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Joy of Science
Experience the evolution of the
Universe, Earth and Life
•  Review of last class
•  Introduction to Astronomy
•  Contents of today’s lecture
•  Quiz time
Review
Review 1
n  The Sun is currently classified as
A. a main-sequence star
B. a white giant
C. a red giant
D. a white dwarf
E. neutron star
Review
Review 1
n  The Sun is currently classified as
A. a main-sequence star
B. a white giant
C. a red giant
D. a white dwarf
E. neutron star
Review
Review 2
n  The most abundant element in the universe is
A. neutrinos
B. hydrogen
C. carbon
D. helium
E. iron
Review
Review 2
n  The most abundant element in the universe is
A. neutrinos
B. hydrogen
C. carbon
D. helium
E. iron
Review
Review 3
n  The net effect of the three-step process that produces energy in
the Sun is called
A. proton burning
B. deuterium burning
C. hydrogen burning
D. helium burning
E. carbon burning
F. iron burning
Review
Review 3
n  The net effect of the three-step process that produces energy in
the Sun is called
A. proton burning
B. deuterium burning
C. hydrogen burning
D. helium burning
E. carbon burning
F. iron burning
Review
Review 4
n  During the first step of energy production in the Sun
A. three helium-3 nuclei fuse to form a carbon-12 nucleus
B. two protons fuse to form deuterium, a positron and a neutrino
C. deuterium and a proton fuse to form hydrogen atoms
D. two helium-3 nuclei fuse to form deuterium and a neutrino
Review
Review 4
n  During the first step of energy production in the Sun
A. three helium-3 nuclei fuse to form a carbon-12 nucleus
B. two protons fuse to form deuterium, a positron and a neutrino
C. deuterium and a proton fuse to form hydrogen atoms
D. two helium-3 nuclei fuse to form deuterium and a neutrino
May 6, 2011
Edward Hubble and
the expanding universe
- Experience the evolution of the Universe
Today’s Keywords
cosmology, Edward Hubble, galaxy,
cosmic microwave background (CMB),
big bang, dark matter, dark energy
Outline of today’s lecture
0. Introduction
1. Edwin Hubble’s discoveries
2. Big Bang theory
3. Dark side of universe – dark matter, dark energy
Introduction
Cosmology
n  Cosmology: Study of structure and history of entire universe
n  Edward Hubble’s discoveries:
- Galaxies
(before 1920’s, the Milky Way galaxy was thought as the only group
of stars in the cosmos, i.e. Our galaxy = the entire universe )
- Redshift of galaxies
n  From Hubble’s discoveries and other astronomical evidences, a theory
of the beginning of universe has been established è Big Bang!
n  The fate of universe is ???
Hubble’s discovery 1
Galaxies
n  Galaxies: collections of stars
n  Before Hubble, telescopes could see nearby stars to Earth (the
solar system) in the Milky Way
n  Hubble measured the distance of extremely faint stars and
found the distance is far outside the bunds of the Milky Way
è the nebula containing the measured stars is Andromeda
nebula, and now called Andromeda Galaxy
n  Our galaxy (Milky Way)
- Spiral galaxy: 75% of brighter galaxies in sky
- Flattened disk of 100,000 light-year across
- Central bulge has most of our galaxy’s stars
- Sun is located in the halfway out from the center
Spiral galaxy
Our galaxy
Milky Way
Hubble’s discovery 1
Galaxies (cont’d)
n  Other kinds of galaxies
- Elliptical galaxies : 20% of brighter galaxies
- Irregular and Dwarf galaxies: faint but probably most common
in universe
n  Earth (a planet) à Solar system (Sun: a star) à Our galaxy à
Local cluster à Supercluster à Observable universe
n  Number of galaxies in universe: hundreds billions, each with
hundreds billions of stars
n  Optical point-like stars in sky may be a star in the Milky Way
galaxy, may be a light-reflected planet in our solar system, e.g.
Venus, or a cluster of galaxies!
Hubble’s discovery 2
Redshift of galaxies
n  Redshift: lights from stars (or galaxies) have red-shifted spectrum
an example of Doppler effect
à going away object has shorter wavelength
coming close object has longer wavelength
Doppler effect in sound
Doppler effect in light
Blue-shift
Red-shift
Hubble’s discovery 2
Redshift of galaxies
n  Redshift: lights from stars (or galaxies) have red-shifted spectrum
an example of Doppler effect
à going away object has shorter wavelength
coming close object has longer wavelength
è Distant galaxies are moving away from Earth
è The universe is expanding!
n  Hubble’s law:
‘ The farther away a galaxy is, the faster it recedes.’
à Imagine a raisin-bread dough
è We can determine the distance to galaxies by measuring the
redshift of the light we receive
è Hubble constant from Hubble’s law provide the expansion rate of
universe, hence, the age of universe of about 14 billion years
Analogy of expanding universe
: raisin-bread dough
B
B
A
A
C
C
Expanding universe
Big Bang
n  Big Bang theory: idea that universe began at a specific moment in
time and has been expanding ever since
n  Universe began as an infinitely hot, dense concentration of energy
n  Evidences for big bang
- Universal expansion
- Cosmic microwave background (CMB) radiation: microwave
radiations fall on Earth from all directions in a form of background
signals (continuous noise) à hot universe at early time still give
off electronic radiation and it would be characteristic of an object
at a few degrees above absolute zero à confirmed by satellite
telescope data
- Abundance of Light elements: abundance of hydrogen,
helium-3,4 and lithium are already predicted in Big Bang theory
Expanding universe
History of universe
n  All matter heats up when it is compressed
n  Universe began from a hot, dense state and has been
expanding and cooling ever since
n  Time of changes of state in universe according to changes of
temperature è ‘freezing’ (imagine the freezing of water)
Dark universe
Dark matter
n  Visible world in Universe is only 4 % of entire universe!
n  Dark matter: interact only with gravitationally
n  First evidence for dark matter:
galactic rotational curve
observed
Galactic rotational curve
expected
Dark universe
Dark energy - future of universe
Fates of universe
n  Open universe: gravity (mass of universe) < Hubble expansion
n  Closed universe: gravity > Hubble expansion à ‘Big Crunch’
n  Flat universe: gravity = Hubble expansion
Dark energy: a source of force acting to push galaxies apart
à a sort of antigravity
n  Fates of universe depend on amount (or density) dark energy
Quiz 1
n  The presence of CMB radiation is good evidence for
A. the internal makeup of the Sun
B. the account of creation in Genesis
C. the steady-state universe
D. the big bang theory
Quiz 1
n  The presence of CMB radiation is a good evidence for
A. the internal makeup of the Sun
B. the account of creation in Genesis
C. the steady-state universe
D. the big bang theory
Quiz 2
n  The Milky Way, the galaxy within which we live, is
A. an irregular dwarf galaxy
B. a quasar galaxy
C. an elliptical galaxy
D. a spiral galaxy
Quiz 2
n  The Milky Way, the galaxy within which we live, is
A. an irregular dwarf galaxy
B. a quasar galaxy
C. an elliptical galaxy
D. a spiral galaxy
Quiz 3
n  A large assembly of stars together with gas, dust, and other materials
that is held together by the forces of mutual gravitational attraction is
called
A. a solar system
B. a planetary system
C. a galaxy
D. the cosmos
Quiz 3
n  A large assembly of stars together with gas, dust, and other materials
that is held together by the forces of mutual gravitational attraction is
called
A. a solar system
B. a planetary system
C. a galaxy
D. the cosmos