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Old ideas of universe
• Flat Earth: disproved by Aristotle
– Ship sails over horizon
– Earth’s shadow on moon circular
– North star rises and falls
• Some people (uneducated) believed
through 1500’s!
• Geocentric (earth in center): Ptolemy
– Circle orbits (epicycles)
– Couldn’t explain retrograde motion
Old ideas of universe
• Heliocentric: (sun-centered)
– Copernicus
• Still had circular orbits
– Kepler’s 3 Laws
• Elliptical orbits, sun not in center
• Closer to sun, move faster
• Farther from sun, take longer to orbit
Edwin Hubble, 1929
• Noticed that light from stars is shifted
towards red end of spectrum
• Means they’re
moving away
from us
• More it’s shifted,
faster they’re
moving
•Opposite is blue shift (means it’s moving
towards us)
Edwin Hubble, 1929
• Hubble noticed that most galaxies are
moving away from us and the
farther they are, the faster they’re
moving
• If go backwards, means everything
was in a small point and exploded
and is moving from that point out.
• Was called Big Bang to make fun of it
and the name stuck!
Big Bang Theory
• EVERYTHING was in one small point
(singularity) that “blew up” and is
still moving outwards today
• Not really an explosion, so much as a
very rapid expansion…like blowing
up a balloon
• About 13.7 billion years ago
• Microwave radiation detected in the
1960’s supports this theory. It’s left
over energy from the Big Bang
Penzias and Wilson
• In addition to Hubble’s
evidence
• 1960’s discovered
microwave radiation all
throughout the
universe, but couldn’t
explain where it came
from
• Only possible source is it’s
left over from Big Bang
Other evidence
• First elements to form should be
simplest….Hydrogen and Helium,
and that’s what most of the
universe is (>25%)!
• Other heavier elements made by stars
(fusion up to Iron) and supernovas
(other heavier elements)
Timeline
• Inflationary period:
– Rapid expansion, temperature billions of
degrees…way too hot even for atoms to exist!
• 10-43 sec after
– .00000000000000000000000000000000000000
00001 seconds
– 4 forces separate (electromagnetism,
gravity, weak molecular, strong
molecular)
– Rules of physics we know now start to
work
Timeline
• 10-35 seconds after
– Protons and neutrons form
– .00000000000000000000000000000000001
• 10-1 seconds after
– Electrons form
– .01 sec.
• 3 minutes after
– 1st atomic nuclei form (H and He)
– Temperature down to 1 billion degrees
• 300,000 years after
– First atoms and light (background
radiation)
WMAP…background
radiation
What’s next?
• Amount of mass will decide what’s going to
happen to the universe:
– May continue expanding forever
– May continue expanding and then stop at
some size
– May expand to a point and then collapse
(Big Crunch) and explode again
• We see the effects (gravity) of some matter
we can’t see, so we call it dark matter
(e.g. black holes)
What’s next?
• Galaxies don’t seem to be slowing down (so
Big Crunch is doubtful, or we’re still in a
period of expansion)
• Open universe: idea that not enough matter
to keep it all together, and things keep
expanding (like molecules from open jar)
• Closed universe: idea that enough mass to
pull everything back in to Big Crunch
• Oscillating Universe: Bang, Crunch, Bang,
Crunch forever and ever
Limitations
• Where’d it come from?
• What made it “blow up”?
• What’s going to happen next?
• Not a limitation: how did life get
here. BBT does not even care about
this…that’s a topic in Biology next
year.
Not “just a theory”
• Remember, a theory in science is
not just some idea somebody
cooked up with no evidence.
• In science a theory is an attempt
to explain observations and has
a lot of evidence
Not “just a theory”
• BBT explains why:
– the universe is expanding
– has a lot of Hydrogen and Helium
– there is background microwave
radiation
• That’s why it’s a good theory, even
if you personally don’t believe
it’s true.
Steady state theory
• Everything is now as it always has
been and nothing will ever
change…stars, planets, galaxies
are the same as they always have
been and will never change
• Nothing changes, nothing moves in
the universe (not same a solar
system)
Changes to theory/alternates
• Anthropic: attempts to answer where
life came from
– Weak: life exists in small window of time
where conditions are just right
• Suggests life is a “lucky accident” that will
eventually not be possible anymore
– Strong: many different things could have
happened
• Suggests things were planned this way
Changes to theory/alternates
• Background radiation is not perfectly
smooth (as P&W guessed)
• pockets of emptiness became space
and pockets of matter became
galaxies