Announcements

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Announcements

• Abstract of project due Feb 9
• 1st midterm Feb 12
• Review session ?????

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Lecture 11Newtons Cosmology I Absolute Space
and Time
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What do you think about the following questions ?

• What happened before the universe existed?
• What is outside the universe?
• What is the universe like right now ?
• How did it get there ?

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The Physical Universe

• Proposition the universe can be understood in
  terms of natural laws (here Newtons laws)
• physical anything that can be objectively
  observed (at least in principle)
• physical universe the universe of material
  objects, of energy, of space, of time

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The Copernican principle

• Man, Earth, Sun, the Galaxy etc. are not
  special/are not at a special place in the
  universe. This a priori does not exclude that
  there may exist special objects/special places.

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Quiz Quiz Quiz Quiz Quiz Quiz
NameSID
Date
Answer
Signature
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What key ingredient was ignored by Kapteyn and
Shapley so they came to opposite conclusions
regarding the size of the Milky Way and the
location of the Sun ?
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Answer both ignored the effect of dust.
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The anthropic principle

• The weak anthropic principle states that the
  universe must be compatible with our existence.
• The strong anthropic principle states that the
  universe is such as it is because its purpose is
  to create life.

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Example

• Existence of life (as we know it) strongly
  depends on the presence of carbon
• If the nuclear force (the force that keeps atomic
  nuclei together) would have been slightly
  different, no carbon could have been formed.
• Why is the nuclear force as strong as it is
  ?Anthropic principle ? If it had been different,
  we wouldnt ask this question.

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A toy universe

• According to Newton, what is going to happen ?
• The model Universe is going to collapse under its
  own gravity

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Newtons Universe

• In order to avoid collapse
• homogeneous
• isotropic
• infinite size
• no center
• infinite in time
• has always been
• will always be
• ? perfect cosmological principle

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The cosmological principle

• homogeneous the universe looks the same
  everywhere on large scales? there is no special
  place (center)
• isotropic the universe looks the same in all
  directions on the sky
• ? there is no special direction (axis)

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The perfect cosmological principle

• homogeneous the universe looks the same
  everywhere on large scales? there is no special
  place (center)
• isotropic the universe looks the same in all
  directions on the sky? there is no special
  direction (axis)
• unchanging The universe looks the same at all
  times ? there is no special epoch

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Homogeneity and Isotropy
Copernican Principle
Isotropy
?
Homogeneity

Isotropy around another point
Isotropy
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Homogeneity

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Does the cosmological principle apply to our
universe ?
The cosmic microwave background radiation
(CMB) afterglow from the big bang. Its smooth
to 1 part in 105
? yes, the universe appears to be
homogeneous and isotropic
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Does the strong cosmological principle apply to
our universe ?
Galaxies 10 billion years ago
Galaxies today
? no, the universe appears to change with time
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Announcements

• Abstract of project due Feb 9
• 1st midterm Feb 12
• Review session ?????