Astronomy 102' December 6, 2005'

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Astronomy 102.December 6, 2005.
Pigeon Trap Used by Penzias and Wilson Penzias
and Wilson thought the static their radio
antenna was picking up might be due to
droppings from pigeons roosting in the antenna
horn. They captured the pigeons with this trap
and cleaned out the horn, but the static
persisted. Lent by Robert Wilson
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Observational tests direct observation of the
big-bang.

• In the 1940s, George Gamows students, Ralph
  Alpher and Bob Herman, predicted that the blast
  from the big-bang should be detectable someday.
• Specifically light would be seen that arose at
  the time when the Universe had cooled to the
  point that atoms could form.
• The light started off visible, but owing to the
  great distance of its source it would be
  red-shifted into the microwave band (wavelengths
  of a millimeter to a few centimeters), and look
  like a black body with a temperature a few
  degrees Kelvin (above absolute zero).
• Since it was close to a singularity when emitted,
  the light should appear isotropic spread
  uniformly across the sky.

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Observational tests direct observation of the
big-bang.

• In 1965, Bob Wilson and Arno Penzias (ATT Bell
  Telephone Laboratories) were working on a very
  sensitive microwave receiver and antenna they
  built for satellite communication. They were
  trying to tune it up to reach ideal performance,
  but persistently found extra noise power for
  which they couldnt account. They knew nothing
  of Gamows prediction.
• The extra power was like that of a black body
  with temperature 2.7 K (2.7 degrees above
  absolute zero).
• It was the same no matter which direction they
  pointed their antenna.
• They were trying to find an explanation, when
  they were paid a visit by radio astronomer Bernie
  Burke, a professor at MIT.

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The discovery of the micro-wave background.

Image Bob Wilson (left) and Arno Penzias with
the horn antenna they used to discover the cosmic
microwave background.
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Observational tests direct observation of the
big bang.

• Burke knew of efforts at Princeton by Dicke and
  Peebles to build a sensitive microwave receiver
  and antenna to look for the big-bang radiation
  predicted by Gamow, but were having technical
  troubles. He introduced the Bell Labs group to
  the Princeton group.
• It was quickly verified that Penzias and Wilson
  had indeed detected that relict radiation (now
  called the Cosmic Microwave Background).
• Thus the blast from the big-bang was seen
  directly. This was the sturdiest nail in the
  coffin of the steady-state model.
• For this important discovery, Penzias and Wilson
  shared the 1978 Nobel Prize in Physics.

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Cosmic Microwave Backgroundfluctuations in the
early universe.

Microwave background is created when hydrogen
atoms form (about 400,000 years after the
big-bang.
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Sky map of the cosmic microwave background.

• COBE images of the entire sky at a wavelength of
  5.7 mm, with brightness expressed as the
  blackbody temperature (in K) that would produce
  the detected power (NASA/GSFC).
• Plotted on a linear scale the Universe looks very
  uniform (top figure where blue 0 K and red
  4.0 K).
• On a small scale (bottom figure where blue
  2.725 K and red 2.731 K) the Universe does not
  look uniform.

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Spectrum of the cosmic microwave background.

• COBE measurements of the background brightness as
  a function of wavelength (points), compared to
  that expected from a 2.728 K black body (solid
  curve).
• From Ned Wrights Cosmology Tutorial.

Brightness
1/l (cm-1)
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Cosmic Microwave BackgroundFluctuations in
early universe.

Observations by COBE have been confirmed by
BOOMERANG with an improved angular resolution
(factor of 35).
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History of the big-bang the expansion of the
Universe and decoupling.

• Time starts along with the expansion. At the
  singularity, like in a black hole, time does not
  exist, only the four-dimensional space of quantum
  foam, the result of the extreme mixture and
  warping of space-time.
• Therefore the question what existed before the
  big-bang? is meaningless for anyone living in
  the Universe there is no before, because there
  is no such thing as time at the singularity. One
  would have to be outside the universe to ask the
  question sensibly, and there seems to be no
  outside to the Universe, either.

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History of the big-bang the expansion of the
Universe and decoupling.

• As is the case for matter just about to form a
  black hole singularity, the Universe was
  extremely hot and dense shortly after the
  expansion (and time) began. As the expansion
  proceeded, the Universe cooled off.
• The temperature of the early Universe was too
  high for normal matter to exist as such. It
  needed to cool down in the expansion before the
  normal constituents of matter could condense from
  the high-energy soup and not be broken up
  immediately.

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History of the big-bang the expansion of the
Universe and decoupling.

• Early in the expansion, energy in the form of
  radiation was in equilibrium with all forms of
  matter and antimatter, continually producing all
  possible particle-antiparticle pairs, which would
  soon annihilate to produce radiation again.

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History of the big-bang the expansion of the
Universe and decoupling.

