1446 Introductory Astronomy II

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1446 Introductory Astronomy II

• Chapter 18B
• Cosmology II
• R. S. Rubins
  Fall, 2009

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INTRODUCTION 1

• The universe was brought into being in a less
  than fully formed state, but was gifted with
  capacity to transform itself from unformed matter
  into a truly marvelous array of structure and
  life-forms.
• Augustine (5th Century)
• As in music and art, the most appealing patterns
  in the universe are neither completely regular,
  nor totally random.
• There are 92 different types of atoms in nature,
  far more than the three created in the Big Bang
  process, and these are found in complex
  organisms, stars and interstellar gas clouds.
• Temperatures range from tens of thousands of K in
  the cores of large stars to just 3 K above
  absolute zero in the CMB.
• After, Martin Rees, in Just Six Numbers (1999).

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Q Revisited

• The symbol Q 105, which represents the
  relative size of the temperature ripples in the
  3K cosmic microwave background, is a crucial
  parameter in the future development of the
  universe.
• If Q were appreciably larger, regions far larger
  than our galaxies would have formed early in the
  history of our galaxy.
• Stars would not have formed, and the galaxies
  would have collapsed into gigantic black holes.
• If Q were appreciably smaller, the formation of
  stars and galaxies, would have been much slower
  and less efficient.
• If Q were less than 106, the universe would have
  remained forever dark and featureless.
• After, Martin Rees, in Just Six Numbers (1999).

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Evidence for the Big Bang Story

• Following Hubbles redshift measurements, which
  lead to the Big Bang hypothesis, much more
  indirect evidence has been found.
• 1. The remarkably precise fit of the COBE data
  for the CMB to thermal (blackbody) radiation at
  2.73 K.
• 2. No object has been found with a helium
  concentration of less than 23, since there is no
  reverse process to the production of He from H.
• 3. The current distribution of galaxies is
  consistent with the temperature ripples in the 3K
  cosmic microwave background.
• 4. The abundances of deuterium and lithium
  nuclei produced in the Big Bang are in agreement
  with theoretical estimates.

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Chronology of the Universe 1

• Big Bang (t 0)
• At the instant of the big bang, it is thought
  that
• i. mass, energy, space and time came into
  existence
• ii. the universe was a point (or
  singularity).
• Since time came into existence only after the
  Big Bang, the concept of before the Big Bang is
  meaningless.
• The Planck Epoch ( t lt 10?43 s)
• All the forces of today were unified as a
  single force, but at the Planck instant (10?43
  s), the gravitational force separated from the
  other fundamental forces.
• Heavy particles (quarks) and light particles
  (electrons) existed on an equal footing.

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Chronology of the Universe 2
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Chronology of the Universe 3

• Grand Unification Era (from 10?43 s to 10?35 s)
• During the GUT (grand unified theory) era, the
  strong nuclear, weak nuclear, and EM theories
  were unified.
• Symmetry breaking instant (t ? 10?35 s)
• At this instant, the strong nuclear force
  separated from the weak nuclear and EM forces,
  which together are known as the electroweak
  force.
• The popular hypothesis (Guth,1980) is that a
  large amount of energy was released, which caused
  a short-lived but enormously rapid expansion of
  the universe, in a process known as inflation.

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Chronology of the Universe 4

• Inflationary Epoch (from about 10?35 s to 10?32
  s)
• The concept of inflation, proposed by Alan
  Guth in 1980, is that, for in extremely brief
  time-period of about 10?32 s, the radius of the
  universe increased by a factor of possibly 1050,
  a consequence of which is that much of the
  universe cannot be observed with even the most
  ideal telescope.
• Two problems solved by inflation were
• i. the horizon problem, which refers to the
  observation that distant parts of the universe,
  between which signals cannot pass, are extremely
  close in temperature,
• ii. the flatness problem, which is that space
  appears to be flat, not curved, as might be
  expected.
• A problem with inflation, which has not been
  solved, is why did it turn off so soon after it
  started?

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Two-dimensional Representations of Space

• Spherical space would make distant objects look
  larger, while the reverse is true for hyperbolic
  space.
• Observations indicate that space appears flat.

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Inflation Solves the Flatness Problem

• Two-dimensional representation of how a greatly
  expanded curved surface appears flat.

