Cosmology: The Origin and Evolution of the Universe

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Cosmology The Origin and Evolution of the
Universe
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• The universe shows structure at many scales
• subatomic particles
• atoms
• stars and planets
• star clusters and galaxies
• galactic cluster and superclusters
• voids and sheets

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Does this structure ever end?

• While the data is not totally conclusive, it
  appears that on the scale of greater than 200 Mpc
  the structure does ends in other words becomes
  homogenous and isotropic.

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Cosmological Assumptions

• Homogeneous
•    Every region is like every other region
•    This is true over large regions (300 Mpc and
  more)
• Isotropic
•    The Universe has the same properties, no
  matter what direction you look
•    Together these requirements make up the
  cosmological principle
•    This basic assumption is needed to get started

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• Something is isotropic at a particular point if
  it looks the same in all directions when you
  stand at that point. On the largest scales, the
  universe is thought to be isotropic at every point

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The Expanding Universe

• Olbers's Paradox
• The observation that the sky is dark at night
  contrasted with a simple argument that shows that
  the sky should be uniformly bright.
• This haunted Kepler as long ago as 1610.
• 1823 - Heinrich Olbers proposed paradox

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Argument

• Assume universe is infinite and stars are
  randomly scattered.
• Isaac Newton argued that no other assumption made
  sense
• Then - in every direction you will eventually
  come to a star and the sky will be glowing

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Resolution of the paradox

• Stars are moving away so light is red-shift and
  not as bright.
• The universe is not infinitely old - so some
  light hasn't had time to reach us.

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The Hubble law describes the continuing expansion
of space
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The universe is expanding
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The redshifts that we see from distant galaxies
are caused by this expansion, not by the motions
of galaxies through space
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The redshift of a distant galaxy is a measure of
the scale of the universe at the time the galaxy
emitted its light
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The darkness of the night sky tells us aboutthe
nature of the universe

• The Cosmological Principle Cosmological theories
  are based on the idea that on large scales, the
  universe looks the same at all locations and in
  every direction
• It is meaningless to speak of an edge or center
  to the universe or of what lies beyond the
  universe

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The Big Bang

• Explains the features of the universe
• Present data favors this model of cosmology
• Follows Einstein's General Theory of Relativity

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• Early on the universe was compressed to infinite
  density and it has been expanding ever since.
• Was it compressed to a point?
• could it be infinite in size still?
• Hubble's Law tells us this happened about 15
  billion years ago.

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Supporting Data for the Big Bang Theory

• Hubble's Law
• As the universe expands, galaxies move apart
• Quasars (i.e. the universe appears to have an
  early history)
• Cosmic microwave background
• We can reinterpret the cosmological redshift
• As the universe expands, the wavelength of
  radiation is stretched
• Helium abundance (cosmic microwave background)

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Exactly what is expanding?

• The Universe is not expanding into empty space
• This idea comes from watching explosions
• Instead, space-time itself is expanding and
  carrying galaxies along with it
• Another question are we really at the center of
  the expansion?
• critical density is about 5 hydrogen atoms per
  cubic meter, averaged over the entire universe,
• or 10-26 kg/m3 not much!

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The observable universe extends about 14 billion
light-years in every direction from the Earth

• We cannot see objects beyond this distance
  because light from these objects has not had
  enough time to reach us

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Discovery of Cosmic Microwave Background

• 1965 - Penzias and Wilson
• Bell Labs, New Jersey
• radio telescope and receiver (7 cm wavelength
  receiver)

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Tried to fix static problem

• Tried to eliminate all static
• fixed loose joints
• repaired faulty connections
• removed nesting pigeons and "sticky white
  deposits"

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• Still there remained an annoying low level static
  
• coming from all directions
• all times of day
• through all the seasons

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WHAT WAS IT?

• Blackbody Radiation
• All objects emit radiation with the wavelengths
  characteristic of their temperature

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WHAT WAS IT?

• The Cosmic Microwave Background
• Penzias and Wilson soon learned that this was
  predicted by Big-Bang.

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Temperature vs Wavelength
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• The 100,000 year old universe was a 3,000 K
  blackbody.
• peak wavelength 1 micrometer 10-6m
• As we look across space we see it glowing
• we see 1 millimeter 10-3 meters
• The shift of the spectrum of the cosmic microwave
  background from 3,000K to 3K is an enormous
  red-shift (cosmological red shift)
• This "light", which we detect on earth now comes
  from the very early days of the universe
• (Age100,000 years or about 15 billion years ago)

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The microwave radiation that fills all space is
evidence of a hot Big Bang
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The background radiation was hotter and more
intense in the past

• The cosmic microwave background radiation,
  corresponding to radiation from a blackbody at a
  temperature of nearly 3 K, is the greatly
  redshifted remnant of the hot universe as it
  existed about 380,000 years after the Big Bang
• During the first 380,000 years of the universe,
  radiation and matter formed an opaque plasma
  called the primordial fireball

