BIG BANG THEORY
This and other observations has led to the Big
Bang Theory
The Big Bang Theory claims that the universe
has expanded from a very dense, very hot state
that existed at some time in the past.
Studies of red-shifts of distant galaxies show that
the universe is expanding.
Big Bang Model
Running the expansion backward allows us to
calculate the temperature and density of the
universe during its earliest moments.
The known laws of physics can be used to
determine the behavior of matter and energy
at these temperatures and densities.
The model is then used to make predictions
that can be compared to observations.
Where did matter in the universe
come from?
E = mc2
Mass is just “condensed” energy
Energy → mass
A particle – antiparticle pair can be
created if the available energy equals
the mass of both particles times the
speed of light squared
A very BIG number!
Albert Einstein
BIG BANG MODEL
We will begin our discussion at about one
millionth of second after the universe began
its expansion.
It is at this time that the universe had cooled
enough for protons and neutrons, the building
blocks of matter, to exist as individual
particles.
ABOUT A MILLIONTH OF A SECOND…
• Temperature is about 1013 K (ten trillion Kelvin) ≡ a lot
of energy
• Protons, anti-protons, neutrons and anti-neutrons begin
to form
• As a proton or neutron collides with its anti-particle they
annihilate and are converted to energy in the form of
photons
protonanti-proton
anti-neutron
neutron
ABOUT A MILLIONTH OF A SECOND…
• Because of the large amount of energy available, as fast
as these particles annihilate, new protons, anti-protons,
neutrons and anti-neutrons form
• A billion and one protons and neutrons form for every
billion anti-protons and anti-neutrons
protons
+
1 billion 1 billion
anti-protons
About one ten-thousandth of a second . . .
• Almost all particles and anti-particles annihilate and
produce gamma ray photons.
• Temperature has fallen to about 1012 K (one trillion
Kelvin)
• It is no longer hot enough to produce protons and
anti-protons (or neutrons and anti-neutrons)
spontaneously from pure energy to replace those that
annihilate each other.
protonanti-proton
ONE TEN-THOUSANDTH OF A SECOND . . . CONTINUED
• Immediately after annihilation there are equal numbers
of protons and neutrons
• Annihilation results in a billion photons for every
proton or neutron
• Photons are constantly scattered by free particles with
an electric charge like electrons or protons
• These photons increase in wavelength as the universe
expands and will eventually become the majority of
photons that make up the cosmic background radiation
ONE TEN-THOUSANDTH OF A SECOND . . . CONTINUED
• These constantly occurring reactions that transform
protons and neutrons into each other initially maintain
equal numbers of protons and neutrons . . .
p+ + e− ↔ n + e n + e+ ↔ p+ +e
• High energy collisions between protons, neutrons and
other particles like electrons can transform one particle
into another.
About a tenth of a second . . .
• As the temperature (and available energy) drops,
transformation to protons is favored over neutrons
About one second…
• Transformation reactions can no longer occur.
Neutrons begin to decay into protons
n → p+ + e− + e
. . . however, the mass of a proton is slightly less than the
mass of a neutron, so . . .
ABOUT 100 SECONDS …
• Temperature is about 109 K. Neutron decay results in
a 1:7 abundance of neutrons to protons at this point.
• Universe is now cool enough for protons and
neutrons to bind together. This is called fusion.
This process creates new, heavier atomic nuclei and is
called nucleosynthesis.
proton
neutron
deuterium
tritium helium
AT THE BEGINNING OF NUCLEOSYNTHESIS . . .
14 protons 2 neutrons
12 hydrogen nuclei
Atomic mass = 12
At the end of nucleosynthesis . . .
1 helium nucleus
Atomic mass = 4
Mass ratio 75% 25%
• After the temperature drops below about 109 K
(one billion Kelvin), very little happened in
nucleosynthesis for a long time as temperature
and density are too low for fusion.
• It required star formation for the production of
heavier elements.
About 10 minutes . . .the end of big bang nucleosynthesis
• Temperature drops to 3000 K
• Universe is cool enough for electrons to bind with
nuclei and form stable atoms
• With most electrons now bound in atoms, photons can
travel large distances without being scattered by free
electrons. Photons now travel in all directions, resulting
in what is called the cosmic background radiation.
ABOUT 380,000 YEARS …
HHe
NOW …
• With continued expansion, temperature drops
to about 3 K (Three degrees above absolute
zero)
• Photons that make up the cosmic background
radiation are now microwaves – most of these
photons were produced by the particle anti-
particle annihilation at about one ten-
thousandth of a second
Big Bang Model Predictions
The only elements in the early universe were
hydrogen and helium (and a tiny amount of
lithium). The hydrogen-helium mass ratio was
about 75-25%.
Microwaves with an energy corresponding to a
temperature of about 3 K will be found
everywhere in space. From Earth they will be
“seen” across the entire sky.