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Cosmic Inflation

• There is currently no direct evidence supporting
  inflation, although it explains both the horizon
  and flatness problems.

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Chronology of the Universe 5

• Break-up of the electroweak force (t ? 10?12 s, T
  ? 1015 K)
• At this instant, the universe consisted of an
  electron-quark soup, and the four fundamental
  forces became essentially what they are today.

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Chronology of the Universe 6

• Confinement (t ? 10 6 to 10 3 s, T ? 1012 K)
• The lower temperatures finally allow quarks
  to stick together, forming protons and neutrons
  (and their antiparticles), with 1 neutron for
  every 10 protons.
• Particles and antiparticles annihilate (t ? 1 s,
  T ? 1010 K)
• In a process is known as symmetry breaking,
  particles and their corresponding antiparticles
  annihilate, producing EM radiation.
• To explain our universe, which appears to be
  constructed of matter, rather than antimatter, we
  must assume a slight excess of particles over
  antiparticles.
• After annihilation, just the particles remained.
  

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Before Annihilation
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After Annihilation
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Chronology of the Universe 7

• Fusion of 2H and 4He (t ? 3 min, T ? 109 K)
• Much of the heavy hydrogen (2H) nuclei and helium
  nuclei now present in the universe, and a trace
  of Li, were created at this time.
• An additional 3 He and all heavier elements
  have since been created by stellar fusion and
  supernovae.
• According to current ideas, about fifteen 2H
  nuclei were created for every million 1H nuclei
  at this time, although 2006 measurements on dust
  grains have indicated that there is far more 2H
  than previously thought.

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Deuterium and Lithium Concentrations

• For space-time to be flat, the density of matter
  in the universe must be close to the critical
  density .
• Estimates of density of visible matter indicate
  that it is about 4 of the critical density.
• The calculated estimates of the D and Li
  concentrations agree with the visible matter
  estimates (green line).

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Chronology of the Universe 8

• Decoupling the era of recombination
• (t 400,000 y, T 3000 K)
• Up to this time, the universe had been an
  electron-photon soup, in which the electrons
  scattered the EM radiation in all directions,
  making the universe an opaque fog.
• When the temperature dropped below 3000 K,
  electrons and nuclei combined to form the first H
  and He atoms.
• Only those few photons with wavelengths
  corresponding to Bohr transitions interacted
  with atoms, so that matter and radiation were
  effectively decoupled, and the universe became
  transparent to EM radiation.
• The EM radiation, then at 3000 K, has now
  cooled to become the 3K Cosmic Microwave
  Background (CMB) radiation.

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The Era of Recombination
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Day One was 400,000 years
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The Timeline for the 1st Millisecond
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The Timeline from 3 Min. to14 Billion Years
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A Mini Bang Collision at RHIC

• In the Relativistic Heavy Ion Collider
  (RHIC) at Brookhaven National Laboratory,
  thousands of particles stream from the collision
  of two gold nuclei at 99.99 the speed of light.
• These collisions simulate the first few
  microseconds of the Big Bang.

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ATLAS Collaboration at CERN, Geneva 2

• Beams of protons travel 200 m underground around
  a path about 15 km long.

• 1800 scientists from 164 institutions in 35
  countries collaborate on this project.

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The ATLAS Experiment
UTAs Dr. Kausik De was named the US ATLAS
Operations Coordinator.
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The Cosmic Timeline 3

• The Dark Ages supposedly existed between the
  formation of
• atoms after 400,000 years (when the universe
  became
• transparent to EM radiation) until nearly 1
  billion years.

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Timeline Before the Dark Ages
Dark Ages
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The Dark Ages

• Calculations indicate that dwarf galaxies began
  to form at about 100 million years after the Big
  Bang.
• Stars began to form within these galaxies, and
  the ultra-violet radiation from them leaked into
  intergalactic space, creating expanding bubbles
  of ionized gas, which ultimately merged to cover
  all of space.
• Within the embryonic galaxies, gas cooled and
  created stars.
• Modern galaxies, such as the Milky Way, were
  formed by the coelescence of millions of these
  building blocks.
• Observations made in the Sloan Digital Sky Survey
  showed the existence of quasars of more than 1
  billion solar masses existing at just 1 billion
  years after the Big Bang, although the reason for
  their existence at this early date is not known.

