An Introduction to Astronomy Part XIV: Cosmology

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An Introduction to AstronomyPart XIV Cosmology

• Lambert E. Murray, Ph.D.
• Professor of Physics

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Cosmology

• Cosmology is the study of the structure and
  evolution of the Universe on its grandest scale.
  In the study of cosmology we wish to answer
  question like
• What do our observations tell us, if anything,
  about the size and geometry of the Universe?
• What do our observations tell us, if anything,
  about how the Universe came into existence?
• What do our observations tell us, if anything,
  about the future of the universe?

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What Have we Learned about the Nature of the
Universe so far?

• There seems to be an infinite number of galaxies
  (of various types). No matter the direction you
  look, as we build bigger and bigger telescopes,
  you see more and more galaxies.

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They are Everywhere
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Galactic Clusters and Superclusters

• There are enormous distances between galaxies,
  but as we look deeper and deeper into our
  Universe we find that galaxies are grouped
  together by gravitational attraction.
• If we plot the location of galaxies, we begin to
  see large-scale structure within the Universe.
• We find that although there are small scale
  fluctuations within the universe we can think
  of the universe as rather homogeneous and
  isotropic on the very largest scale.

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Structure Within the Universe
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Computer Modelof Universe
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What Else Have we Learned about the Nature of the
Universe so far?

• Hubbles work indicates that the farther away a
  galaxy is, the faster it moves away from us.
  This is true in every direction we look.

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The Hubble Law
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Are we in a Special Place?The Cosmological
Principle

• Since we see a large number of galaxies in all
  directions, and these are all moving away from us
  are we at the center of the universe (a very
  special place)?
• A fundamental assumption in the study of
  cosmology is that we are not located in a unique
  region within the universe this is the
  Copernican Principle.
• In addition, we assume that the universe at the
  largest scale is isotropic and homogeneous, i.e.,
  that the universe looks the same in all
  directions and that this is true no matter where
  you are. This is the cosmological principle.
• This principle assumes that our location within
  the universe is characteristic of any other
  location within the universe and is the only
  assumption that will allow us to make any
  progress in understanding our Universe as a whole.

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An Expanding Universe

• If we are not located at a special spot in the
  Universe, and
• If we would expect to obtain the same Hubble plot
  of the recessional speeds of galaxies from any
  arbitrary location within the Universe,
• This means that the distance between any two
  galaxies (on average) is increasing all over the
  Universe.
• Thus, the Universe is expanding like a raisin
  cake or an expanding balloon.

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Is the Universe Infinite?Olbers Paradox

• Olbers paradox If our universe is infinite in
  time and space, and if we see galaxies wherever
  we look, why is the sky dark?
• Although the light intensity drops off as 1/R2,
  the number of stars you can see in an infinite
  universe increases like R2 so these two effects
  cancel.
• Likewise, if the universe is filled with dark
  material which would absorb the light, that
  material would heat up and eventually glow
  especially if there were an infinite number of
  stars.
• Conclusion The universe must not be infinite in
  space and time!

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The Big Bang

• Initially many astronomers believed that the
  Universe was static and infinite. Olbers
  paradox raised serious questions about this
  assumption.
• Hubbles observations of an expanding Universe
  seems to indicate that the Universe had a
  beginning in time and perhaps in space.

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The Hubble Age of The Universe

• Hubbles Law can be written as velocity of
  recession (constant) x distance
• the constant is Hubbles Constant, H0
• We believe that Hubbles constant is constant
  over all the universe at any instant of time, but
  it may actually change over time.
• Running the expansion backwards
• Since H0 velocity/distance,
• 1/H0 distance/velocity time
• Thus, 1/H0 is a measure of the age of the
  universe.
• Using a value for H0 of 50 km/sec/Mpc (kilometers
  per second per megaparsec) leads to an age of
  about 19.7 x 1010 years, or roughly 20 billion
  years, while using a value of 100 km/sec/Mpc
  leads to an age of only about 10 billion years.

