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I. Paradigm Shifts in the History of Astronomy
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 2
What is a Paradigm?
• “A philosophical and theoretical framework of a scientific school or discipline within which theories, laws, and generalizations and the experiments performed in support of them are formulated.” (Merriam-Webster Dictionary)
• In short: a way of understanding the world – a world view
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 3
History of Astronomy : Ancient Times
• All cultures have myths and legends about the night sky, the Sun, the Moon, the seasons, etc.
• Science in Ancient Greece: – Principle of cause and effect – Observe phenomena, seek explanations – World can be understood in precise geometrical
terms – Rational thought (“theory”) held above observation
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 4
Science in Ancient Greece
• Discoveries of Greek Science: – The Moon is a sphere – The Earth is a sphere – The Moon is between the Earth and the Sun – Measurement of the Earth’s circumference – (Aristarchus: the Sun is the centre of the Universe – not
widely accepted*)
• But: bodies spherical, moving in circles (geometry!), Earth at centre of Universe*
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 5
Ptolemy to Copernicus
• Ptolemy (~100 AD): – Almagest: Geocentric model with planets and
sun moving in epicycles on circular orbits, Earth stationary at centre
– Basis of European and Arabic world view for next ~1500 years
• Copernicus (1473-1543): – Heliocentric model with rotating Earth, still
with epicycles – Challenged the ancient world view of Earth at
the centre of the Universe
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 6
Geocentric vs. Heliocentric
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 7
Occam’s Razor
• William of Ockham (14th century England): the simplest explanation tends to be the best one
• Not a scientific or logical result, but used as a general rule of thumb in modern science
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 8
Why Wasn’t the Heliocentric Model Accepted?
Problems with the model
• Against Church teachings • New (vs. accepted wisdom) • Predicts parallaxes (≠
observations) • Rotating Earth (≠ Aristotle,
observations?) • Less accurate than Ptolemaic
model (required more epicycles)
A good idea isn’t always enough
• Getting better data (Tycho Brahe)
• Fixing the model (Johannes Kepler)
• Promoting the new model (Galileo Galilei)
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 9
The Renaissance: Brahe and Kepler
• Tycho Brahe (1546-1601): – Last great “naked eye”
observer – very meticulous in collecting
data
• Johannes Kepler (1571-1630): – None of the earlier models is
correct -- planets move in ellipses, not circles
– 3 Laws: ellipses, planets move faster near Sun, (period)2 is proportional to (distance)3
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 10
The Renaissance: Galileo
• Galileo Galilei (1564- 1642): – First (?) to point a telescope at the night
sky – Tested and rejected Aristotle’s physics –
inertia and momentum, objects of different composition fall at the same rate
– Saw mountains on Moon, phases of Venus, moons of Jupiter, Sunspots, stars of the Milky Way
– Tried for heresy!
• Still missing: someone to put all the pieces together into a coherent physical theory...
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 11
The Newtonian Revolution
• Isaac Newton (1643-1727): invented calculus*, reflector telescope, optical spectrum, Theory of mechanics, Theory of gravity, etc....
• Mechanics: – A body at rest/uniform motion will stay that
way without external force – Every action equal and opposite reaction
• Gravity:
• Equivalence:
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 12
€
F =GMmr2
F = ma =GMmr2
⇒ a =GMr2
The Relativistic Revolution
• Albert Einstein (1879-1955): – 1905: Special Relativity, Photoelectric Effect,
Brownian Motion... – 1907: Equivalence Principle: gravity = acceleration,
gravitational effects on light (bending/redshift) – 1915: General Relativity (GR) – mass tells space-
time how to curve, the curvature of space-time tells mass how to accelerate
• GR predicts directly observable effects – Precession of Mercury’s orbit – Bending of starlight by Sun’s gravity (also
predicted by Newtonian gravity, but amount of bending different)
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 13
Observational Tests of General Relativity
• Precession of Mercury’s orbit: – Perihelion (closest approach to the Sun) advances 43
arcseconds/century – Other solutions: Venus more massive, another planet
closer to sun (“Vulcan”), Sun more flattened than observed, Mercury had a moon, or... Newtonian gravity incorrect
– Precession directly explained by GR
• Bending of starlight by Sun – Predicted to be 1.74 arcseconds at the Sun’s surface – 1919 Solar Eclipse: Eddington Africa, Crommelin
Brazil, observational data not optimal for either expedition
– However, Eddington claimed results supported GR
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 14
1922 Eclipse in Western Australia: much better data, but credit still goes to Eddington...
The Next Great Paradigm Shift: The Size of the Universe
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 15
• What is the size of the Milky Way?
• What is the size of the Universe?
