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Goal: To understand the big
bang.
Objectives:
1) To explore the History of the
Universe
2) To understand How we know it
exists
3) To learn about What the big bang
tells us
Hints at the big bang
• By the 1950s it was pretty well established
that our universe was expanding.
• But what did that mean?
• Was it possible everything was once all
together (just like our conga line)?
• However, what this meant for our universe
in the past was not known yet.
• There were many theories around.
“Big Bang” coined
• One of the theories was that the universe
started at a point as a single atom.
• In a radio show in 1949 Fred Hoyle coined
the term “Big Bang”.
• However, support was half towards this
theory and half towards the other.
• We needed evidence to show who was
right!
Cosmic Microwave background
• Was first predicted in 1948 (although done
very incorrectly).
• In 1965 Arno Penzias and Robert
Woodrow Wilson were attempting to
construct a radio telescope but kept
finding a 3 degrees K excess temperature.
• At first they though it was the equipment.
However
• By testing every possible orientation they
discovered that this 3 degrees came from
every direction!
• They had inadvertently found the cosmic
microwave background!
• This proved that the hot big bang model
was the correct one.
Where it comes from?
• Lets hit the rewind button for a minute here.
• Instead of galaxies flying away from us they now
come towards us.
• All the galaxies would meet in 1 region of space
(all the ones we can see anyway).
• As that happens the temperature of the gas
would be higher (simple gas law).
• If the gas gets too hot the atom becomes
ionized.
• What that means is that the electron and proton
are seperated.
Oops I rewound too far
• If everything is ionized all the light gets scattered
because there are now an infinite amount of
energy levels for the electron.
• So, the electrons scatter all the available light.
• However, as the universe expands, the gas will
cool, and therefore reach a point where the
electrons combine with the protons.
• This is called recombination.
• Once this happens, the resulting photons are
free to travel through the universe undisturbed
until they reach us.
Proof is in the (why would it be in
pudding?)
• So we have a blackbody spectrum – CHECK!
• But it is at 2.7 degrees K not the 6000 K that
recombination come out at.
• So, why the difference?
• Well, because of the expansion of the universe.
• So, the light is Doppler shifted so that it seems
cooler.
• And we have our big bang!
Big bang unveiled
What can we learn?
• We can start to see the structure of the
universe as we have it today.
• We can also see other things such as how
much mass the universe has (but more on
that next week).
• Finally, we get to see how OLD it is!
How age?
• Well if we know how fast things are moving
away from us based on their distance then we
know how long it took to get there.
• Back to the conga line analogy suppose the
closest person to you was 5 m away. If they go
1 m per song then you know the age of the
conga line is 5 songs.
• In astronomy if you know Hubble’s constant you
have some idea of the age of the universe.
Hubble Constant
• V = Ho * D (or D = V / Ho)
• Ho is currently estimated to be 71 km/s / Mpc
(with an error bar of about 3).
• So, if D = 1 Mpc then the velocity is 71 km/s.
• How long does it take to go 1 Mpc at a constant
velocity of 71 km/s?
• T = D/V (basic physics) = 1 / Ho
• If Ho is 71 km/s / Mpc then T is 13.7 billion
years.
Conclusion
• The universe expands, so that means everything
once started at the same point.
• This point and time was the big bang.
• From the universe know we can retrace the
steps to the big bang.
• One of the major steps is the time of
recombination which created the cosmic
microwave background which is now “cooled” to
2.7 K.
• From the structure of this we can find other cool
stuff – but that is a lesson for next week!