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Timeline After the Dark Ages
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Stellar Birth Rates 1

• Following the dark ages, star formation is
  thought to have begun quickly, and has steadily
  tapered off, as the amount of interstellar
  hydrogen has decreased.

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The First Stars and Galaxies

• A burst of star formation is estimated to have
  begun about 200 million years after the Big Bang.
• In 2007, a star (HE 1523) estimated to be about
  13.2 billion years old (created just 500 million
  years after the Big Bang) was observed by U.
  Texas scientists.
• In 2008, a galaxy (A1689-zD1), born about 700
  million years after the Big Bang, was observed
  with the Hubble telescope, through the
  gravitational lensing produced by a galactic
  cluster.
• As mentioned in an earlier chapter, the oldest
  (and most distant) object yet observed was a
  gamma-ray burst, which occurred about 13.0
  billion years ago, or about 700 million years
  after the Big Bang.

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Structure Formation with Dark Matter 1

• Current calculations indicate that without
  (exotic) dark matter, gravity is insufficient to
  hold galaxies together.
• Since about 85 of the matter in the universe
  appears to be in the form of dark matter, which
  interacts only through gravity, the formation of
  galaxies must depend on dark matter clumping
  together in spherical blobs, known as (galactic)
  halos.
• In the early universe, the normal matter does not
  clump, because it interacts with the EM
  radiation, and is too hot to form stars.
• Only after about 200 million years has the
  universe cooled sufficiently for the first stars,
  and later, galaxies to form near the centers of
  the dark matter halos.

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Structure Formation with Dark Matter 2
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The Search for Dark Matter 1

• Since over 80 of the mass found in the universe
  is thought to be in the form of exotic dark
  matter, which interacts with gravity, but not
  with EM radiation, a major effort is underway to
  find its source.
• There is much indirect evidence for the existence
  of dark matter, but we still await direct
  evidence.
• A 2008 study which combined observations made
  with the Chandra X-ray telescope and the Hubble
  space telescope on a gigantic collision between
  two galactic clusters about 5.7 billion ly away
• The X-ray telescope showed a normal matter signal
  from the very hot gases produced by the
  collisions, while the Hubble telescope mapped the
  dark matter from the gravitational lensing of
  light from more distant galaxies.

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The Search for Dark Matter 2
X-ray signal from normal matter in red (false
color).
Optical signal from normal matter in blue (false
color), produced by gravitational lensing.
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Indirect Evidence for Dark Matter

• There is considerable indirect evidence for the
  existence of dark matter, but most does not
  preclude other explanations.
• In 2009, this type of evidence was obtained from
  a Hubble study of the heart of the Perseus Galaxy
  Cluster, which is about 250 million ly away.
• In this cluster, a large population of older
  small dwarf galaxies has remained intact, while
  the larger galaxies around them are being pulled
  apart by the gravitational attraction of
  neighboring galaxies.
• One conclusion that can be made from these
  results is that the centrally located small
  galaxies are held together by a higher
  concentration of dark matter.

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Fate of the Universe pre-1998

• Only the bound universe has sufficient mass to
  reverse the velocity, resulting in the big
  crunch.
• The unbound universe will expand for ever at a
  constant rate.
• The marginally bound will slow down, but never
  reverse.

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The Accelerating Universe 1

• In 1998, independent measurements of Type Ia
  supernovae at widely different distances,
  indicated that the rate of expansion of the
  universe is increasing with time.
• This increase of the expansion rate suggests the
  presence unknown repulsive force, which is known
  as dark energy.
• The 1998 measurements showed that gravitation was
  the dominant force in the universe only for the
  first few billion years.
• The initial dominance of gravity is not
  surprising, since an attractive force was needed,
  early in the life of the universe, to produce
  planets, stars and galaxies from the gas clouds.
• After about 8 billion years, dark energy became
  dominant.

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The Accelerating Universe 2

• Only in the last 5 billion years, has the
  repulsive force become dominant, producing an
  accelerating expansion rate
• A dominant repulsive force would have spread the
  matter out smoothly everywhere.
• A 2008 study of 86 galactic clusters using the
  Chandra X-ray telescope has indicated that the
  clusters are growing appreciably more slowly than
  they would in a universe not containing dark
  energy.
• As the universe continues to expand, the visible
  universe is expected to empty itself of matter.
• As dark energy becomes more dominant, it should
  ultimately pull apart galaxies, stars and
  planets.

