The Hot Big Bang Andrew Liddle T H E O F E U N D I I V E R S N U B R G I T Y H Image: NASA/WMAP Science Team
The Standard Model The discovery of the Higgs particle completes the Standard Model of Particle Physics. Its particles and forces are supposed to explain all the physics of our everyday lives, except for gravity.
The Standard Model Combined with Einstein s theory of gravity - general relativity - it should allow us to describe the entire Universe.
Expanding Universe The Universe is expanding and is filled with radiation whose temperature is just 2.7 degrees above absolute zero. But any gas which is allowed to expand cools down as it does so. Temperature 1/size
Temperature and time During its early radiation-dominated phase (which lasts about 100,000 years), the temperature and time are related by ( ) 1/2 1 second t T 2 10 10 Kelvin This says that when the Universe was one second old, its temperature was over ten trillion Kelvin. At earlier stages it was even hotter.
Energy and time Temperature is simply a measure of the typical energy of particles in a gas. The young Universe is a sea of interacting particles, and the corresponding energy is ( 1 second t ) 1/2 E 2 MeV MeV = Mega electron-volts is a measure of particle energy. For comparison, a photon of visible light has an energy of about one electron-volt. An MeV is the energy characteristic of nuclear interactions.
Energy and CERN The energy of collisions at CERN is about 10 TeV, ie 10 7 MeV. Using our relation ( 1 second t ) 1/2 E 2 MeV we find this was achieved when the Universe was about 4 10 14 seconds old. At that time, each particle collision in the Universe was like an event at CERN!
Successes of the Hot Big Bang There are two main pillars of the Hot Big Bang theory. Cosmic microwave background: radiation left over from the Hot Big Bang and explained as due to atomic processes when the Universe was about 400,000 years old. Nucleosynthesis: The abundance of light elements such as hydrogen, deuterium, helium and lithium are explained by nuclear interactions when the Universe was a few minutes old. Combined with the observed expansion of the Universe, these led to the widespread acceptance of the Hot Big Bang model by the 1970s.
Things the Standard Model does not explain Despite those successes, there are quite a few features of the Universe not explained by the Standard Model. Why does the Universe contain matter but no antimatter? Why is the dark matter, whose presence is needed to allow galaxies to form and to hold them together once they do? Why do observations show that the Universe s expansion is presently accelerating? What causes irregularities in the density field of the primordial Universe, seen directly in the cosmic microwave background? These may be evidence of the need for new physics beyond the Standard Model.
Things the Standard Model does not explain There is an active search for possible explanations for all of these. The matter-antimatter asymmetry may be due to unification of the strong and electroweak interactions at very high energies.
Things the Standard Model does not explain There is an active search for possible explanations for all of these. The dark matter may be comprised of fundamental particles associated with an extension of the Standard Model. Examples are Weakly Interacting Massive Particles (WIMPs) associated with supersymmetry, and the axion which is associated with a global symmetry breaking.
Things the Standard Model does not explain There is an active search for possible explanations for all of these. The acceleration of the Universe may be caused by a non-zero energy of quantum fields even in their vacuum state.
Things the Standard Model does not explain There is an active search for possible explanations for all of these. Density irregularities may have been created during a period of very rapid expansion, known as inflation, in the very young Universe. During an inflationary epoch, quantum processes governed by Heisenberg s uncertainty principle inevitably lead to some level of irregularity.
Summing up The young Universe is a testing ground for ideas in fundamental physics. In its early stages the energies exceed those that we can create on Earth. Processes during these stages may leave evidence that can give us clues to possible physics beyond the Standard Model. There are already several phenomena that apparently cannot be explained by the Standard Model alone.