Cosmic Microwave Background Radiation (CMBR)

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Cosmic Microwave Background Radiation (CMBR)

• Relic of the Big Bang (afterglow of initial
  fireball) predicted in late 1940s
• Discovered by Penzias Wilson in 1965 they won
  the Nobel Prize for this discovery
• CMBR studied in detail by satellites (COBE, WMAP)
• Radiation comes from era of decoupling of matter
  and radiation in the early Universe (300,000
  years old) when neutral H atoms first formed

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Observations of the CMBR

• CMBR very smooth photons from different
  directions have the same properties
• Earths motion with respect to the CMBR is
  detectable one half of sky hotter by one part
  in 1000
• Satellite observations detected tiny fluctuations
  in CMBR (1 part in 100,000) that represent seeds
  of density fluctuations from which galaxies arose

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igtclicker quiz 25

• Which of the following statements is FALSE?
• The Cosmic Microwave Background Radiation is
  considered to be strong evidence in favor of the
  Big Bang theory
• The Steady State theory is founded on the
  Cosmological Principle
• Most astronomers believe that the Universe
  satisfies the Cosmological Principle but not the
  Perfect Cosmological Principle
• The Steady State theory postulates that, as the
  Universe expands, matter in the form of galaxies
  is created out of the energy in the C-field
  (Creation field)

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Major Epochs in the Early Universe

• tlt3x105 years Universe radiation dominated
• tgt3x105 years Universe matter dominated
• Why?
• Let R be the scale length of the Universe (the
• separation between your favorite pair of
  galaxies, say).
• Energy density of matter a 1/R3 since volume a R3
• Energy density of radiation a 1/R4 since ?
  stretched
• out a R. By Wien's Law, T decreases as
  1/R, and by the
• blackbody eqn. energy density decreases as
  T4 a 1/R4

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Unification of Forces

• All four fundamental forces of Nature unified at
  tlt10-43 s, the Planck time.
• Gravity froze' out separate from the other three
  forces at this time.
• Next the strong nuclear force froze out at
  t10-35 s
• Weak and electromagnetic forces unified until
  t10-12 s
• Electroweak unification confirmed in the
  laboratory during the 1980s at CERN particle
  accelerator in Europe.

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igtclicker quiz 26

• Which of the following statements is FALSE?
• Most of the mass in the Milky Way galaxy is
  thought to be in the form of non-luminous dark
  matter whose exact nature is unknown
• Albert Einstein introduced the Cosmological
  constant ? to obtain a static solution to the
  field equations of General Relativity because he
  believed at that time that the Universe was not
  expanding
• Hubbles law v H0d indicates that the
  Universe is expanding uniformly
• Edwin Hubble discovered that the expansion rate
  of the Universe is speeding up because of a
  repulsive anti-gravity force associated with
  dark energy

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Baryon Asymmetry

• Extremely hot radiation in the few seconds after
  the Big Bang
• Very energetic photons ? continuous interchange
  of radiation into matter and vice versa (via pair
  production and pair annihilation).
• Observable Universe is made up of mostly matter
  (as opposed to anti-matter)
• Implies a slight asymmetry between matter and
  anti-matter in the very early Universe (a little
  more matter than antimatter)
• This is referred to as the baryon asymmetry' of
  the Universe

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Confinement and Recombination

• Quarks are the basic particles that protons and
  neutrons are
• thought to be composed of.
• t10-6 sec (T1013 K), quarks were able to
  combine to form protons and neutrons ? the epoch
  of confinement.
• After t 3x105 years the temperature dropped to
  T3000 K
• Protons and electrons (and neutrons) were able to
  combine to form neutral atoms.
• Matter and radiation practically ceased to
  interact with each other (i.e., the Universe
  became transparent to radiation ? CMBR).
• The epoch of decoupling of matter and radiation
  or the epoch of recombination.

