Lecture 2: History of Cosmology

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Lecture 2 History of Cosmology

• Aug 23, 2007

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Plan for Todays lecture

• Scientific thinking and scientific method
• When Cosmology became Science from ancient
  Greeks to the Renaissance
• Movie History of Hubble Space Telescope
• History of Modern Cosmology from Newtons
  Gravity to the Big Bang

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Let the Journey Begin

• We especially need imagination in science. It
  is not all mathematics, nor all logic, but is
  somewhat beauty and poetry.

Maria Mitchell (1818 1889) Astronomer and first
woman elected to American Academy of Arts
Sciences
The most incomprehensive thing about the Universe
is that it is comprehensible. -- Albert Einstein
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In the Beginning

• Modern cosmology is scientific
• Based on detailed observations and complicated
  theory
• But the quest of the origin and fate of the
  universe lasted as long as the human civilization
  existed
• The earliest cosmological models are based on
  human experience, and are strongly influenced by
  culture, mythology and religion
• But what makes Cosmology a Science?

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Scientific Thinking

• It is a natural part of human behavior.
• We draw conclusions based on our experiences.
• Progress is made through trial and error.

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The Scientific Method

• Question
• Hypothesis
• a tentative explanation
• Prediction
• Test
• Result
• confirm, reject, or modify

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Hallmarks of Good Science

• Science seeks explanations for observed phenomena
  that rely solely on natural causes.
• Science progresses through the creation and
  testing of models of nature that explain the
  observations as simply as possible.
• Occams Razor
• A scientific model must make testable predictions
  that could force us to revise or abandon the
  model.

Theory
-- a model which survives repeated testing
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Scientific Theory Can be WRONG!

• In fact, the very nature of scientific theory is
  that it has to be testable, I.e., it has to be,
  in principle, falsifiable
• Scientific theory first tries to explain existing
  data
• Then seeks to make testable prediction and seeks
  to reach its limitation in application
• Then either the hypothesis can be improved, or it
  has to be abandoned
• All theories has its limitation, to put it in
  another way, all theories was, or will be WRONG!
• But at each step, we are getting closer to the
  truth!
• So, do you believe in the Big Bang???

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Do I believe in Big Bang ??

• Yes the currently observable universe started
  in and expanded from a very hot, dense state and
  it is still expanding the big bang theory has
  passed all the tests we have so far
• No the Big Bang theory can only take us to the
  limit where current physics theory (relativity
  and quantum theory) can be applied it is not the
  end of the story, and will be improved upon
• Irrelevant the Big Bang theory explains well
  the physical universe but it has little to do
  with belief or faith, and it provides no direct
  answer to your more personal universe
  spiritual, religious, ethical etc.

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Ancient Greeks First Cosmologists

• Greeks built first cosmological models based on
  reasoning and observations
• They used these models to explain, and to predict
  planetary motions
• Most of Greek models are Geocentric, but even
  Heliocentric models (e.g. Aristarchus) was
  proposed (2000 years ahead of his time)
• In Greek Cosmology, we see the close relation
  between science and philosophy

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Plato (428 - 348 BC)

• All natural motion is circular
• Reason is more important than observation

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Claudius Ptolemy (AD 100-170)

• star catalogue
• instruments
• motions model of planets, Sun, Moon

His model fit the data, made accurate
predictions, but was horribly contrived!
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Ptolemys Geocentric Model

• Earth is at center
• Sun orbits Earth
• Planets orbit on small circles whose centers
  orbit the Earth on larger circles
• the small circles are called epicycles

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Nicolaus Copernicus (1473-1543)

• He thought Polemys model was contrived
• Yet he believed in circular motion
• De Revolutionibus Orbium Coelestium

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Copernicus Heliocentric Model

• Sun is at center
• Earth orbits like any other planet
• Inferior planet orbits are smaller
• Explained the available observations better than
  Polemys model

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Johannes Kepler (1571-1630)

• Greatest theorist of his day
• a mystic
• forces made the planets move
• Keplers three laws (assignment learn Keplers
  laws at masteringastronomy.com)

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Galileo Galilei (1564-1642)

• First man to point a telescope at the sky
• wanted to connect physics on earth with the
  heavens
• Galileo discovered that Jupiter had four moons of
  its own.
• Jupiter was the center of its own system.
• Heavenly bodies existed which did not orbit the
  earth.
• Dialogue Concerning the Two Chief World Systems

