BIG BANG SUMMARY NOTES

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 TIME LINE

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.