Download Astronomy 101 Course Review and Summary

Astronomy 101
Course Review and Summary
Main Topics:
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The Night Sky
History of Astronomy & Science
Light and Matter
The Solar System
Structure and Evolution of Stars
Structure and Evolution of Galaxies
Structure and Evolution of the Universe
The Night Sky
The Celestial Sphere
The sky as seen from Earth is divided into 88 constellations.
It is convenient to pretend the stars are attached to a celestial
sphere.
The celestial sphere appears to rotate about the celestial
poles (1 day).
The Sun appears to move west to east relative to stars (1
year).
The Moon appears to move west to east relative to stars (1
month).
Celestial Sphere: A large imaginary sphere centered on Earth
Season & Calendars
The cause of the seasons is the tilt of the
Earth’s rotation axis relative to its orbit
around the Sun.
The day is based on the time between one noon
and the next.
The year is based on the time between one
vernal equinox and the next.
The moon (month) is based on the time
between one new moon and the next.
Moon Phases & Eclipses: Key Concepts
Lunar phases change as we see more or less of the Moon’s
sunlit half.
The Moon rotates about its axis as it revolves around the
Earth.
The sidereal month=27.3 days; the synodic month=29.5
days.
A lunar eclipse occurs when the Moon passes through the
Earth’s shadow.
A solar eclipse occurs when the Earth passes through the
Moon’s shadow.
Solar eclipses occur when Moon
is between Sun and Earth.
Solar eclipses occur at NEW MOON.
A lunar eclipse occurs when the Moon
passes through the Earth’s shadow.
Lunar eclipses occur when Earth is between
Sun and Moon.
Lunar eclipses occur at FULL MOON.
From Ptolemy to Copernicus: Key Concepts
Aristotle (4th cent BC) showed that the Earth is round.
Greek astronomers developed a geocentric model for the
universe.
Ptolemy (2nd cent) used epicycles to explain retrograde
motion of planers.
Copernicus (16th cent) proposed a heliocentric model for the
universe.
In the model of Copernicus, retrograde motion is easily
explained.
The combination of small and
large circles produces
“loop-the-loop” motion.
Tycho, Kepler, & Galileo: Key Concepts
Tycho Brahe made accurate measurements of planetary
motion.
Planetary orbits are ellipses with the Sun at one focus.
A line between planet & Sun sweeps out equal areas in equal
times.
The square of a planet’s orbital period is proportional to the
cube of its average distance from the Sun.
Galileo made telescopic observations supporting the
heliocentric model.
Kepler’s First Law
of planetary motion
The orbits of planets around the Sun are
ellipses with the Sun at one focus.
Kepler’s Second Law
of planetary motion
A line
from the
Sun to a planet
sweeps
out equal areas in equal time intervals.
Kepler’s Third Law
of planetary motion
The square of a planet’s orbital period is
proportional to the cube of its average
distance from the Sun*:
*A planet’s average distance from the Sun is equal to the
semimajor axis of its orbit.
Newton’s Laws
Three Laws of Motion:
(1)An object remains at rest, or moves in a straight line at
constant speed, unless acted on by an outside force.
(2) The acceleration of an object is directly proportional to
force, and inversely proportional to mass.
(3) For every action, there is an equal and opposite reaction.
Law of Gravity:
The gravitational force between masses M and m, separated
by distance r, is proportional to the product of the masses
divided by the square of the separation
Applying Newton’s Laws
Newton modified and expanded Kepler’s Laws
of Planetary Motion.
Kepler described how planets move; Newton
explained why they move.
Tides are caused by the difference between the
Moon’s gravitational force on different sides
of the Earth.
Tidal forces are slowing the Earth’s rotation &
enlarging the Moon’s orbit.
Newton’s First Law of Motion:
An object remains at rest, or moves in a
straight line at constant speed, unless acted
on by an outside force.
Precise mathematical laws require precise definitions of
terms:
SPEED = rate at which an object changes its position.
Example: 65 miles/hour.
VELOCITY = speed plus direction of travel.
Example: 65 miles/hour to the north.
Newton’s Second Law of Motion:
The acceleration of an object is directly
proportional to the force acting on it, and
inversely proportional to its mass.
In mathematical form:
Or alternatively:
Newton’s Third Law of Motion:
For every action, there is
an equal and opposite reaction.
Whenever A exerts a force on B, B exerts a
force on A that’s equal in size and opposite
in direction.
All forces come in pairs.
