EPSC201 - Lecture 3 Notes
Universe – Big Bang:
The universe initially had infinite energy. Energy converted to mass, temperature decreased. Universe ex-
panded, and temperature decreased further.
• 2 up quarks + 1 down quark = proton
2 down quark + 1 up quark = neutron
• Proton + neutron = hydrogen nucleus
Energy throughout the early universe was being bounced around, and made it impossible to discern any-
thing. It was very dense and hot. Impossible to see before.
379,000 years after Big Bang. From this point on, we can observe the universe. At 6000 degrees, it was
cold enough to start observing.
At about 379 years after the big bang, the background scattering effectively stopped. Atoms have formed
and matter has spread out. The universe is observable. Cannot observe before 379 years due to energy
scattering. This all occurred when the uni-
verse was about 6000 degrees Kelvin. Now
the temperature is 2.75 degrees Kelvin.
Things have cooled off. Bell lab scientists observed
this radiation in the sixties. Scientists
calculated that there would be a specif-
ic background radiation at 2.75 de-
grees, and if the background radiation
is measured from space, it is found to
be the same wavelength.
Black Body Radiation – absorbs all
forms of radiation and reemits all
forms too. The early universe was
nearly a perfect blackbody; all radia-
tion was re emitted and absorbed.
At 379,000 years after Big Bang, the universe was heterogeneous. Matter
was already clumped throughout the universe, which gives rise to hot and cold spots. The first stars were
massive, around a million years after the Big Bang. There were clouds of material, largely of hydrogen
and small elements, spread throughout the universe. These patches drag in more matter, and gravita-
tional collapse occurs. This is how stars are made.
Stars shine because they are emitting energy. Once they reach 10 million degrees, nuclear fusion occurs.
Deuterium is quickly formed, and it transforms into helium. The formation of helium releases a lot of ener-
gy. The core temperature of our sun is about 15 million degrees, the surface is 6000 degrees. Star forma-
tion requires that you have enough mass to reach the threshold energy, for nucleosynthesis to occur. If
there is not enough mass present, the star will not be formed, it won’t be hot enough. Nucleosynthesis of helium is why stars emit energy. As the temperature increases, heavier elements
are formed. In theory, a billion degrees can be reached before the star explodes.
If the cloud is large enough, many stars will be formed. As mat-
ter is dragged inwards, it starts to spin (true for
galaxy and star formation). Most
stars and galaxies spin. At the
core of the galaxy lies the ma-
jority of the mass. The more it
spins, the tighter ring it will form.
Slow spinning galaxies have
Our galaxy is 90,000 light
years across. Our galaxy
does spin; every 250 million
years our galaxy completes a
rotation. At the center of our
universe, there is a black hole.
Black hole – an object that is
so massive, that nothing can escape it (not
light or mass). That’s why they don’t emit light. We cannot see black holes for this reason, but we can de-
tect them by gravitation field distortions.
Our star was made roughly three and a half billion years ago, and has roughly the same time to live.
Our star is middle aged. It is also a 2nd or 3rd genera-
tion star. It is in equilibrium where gravity is
pulling it together, and fusion is
exploding it outward. It’s com-
position is much different then
a 1st generation star, which
are much older. The material
in the outside of the star has an
easier time to overcome gravity
and explode outward to become
a red giant when nuclear fusion
starts to consume most of the