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The Big Bang Theory
and the evidence that supports it
Some Preliminary Information
What is everything made of?
Atoms!
Image courtesy of Particle Data Group of Lawrence Berkeley National Laboratory.
What makes one atom different
from another atom?
Atoms are made of:
Protons
+
Neutrons
Electrons
_
Atom #
1
1 proton
5
6
5 protons
2
2 protons
3
3 protons
7
7 protons
4
4 protons
8
8 protons
6 protons
Atom #
1
H
2
He
3
Li
4
Be
5
6
B
7
N
8
O
C
An atom with extra neutrons does not
change the atomic number
H
Number
of
protons
+
neutrons
Atomic number = 1
2H
Hydrogen-2 or Deuterium
Atomic number = 1
3H
Hydrogen-3 or Tritium
Atomic number = 1
Neutrons
Neutrons are generally stable within the
nucleus, but are unstable by themselves.
Neutrons are also unstable if there are too many
of them in a particular nucleus.
If a neutron is by itself, it will eventually change
(or decay) into a proton, electron and antineutrino.
n → p+ + e− +e
Neutrons
If a neutron is unstable in a particular nucleus
(like tritium), it will eventually decay, changing
the nucleus into the element with the next
atomic number.
3H
Hydrogen-3 or Tritium
Atomic number = 1
n → p+ + e− +e
3He
Helium-3
Atomic number = 2
Helium-3 is stable and does not decay
Big Bang Theory
Studies of redshifts of distant galaxies show that
the universe is expanding.
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.
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.
Big Bang Model
The laws of physics (as we understand them
today) are able to describe conditions and
events in the universe for times after 10-43
seconds following the Big Bang.
The earliest time that can be probed
experimentally is roughly one hundred
billionths of a second (10-11 seconds) when
the temperature exceeded about 2 x 1015 K
Big Bang Predictions
The following timeline is mainly concerned with
events that lead to predictions that can be tested
by observations.
This simplified treatment leaves out some
interesting details including many events that
involve subatomic particles like quarks,
electrons, positrons, neutrinos and anti-neutrinos.
Big Bang Model
Collisions between particles are VERY
common in the extremely hot and dense
environment that exists in the universe during
these early times.
Where did matter in the universe
come from?
E=
2
mc
Mass is just “condensed” energy
Energy → mass
Albert Einstein
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!
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
neutron
proton
anti-neutron
anti-proton
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
anti-protons
+
1 billion
1 billion
About one ten-thousandth of a second . . .
• 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.
• Almost all particles and anti-particles annihilate and
produce gamma ray photons.
anti-proton
proton
One ten-thousandth of a second . . . continued
• 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
• Immediately after annihilation there are equal numbers
of protons and neutrons
One ten-thousandth of a second . . . continued
• High energy collisions between protons, neutrons and
other particles like electrons can transform one particle
into another.
• 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
. . . however, the mass of a proton is slightly less than the
mass of a neutron, so . . .
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
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.
proton
deuterium
tritium
helium
neutron
This process creates new, heavier atomic nuclei and is
called nucleosynthesis.
At the beginning of nucleosynthesis . . .
14 protons
2 neutrons
At the end of nucleosynthesis . . .
12 hydrogen nuclei
1 helium nucleus
Atomic mass = 12
Mass ratio
75%
Atomic mass = 4
25%
About 10 minutes . . .
the end of big bang nucleosynthesis
• 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 380,000 years …
• Temperature drops to 3000 K
• Universe is cool enough for electrons to bind with
nuclei and form stable atoms
H
He
• 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.
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 antiparticle annihilation at about one tenthousandth 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.
Comparison to Observations
Observations of large far away clouds of gas that
have few heavy elements allow astronomers to
measure the relative amounts of helium and
hydrogen in the clouds.
Estimates of initial hydrogen and
helium abundances based on these
and other observations agree with
predictions.
Gas cloud in the LMC, a nearby galaxy
Image Credit ESO
Comparison to Observations
In 1964, microwave photons were detected (from
all directions in the sky) with this antenna by
Penzias and Wilson.
They were doing
telecommunications
research and were not
aware of the prediction
of a microwave
background radiation
Comparison to Observations
“Snow” that can be
seen on an un-tuned
analog TV is due in
part to photons that
make up the cosmic
microwave
background
radiation