Prof. Mel Ulmer

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Prof. Mel Ulmer DEPT OF PHYSICS
ASTRONOMY
Next week begin observing, no signups necessary
http//www.astro.edu/astro/faculty/ulmer/a01_fall2
002.html
No discussion section meetings this week
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Big Bang Myth

• 15 billion years ago universe was hot and dense
  and expanding

• It cooled and got less dense until it got to
  about 3000 K 273 K 0 Celsius

• Then the gas became transparent emits light

• We see this light today as the Cosmic Microwave
  Background (CMB) really cold 3 K

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Brick Wall, 15 billion lt yrs
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Basic Concepts

• Space and time are interrelated
• Far away look back in time. gt
• Brick wall about 15 billion light years away from
  us gt
• far away means looking at younger universe
• Brick wall 500,000 to 1 million years old.
• The further away, more rapid expansion
• The larger the shift to the red
• redshift 1z gt

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Big Bang Myth Details

• Seeing to the brick wall rushing away from us so
  it has aredshift of 1z 1000 and TT_o(1z)
• Ttemp of brick wall, T_oObserved today

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• Far away means long ago and high velocity of
  recession goes with z and redshift
• Long ago means the universe was hot and dense
  then
• But hot then but looks cold now

• Look far away, we see universe when it was
  denser!
• Too dense to see through
• The brick wall concept we see back until the
  universe is too dense for light to penetrate!

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• Key concepts
• Expansion, distance, and time travel are all
  related
• Far way long ago objects moving away from us
  faster the further away they are.

• Long ago means (in our Myth) hot and dense
• Today means old and sparse

• At Brick Wall density about 10-21 g/cm3)

• Today, density about 10-30 g/cm3

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Brick Wall Facts

• If Brick Wall density much less than air (about a
  billion, billion times less, why cant we see
  through it?)

• Because it is about 1,000 light years thick and
  physics (deferred to later in the course)

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• Brick Wall (a.k.a. the CMB) was found to be too
  smooth to be explained by ordinary matter. Why?
  (again more deferred to later)

• Ordinary matter affects light with electrons
  (think electricity, radio, TV etc).

• If we assume initial lumps made of of enough
  ordinary matter to make clusters, CBM would have
  been lumpier

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• How much?
• About 10-100 times too much!

• Simple calculation to see this
• Go backwards in time.
• Clusters dont contract (arent expanding now,
  this is reverse)
• Universe does get more dense take ratio of
  densities cluster lump to average about
  5/10,000 500/1,000,000 versus observed
  smoothness to 50/1,000,000

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Model Universe Based on GR

• Model is that we live on a 4-dimensional
  surface

• The more matter, the more tightly curved the
  surface
• Works locally, and for the entire universe gt
• Concept of curvature of Universe
• It is related to the mass density of the
  universe

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• Curvature (k)
• k 1, closed (balloon)
• k 0 flat (sheet)
• k -1 open (saddle)

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k-1
k0
k1
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GR Concepts, Cont.

• Additional concept density parameter W0 the
  subscript 0 means today
• W stands for a ratio of the density of the
  universe to some critical value (later)

W0 and k are intimately related
W0 gt 1 k 1 W01 k 0 W0lt1 k -1
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• Basic Holy grails of our Myth
• k and W0

• Other one, does universe expand forever or not?

• knowledge of value of W0 ALONE used to
  guarantee we could predict the future

• Aint necessarily so!!!

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• Because of dark energy!

• Energy and mass are equivalent through E Mc2
• Dark Energy contributes to W

but

• Dark Energy can act as a pressure to push the
  universe apart more strongly than the Dark Energy
  mass pulls things in

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Basic Concepts Cont.

• Additional holy grail amount of Dark Energy gt

• We need to know
• WL (Dark Energy mass ratio),
• Wm (total matter stuff ratio),
• Total W gives us k
• Generally, WL positive means will expand forever.

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Bottom Line WL 0.7, Wm 0.3, k ?
0
Why do we believe this?
Observations plus faith in GR