|Where did all the matter
in the universe come from? To answer this question cosmologists have
turned to the microscopic world of particle physics. Particle physics
is the study of matter and energy at its most fundamental level and particle
physicists seek to understand the basic structure of matter and the fundamental
forces or interactions existing between matter in the universe.
There are four basic interactions
that govern the interaction of matter:
All these four forces are mediated by carrier particles which are elementary particles that 'carry' the force from one particle to another. The W and Z particles were discovered in 1984 using the Super Proton Synchrotron particle accelerator at CERN in Geneva. The Gluon has been detected by indirect methods but the graviton awaits a quantum theory of gravity and has so far not been observed.
|Interaction||Strength (relative to strong)||Range/m||Carrier Particle|
Cosmologists believe that
in the universe's early history it was radiation that dominated
and in order to understand why, we need to look at a key concept in particle
physics called pair production We can write pair production symbolically
photon + photon = particle + antiparticle
The reverse is also possible.
A particle and antiparticle can collide and annihilate each other producing
two high energy gamma ray photons:
particle + antiparticle = photon + photon
For pair production to occur the following conditions must be met:
(1) The energy of the photons must be greater than the mass-energy of the created particles.
(2) Pair production must obey the law of conservation of energy.
Cosmologist's model the early universe as a hot, high energy photon gas and the kinetic energy of the photons depends on their temperature. Creation and annihilation of particles occurs in equal numbers so that as many particles are made per second as are being destroyed and the universe is in a state of thermal equilibrium. Particle physicists call this state of balance between matter and antimatter symmetry . The average energy of the photons in a gas can be approximated by the equation E = kT. It is important to understand that the creation of different types of particles by pair production depends on the temperature of the photons.
of the Universe
|Cosmologists divide the
history of the universe into four periods or eras of time each one
corresponding to a particular range of temperatures. These are:
Heavy particle era:
temperature of the universe: <
1033 K, time after Big Bang: >
Light particle era:
temperature of the universe: <
1010 K, time after the Big Bang: >
Radiation era: temperature
of the universe: < 1010
K, time after the Big Bang: >
Matter era: temperature
of the universe: < 3000 K,
time after Big Bang: > 106
Origin of the Four Basic Interactions
|Particle physicists believe
that, despite their different characteristics, if two particles were to
collide at very high energies, these forces would be indistinguishable
from each other and have the same strength. In other words, they would
all be unified into a single super force and theories which attempt
to unify the four interactions in this was are called Grand Unified
Theories or GUT's for short.
In the 1970's particle physicists were able to show theoretically that the weak and electromagnetic force, collectively called the electroweak force, could be unified at energies of about 100 GeV corresponding to temperatures of 1015 K. The unification of the electroweak and strong forces does not occur until about 1014 GeV and, according to GUT's, all forces are unified at 1019 GeV. Particle accelerators have been built that can operated at a few 100 GeV, but is simply not possible to construct them capable of producing the full range of unification energies.
The Big Bang model though, offers a way of testing GUT's. According to GUT's during the first 10-43 s of the universe's existence when it was at a temperature exceeding 1032 K, all the forces were unified as a single force. When it cooled, the forces of nature 'froze out' to the four interactions that we know today. This would account for the balance of matter over antimatter as before the strong force froze out, equal numbers of particles and antiparticles were being created and destroyed. After the strong force froze out this symmetry was broken, leading to an excess of matter.
GUT's also predict that neutrinos created in the Big Bang should also have a small mass. If this is so then they would be a candidate for dark matter and the value of the mass-energy density might be altered so that:
P = Pc
Where Pc is the critical density.
with the Big Bang
|Despite its successes the
standard big bang theory still leaves some unanswered questions. The first
concern is the geometry of space and time. Observations suggest that Omega0
is very close to 1 which would suggest that we are living in a flat universe.
special conditions would have led to the mass-energy density being exactly
equal to the critical density
It turns out that if the average density of matter even slightly deviated from Pc then this variation would have rapidly multiplied as the universe expanded leading to a value very far from Omega0 = 1. The fact that Omega0 is close to 1 now, means that some mechanism in the big bang ensured the near flat universe we see today. This is called the flatness problem and it is explained by introducing a concept called inflation.
Inflation explains why the cosmic microwave background is isotropic. Before inflation, the universe was small enough for all parts to reach the same temperature within the light horizon. After inflation, this uniformity of temperature is maintained at larger distances even though it now takes considerably longer for light to travel across the universe. Note that inflation does not violate special relativity's postulate that the speed of light is constant since it is the expansion of space and not expansion through space that is at work here.
10-43 s - The Planck Time
|General relativity regards
space and time as linked together in a smooth continuum but, at very small
scales of length and time, quantum mechanics says that this representation
of space-time breaks down.
This is because of a law in quantum mechanics
called the Heisenberg Uncertainty Principle named after a German
physicist Werner Heisenberg (1901-1976). The uncertainty principle tells
us that there always exists a degree of uncertainty between the position
of a particle and its momentum. The more accurately you measure the speed
of a particle the less certain you are of its position and the uncertainty
principle makes it impossible to measure the exact position and the exact
momentum simultaneously. The principle also extends to energy and time.
By using the uncertainty principle, cosmologists are able to arrive at an interval after which space and time are measurable called the Planck time tp which is made up of the fundamental constants G, h and c as
tp = G1/2h1/2c-5/2 = 1.35 x 10-43 s
From the beginning of the Big Bang at t = 0 to t = tp, we do not know how the universe behaved and we have no theory that can explain what happened using quantum mechanics or general relativity. Time itself began at the Big Bang and to say what came 'before' is a meaningless statement as time didn't exist then!
Particle physicists believe that the vacuum of space is not empty but is seething with pairs of particles and antiparticles that are constantly being made and destroyed. These particles exist for so brief a time (less than 10-21 s) that they hardly exist at all, and are called virtual particles. These particles may exist for a time that satisfies the uncertainty principle , but as the universe rapidly expanded they became separated and appeared as real particles after the universe was older than the Planck time. The gravitational energy associated with the Big Bang singularity provided enough energy to fill the universe as it expanded. The main point about all these concepts is that the Planck time sets a limit to what we can know about the very early universe according to our current knowledge of physics, however theoretical particle physicists are currently working on a quantum theory of gravity and progress continues.
HEISENBERG PAUL DIRAC FURTHER INFORMATION LINKS
A biography of Werner Heisenberg
The St. Andrew's Paul Dirac pages.
CERN - At the CERN homepage
Electro Weak Force
Neutrinos and the Solar Neutrino Problem:
PAUL DIRAC FURTHER INFORMATION LINKS
FURTHER INFORMATION LINKS