1446 Introductory Astronomy II Chapter 18A

1
1446 Introductory Astronomy IIChapter 18A

• Cosmology I
• R. S. Rubins
  Fall, 2009

2
(No Transcript)
3
Prologue 1

• Cosmologists are addressing some of the problems
  that people attempted to resolve over the
  centuries through philosophical thinking, but we
  are doing so on systematic observation and
  quantitative methodology.
• As citizens of the universe we cannot help but
  wonder how the first sources of light formed, how
  life came into existence, and whether we are
  alone as intelligent beings in this vast space.
• Abraham Loeb in Scientific American, Nov. 2006

4
Prologue 2

• What makes modern cosmology an empirical science
  is that we are literally able to peer into the
  pastcosmologists do not need to guess how the
  universe evolved we can watch its history
  through telescopes.
• We have a snapshot of the universe as it was
  400,000 years after the big bang as well as
  pictures of individual galaxies a billion years
  later.
• Abraham Loeb in Scientific American, Nov. 2006

5
A Simplified Chronology 1

• Cosmology is the study of the origin, structure
  and evolution of the universe
• ca. 300 BCE
• Aristarchus proposes a Sun-centered universe,
  after deducing that the Sun is appreciably larger
  than the Earth.
• 1530
• Copernicus revives the heliocentric universe.
• 1610 1632
• Galileo makes the first telescopic
  observations of the Sun, planets and the milky
  way, and later publishes a book supporting the
  Copernican model.

6
A Simplified Chronology 2

• 1666 1687
• Newton introduces his Laws of Motion and
  Theory of gravity.
• He believed that the universe was infinite
  and static, with every star held in place by
  a uniform gravitational pull from all sides.
• Without the tools needed to apply his equations
  to very complicated systems, he invoked the
  idea of intelligent design in writing
• This most beautiful system of the sun,
  planets and comets, could only proceed from the
  counsel and dominion of an intelligent and
  powerful Being
• Now, with high-powered computers, the structure
  of the solar system can be calculated by physics
  alone, without assuming the help of a powerful
  Being.

7
A Simplified Chronology 3

• 1755
• Kant and Wright separately propose that the
  Sun is part of a larger universe of stars.
• 1823
• Olbers paradox why is the night sky dark?
• For an infinite universe, every line-of-site
  should meet a star.
• 1845
• Lord Rosse constructs the worlds largest
  telescope in Ireland, and identifies island
  nebulae.

8
A Simplified Chronology 4

• 1905
• Einstein introduces the special theory of
  relativity, which shows the following
• i. that we live in four-dimensional
  universe, with space and time are linked
  inextricably
• ii. the speed of light is the limiting velocity
  at which matter and energy can travel
• mass is a form of energy, given by E mc2.
• 1912
• Henrietta Leavitt discovers the relationship
  between luminosity and period for Cepheid
  variables, thus extending distance measurements
  well beyond the range of the parallax method.

9
A Simplified Chronology 5

• 1915
• Einstein introduces his General Theory of
  Relativity, which superseded Newtons theory of
  gravity, and could be applied to ultra-strong
  gravity and objects traveling at speeds close to
  c.
• 1917
• Believing in a static universe, Einstein
  introduced a repulsive anti-gravity force, termed
  the cosmological constant, since without it, his
  calculations showed that a static universe
  immediately began to contract.
• By studying the orbits of globular clusters,
  Shapley deduces that the Sun is far from the
  center of our universe.
• 1920
• Shapley Curtis debate are island nebulae
  small objects within our universe (Shapley) or
  island universes beyond our universe (Curtis)?

10
A Simplified Chronology 6

• 1922
• Friedman (1922) and Lemaitre (1927)
  independently deduced that Einsteins general
  theory of relativity could lead to an expanding
  universe.
• 1924
• Hubble applied Leavitts relationship for
  Cepheid variables to show that Curtis was
  correct showing (for example) that Andromeda is
  a separate galaxy, outside the Milky Way.
• 1927
• Humason, a former Minnesotan mule driver,
  working with Hubble, discovered Hubbles Law,
  which states that the recessional velocities of
  galaxies are proportional to their distances from
  us.

11
A Simplified Chronology 7

• 1927
• Lemaitre deduced that the universe must have
  evolved from a smaller and denser state.
• Three years before his death in 1955,
  Einstein told George Gamow that his cosmological
  constant was the greatest blunder of his life.
• The connection of Hubbles Law with Einsteins
  general theory of relativity lead to its
  reinterpretation as a cosmological redshift
  i.e., an expansion of space itself, rather than
  aDoppler effect, which describes the motion of
  objects in a fixed space).
• 1934
• Following the discovery of the neutron by
  Chadwick and the prediction of the neutron star
  by Landau, both in 1932, Zwicky and Baade
  suggested independently that the most luminous
  novae were supernovae, with neutron stars at
  their centers.

12
A Simplified Chronology 8

• 1948-9
• Gamow suggested that we should be able to
  observe the remnant of the intensely hot
  radiation emitted at the Big Bang, while Alpher
  and Herman suggested this radiation should now be
  at about 5K, giving thermal radiation in the
  microwave range.
• Hoyle (who gave the Big Bang its name), Bondi and
  Gold proposed a rival steady-state theory, in
  which matter is continually receding, with new
  matter being created to replace it.
• 1960s
• Penzias and Wilson at Bell labs realized that
  apparent noise, occurring in their
  measurements with a sensitive microwave antenna
  designed for satellite communications, was in
  fact the radiation from the Big Bang.