• As the temperature fell, the highest energies
  available in photons, gravitons and the like
  decreased therefore higher-energy
  particle-antiparticle pairs ceased to be created.
• When it became too cold for the most massive
  particle-antiparticle pairs to be produced, these
  pairs annihilated each other and turned back into
  photons.

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History of the big-bang the expansion of the
Universe and decoupling.

• However, it seemed that a slight asymmetry
  developed early on that left what we call the
  particles slightly outnumbering the
  anti-particles, so that not everything
  annihilated there was still some matter left
  over, as well as lots and lots of photons.

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History of the big-bang the expansion of the
Universe and decoupling.

• Combinations of particles, bound together by
  electromagnetic or nuclear forces, could also
  form in the early universe, but when the
  temperature was high enough, the combinations
  were immediately broken up by the photons.
  Examples

Quarks and gluons
Protons and neutrons and photons
Protons and neutrons
Atomic nuclei and photons
Nuclei and electrons
Atoms and photons
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History of the big-bang the expansion of the
Universe and decoupling.

• When the temperature got sufficiently low, the
  density of high-energy photons decreased
  significantly, and the particle combinations
  stopped being broken up by the photons.

Quarks and gluons
Protons and neutrons and photons
T lt 1012 K
Protons and neutrons
Atomic nuclei and photons
lt 106 K
Nuclei and electrons
Atoms and photons (Decoupling)
lt 4000 K
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Expansion of the Universe
Time
Us (t 1010 years)
Distance
1010 light years
Note means approximately equals.
Decoupling Atoms (t 2?105 years)
Protons, neutrons, nuclei (t 200 sec)
Electrons (t 1 sec)
See Silk, page 111.
Quarks (t 10-6 sec)
Big Bang
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Decoupling.
Proton
H atom
Electron
Photon

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Decoupling.

• Before decoupling, typical photons could destroy
  atoms, and so were coupled to matter in the sense
  that they were constantly being created and
  destroyed as atoms were being destroyed and
  created.
• Any photon trying to get out get absorbed and
  re-emitted many times on the way the Universe
  was opaque before decoupling.
• After decoupling, the average energy of the
  photons is insufficient to break up an atom.
• All the electrons and protons combined to form
  atoms and emitted photons, which then lead
  completely separate lives.
• Now photons can travel without being absorbed and
  re-emitted constantly the Universe became
  transparent after decoupling.

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Decoupling.

• Light coming from the surface where decoupling
  occurred is the cosmic microwave background.
• Because its opaque before decoupling, we cannot
  see any closer to the singularity, using light.
  Neutrinos could be used to see deeper.
• However, because all particles experience a
  similar decoupling, nothing can be used to see
  the big-bang singularity itself.

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Appearance of the decoupling surface why is the
cosmic microwave background isotropic?

• Because it was emitted so close to a singularity
• Compare our situation to that of an observer
  inside a black hole. Light emitted within a black
  hole horizon cannot escape (and therefore must
  fall into the singularity), no matter what
  direction it is emitted all light paths end at
  the singularity.
• By the same token -- since light can travel in
  either direction along these paths -- light
  emitted from the surroundings of the singularity
  would seem to the observer within the horizon to
  arrive from all directions, rather than one
  particular direction. It would look as if the
  singularitys surroundings filled the sky.
• As we have seen, this is precisely the way the
  cosmic microwave background looks.

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Why is the cosmic micro-wave background isotropic?
Us (emitting light)
Paths of light through warped space
Singularity
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Why is the cosmic microwave background isotropic?
Us (looking at the sky)
Paths of light through warped space
Singularity
Decoupling surface
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Why does the spectrum of the cosmic microwave
background look like that?

• The universe before decoupling was opaque and had
  a nearly constant temperature of about 4000 K, so
  the decoupling surface looks like a 4000 K
  blackbody. Note opaque and constant
  temperature is the very definition of a
  blackbody.

Sun (6000 K)
Brightness
A 4000 K blackbody
10-4 cm
10-5 cm
10-3 cm
Wavelength
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Appearance of the decoupling surface.

• Why does the microwave background change as
  function of time?
• Because he decoupling surface lies so far in the
  past, it lies at a great distance.
• Because of its great distance and the Universes
  expansion, the decoupling surface appears to us
  to be greatly red-shifted. The velocity of the
  surface will move with a velocity given by
  Hubbles Law, V H0D.
• In the expansion, all distance intervals not
  ruled by local gravity grow in the same
  proportion. This means that the cosmic microwave
  backgrounds wavelengths will all be red-shifted
  the same way.
• Thus the spectrum of the cosmic microwave
  background should always look like a black body,
  at ever lower temperatures as the Universe
  expands. This is a strong prediction of all
  big-bang models. And so it does, as we have seen.

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Inflation the cosmic microwave background is
almost too isotropic.

• The results of the cosmic microwave background
  studies show that no part of the cosmic microwave
  background differs in brightness from the average
  by more than 0.001. It is hard to make the
  emission of the cosmic microwave background that
  smooth and uniform. Consider for example the
  surface of the sun with its sun spots.