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• When the temperature of the radiation fell below
  3000 K, protons and electrons could combine to
  form hydrogen atoms and the universe became
  transparent

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The abundance of helium in the universe is
explained by the high temperatures in its early
history
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History
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Steady-State Theory

• The steady state theory pre-dates the Big Bang
• Consistent with Hubble's Law
• Assumption -
• universe is homogeneous, isotropic, and
  unchanging in time
• New matter is continuously created as universe
  expands to keep universe unchanged

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BB/SST

• Both BB and ST assume a homogeneous, isotropic
  Universe
• (i) no edge
• (ii) no centre
• (iii) every part looks the same (on average, i.e.
  over a large enough volume)

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Big Bang assumes also

• (i) initial 'singularity' (beginning)
• (ii) expansion
• (iii) evolution
• (iv) cooling, transformation of energy into
  matter, formation of structure, all through
  normal physical processes.

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Steady State also

• (i) expansion (in current theory original
  theory did not!)
• (ii) no evolution
• (iii) no beginning
• (iv) matter (Hydrogen) spontaneously created "in
  between" Galaxies
• (v) all radiation is from stars

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How much matter must be created?

• 1 hydrogen atom/cubic cm every 1015 years
• (1,000,000,000,000,000 years)
• or 1,000 atoms per year in the Astrodome (cannot
  be detected)
• not much

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• However, this theory is not in favor since Big
  Bang explains so much and Steady State Theory
  cannot explain the cosmic microwave background
  nor how matter or energy is created.

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• Einstein found that he couldn't allow universe to
  be static in his theory
• it either expands or contracts
• Einstein was unhappy with these two and invented
  a fudge factor (the cosmological constant) to
  force a static universe
• Basically the force of expansion is equaled by
  the gravitational force of the mass

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Hubble later tells us it's expanding

• 3 Possibilities in Einstein's General Theory of
  Relativity
• infinite and unbounded - open
• infinite and unbounded - flat
• finite - closed (need not be bounded)

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If ?0 is greater than ?c, the density parameter
O0 has a value greater than 1, the universe is
closed, and space is spherical (with positive
curvature)
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If ?0 is equal to ?c, the density parameter O0 is
equal to 1 and space is flat (with zero curvature)
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If ?0 is less than ?c, the density parameter O0
has a value less than 1, the universe is open,
and space is hyperbolic (with negative curvature)
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Future of the Universe

• Major Question - Will it continue to expand or
  will it collapse to big crunch?
• Open, closed, or flat?

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Same Question -

• Is there enough gravity (or matter) to bend
  universe back in on itself
• to "close" the universe
• the amount of mass needed to close the universe
  is call the critical density

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• Need to Determine Average Mass in a Given Volume
  (Density)
• Finding all mass is tricky because of the Dark
  Matter
• Neutrino Mass - very many neutrinos in universe
  so very small mass for each would add up to a lot
  open or closed??

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The expanding universe emerged from a cataclysmic
event called the Big Bang

• The universe began as an infinitely dense cosmic
  singularity which began its expansion in the
  event called the Big Bang, which can be described
  as the beginning of time
• During the first 1043 second after the Big Bang,
  the universe was too dense to be described by the
  known laws of physics

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The shape of the universe indicates its
matterand energy content

• The curvature of the universe as a whole depends
  on how the combined average mass density ?0
  compares to a critical density ?c

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Observations of temperature variations in the
cosmic microwave background indicate that the
universe is flat or nearly so, with a combined
average mass density equal to the critical density
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Observations of distant supernovae reveal that
welive in an accelerating universe

• Observations of galaxy clusters suggest that the
  average density of matter in the universe is
  about 0.27 of the critical density
• The remaining contribution to the average density
  is called dark energy
• Measurements of Type Ia supernovae in distant
  galaxies show that the expansion of the universe
  is speeding up
• This may be due to the presence of dark energy in
  the form of a cosmological constant, which
  provides a pressure that pushes the universe
  outward

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Key Words

• average density of matter
• Big Bang
• closed universe
• combined average mass density
• compression
• cosmic background radiation
• cosmic microwave background
• cosmic light horizon
• cosmic singularity
• cosmological constant
• cosmological principle
• cosmological redshift
• cosmology
• critical density
• dark energy
• dark energy density parameter
• dark-energy-dominated universe
• density parameter
• era of recombination

• homogeneous
• hyperbolic space
• isotropic
• lookback time
• mass density of radiation
• matter density parameter
• matter-dominated universe
• negative curvature
• observable universe
• Olberss paradox
• open universe
• Planck time
• plasma
• positive curvature
• primordial fireball
• radiation-dominated universe
• rarefaction
• relativistic cosmology
• spherical space

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• If we accept 14 billion years as the age of the
  Universe, then we can only see 14 billion light
  years into space
• Light from anything more distant hasnt had time
  to reach us yet
• There is a sphere around us with a 14 billion
  light year radius beyond which we cant see
• This sphere is called the cosmic particle horizon
  
• The observable Universe is located inside this
  sphere