Chapter 28 Cosmology The Creation and Fate of the Universe

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Chapter 28 CosmologyThe Creation
and Fate of the Universe
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Guiding Questions

• What does the darkness of the night sky tell us
  about the nature of the universe?
• As the universe expands, what, if anything, is it
  expanding into?
• Where did the Big Bang take place?
• How do we know that the Big Bang was hot?
• What was the universe like during its first
  300,000 years?
• How is it possible to measure how the universe is
  curved?
• What is dark energy?
• Has the universe always expanded at the same
  rate?
• What will happen if the universe keeps expanding
  forever?

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Some Vocabulary

• Universe
• All matter, energy, and spacetime -- in other
  words, everything that is.
• Cosmogony
• A belief system that describes where the universe
  came from and why it is here.
• Cosmology
• The scientific investigation of the origin,
  structure, and evolution of the universe.
• Cosmetology

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The darkness of the night sky tells us about the
nature of the universe.

• If space goes on forever, then eventually every
  space should be filled with light, yet thats not
  what we see when we look up.
• Olbers paradox is that the sky is actually dark
  in places.
• We live in an expanding universe and all of the
  light in the universe has not yet reached us.

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The universe is expanding

• Hubbles law shows that the more distant galaxies
  have higher recessional velocities.
• Hubbles law is the same in all directions
  (called isotropic).
• Hubbles law allows to play the scenario
  backwards and determine an age of the universe.
• Doppler red shifts are caused by an objects
  motion whereas cosmological redshifts are caused
  by the expansion of spacetime.

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The 3D universe is expanding rather like the 2D
surface of the balloon.
As there is no central point on the 2D surface,
there is no center to our 3D space volume.
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Greater separation ? faster separation
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The expanding universe emerged from an event
called the Big Bang.

• In the 1940s, based on Hubbles Law, George Gamow
  proposed the universe began in a colossal
  explosion.
• In the 1950s, the term BIG BANG was coined by an
  unconvinced Sir Fred Hoyle.
• In the 1990s, there was an international
  competition to rename the BIG BANG with a more
  appropriate name, but no new name was selected.
• Big STRETCH is better, as the expansion is NOT
  really an explosion.

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BIG BANG is a relatively simple idea

• If the universe is expanding, it must have been
  smaller in the past.
• If it was smaller in the past, then something
  must have made it begin to expand.
• This event is called the BIG BANG.
• The age of the universe is the distance to the
  furthest galaxies divided by their recessional
  speed
• Current figures date the universe at about 13
  billion years.
• What caused the expansion? A quantum fluctuation
  in the primordial vacuum?

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The farther we look into space, the farther back
in time we are seeing.
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The microwave radiation that fills all space is
evidence of a hot Big Bang.
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The spectrum of the Cosmic Microwave Background
Radiation reveals a temperature of 2.73K.
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Variations in the microwave sky are due to the
motion of Earth through the cosmos.
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Earth orbits Sun, Sun orbits in the Milky Way,
and our Galaxy moves too.
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The universe was a hot, opaque plasma during its
first 300,000 years.

• Everything in the early universe falls into two
  categories matter or energy.
• Mass density of radiation, rad
• rrad 4sT4/c3 s 5.67 x 10-8Wm-2K-4
• for T 2.73K, rrad is 4.6 x 10-31 kg/m3
• Average density of matter, rmass
• present day rmass is about 2 to 4 x 10-27 kg/m3

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The universe was a hot, opaque plasma during its
first 300,000 years.
The early radiation dominated universe became
todays matter dominated universe.
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Light could not stream freely through matter
until 300,000 years after BB.
At t 300,000 years, the universe was finally
cool enough from its initial primordial fireball
that electrons and protons could combine to form
atoms (era of recombination).
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The shape of the universe indicates its matter
and energy content.

• The shape of our universe depends on the combined
  average mass density of all forms of matter and
  energy. The three possibilities are
• ZERO CURVATURE Two parallel beams of light
  never intersect the universe is FLAT.
• POSITIVE CURVATURE Two initially parallel beams
  of light gradually converge the universe is
  spherical and is CLOSED.
• NEGATIVE CURVATURE Two initially parallel beams
  of light gradually diverge the universe is
  hyperbolic and is OPEN most likely.

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Tiny temperature variations of about 3 x 10-4 K
in the CMB may have provided the seeds of the
large-scale structure of the Universe as observed
by Geller and Huchra.
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The shape of the universe indicates its matter
and energy content.

• Critical density of the universe
• rc 3H02 / 8pG
• H0 is the Hubble constant and G is the universal
  constant of gravitation.
• For H0 70 km/s/Mpc, rc 9.2 x 10-27 kg/m3

• Density parameter W0
• W0 r0/rc
• r0 is the combined average mass density.

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The shape of the universe indicates its matter
and energy content.
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The universe appears to be filled with dark
energy.

• Our observations suggest that the universe is
  flat or slightly open.
• This conflicts somewhat with our observation that
  all known radiation, matter, and dark matter only
  account for 20 to 40 of the total density of
  the universe.
• There must be an additional source somewhere
  dark energy.

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Observations of distant supernovae indicate that
we live in an accelerating universe.
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The matter and dark energy in the universe
determine its future evolution.

• Deceleration parameter (q0)
• If q0 0, then the universe expands forever at a
  constant rate.
• If q0 ½, then the universe is marginally
  bounded and just barely is able to continue
  expanding.
• If q0 lt ½, then the universe is unbounded when
  the universe expands at a decreasing rate, but
  never stops.
• If q0 gt ½, then the universe is bounded and will
  eventually collapse in on itself ending in a big
  crunch.

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Guiding Questions

• What does the darkness of the night sky tell us
  about the nature of the universe?
• As the universe expands, what, if anything, is it
  expanding into?
• Where did the Big Bang take place?
• How do we know that the Big Bang was hot?
• What was the universe like during its first
  300,00 years?
• How is it possible to measure how the universe is
  curved?
• What is dark energy?
• Has the universe always expanded at the same
  rate?
• What will happen if the universe keeps expanding
  forever?