Olbers paradox

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Olbers paradox

• Why is the sky dark at night?
• Of course, the Suns gone down! But more
  careful consideration of this simple fact led
  early astronomers to get the first constraints on
  cosmological models

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Olbers paradox

• Why
• If the universe were infinite in size
  contained an infinite number of stars that live
  forever, then every line-of-sight would
  eventually lead to a star
• Stars light dims as 1/r2 but the volume of space
  sampled increases by the same factor
• So, the night sky should be as bright everywhere
  as the average surface of a star

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Olbers paradox

• How can it be dark then?
• Pondered as early as Kepler and Newton.
• Newton wanted the universe to be infinite to
  avoid collapse under his theory of gravity
• One of the scientists associated with discussing
  the puzzle was Heinrich Olbers, and his name
  remained associated with Olbers paradox
• Some people suggested the distant stars light to
  be absorbed and thus diminished before reaching
  us
• No good - why?

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Olbers paradox

• Any material which absorbed the starlight should
  heat up and re-emit it, we would see this gas
  glowing !
• Flaw in arguments was assumption stellar
  lifetimes are infinite
• In fact, if we look far enough we look back to a
  time when no stars existed
• Further, an universe with finite age or which is
  expanding has a limit to how far we can see,
  light has to have had time to reach us

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Olbers paradox

• Anyway, number of stars is too small, and stellar
  lifetimes to short to fill space with light
• The darkness of the night sky rules out the
  simplest idea that the universe is infinite and
  filled with unchanging stars

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The Cosmic Microwave Background (CMB)

• However, there is more radiation filling the
  universe than that from stars
• Something called the Cosmic Background Radiation
  (CBR) fills the sky in all directions at
  wavelengths too long for our eyes to see

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The Cosmic Microwave Background (CMB)

• The expanding universe does tell us something
  about this new version of Olbers paradox
• The expansion has caused CBR to redshift to
  longer wavelengths from its original energy!
• The cosmos must have once been ablaze with this
  radiation-this is the radiation predicted to have
  been produced in the Big Bang!

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The Cosmic Microwave Background (CMB)

• Observational discovery of the CMB
• The Big Bang model
• What can we learn from the CMB?

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THE OBSERVATIONAL DISCOVERY OF THE COSMIC
MICROWAVE BACKGROUND
1964 Penzias Wilson (Bell-Labs) and antenna
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CBR

• Arno Penzias Robert Wilson (1964)
• Attempted to study radio emission from our Galaxy
  using sensitive antenna built at Bell-Labs
• Needed to characterize and eliminate all sources
  of noise
• They never could get rid of a certain noise
  source noise had a characteristic temperature of
  about 3 K
• They figured out that the noise was coming from
  the sky, and was approximately the same in all
  directions

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THE HOT BIG BANG MODEL

• Penzias Wilson had discovered radiation left
  over from the early universe
• The big bang model
• Independently developed by James Peebles and
  George Gamov
• They suggested that the universe started off in
  an extremely hot state
• As the universe expands, the energy within the
  universe is spread over in increasing volume of
  space
• Thus the Universe cools down as it expands

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CBR

• Why did they suggest this model?
• If the early universe was hot (full of energy), a
  lot of features of the current universe could be
  explained
• Could explain where the matter that we see around
  us came from (H, He, Li created by the big bang)
• Could explain the observed ratio of elements
  (nucleosynthesis occurred within first few
  minutes)
• Predicted that there should be left over
  radiation in the present universe

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A brief look at the stages of the Universes life

• We will discuss this diagram in detail in future
  classes.
• Most crude description
• t0 The Big Bang
• For first 380,000ys, universe is an expanding
  soup of tightly coupled radiation matter
• After 380,000yrs, radiation matter decouple.
  Radiation field reduced enough (due to expansion)
  that protons can now capture electrons hang on
  to them
• Left over radiation that we see known as the CMB

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THE COSMIC BACKGROUND EXPLORER (COBE)
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COBE

