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The Hot Big BangAndrew Liddle
TH
E
UN I V E R S I TY
OF
E D I N B U
RG
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Image: NASA/WMAP Science Team
The Standard Model
The discovery of the Higgsparticle completes theStandard Model of ParticlePhysics.
Its particles and forces aresupposed to explain all thephysics of our everydaylives, except for gravity.
The Standard Model
Combined with Einstein’stheory of gravity - generalrelativity - it should allowus to describe the entireUniverse.
Expanding UniverseThe Universe is expanding and is filled with radiation whosetemperature is just 2.7 degrees above absolute zero.
But any gas which isallowed to expand coolsdown as it does so.
Temperature ∝ 1/size
Temperature and timeDuring its early radiation-dominated phase (which lastsabout 100,000 years), the temperature and time are relatedby (
1 second
t
)1/2
' T
2× 1010 Kelvin
This says that when the Universe was one second old, itstemperature was over ten trillion Kelvin. At earlier stagesit was even hotter.
Energy and timeTemperature is simply a measure of the typical energyof particles in a gas. The young Universe is a sea ofinteracting 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 ofabout one electron-Volt. An MeV is the energy characteristicof nuclear interactions.
Energy and CERNThe energy of collisions at CERN is about 10 TeV, ie107 MeV. Using our relation(
1 second
t
)1/2
' E
2 MeV
we find this was achieved when the Universe was about4×10−14 seconds old. At that time, each particle collisionin the Universe was like an event at CERN!
Successes of the Hot BigBang
There are two main pillars of the Hot Big Bang theory.
Cosmic microwave background: radiation left over from theHot Big Bang and explained as due to atomic processes whenthe Universe was about 400,000 years old.
Nucleosynthesis: The abundance of light elements such ashydrogen, deuterium, helium and lithium are explained bynuclear 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 BigBang model by the 1970s.
Things the Standard Model does not explain
Despite those successes, there are quite a few features of the Universenot 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 galaxiesto form and to hold them together once they do?
Why do observations show that the Universe’s expansion ispresently accelerating?
What causes irregularities in the density field of the primordialUniverse, seen directly in the cosmic microwave background?
These may be evidence of the need for new physics beyond theStandard Model.
Things the Standard Model does not explain
There is an active search for possible explanations for allof these.
The matter-antimatter asymmetry may bedue to unification of the strong andelectroweak interactions at very high
energies.
Things the Standard Model does not explain
There is an active search for possible explanations for allof these.
The dark matter may be comprised offundamental particles associated with an extension
of the Standard Model. Examples are WeaklyInteracting Massive Particles (WIMPs) associated
with supersymmetry, and the axion which isassociated with a global symmetry breaking.
Things the Standard Model does not explain
There is an active search for possible explanations for allof these.
The acceleration of the Universe may becaused 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 allof these.
Density irregularities may have been createdduring a period of very rapid expansion,
known as inflation, in the very youngUniverse. During an inflationary epoch,
quantum processes governed by Heisenberg’suncertainty principle inevitably lead to some
level of irregularity.
Summing upThe young Universe is a testing ground for ideasin fundamental physics.
In its early stages the energies exceed those thatwe can create on Earth.
Processes during these stages may leave evidencethat can give us clues to possible physics beyondthe Standard Model.
There are already several phenomena thatapparently cannot be explained by the StandardModel alone.
