Lecture 10 Cosmological Concepts

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Lecture 10 Cosmological Concepts

• ASTR 340
• Fall 2006
• Dennis Papadopoulos

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The Universe

• The Physical Universe
• Material objects, energy, space, time, forces

Are space and time physical ? What is space, what
time?
Space is what keeps everything from happening at
the same location
Time is what prevents everything from happening
at once
Any physical model should include and explain
space and time along with every other physical
phenomenon. We cannot invoke a preexisting space
and time into which we construct the universe. We
cannot ask what happened before the universe
existed or what is outside the universe. Space
and time are not properties distinct from the
universe. Time did not exist before the universe
and space does not exist outside it. The universe
is not expanding into space or spacetime.
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Scientific Cosmology

• What is the universe now? How did it reach this
  state?
• If we trace the evolution of the universe
  backward in time we can ask whether or not there
  was a t0 point.
• If there was we must arrive at the question of
  initial conditions, how things were at the
  earliest moment we can consider.
• Cosmology aims at describing these initial
  conditions and describing how it evolved from
  them
• Initial conditions must simple (Occams razor)
  e.g. big bang -gt a certain amount of energy and
  matter, certain physical laws and certain
  fundamental constants.
• The complexity we observe existed as a
  potentiality and developed naturally in the
  subsequent evolution

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Fundamental constants of nature

• G, c, h, e, me, mp

• Values of fundamental constants and basic laws
  of physics determine what is possible in the
  universe
• If any of these changed even slightly the
  universe would be different than the one we
  observe
• e.g. if nuclear reactions were not possible at
  the densities and temperatures prevailing at the
  cores of gravitationally bound gaseous nebulae
  would they be stars? Life?

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The Cosmological Principle

• Universe -gt sum of all particles and energy,
  physical laws and spacetime
• Humans do not occupy a privileged position in
  universe
• Copernican principle -gtEarth and solar system do
  not occupy special place in universe
• Does not claim that there is no center
• Is there a center somewhere?
• Humans as observers
• Cosmological principle -gt

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Cosmological Principle (cont)
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Isotropy and Homogeneity

• Homogeneous -gt we see no difference when we
  change position there is no preferred position
  in the universe (translational invariance)
• Isotropic -gt no difference when we look at a
  different direction
• Examples Surface of uniform cylinder is
  homogeneous but not isotropic- what about the
  surface of a sphere or chessboard ?
• Cosmological Principle (CP)-gt universe is
  homogeneous and isotropic (at a given
  cosmological time)

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CP

• Cosmological principle means that physical laws
  are assumed to be the same everywhere, too
• The cosmological principle of isotropy and
  homogeneity, like other scientific hypotheses, is
  testable by confrontation with data.
• So far, observations support this hypothesis

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Tests
Galaxies arranged in superclusters that appear
as long sheets surrounded by voids
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Cosmological Principle Tested
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The Perfect Cosmological Principle
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The role of the observer

• Anthropic principle weak and strong
• We dont know why the constants have these values
  or if it is possible to have other values but out
  of all possible universes ours is special because
  we exist
• Weak Anthropic Principle (WAP) The conditions we
  observe in the universe (constants, laws, etc)
  must be compatible with our existence.

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Weak Anthropic Principle (WAP)
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Against WAP
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Other Anthropic Principles
Non-Defendable
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Cosmological Principles