• The COBE mission
• Built by NASA-Goddard Space Flight Center
• Launched Nov. 1989
• Purpose was to survey infra-red microwave
  emission across the whole sky (BB predicts 3 K
  black body)
• Primary purpose to characterize the CMB
• Had a number of instruments on it
• FIRAS (Fair infra-red absolute spectrophotometer)
• DMR (Differential Microwave Radiometer)
• DIRBE (Diffuse Infrared background Experiment)

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The CMB DMR map of the microwave sky
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COBE Results

• Map of the microwave sky (frequency of 50GHz)
• Were looking at the CMB
• The map is very uniform.
• Means that the CMB is extremely isotropic (i.e.
  the same in every direction we look)
• Supports the idea that the universe is isotropic
  (one of the basic cosmological principles).
• In fact, if we measure the universe to be
  isotropic, and were not located at a special
  place in the Universe, we can also deduce that
  the Universe is homogeneous!

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The spectrum of the CMB (FIRAS)
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CBR-Spectrum

• Spectrum has precisely the shape predicted by the
  theory
• So-called Blackbody spectrum
• Characteristic temperature of 2.728K

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Subtract off average level
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CBR

• What causes this pattern of redshift and
  blueshift?

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CBR

• What causes this pattern of redshift and
  blueshift?
• Earths motion through space causes this dipole
  (anisotropy with 2 well-defined/opposite points)
• (Earth orbits Sun at 30 km/s, Sun orbits MW at
  220 km/s, MW has motion around center of local
  group etc)

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Subtract the dipole
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CBR

• Subtract off the dipole resulting from the
  Earths motion
• Are left with
• Bright ridge corresponding to microwave emission
  from our Galaxy
• Pattern of random fluctuations in the CMB

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Subtract off the emission from our Galaxy
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CBR fluctuations

• Can use the different spectrum of the Galaxys
  emission and the CMB to distinguish them.
• So, can subtract off the emission from our
  Galaxy
• Left with a random pattern of fluctuations in the
  CMB correspond to temperature differences of 30
  millionths of a Kelvin

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CBR fluctuations

• What are these fluctuations
• The early universe was very close to being
  perfectly homogeneous
• But, there were small deviations from
  homogeneity some regions were a tiny bit colder
  and some were a tiny bit hotter.
• When matter and radiation decoupled, this pattern
  of fluctuations was frozen into the radiation
  field.
• We see this nowadays as fluctuations in the CMB.

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CBR Fluctuations

• Why are the fluctuations important?
• Before decoupling, fluctuations in the radiation
  field also meant fluctuations in the mass density
• After decoupling, these small fluctuations in
  density can get amplified (slightly dense regions
  get denser and denser due to gravity).
• These growing fluctuations eventually collapse to
  give galaxies and galaxy clusters.
• So, by studying these fluctuations, we are
  looking at the seeds that grow to become
  galaxies, stars, planets

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Microwave Anisotropy Probe (MAP)
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Microwave Anisotropy Probe (MAP)

• NASA mission to map out the fluctuations in the
  CMB in fine detail
• Will characterize these seeds for structure
  formation
• Will determine fine detail of the CMB
  fluctuations that depend upon the curvature of
  space (k) and ?.
• Launched last year

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New WMAP results - Tues Feb 11
A NASA satellite has captured the sharpest-ever
picture of the afterglow of the big bang Dates
universe to 13.7 billion years.
NASA's Wilkinson Microwave Anisotropy Probe (WMAP)
Patterns in the big bang afterglow were frozen in
place only 380,000 years after the big bang, a
number nailed down by this latest observation.
These patterns are tiny temperature differences
within this extraordinarily evenly dispersed
microwave light bathing the universe, which now
averages a frigid 2.73 degrees above absolute
zero temperature. WMAP resolves slight
temperature fluctuations, which vary by only
millionths of a degree. Theories about the
evolution of the universe make specific
predictions about the extent of these temperature
patterns