Download Particle Production In The Early Universe

Objectives
• Describe the characteristics of the
universe immediately after its birth.
• Explain how matter emerged from the
primeval fireball.
Early Origins
• Astronomers are unable to observe the universe
when it was very young, because truly far-away and
long-ago events were engulfed in a sea of intense
radiation
• Only subatomic particles existed—not only the
protons, neutrons and electrons we know today, but
also, we think, various strange and exotic elementary
particles predicted by current theory.
• Surprisingly, part of this group of particles that
characterized the early universe can now be studied
here on Earth, in huge particle accelerators
Early Origins
• Only subatomic particles existed
Density
• On the very largest scales we can
regard the universe as a mixture of
matter and radiation.
• The overall density of matter is not
known with certainty, but it is thought to
be at least a few tenths of the critical
density, about 10-26 kg/m3
Density
• Most of the radiation in the universe is in the form of
the cosmic microwave background, the lowtemperature (3 K) radiation field that fills all space.
• For our current purposes, then, we can regard the
cosmic microwave background as the only significant
form of radiation in the universe
• The reason for this is that stars and galaxies, though
very intense sources of radiation, occupy only a tiny
fraction of space.
Density
• We can express the energy in the microwave
background as an equivalent density by first
calculating the number of photons in any
cubic centimeter of space, then converting
the total energy of these photons into a mass
using the relation E = mc2.
• we arrive at an equivalent density for the
microwave background of about 5 x 10-31
kg/m3
Density
• Density for the microwave background
of about 5 x 10-31 kg/m3
• at the present moment :
– the density of matter (around 10-26 kg/m3)
in the universe far exceeds the density of
radiation.
– Matter dominated
Density
matter-dominated universe
A universe in which the density of matter exceeds the
density of radiation. The present-day universe is
matter-dominated.
Density
Even though today the radiation density is much less
than the matter density, there must have been a time
in the past when they were equal.
Before that time, radiation was the main constituent
of the cosmos. The universe is said to have been
radiation-dominated then.
Density
radiation-dominated universe
Early epoch in the universe, when the density of
radiation in the cosmos exceeded the density of
matter.
Density
As the universe
expanded, the number of
both matter particles and
photons per unit volume
decreased.
However, the photons
were also reduced in
energy by the
cosmological redshift,
reducing their equivalent
mass, and hence their
density, still further.
As a result, the density of radiation fell faster than the density of matter as the
universe grew. Tracing the curves back from the densities we observe today,
we see that radiation must have dominated matter at early times—that is, at
times before the crossover point.
Particle Production In The Early Universe
pair production
The process in which two photons of electromagnetic
radiation give rise to a particle—anti-particle pair.
Particle Production In The Early Universe
(a) Two photons can
produce a particle—
antiparticle pair—in
this case an
electron and a
positron—if their
total energy
exceeds the mass
energy of the
particles produced.
(b) The reverse process is particle—antiparticle annihilation, in which
an electron and positron destroy each other, vanishing in a flash of
gamma rays.
Particle Production In The Early Universe
(c) Tracks in a particle detector allow us to visualize pair
creation. Here a gamma ray, whose path is invisible
because it is electrically neutral, arrives from the left; it
dislodges an atomic electron and sends it flying (the
longest path).
At the same time it provides the
energy to produce an
electron—positron pair (the
spiral paths, which curve in
opposite directions in the
detector's magnetic field
because of their opposite
electric charges).
Particle Production In The Early Universe
As an example of how pair production
affected the composition of the early
universe, consider the production of
electrons and positrons as the universe
expanded and cooled.
At high temperatures—above about 1010
K—most photons had enough energy to
form an electron or a positron, and pair
production was commonplace.
Particle Production In The Early Universe
• As a result, space seethed with electrons
and positrons, constantly created from the
radiation field and annihilating one
another to form photons again.
• Particles and radiation are said to have
been in thermal equilibrium
• new particle—antiparticle pairs were
created by pair production at the same
rate as they annihilated one another.
Particle Production In The Early Universe
As the universe expanded and the temperature
decreased, so did the average photon energy.
By the time the temperature had fallen below a
billion or so kelvins, photons no longer had
enough energy for pair production to occur, and
only radiation remained.
Particle Production In The Early Universe
(a) At 10 billion K most photons have enough
energy to create particle—antiparticle (electron—
positron) pairs, so these particles exist in great
numbers, in equilibrium with the radiation.
(b) Below about 1 billion K, photons have too little
energy for pair production to occur.
Particle Production In The Early Universe
Pair production in the very early universe was
directly responsible for all the matter that exists
in the universe today. Everything we see around
us was created out of radiation as the cosmos
expanded and cooled
Particle Production In The Early Universe
The first few hundred seconds of the universe's
existence saw the creation of all of the basic "building
blocks" of matter we know today
protons and neutrons froze out when the temperature
dropped below 1013 K, when the universe was only
0.0001 s old.
The lighter electrons froze out somewhat later, about
a minute or so after the Big Bang, when the
temperature fell below 109 K. This "matter-creation"
phase of the universe's evolution ended when the
electrons—the lightest known elementary particles—
appeared out of the cooling primordial fireball