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A Correction to the Hubble Age of the Universe

• Most astronomers believe that Hubbles constant
  has changed with time. Gravity should be
  slowing the expansion of the Universe, causing
  a deceleration.
• Thus, the initial expansion should have been
  faster, leading to a younger universe one with
  an age of about 2/3 of the age determined from
  the present value of Hubbles constant.
• Astronomers are using several different
  techniques to determine an accurate value of
  Hubbles constant so that we can get a handle of
  the actual age of the Universe.

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Measurements of Hubbles Constant

• Hubbles initial measurements of H0 was 550
  km/sec/Mpc, but by the 1990s the most frequently
  quoted values were 50 70 km/sec/Mpc.
• Hubbles measurements did not take into account
  several effects now known to astronomers that
  would have influenced his measurements.
• 2/3 of the Hubble age calculated from these
  values would give 9 13 billion years for the
  approximate age of the Universe.

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An Alternate Age Measurement

• An alternate method of determining the age of the
  Universe is to determine the age of objects
  within the Universe, since the Universe cannot be
  younger than the oldest objects in the Universe.
• We can get an estimate of the age of globular
  clusters based upon their HR diagrams observing
  their turning points.

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HR Diagram for M13
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An Age Crisis

• Before the mid 1990s, the oldest globular
  clusters were thought to be 14 17 billion years
  old, so the Universe must be older than this.
• If the age of the Universe is only 9 13 billion
  years according to Hubbles expansion constant,
  we clearly have an age crisis.

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Measuring the Change in the Expansion Rate of the
Universe

• Since the Hubble constant is probably not
  constant in time, a measure of the Hubble
  constant at any instant of time gives an
  incomplete picture of the situation.
• The answer to this problem would be to measure
  the Hubble constant for different times in the
  life of the Universe but how can be do that?
• By looking at the redshifts of galaxies at
  different distances (times) from us, we might be
  able to determine how the Hubble constant changes
  in time.

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Measuring the Change in the Expansion Rate of the
Universe II

• Measuring the redshift of nearby galaxies is
  problematic because gravitational attraction
  between the nearby galaxies induces peculiar
  motions of these galaxies.
• These problems should be less pronounced as we
  move further away (back in time), but the use of
  Cepheid variables as standard candles is limited
  to the nearer galaxies, where they can be
  resolved.
• In the 1990s it was discovered that Type Ia
  supernovae all have nearly the same peak
  luminosity, and these can be seen at distances
  thousands of times more distant than Cepheids.
• Measurement of these supernovae give the present
  preferred value of 65 km/sec/Mpc.

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Resolving the Age Crisis

• After corrections were made to the Hubble Space
  Telescope, measurements of H0 based primarily on
  Cepheid variable stars in 18 galaxies (1999) gave
  a value of 70 to 80 km/sec/Mpc with an
  uncertainty of 7 km/sec/Mpc.
• Type Ia supernovae measurements indicate a value
  for H0 of about 65 km/sec/Mpc, giving an age of
  the Universe of 15 billion years if there is no
  deceleration, or about 12 billion years with
  deceleration.
• Better measurements of the age of globular
  clusters now gives an age estimate of 11 14
  billion years for the oldest globular clusters.

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Additional Evidence for the Big Bang?

• Cosmic Microwave Background
• In the 1940s Russian physicist George Gamov
  worked out a theory for the creation of elements
  (a nucleosynthesis) in a big-bang type event.
• He predicted that copious amounts of radiation
  would be emitted from the hot gas of this Big
  Bang with a black-body spectrum of about 3000K
  when the Universe became transparent to photons.
  (Prior to this the photons were trapped by the
  non-transparent universe.)
• Later it was realized that this radiation would
  have been red-shifted over time and would look
  like emissions from a very cool gas.
• In the mid-60s Robert Dicke at Princeton began
  building a receiver to detect this radiation.

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Penzias Wilsons Microwave Experiment

• 1964-65 Bell Labs scientists Arno Penzias
  Robert Wilson were working on a microwave horn
  antenna for satellite communication.
• They were troubled by a persistent background
  noise. After trying everything to eliminate the
  background
• recalibrated antenna
• cooled their detectors
• removed nesting pigeons from horn
• cleaned antenna
• could not eliminate the static
• they found that the background persisted.

