The most widely accepted theory for the origin of the Universe is called the Big Bang Theory. This argues that the Universe began in a state of near infinite temperature and density from which it has been expanding ever since. The first 10-43 seconds are known as the era of quantum gravity. We know almost nothing about this period because the theory of general relativity is not valid in such extreme conditions.
At about 10-35 seconds, the Universe is thought to have undergone a short period of very rapid expansion called inflation. When this episode of inflation ended a fraction of a second later, the scale of the Universe had been increased by a factor of at least 1027. At the end of inflation the normal expansion of the Universe took over, and photons and particles were created. As the expansion continued, the radiation and matter cooled until, at a temperature of about 1012 K, the Universe contained radiation, protons, neutrons and electrons together with three sorts of neutrinos. When the temperature had dropped to about 109 K, the protons and neutrons combined to form the nuclei of deuterium, 3He and 4He. About 25% of the mass was converted to 4He at this time. The observation of 25% by mass in the form of 4He today is one of the strongest pieces of evidence in support of the hot big bang theory. These simple nuclei are the building blocks from which all other elements are created in stars.
In the Universe today, matter in the form of gas, stars, galaxies and dark matter contributes more to gravity than radiation does. Before this the Universe was so hot that the radiation and matter were tied closely together and it was the radiation that drove the expansion. During this period the Universe is said to have been radiation-dominated.
Today the expansion of the Universe is matter-dominated, but recent observations have detected a form of mass-energy that may be due to a negative pressure called the cosmological constant. This acts together with the matter to control how the Universe expands. If the Universe was under the influence of matter alone, the expansion would be slowing down, but the effect of the cosmological constant is causing the expansion of the Universe to accelerate.
At 300,000 years after the Big Bang, when the Universe was at a temperature of 3000K, all of the protons and electrons finally combined to form neutral atoms. A snapshot of this time can be seen in the Cosmic Microwave Background (see below). After the formation of neutral atoms, regions of overdensity were able to collapse under their own gravity to form stars and galaxies.
The earliest of these galaxies can be seen as faint blue dots in the Hubble Deep Field. This image covers a tiny patch of sky - only 1/30 the size of the full Moon - which seemed to be completely empty before the Hubble Space Telescope took this picture.
This is an alternative model for the Universe that once rivalled the Big Bang theory. It states that the Universe is expanding, but has a constant density because matter is continuously being created. It is based on the perfect cosmological principle, which states that the Universe is isotropic and homogeneous in space, and is also homogeneous in time. It is no longer a serious rival to the Big Bang theory because it cannot explain the Cosmic Microwave Background and the evolution of structure in the observed Universe.
The true extent of the Universe isn’t really known because it is only possible to see to the edge of the visible Universe. Galaxies can only be seen because of the light coming from them. Although light moves very quickly, it still takes a long time to reach us from distant galaxies. The farthest that can be seen is the distance light has travelled in the age of the Universe, i.e. 15 billion years. Light from more distant objects hasn’t had time to reach us yet.
Observing the light from these distant galaxies is like looking back in time to the early Universe. The starlight that is seen in the night sky left the stars millions of years ago, so each star does not appear as it is now, but as it was when the light set out from it, a snapshot of history, several million years ago. It will be another few million years before the stars can be seen as they look today.
If there is enough matter in the Universe so that the effect of gravity is great enough to slow and reverse the expansion, it could collapse back in on itself and end in a huge collision called the “Big Crunch.” This happens if the density exceeds the critical density and is called a “closed Universe.”
If there is not enough matter to slow the acceleration, the Universe will continue to accelerate outwards forever, with the galaxies becoming further and further apart and slowly cooling down. This happens if the density is less than the critical density and is called an “open Universe.”
Current measurements indicate that the Universe is expanding at an accelerating rate under the influence of the cosmological constant. This implies that the Universe will expand forever. A consequence of this is that galaxy redshifts will increase with time. Eventually the light will be shifted to such long wavelengths that it can no longer be seen and galaxies will disappear from the visible universe.
If the cause of the accelerating expansion is not the cosmological constant, but some other form of “dark energy,” then the accelerating expansion may cease and the long-term future is probably impossible to predict.
Click on the links below to find out more about the Universe.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001