The diagram here illustrates the main events occurring in the history of our Universe. The vertical time axis is not linear in order to show early events on a reasonable scale. The temperature rises as we go backwards in time towards the Big Bang and physical processes happen more rapidly. The timescales and temperatures indicated on this diagram span an enormous range.
The Universe began about fourteen billion years ago in a violent explosion; every particle started rushing apart from every other particle in an early super-dense phase. The fact that galaxies are receding from us in all directions is a consequence of this initial explosion and was first discovered observationally by Hubble.
The Copernican or cosmological principle states that the Universe appears the same in every direction from every point in space. It amounts to asserting that our position in the Universe - with respect to the very largest scales - is in no sense preferred. There is considerable observational evidence for this assertion, including the measured distributions of galaxies and faint radio sources, though the best evidence comes from the near-perfect uniformity of the relic cosmic microwave background radiation. This means that any observer anywhere in the Universe will enjoy much the same view as we do, including the observation that galaxies are moving away from them.
The fact that the Universe is expanding - about every point in space - can be a difficult concept to grasp. The analogy of an expanding balloon may be helpful: imagine residing in a curved flatland on the surface of a balloon. As the balloon is inflated, the distance between all neighbouring points grows; the two-dimensional Universe grows but there is no preferred centre.
About 100,000 years after the Big Bang, the temperature of the Universe had dropped sufficiently for electrons and protons to combine into hydrogen atoms, p + e ⇒ H. From this time onwards, cosmic radiation was effectively unable to interact with the background gas; it has propagated freely ever since, while constantly losing energy because its wavelength is stretched by the expansion of the Universe. Originally, the radiation temperature was about 3000 degrees Kelvin, whereas today it has fallen to only 3K.
Observers detecting this radiation today are able to see the Universe at a very early stage on what is known as the ‘surface of last scattering’. Photons in the cosmic microwave background have been travelling towards us for over thirteen billion years, and have covered a distance of about a million billion billion miles.
Prior to about one second after the Big Bang, matter - in the form of free neutrons and protons - was very hot and dense. As the Universe expanded, the temperature fell and some of these nucleons were synthesised into the light elements: deuterium (D – a hydrogen atom with a neutron and a proton inside its nucleus), helium-3 (helium with only one neutron in its nucleus), and helium-4. Theoretical calculations for these nuclear processes predict, for example, that about a quarter of the Universe consists of helium-4, a result which is in good agreement with current stellar observations.
The heavier elements, of which we are partly made, were created later in the interiors of stars and spread widely in supernova explosions.
The standard Hot Big Bang model also provides a framework in which to understand the collapse of matter to form galaxies and other large-scale structures observed in the Universe today. At about 10,000 years after the Big Bang, the temperature had fallen to such an extent that the energy density of the Universe began to be dominated by massive particles, such as protons, neutrons and electrons, rather than the light and other radiation which had predominated earlier. This change in the form of the main matter density meant that the gravitational forces between the massive particles could begin to take effects, so that any small perturbations in their density would grow. Over ten billion years later we see the results of this collapse.
Despite the self-consistency and remarkable success of the standard Hot Big Bang model in describing the evolution of the Universe back to only one hundredth of a second, a number of unanswered questions remain regarding the initial state of the Universe.