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A Cosmic Background?

• The peak in the background radiation from
  Penzias and Wilsons experiment occurred at 7.35
  cm (4080 MHz)
• They found that the background was constant
  regardless of time of day, season of year,
  direction in the sky, etc.
• When they learned of Dickes work, they realized
  that they might be seeing this residual radiation
  remnant from the Big-Bang now called the Cosmic
  Microwave Background Radiation.
• If a blackbody curve is assumed, the 7.35 cm peak
  corresponds to a background temperature of
  roughly 3.5 Kelvin.
• 1976 Penzias Wilson received the Nobel Prize
  for their work.

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Experimental Measurements of the Cosmic
Background Radiation

• Very little of the 3 K background radiation can
  penetrate the atmosphere
• This made it difficult to measure radiation curve
• Early attempts used balloons, rockets, aircraft,
  etc.
• In 1989 NASA launched the Cosmic Background
  Explorer (COBE) which orbited the Earth.
• Within two months it was determined that this
  background followed (within 1) a blackbody
  radiation curve corresponding to a temperature of
  2.735 Kelvin (within 0.06K).

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Blackbody Spectrum of COBE Satellite and fit to a
2.73 K Curve
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Variations in the Background Radiation Pattern

• COBE data is accurate enough to measure the
  motion of the earth relative to the background
  radiation by analyzing variations in the data to
  1 part in 10,000. This is shown in the next
  image.
• A more detailed analysis, with the dipole effect
  of the Earths motion subtracted out, shows small
  scale variations in the background radiation data.

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COBE Dipole Speeding Through the Universe
Credit NASA, COBE, DMR, Four-Year Sky Map
Astronomy Picture of the Day, February 5, 1996
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A map of the brightness of the cosmic microwave
background made by the cosmic background explorer
(COBE) satellite. Notice the patchiness of the
brightness. Each pink patch may represent a
"lump" of matter from which groups of galaxies
ultimately grew. The patches were approximately
one half billion light years across when they
emitted the radiation. (NASA GSFC and the COBE
Science Working Group.)
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Confidence in the Big Bang

• There seems little doubt that the Universe as we
  know it is expanding, and that it was the result
  of a Big Bang.
• Small fluctuations in the cosmic background
  radiation are consistent with our observations of
  the grouping of galaxies within the Universe.

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Recent Confirmation

• The excellent agreement between the current
  theories of cosmology and the most recent data
  from Cosmic Background Radiation (WMAP) indicate
  that
• The age of the Universe is 13.7 billion years to
  within 1.
• That the Universe is flat and that the visible
  universe makes up only 4 of the Universe.

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WMAP Composition of the Universe
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Theoretical Models for the Expansion of the
Universe

• Until recently, precise, unambiguous,
  experimental measurements of H0 with distance did
  not exist.
• Theory indicates that there are three basic
  models for our Universe
• Closed Universe (WM gt 1)
• Flat Universe (WM 1)
• Open Universe (WM lt 1)

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Diagram of Theoretical Models for the Expansion
of the Universe
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The Future in Different Models

• In a closed universe, the recessional speed of
  galaxies would decrease to zero and that universe
  would begin to collapse as gravity overcame the
  initial expansion from the Big Bang. This
  universe would eventually collapse completely
  or perhaps oscillate.
• In a flat universe, the recessional speed
  continues to decrease and approach zero only as
  time approaches infinity.
• In an open universe, the recessional speed
  decreases, but never reaches zero.

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A Surprise

• Recent measurements of large-z Type Ia Supernovae
  indicate that the recession rate follows none of
  the previous patterns, but actually are
  consistent with an accelerating universe which
  would require some sort of anti-gravity
  repulsive force which is now associated with
  Einsteins cosmological constant L.
• Many suggest that the source of this
  anti-gravity is a vacuum energy or dark
  energy due to quantum fluctuation.

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End of Part XIV