EPSC201 - Lecture 2 Notes
• What our eyes can see is only a small part of larger spectrum of radiation
• Wavelength = the distance between peaks
• Red light – low in energy – long wavelength
• Blue light - high in energy – short wavelength
• Red shift – wavelength of light becomes longer if light source is moving away
• Blue shift – wavelength of light becomes shorter if light source is moving towards you
Hydrogen and the Red
The universe is mostly made of
hydrogen. Hydrogen through-
out the universe absorbs ener-
gy, and emits it back out as
light. It only emits very specific
wavelengths. These particular
wavelengths of hydrogen have
been identified in the lab. We
know the wavelengths that hy-
drogen should emit when sta-
tionary. But the hydrogen
around the universe emits
longer wavelengths. It has been
red shifted. We know that it is moving away from us. Thus, we know the universe, and the distance be-
tween stars, is expanding.
This effect of the wavelengths changing due to movement away/towards is called the Doppler Effect. • Key - using the concept of the Doppler effect, translating to the hydrogen spectrum and the red shift
can conclude that the universe is expanding
Hubble – asked questions such as: How old is the universe? How fast is it expanding? Will the universe
eventually stop expanding and start contracting?
Hubble determined our distance from other stars using luminosity of stars. The further a star is, the less
luminous is. He used the red shift to determine how fast the stars were moving away from us. He used rel-
ative luminosities by comparing different stars. Had to assume every star was the same brightness and
speed was constant. He also used groups of stars to account for the fact that their brightness is NOT con-
Using Velocity = Distance / time Hubble was able to predict the age of the universe. He calculated that it
was 13.4 billion years old. Current models suggest it is actually 14.67 billion years old – Hubble was very
The Big Bang Theory:
If the universe is expanding now, then it must have originally been very small. The universe started wit ha
slow expansion, followed by a rapid expansion, followed again by a slow expansion (now).
E = mc^2 Energy equals mass times speed of light squared
This equations basically says that energy and mass are the same thing. At the beginning of the universe,
there was a lot of energy. It was infinitely small, with an infinite amount of energy. Things were hot. Since
then, we’ve been expanding. As the universe expanded, things cooled off, and mass was created.
At the beginning of the universe there were 6 types of quarks. These quarks will eventually become the
building blocks of bigger products. The six types of quarks are up, down, charm, strange, bottom, top.
These particles eventually become building blocks for larger particles that are physically more important to us. This is a soup of subatomic particles → crashing into each other → anti matter hits matter and wipe
each other out.
At 10^-5 seconds after the big bang, things have cooled down a bit. Protons have been made by combing
2 up quarks with 1 down quark. Neutrons have
been made from 2 down quarks and 1 up
Hydrogen is 1 proton and 1 electron. This is the
simplest atom. Deuterium, heavy hydrogen, is 1
proton, 1 electron and 1 neutron.
At the beginning stages of the universe, energy
and mass were constantly changing back and
for, via E=mc^2. Once the universe had ex-
panded, and therefore cooled down, matter be-
come much more dominant, than in the early
stages. As the universe cools, larger particles