• What are the “nebulae”? • Definition – novae
(“new”): stars which brighten dramatically
The Andromeda “Nebula”, M31
The Size of the Universe: The Great Debate (1920)
Harlow Shapley: “Nebulae” part of our Galaxy
• The Galaxy is large (100 kpc), and we are not at its center
• If “Nebulae” are other galaxies, must be far away, but “novae” in them are ~as bright as novae in our Galaxy
• van Maanen: claimed to measure rotation of M31
• Nebulae “avoid” the Milky Way • Nebulae all appear to be moving
away from us
Heber Curtis: “Nebulae” other galaxies, like our Milky Way
• The Galaxy is only 10 kpc across and we’re at the centre – hard to see what’s going on
• Spectra of Nebulae look like integrated light of many stars
• Many Nebulae have rings of dust, which could block the view of Nebulae in the Galactic plane
• Most “novae” in Andromeda are much fainter than in the Milky Way
• The Milky Way seems to have spiral structure
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 16
• The Galaxy is large (100 kpc 30 kpc), and we are not at its center
• If “Nebulae” are other galaxies, must be far away, but “novae” in them are ~as bright as novae in our Galaxy
• van Maanen: claimed to measure rotation of M31
• Nebulae “avoid” the Milky Way • Nebulae all appear to be moving
away from us
• The Galaxy is only 10 kpc across and we’re at the centre – hard to see what’s going on
• Spectra of Nebulae look like integrated light of many stars
• Many Nebulae have rings of dust, which could block the view of Nebulae in the Galactic plane
• Most “novae” in Andromeda are much fainter than in the Milky Way
• The Milky Way seems to have spiral structure
New Technology, New Techniques, and a Solution
• Edwin Hubble (1889-1953): – 1922 – 1923: used new 100”
telescope at Mt. Wilson, California to discover Cepheid variable stars in several “Nebulae”, including Andromeda
– 1925: established that the “Nebulae” were much too far away to be part of the Milky Way, and were in fact other galaxies
• The Universe is a very big place
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 17
Another Paradigm Shift: The Expanding Universe
• When Einstein published GR (~1915), the Universe was thought to be static (not expanding or contracting), based on observational data
• But GR says a Universe with matter in it must be expanding or contracting introduce a “fudge factor”, Λ: a “cosmological constant”
• 1929: Hubble paper reveals the Universe is expanding no need for Λ(Einstein’s “greatest blunder”)
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 18
1998: The Year Λ Came Back
• Type Ia Supernovae (SNe Ia): explosion of a white dwarf star
• Standard Candles: have ~same luminosity (intrinsic brightness) can get distances
• In 1998, two competing teams observing SNe Ia to measure the expansion of the Universe
• Expectation: Universe’s expansion slowing down
• Discovery: Universe’s expansion is speeding up!
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 19
Two Competing Teams, Same Result
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 20
S. Perlmutter, G. Aldering, S. Deustua, S. Fabbro, G. Goldhaber, D. Groom,A. Kim, M. Kim, R. Knop, P. Nugent, (LBL & CfPA)N. Walton (Isaac Newton Group)A. Fruchter, N. Panagia (STSci)A. Goobar (Univ of Stockholm)R. Pain (IN2P3, Paris)I. Hook, C. Lidman (ESO)M. DellaValle (Univ of Padova)R. Ellis (CalTech)R. McMahon (IofA, Cambridge)B. Schaefer (Yale)P. Ruiz-Lapuente (Univ of Barcelona)H. Newberg (Fermilab)C. Pennypacker
• Brian Schmidt (ANU)• Nick Suntzeff, Bob Schommer, Chris Smith (CTIO)• Mark Phillips (Carnegie)• Bruno Leibundgut and Jason Spyromilio (ESO)• Bob Kirshner, Peter Challis, Tom Matheson (Harvard)• Alex Filippenko, Weidong Li, Saurabh Jha (Berkeley)• Peter Garnavich, Stephen Holland (Notre Dame)• Chris Stubbs (UW)• John Tonry, Brian Barris (University of Hawaii)• Adam Reiss (Space Telescope)• Alejandro Clocchiatti (Catolica Chile)• Jesper Sollerman (Stockholm)
Another Paradigm Shift: The Accelerating Universe
• Many different possible explanations were explored: – Dust affecting observations? – Are today’s SNe the same as those far in the past? – Other observational effects...?
• Eleven years later, however, the general consensus is that the data are best fit by an accelerating expansion of the Universe, usually written as: Λ (!)
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 21
Sneak Peek: What is the Universe Made Of? The Current Paradigm
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 22
Only 0.6% in Stars!
Some Lessons from the History of Astronomy
• The evolution of our understanding of the Universe has not always been moving smoothly towards the “truth”
• The development of astronomy has been hindered by: – theoretical “wrong turns” and misinterpretation of data – bad observational data – missing / unavailable information
• Progress has often happened through a combination of new theories and new observations
• Paradigm shifts happen!
ASTR170 Introductory Astronomy: I. Paradigm Shifts in the History of Astronomy 23
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