Chapter 29
Exploring the Early Universe
Guiding Questions
1. Has the universe always expanded as it does today?
2. What is antimatter? How can it be created, and how is
it destroyed?
3. Why is antimatter so rare today?
4. What materials in today’s universe are remnants of
nuclear reactions in the hot early universe?
5. How did the first galaxies form?
6. How were the fundamental forces of nature different
during the first second after the Big Bang?
7. Are scientists close to developing an all-encompassing
“theory of everything”?
8. How many dimensions do unified models predict?
The Isotropy or Horizon problem: why should the universe
look the same in all directions?
The Flatness problem: why should the density of the
universe be just barely enough to prevent recollapse?
Before the first second, the universe may have
expanded 1050 times in 10-24 s.
The newborn universe may have undergone a
brief period of vigorous expansion.
• PROBLEM: If the universe is at least 26 billion light years
across (13 billion in each direction), how could both sides have
exactly the same temperature if they couldn’t “communicate?”
• SOLUTION: During the first second, the universe underwent a
rapid but even expansion, called the inflationary epoch, in
which it became many times larger than its original size.
Inflation also explains the flatness problem.
During inflation, all the mass and energy in the
universe burst forth from the vacuum of space.
• Quantum mechanics explains the behavior of nature
on the atomic scale and smaller.
– QM tell us how to calculate the structure of atoms and
interactions between atomic nuclei.
– Elementary particle physics is the branch of QM that
explains how subatomic particles interact.
• Heisenberg’s uncertainty principle: one cannot
know the exact position and momentum (mass and
velocity) of a particle simultaneously.
– In cosmology, we cannot simultaneously know the
energy precisely for every moment in time.
During inflation, all the mass and energy in the
universe burst forth from the vacuum of space.
• Over brief intervals of
time, it is impossible to
know exactly how much
energy or matter exists in
a particular space.
• Virtual pairs of particles
and antiparticles can
virtually exist
momentarily
As the primordial fireball cooled off, most of the
matter and antimatter in the early universe
annihilated each other.
These pairs of particles are spontaneously created,
and then usually annihilate each other.
Why didn’t all matter annihilate with antimatter?
1 proton survived for every billion protons
annihilated with antiprotons: an example of
“symmetry breaking”
Nucleosynthesis during the first few minutes
of the Big Bang created most of the Helium
atoms in the universe.
Galaxies formed from density
fluctuations in the early universe.
Clumps bigger than the Jeans
length can grow.
LJ = [pkT/mGrm]½
k = 1.38x10-23 J/K
T = gas temperature
m = mass of one particle of gas
G = universal gravitation constant
rm = average density of matter in
the gas
In grand unified theories, all physical forces had the
same strength immediately after the Big Bang.
• Gravitational force - attraction between any two objects
with mass.
• Electromagnetic force - attraction/repulsion between
charges, and forces from currents and magnetic fields
• Strong nuclear force - holds protons in the nucleus of an
atom together.
• Weak nuclear force - governs the radioactive decay of
neutrons into protons.
A theory that correctly combines all forces in the
early universe is known as a supergrand
unification theory or theory of everything (TOE).
GUT unifies 3 out of 4 forces.
Standard model explains electroweak unification (2/4).
Relative strengths of four fundamental forces
in the present time: Gravity is the weakest.
Theories attemting to unify the physical forces predict
that the universe may have 11 dimensions.
• We experience 3 spatial
dimensions (up/down,
right/left, forward/back).
• Plus one time dimension:
3x + 1t = 4 D
• Kaluza-Klein theory uses
multiple dimensions to
describe electricity and
magnetism.
• String theorists we use 11 D to
describe the universe
10x + 1 t = 11 D
Guiding Questions
1. Has the universe always expanded as it does today?
2. What is antimatter? How can it be created, and how is
it destroyed?
3. Why is antimatter so rare today?
4. What materials in today’s universe are remnants of
nuclear reactions in the hot early universe?
5. How did the first galaxies form?
6. How were the fundamental forces of nature different
during the first second after the Big Bang?
7. Are scientists close to developing an all-encompassing
“theory of everything”?
8. How many dimensions do unified models predict?