Kepler’s
Third
Law:
Light
Visible light is just one form of
electromagnetic radiation.
Light can be though of as a wave or as a
particle.
Light forms a spectrum from short to long
wavelengths.
A hot, opaque object produces a continuous
blackbody spectrum.
Light forms a spectrum from short to
long wavelength
Visible light has wavelengths from 400 to 700
nanometers. [1 nanometer (nm) = 10-9
meter]
Color is determined by wavelength:
Blue: 480 nm
Green: 530 nm
Red: 660 nm
Visible light
occupies
only a tiny
sliver of the
full
spectrum.
Matter and Forces
Matter can come in various forms that are
composed of fundamental particles
An element is known by it number of protons
Isotopes of an element contain different
number of neutrons
Isotopes can be radioactive and spontaneously
decay
There are four fundamental forces (Gravity,
Electromagnetism, Strong, and Weak)
Hydrogen
1 proton
1H
2H
3He
4He
6Li
7Li
Helium
2 protons
Lithium
3 protons
Proton:
Neutron:
3H
Spectra
A hot, transparent gas produces an emission
spectrum.
A cool, transparent gas produces an
absorption spectrum.
Every type of atom, ion, and molecule has a
unique spectrum.
The most abundant elements in the universe are
hydrogen and helium.
The radial velocity of an object is found from
its Doppler shift.
Continuum
Source
Cloud
Solar System
Solar System Constituents
The terrestrial planets are made primarily of
rock and metal.
The Jovian planets are made primarily of
hydrogen and helium; also have large
amounts of water, methane, and ammonia
Moons (a.k.a. satellites) orbit the planets; some
moons are large.
The terrestrial planets are made primarily of
rock and metal.
Mercury, Venus, Earth, &
Mars.
The terrestrial planets are:
low in mass (< Earth mass)
high in density (> 3900 kg/
m3).
Water = 1000 kg/m3
Air = 1 kg/m3
Rock = 3000 kg/m3
The Earth
The study of seismic waves tells us about the
Earth’s interior.
The Earth is layered into crust, mantle, inner
core, and outer core.
The Earth is layered because it underwent
differentiation when molten.
The crust is broken into plates that move
relative to each other.
Seismic waves
radiating
through the
Earth after an
earthquake:
Note: S waves do
not travel
through the
outer core!
The Moon
The Moon’s surface has both smooth maria
and cratered highlands.
The surface was shaped by heavy
bombardment, followed by lava floods.
The Moon has a thick crust but a tiny ironrich core.
The Moon may have been ejected when a
protoplanet struck the Earth.
Computer
simulation of
impact:
Mantle of the
colliding body
was ejected to
form the Moon.
Iron core of the
colliding body
sank to the
Earth’s center.
Mercury
Mercury has a 3-to-2 spin-orbit coupling (not
synchronous rotation).
Mercury has no permanent atmosphere
because it is too hot.
Like the Moon, Mercury has cratered
highlands and smooth plains.
Mercury has an extremely large iron-rich
core.
Radius of
Mercury =
2400 km.
Radius of iron
core =
1800 km.
Venus
The surface of Venus is hidden from us by
clouds of sulfuric acid.
The atmosphere of Venus is hot because of a
runaway greenhouse effect.
The surface of Venus shows volcanic activity
but no plate tectonics.
The interior of Venus is similar to that of the
Earth.
The interior of Venus is similar to that of
the Earth.
Uncompressed
density of Venus
= uncompressed
density of Earth
= 4200 kg/m3.
Venus probably
has a metal core
and rocky
mantle, like the
Earth.
Mars
Mars has a tenuous atmosphere, with little
water vapor and few clouds.
(Mars has large volcanoes and a huge rift
valley, but no plate tectonics.
Robotic “rovers” have given us a close-up
look at Mars.
Mars has two small irregular moons, Phobos
and Deimos.
Mars
interior
:
Jupiter and Saturn
Jupiter and Saturn consist mainly of hydrogen
and helium.
Jupiter and Saturn have belts and zones of
clouds, plus circular storms.
Jupiter and Saturn have magnetic fields
created in metallic hydrogen.
Differences between Jupiter and Saturn are
due to Jupiter’s higher mass.
All Jovian planets have rings.
Jupiter and
Saturn are
differentiated.