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igtclicker quiz 27

• Which of the following statements is FALSE?
• Black holes emit Hawking radiation which can be
  characterized as thermal radiation and it is
  related to pair-production/annihilation from
  vacuum energy near the Schwarzschild radius or
  event horizon
• The event horizon or Schwarzschild radius of a
  black hole, the region over which it is capable
  of trapping light (radiation), is proportional to
  the black holes mass
• A black hole has no hair is a statement that
  describes the loss of identity of matter when it
  is swallowed by a black hole
• The gravitational field of a black hole extends
  only out to its event horizon or Schwarzschild
  radius

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Big Bang Nucleosynthesis

• Almost all the hydrogen we see in the present
  Universe was formed at the epoch of recombination
  
• Most of the light elements (helium, deuterium,
  lithium, etc.) were formed shortly thereafter
• The efficiency with which these light elements
  were formed depends on what the density of
  protons and neutrons was (baryonic matter).
• Studying the abundance of light elements
  (relative to hydrogen) is a good way of
  determining the baryon content of the Universe.
• There is a fairly strong indication that most of
  the matter in the Universe is non-baryonic, in
  addition to being non-luminous.

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The CMBR Horizon Problem

• The CMBR has the same properties in all
  directions.
• Consider two portions of the Universe from
  opposite ends of the sky.
• These two portions are within our observable
  Universe (horizon), but they are outside each
  other's horizons.
• Light has not yet had time to travel from one of
  these portions to the other.
• If they have never been in communication, how do
  they know to be at the same temperature?

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Inflation

• Very early phase of extremely rapid expansion
  (Guth, Linde, 1980s).
• During this inflationary phase, the Universe
  expands by a factor of 1050 in the time span t
  10-35 sec to t 10-24 sec.
• Inflationary phase is immediately after the epoch
  at which the strong nuclear force froze out, and
  before the weak nuclear force and electromagnetic
  force froze apart from each other.
• All of our observable Universe was an
  infinitesimally small volume 1050x1050x1050
  10150 times smaller than we would have guessed
  from a simple extrapolation of the expansion we
  observe today.

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Solving the Horizon Problem

• Two parts of the Universe on opposite sides of
  the sky now outside each other's horizons.
• Prior to inflationary epoch, these two patches
  would have been within each other's horizons and
  therefore known' to acquire the same
  temperature.
• Inflation caused them to expand out of each
  other's horizon.
• Inflation requires the universe to expand faster
  than the speed of light.
• Does not violate relativity STR only applies in
  flat spacetime (i.e., in weak gravitational
  fields).
• Special relativity is a special case of General
  relativity inflation does obey the equations of
  General relativity.

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Inflation, continued

• Why is the density of the present Universe so
  close to
• critical (or why is the geometry of the
  observable Universe so close to flat)?
• The scale of the observable Universe is much
  smaller than its radius of curvature'.
• What causes the rapid expansion during the
  inflationary era?
• Inflation may be thought of as a phase transition
  in the Universe (as in a transition from a liquid
  to solid phase).
• The latent heat' in this phase transition builds
  up into an extremely high vacuum energy density,
  and this drives the expansion (analogous to the
  repulsive effect of Einstein's
• cosmological constant ?).

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igtclicker quiz 28

• Which of the following statements is TRUE?
• Most of the mass in the Milky Way galaxy is
  thought to be in the form of ordinary luminous
  stars
• Albert Einstein introduced the Cosmological
  constant ? to obtain a static solution to the
  field equations of General Relativity because he
  believed at that time that the Universe was not
  expanding
• Hubbles law v H0d indicates that the
  Universes expansion is accelerating
• Edwin Hubble discovered that the expansion rate
  of the Universe is slowing down because of
  gravitational forces between galaxies

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igtclicker quiz 29

• Which of the following statements about the EPOCH
  OF CONFINEMENT is TRUE?
• At this instant, quarks became bound in sets of
  three to produce protons and neutrons, while
  matter and radiation continued to interact
  strongly
• The Universe was matter dominated at this epoch
• Protons and electrons formed stable hydrogen
  atoms for the first time at this epoch, and the
  matter in the Universe became mostly transparent
  to radiation
• This epoch was immediately followed by Inflation