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Inflation the cosmic microwave background is
almost too isotropic.

• To do so would usually require that all parts of
  the gas be interacting with each other strongly,
  or that the gas be well mixed.
• This would not seem possible for different parts
  of the decoupling surface. We were once part of
  that surface, and the parts of it that we see
  today have been out of contact with us (and each
  other) since the Big Bang, since were only now
  receiving light from these parts and no signal or
  interaction can travel faster than light.

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Inflation the cosmic microwave background is
almost too isotropic.

• One theoretically-popular way out of this problem
  is to postulate a brief period of inflation early
  in the Universes history. Briefly, this is
  thought to happen as follows.
• Shortly after the Big Bang, the vacuum could have
  had a much larger energy density, in the form of
  virtual pairs, than it does today. This
  possibility is allowed under certain theoretical
  models of numbers and interactions of elementary
  particles.
• At some time during the expansion, the vacuum
  underwent a phase transition (like freezing or
  condensing) to produce the lower-energy version
  we have today.

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Inflation the cosmic microwave background is
almost too isotropic.

• While the vacuum was in its high-energy-density
  state, it gave a large additional impulse to
  Universal expansion.
• Recall vacuum fluctuation energy density is
  actually negative in strongly curved space-time
  virtual pairs were exotic in the newborn
  Universe. Thus the vacuum acts
  anti-gravitationally early in the expansion.
• Accounting for the vacuums influence in general
  relativity leads to a very much smoother and
  faster expansion. During this period,
  space-times radius of curvature increases more
  like a bubble blowing up, than like a blast wave
  - hence the name inflation for the process.
• During inflation, the vacuum would appear in the
  field equations as a cosmological constant.

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Inflation the cosmic microwave background is
almost too isotropic.

• The inflationary era would have been relatively
  brief, much shorter than the time between Big
  Bang and decoupling.
• If it lasted through 100 doublings of the
  Universes size, that would do it, and this takes
  only about 10-35 seconds.
• During the remaining normal expansion between
  the end of inflation (decay of the vacuum to its
  low energy density state) and decoupling, the
  bumps and wiggles normally present in blast waves
  still wouldnt have had enough time to develop.
• We know of course that the Universe has become
  much less smooth since decoupling. The seeds for
  inhomogeneities like galaxies, stars and people
  were not sown before decoupling, however.

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Expansion of an inflationary Universe
Time
Us (t 1010 years)
Distance
1010 light years
Note means approximately equals.
Decoupling Atoms (t 2?105 years)
Protons, neutrons, nuclei (t 200 sec)
Electrons (t 1 sec)
Quarks (t 10-6 sec)
Inflation (first 10-35 sec)
Big Bang
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The age and fate of the Universe.

• The expanding Universe resembles the interior of
  a black hole. Is the Universe a black hole?
• That is, is the universe open, marginal, or
  closed? If its not open, it really can be
  thought of as a black hole.
• Related question how old is the Universe? That
  is, how long has it been since the expansion (and
  time) began?
• If the Universes total energy is
  matter-dominated (that is, if the cosmological
  constant is zero), the age, expansion rate,
  curvature and fate all turn out to be determined
  by one factor how much density (mass per unit
  volume) there is in the Universe.
• We usually illustrate this by general-relativistic
  calculation of the typical distance between
  galaxies as a function of time elapsed since the
  present day

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The age and fate of the Universe.
Here are some results of such calculations, for
matter-dominated universes with three different
present-day densities. Labels indicate
boundedness and the sign of the space-time
curvature.
Open,negative
Marginal,flat
Typical distance between galaxies, in units of
the present typical distance
Closed,positive
?
Region expanded on next page.
Time from present (years)
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The age and fate of the Universe.
Fate
Open Marginal Closed All matched to observed
expansion rate at present time.
Age
Typical distance between galaxies, in units of
the present typical distance
Time from present (years)
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How can we tell which universe is our Universe?

• Several ways are possible, all with substantial
  and different degrees of difficulty
• Measure the density directly, using observations
  of the motions of galaxies to determine how much
  gravity they experience. This is much like our
  way of measuring black-hole masses by seeing the
  orbital motion of companion stars.
• Measure the ages of the oldest objects in the
  Universe.
• Measure the Universes curvature directly, by
  observing very distant objects with
  well-determined size and distance.
• Measure the acceleration or deceleration of
  galaxies the rate of change of the Hubble
  constant.
• The first two ways are least difficult and
  provide most of our data. We will discuss this
  in more detail on Thursday.

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End-lecture break.

• Today we skipped the mid-lecture break since I
  need to distribute the course opinion
  questionnaires.
• Please complete this questionnaire and return the
  forms to one of our TAs who will deliver them to
  the Deans office.
• The course number for this course is 13860.