13
A Simplified Chronology 9

• 1980s
• Vera Rubin and Ford found overwhelming
  evidence in favor of the existence of dark
  matter.
• 1989-1994
• A NASA satellite, the Cosmic Background
  Explorer (COBE), showed that the cosmic microwave
  background radiation (CMB), fits a thermal
  spectrum at 2.73 K (exactly).
• 2003
• Measurements of Type Ia supernovae indicated
  that the rate of expansion of the universe has
  been increasing in the last 5 billion years,
  indicating the presence of dark energy, an
  anti-gravity force, related to Einsteins
  cosmological constant.

14
The Cosmological Principle

• At the very largest scales (over distances of
  about a billion ly), the universe appears to be
• i. homogeneous (the same at all places)
• ii. isotropic (the same in all directions at any
  point).
• The cosmological principle implies that there is
• i. no edge to the universe
• ii. no center to the universe.
• The proponents of the discredited steady state
  universe (Hoyl et al.), believed in a universe
  (on the largest scales) that was independent of
  time.
• An additional assumption, continually being
  tested, is that physical theories are unchanged
  over time and space.

15
A Universe, Finite and Unbounded

• We imagine 2-dimensional objects on a
  2-dimensional surface.
• While all the coins move away from each other as
  the surface expands, the sizes of the coins do
  not change because of the strong EM forces
  holding each coin together.

16
Cosmological Redshift

• As the balloon expands, the wavelength of an EM
  signal increases i.e. it is redshifted.
• The further away the emitting source, the more
  the received signal will be redshifted.

17
Doppler Redshift vs Cosmological Redshift
18
COBE Data Fits T 2.73 K
From 1989 to 1994, a precise value of 2.73 K for
the cosmic microwave background (CMB) was
obtained with a far IR spectrometer on the
COBE satellite.
19
Motion of the Milky Way 2

• More precise measurements made by COBE showed
  temperature variations of up to 3 mK, with the
  warmer region in the direction of Leo and the
  cooler region in the direction of Aquarius,
  indicating Doppler effects.

20
Motion of the Milky Way 1

• The COBE measurements showed that the Earth is
  moving towards Leo at about 380 km/s.
• Taking into account the motion of the Sun around
  the galactic center, astronomers deduced that the
  Milky Way Galaxy is moving at 600 km/s relative
  to the CMB.

21
Ripples in the Background Radiation 1

• To produce the sudsy structure of galactic
  clusters and observed in the universe,
  cosmological models predicted that there should
  have been tiny ripples in the temperature of the
  CMB before the appearance of stars.
• The lead investigators of the COBE scientific
  team, George Smoot and John Mather, made the
  first measurements of the ripples, and shared the
  2006 Nobel Prize in physics.
• Improved measurements were made with specialized
  telescopes, on the ground and in balloons flying
  high above Antarctica, but the most extensive
  investigations were those of the Wilkinson
  Microwave Anisotropy Probe (WMAP), which was an
  orbiting robotic infra-red telescope.
• The ripples in the 3K CMB were found to be
  roughly10 µK (one hundred thousandths of a
  degree).

22
The Boomerang Experiment, 1998

• Following circumpolar winds at almost 40 km
  above Antarctica for 10 days, microwave detectors
  cooled to 0.3 K obtained images 40 times sharper
  than COBE.

23
Ripples in the CMB

• In the map of the sky after 5 years of collecting
  WMAP data, the red regions are warmer than
  average, and the blue regions cooler, by about 3
  x 105 K.
• The warmer regions are slightly denser, because
  gravitational collapse causes heating.

24
Ripples and Q

• The tiny temperature ripples in the WMAP figure
  are of fundamental importance, because they are
  the origins of the galaxies formed over a million
  years later.
• The density contrasts observed in the WMAP figure
  are amplified during the expansion of the
  universe, because the gravitational force slows
  the expansion of the denser regions.
• The symbol Q 105 represents the size of the
  temperature ripples compared to the absolute
  temperature (3 K) of the CMB.
• If applied to a sphere the size of the Earth, it
  would be equivalent to ripples on the Earths
  surface of less than the length of a football
  field, which is another way of justifying the
  cosmological assumption of homogeneity.

25
Development of Structures

• The pictures show a computer simulation of how
  structures emerge in an expanding universe.
• For practical reasons, the expansion has been
  subtracted out, so that the boxes remain the same
  size.
• During expansion, the denser regions expand more
  and more slowly, compared to less dense regions,
  so that the density contrasts grow.

26
Galactic Distributions

• Observed

• Simulated

27
Questions about Big-Bang Theory

• 1. Was the Big Bang an explosion in previously
  empty space?
• 2. What happened before the Big Bang?
• 3. Do the restrictions of Einsteins Special
  Theory of Relativity mean that galaxies cannot
  recede faster than c, the speed of light in free
  space.
• 4. Does the expansion of the universe cause
  everything within it to grow proportionately in
  size?
• 5. Do those measurements, which show the
  universe to be about 13.7 billion years old, mean
  that the radius of the observable part of the
  universe is 13.7 light years?

28
Misconceptions Answered

• 1. The big bang did not occur at a point in
  space rather, space itself came into existence
  with the big bang.
• 2. There was no before the big bang time
  itself, along with space and matter came into
  being with the big bang.
• 3. The limiting speed c of special relativity
  applies to the motion of matter or energy through
  space, but not to the expansion of space itself.
• 4. Because of the gravitational forces between
  them, the sizes of planets, stars, galaxies, and
  their clusters remain unchanged by the expansion
  of space.
• 5. By the time a signal reaches us from a
  distant galaxy, it will be much further away
  because of the expansion of space.

29
Lookback Time 1

• The lookback time refers to the time when a
  signal we see today was emitted by its source.