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igtclicker quiz 30

• Which of the following statements is TRUE?
• Inflation ended right at Planck time when the
  Universe was 10-50 seconds old
• Gravity was the last of the four fundamental
  forces of Nature to freeze out (i.e. develop a
  unique identity)
• Radiation dominated over matter at very early
  times in the Universes history, but matter has
  dominated over radiation ever since the
  Universes 300,000th birthday
• Most of the elements in the periodic table that
  we learn about in school/college today were
  synthesized in the early Universe and this is
  known as Big Bang Nucleosynthesis

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Solar system 9 light hours diameter
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Spiral galaxy 80,000 light years diameter
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Coma cluster of galaxies 2.5 million light years
across
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Survey of distant galaxies 5 to 9 billion
light-years away
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Cosmic Microwave Background Radiation (after-glow
from the Big Bang) - edge of the observable
Universe 14 billion light years away
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The Detailed Structure of a Spiral Galaxy
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igtclicker quiz 31

• Which of the following statements about INFLATION
  is FALSE?
• The Universe expands faster than the speed of
  light during Inflation
• The rapid expansion during Inflation naturally
  explains why spacetime appears to be flat in
  terms of its geometric properties
• Inflation solves the CMBR horizon problem
• Inflation violates the principles of Einsteins
  General Theory of Relativity
• The theory of Inflation predicted that there
  should be fluctuations at the level of 1 part in
  105 in the CMBR

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igtclicker quiz 32

• Which of the following statements is FALSE?
• The Michelson-Morley experiment established that
  light is bent in a strong gravitational field
• Einstein received the Nobel Prize in Physics for
  explaining the photo-electric effect and the
  particle (photon) nature of light
• Keplers laws can be used to describe the motion
  of planets around the Sun
• Hubble is credited with the discovery of the
  expansion of the Universe
• Guth and Linde formulated the theory of Inflation

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Introduction to Galaxies

• Basic Structure
• How densely packed are stars in a galaxy?
• ? Size (diameter) of a typical star
  106 km
• ? Distance between stars 1 pc 3 x
  1013 km
• ? Analogy 1 cm sized marbles
  separated by 300 km!
• What fills in the space between stars?
• ? Interstellar medium gas, dust

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Disk Galaxies Structural Components

• Flattened differentially-rotating disk
• Dense centrally-concentrated bulge with mostly
  disordered orbits
• Extended, not centrally concentrated, mostly dark
  halo
• Bulge Halo Spheroid

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Spiral Galaxy Properties

• Bulge stars are older on average than disk stars
• Youngest disk stars lie in very thin plane
• Older disk stars lie in a thicker disk
• Disk stars, particularly young ones, are
  organized into spiral arms
• Spiral density waves in the disk the most
  successful explanation of spiral structure

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Globular Clusters
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Globular Clusters

• Most galaxies, including our own, contain dense
  clusters of 103 106 stars known as globular
  clusters
• The observed
• distribution of
• globular clusters
• tells us that the
• Sun is NOT at
• the center of the
• Milky Way
• galaxy

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Galaxy Types

• Spirals irregulars (disk galaxies) ellipticals
• Morphological (structural) features
• Disk, bulge, bulgedisk, presence/absence of
  central bar
• Nature of kinematics (internal motion of stars
  and gas)
• Coherent rotation of stars and gas in a disk
  differential rotation
• Random motion of stars in the bulge of a
  spiral galaxy or elliptical

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Hubble Sequence of Galaxies

• Tuning fork diagram
• E0-E7, S0
• Sa-Sd / SBa-SBd, Irr
• Morphological trends
• along the sequence
• Shape (flattening)
• Bulge-to-disk ratio
• Spiral arms
• Kinematical trends along the sequence
• Ellipticals mostly random motion,
  hardly any rotation
• Spirals mostly rotation, hardly any
  random motion
• Trends in the stellar mix
• Ellipticals mostly cool (old) stars
• Spirals dominated by hot (young) stars