This book got him in trouble with the Church!
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Sir Isaac Newton (1687)

• Published The Mathematical Principles of Natural
  Philosophy
• Three laws of motion
• Low of universal gravitation
• The basic principles of modern physics
• Newton is, arguably, the greatest scientist ever
  in human history
• Physics
• Calculus
• Optics

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Immanual Kant (1755)

• Idea of Island Universe
• Spiral Nebulae are other Milky Ways
• Very distant and external to our Galaxy

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William Hershel, 1785

• First attempt to map out the Milky Way Galaxy
• Beyond the solar system
• Extend the scope of the universe by 10,000 times
• Hershel assumed to the solar system is at the
  center of the Milky Way Galaxy

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Early 20th Century Great Physics Evolution

• 1900 German physicist Max Planck proposed that
  energy radiates in quanta ? start of quantum
  theory
• 1905 Einsteins special relativity ? matter,
  space and mass are closely related
• 1915 Einsteins general relativity ? gravity and
  space/time are closed linked
• 1916 German astrophysicist Karl Schwarzschild
  proposed black holes
• 1919 English astrophysicist Sir Arthur Eddington
  confirmed Einsteins prediction from relativity

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Late 19th to Early 20th century Great Leap in
Technology

• 1858 American astronomer Henry Draper invented
  astronomical photography
• Early 1920th, the new generation of telescopes,
  in particular the 100 inch telescope on Mt.
  Wilson outside LA, began to survey the heavens.

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1920s Birth of Modern Cosmology

• 1920 Great Shapley vs. Curtis debate are spiral
  nebulae island universe?
• 1925 Hubble measured the distance to Andromeda
  the extragalactic astronomy is born, the size of
  our observed universe is again increased by
  10,000
• 1922 Russian physicist Aleksander Friedmann
  discovered the expanding solution of Einsteins
  general relativity ? theoretical prediction of
  the expansion of the universe
• 1929 Hubble discovered the expansion of the
  universe

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The most importance figure of modern cosmology
The further the galaxy, The faster it is moving
away ? The universe is expanding
Velocity
Distance
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The Big Bang

• 1948 Russian-American physicist George Gamow
  published the first model about the universe
  started from a hot and dense fireball ? the hot
  big bang the predicted this creation left
  behind a faint glow that suffuses the universe
  today, the cosmic microwave background

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The discovery of Cosmic Microwave Background

• Bell lab physicists Arno Penzias and Robert
  Wilson discovered CMB by chance when building
  their low-noise communication antenna. This
  confirmed the Big Bang theory. The two key pieces
  of modern cosmology expansion of the universe
  and hot big bang, are firmly established.

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Recent History (1964 - now)

• Discovery of quasar - Marteen Schmidt, 1964
• Opened up the distant universe
• Discovery of microwave background - Penzias and
  Wilson 1965
• verified the big bang cosmology
• Establishment of modern cosmology theory (1970s)
• Zeldovich in Russian
• Peebles in Princeton
• Surveys of the Universe
• 1985, Center of Astrophysics survey
• Observational tests of cosmology theory

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Recent History

• 1970s discovery of dark matter in galaxies
• 1980s inflation cosmology towards the Big Bang
• 1990 Hubble Space Telescope
• 1992 COBE satellite to map the microwave
  background
• 2000 discovery of dark energy and accelerating
  Universe

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What have we learned?

• How did the Greeks lay the foundations for modern
  science?
• By developing the notion of models and putting
  emphasis on the importance of having models agree
  with observed reality.
• What was the Ptolemaic model?
• Ptolemys synthesis of earlier Greek ideas about
  the geocentric universe, which was a
  sophisticated model that allowed prediction of
  planetary positions.

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What have we learned?

• Briefly describe the roles of Copernicus, Kepler,
  and Galileo.
• Copernicus created a Sun-centered model of the
  solar system designed to replace the Ptolemaic
  model, but it was no more accurate because he
  still used perfect circles. Kepler refined the
  model by introducing orbits with the correct
  characteristics. Galileos experiments and
  telescopic observations overcame remaining
  objections to the Copernican idea of the Earth is
  a planet orbiting the Sun.
• What is a theory in science?
• A model that explains a wide variety of
  observations in terms of just a few general
  principles, which has survived numerous tests to
  verify its predictions and explanations.

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History of Modern Cosmology

• Two most important discoveries of modern
  cosmology
• Hubbles Law
• Observations of Microwave Background