 25% helium and trace amounts of deuterium, lithium, beryllium, and boron. Elements heavier than this do not have time to form before nuclear reactions stop. By looking at conditions between 1 second and 3 minutes after the Big Bang, one can predict the elemental abundance of the Universe. These predictions are broadly in agreement with observations.
For later events, see Timeline of the Universe.
- The temperature of the Universe is approximately 10,000 Kelvin. At this temperature hydrogen nuclei capture electrons to form stable atoms. This is particularly significant because free electrons are effective at scattering light, which is why fire is not transparent, while hydrogen atoms will allow light to pass through.
- This implies that this is the time at which space becomes transparent to light, since photons no longer interact strongly with atoms. This means that what we normally think of as matter and what we normally think of as energy become separate.
- The light from the moment at which the universe became transparent has been redshifted to radio waves and makes up the cosmic microwave background.