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Wide Field Experiments in Texas and Hawaii

• In the HETDEX (Hobby-Ebberly Telescope Dark
  Energy Experiment) project at McDonald
  Observatory in west Texas, over a million
  galaxies will be surveyed, and should allow the
  expansion rate of the universe to be measured ten
  times more accurately than at present.
• The Pan-STARRS (The Panoramic Survey Telescope
  and Rapid Response System) in Hawaii belongs to
  the next generation of wide field instruments,
  and is expected to provide huge quantities of
  data relating to dark energy, dark matter,
  extrasolar planets, in addition to mapping the
  solar system in unprecedented detail.
• From these set of measurements, we should be able
  to tell far more precisely how the effect of dark
  energy is varying in time.

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Virtual Particles

• The conventional view of a vacuum is that it is
  an absence of everything, as proposed by Hero of
  Alexandria in the 1st Century, who concluded that
  air consists of atoms moving through a void .
• The Heisenberg uncertainty principle of quantum
  mechanics , which may be written as
• uncertainty in energy ?E x uncertainty in time ?t
• is of the order h (Plancks constant),
• permits virtual particle-antiparticle pairs
  to appear and annihilate spontaneously within a
  time ?t h/?E, where ?E is the total energy of
  the pair.
• One of the strangest consequences of quantum
  mechanics is that a vacuum is filled with virtual
  particles, which pop rapidly into and out of
  existence - an idea which would have appeared to
  be nonsense less than a hundred years ago.

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The Vacuum Energy

• The vacuum energy is the sum of the energies of
  all the virtual particles in existence.
• When Einsteins general theory of relativity is
  applied to the virtual particles of quantum
  mechanics, it is found that the vacuum energy
  produces a repulsive antigravity force, which
  would have the effect of dark energy i.e. causing
  an increase in the expansion rate of the
  universe.
• However, calculations based on the current
  theories of elementary particles give values of
  the acceleration many orders of magnitude greater
  than that observed.
• Thus, the question as to whether dark energy is
  related to the vacuum energy remains unanswered.

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Fate of the Universe post-1998 (in green)
An accelerating universe provided an answer to
the age crisis i.e. that the universe appeared
to be younger than its oldest stars.
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Dark Energy 1

• The density of matter in the early universe was
  so high that galaxies collided and merged
  frequently, pulling each other out of shape, so
  that for the first 3 billion years most galaxies
  were neither elliptical of spiral, and could be
  classified as peculiar.
• After about 5 billion years, galactic collisions
  became far less frequent, so that over 90 of the
  visible galaxies were elliptical or spiral an
  effect which would be enhanced by dark energy.

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Dark Energy 2
Dark energy is supposed to be distributed
smoothly throughout space, while the expansion of
space causes the density of matter to decrease
with time. One result of this effect is to
reduce the rate of star formation, as shown
below.
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Dark Energy 3

• 0.99,
• universe
• remains
• amorphous.

Early universe
Transition period
Today

• Actual ? 0.75,
• cobweb structure remains frozen.
• 0.99
• 0.9 9

• 0.00, cobweb structure
• continues to develop.

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Future of the Universe 1

• With time, the visible region (yellow) grows, but
  the universe (blue) grows faster, while gravity
  pulls the nearer galaxies together.

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Future of the Universe 2

• In 100 billion years, all visible galaxies will
  have merged.

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Future of the Universe 3

• In 5 billion years, the Sun will have become a
  red giant,
• and the galaxy Andromeda will fill the night sky.

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Future of the Universe 4

• In a 100 billion y, the Earths remains will
  orbit the supergalaxy.
• In a 100 trillion years, lights out!

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Alternative to Dark Energy 1
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Alternative to Dark Energy 2
We are here.

• This idea depends on the possibility that the
  cosmological principle does not hold i.e. the
  universe on the largest scale is inhomogeneous.
• If the solar system is near the center of a giant
  void, which expands faster than the more densely
  populated regions of space, the effects of dark
  energy could be produced.

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Alternative to Dark Energy 3
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The Energy Pie

• Current estimates lead to the strange conclusion
  that dark energy accounts for roughly 73 of the
  energy in the universe, with dark matter
  accounting for about 23, leaving only 4
  attributable to normal matter and radiation.

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James Peebles Report Card, 2002