Objectives

1
Objectives

• Describe the characteristics of the universe
  immediately after its birth.
• Explain how matter emerged from the primeval
  fireball.

2
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 existednot 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

3
Early Origins

• Only subatomic particles existed

4
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

5
Density

• Most of the radiation in the universe is in the
  form of the cosmic microwave background, the
  low-temperature (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.

6
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

7
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

8
Density

• matter-dominated universe
• A universe in which the density of matter
  exceeds the density of radiation. The present-day
  universe is matter-dominated.

9
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.

10
Density

• radiation-dominated universe
• Early epoch in the universe, when the density of
  radiation in the cosmos exceeded the density of
  matter.

11
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 timesthat is, at times
before the crossover point.
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Particle Production In The Early Universe

• pair production
• The process in which two photons of
  electromagnetic radiation give rise to a
  particleanti-particle pair.

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Particle Production In The Early Universe

• (a) Two photons can produce a particleantiparticl
  e pairin this case an electron and a positronif
  their total energy exceeds the mass energy of the
  particles produced.

(b) The reverse process is particleantiparticle
annihilation, in which an electron and positron
destroy each other, vanishing in a flash of gamma
rays.
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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 electronpositron pair (the spiral
  paths, which curve in opposite directions in the
  detector's magnetic field because of their
  opposite electric charges).

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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 temperaturesabove about 1010 Kmost
  photons had enough energy to form an electron or
  a positron, and pair production was commonplace.

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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 particleantiparticle pairs were created by
  pair production at the same rate as they
  annihilated one another.

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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.

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Particle Production In The Early Universe

1. At 10 billion K most photons have enough energy
   to create particleantiparticle
   (electronpositron) 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.

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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

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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 electronsthe lightest
  known elementary particlesappeared out of the
  cooling primordial fireball