Moons of Jupiter and Saturn
The Galilean Moons of Jupiter:
Callisto: heavily cratered
Ganymede: larger then Mercury
Europa: covered with smooth ice
Io: volcanically hyperactive
The Giant Moon of Saturn:
Titan: wrapped in an atmosphere
4 of the moons of Jupiter are large (> 3000 km
across) and spherical (like our Moon).
These are the four Galilean moons:
Io, Europa, Ganymede, Callisto
Titan: Saturn’s ATMOSPHERIC moon
Nearly the same size as
Ganymede: escape
speed is the same.
Twice as far from
the Sun as
Ganymede:
temperature is
lower.
Titan, alone among
moons, has a
Uranus and Neptune
Uranus and Neptune are nearly identical in
their internal structure.
The rotation axis of Uranus is tilted by about
90 degrees, causing extreme seasons.
Neptune has surprisingly strong storms,
driven by internal heat.
Triton, the giant moon of Neptune, is a cold
world with nitrogen geysers.
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Interiors of Uranus and Neptune
Gaseous atmosphere: hydrogen, helium, methane
Liquid outer layer: hydrogen, helium
Liquid or slushy mantle: water, ammonia
Solid core: rock, metal
Triton: Neptune’s Frosty Moon
Surface
temperature =
38 Kelvin.
Covered with
“frost” at
poles: frozen
methane,
frozen
nitrogen.
Pluto and its moon Charon are icy worlds that resemble Triton.
Eris, the troublemaker (Greek goddess of strife).
The Kuiper belt, beyond Neptune, contains small, icy, Plutolike objects.
The icy Kuiper Belt Objects are leftover planetesimals.
Comets are “dirty snowballs”: ice mixed with dust & carbon
compounds.
Most comets are in the Kuiper belt or the Oort cloud, far from
the Sun.
A comet or asteroid impact may have caused the extinction of
dinosaurs
Studies of the Outer Solar System continue.
Pluto and Charon have many properties in
common with Neptune’s moon Triton.
•  Cold surfaces (about 40 Kelvin)
•  Icy mantles and rocky cores (about
2000 kg/m3)
•  Pluto has a thin atmosphere (like Triton);
Charon has none.
Eris (“Xena”), the troublemaker.
Discovered in 2005 by Mike
Brown and collaborators.
It has a moon.
It is BIGGER than Pluto!
Led to re-definition of
what a Planet is
Created new class of
object called “Dwarf
Planets
Most comets are in the Kuiper belt or the
Oort cloud, far from the Sun.
Comets with short orbital periods come from
the Kuiper belt,
30-50 A.U.
from the Sun.
We know the Kuiper
belt is full of icy
objects – we
have seen them!
Origin of the Solar System: Key
Concepts
How the Solar System formed:
A cloud of gas & dust contracted to form a
disk-shaped solar nebula.
The solar nebula condensed to form small
planetesimals.
The planetesimals collided to form larger
planets.
When the Solar System formed:
Radioactive age-dating indicates the Solar
System is 4.56 billion years old.
The contraction of the solar nebula made it spin
faster and heat up. (Compressed gas gets hotter.)
Temperature of solar nebula:
> 2000 Kelvin near Sun; < 50 Kelvin far from
Sun.
How does this “nebular theory” explain the
current state of the Solar System?
Solar System is disk-shaped:
It formed from a flat solar nebula.
Planets revolve in the same direction:
They formed from rotating nebula.
Terrestrial planets are rock and metal:
They formed in hot inner region.
Jovian planets include ice, H, He:
They formed in cool outer region.
Radioactive age-dating
Radioactive decay: Unstable atomic nuclei emit
elementary particles, forming a lighter, stable
nucleus.
Example: Potassium-40 (19 protons + 21 neutrons = 40)
89% of the time, Potassium-40 decays to
Calcium-40.
11% of the time, Potassium-40 decays to
Argon-40.
Age of oldest Earth rocks = 4 billion years
Age of oldest Moon rocks = 4.5 billion years
Age of oldest meteorites (meteoroids that
survive the plunge to Earth) = 4.56 billion
years
This is the age of the Solar System
The Structure and
Evolution of
Stars
•  Distance is important but hard to measure
•  Trigonometric Parallaxes
–  direct geometric method
–  only good for the nearest stars (~500pc)
•  Units of distance in Astronomy:
–  Parsec (Parallax second)
–  Light Year
Parallax decreases with distance
Closer stars have larger parallaxes:
Distant stars have smaller parallaxes:
Parallax Formula
p = parallax angle in arcseconds
d = distance in “Parsecs”
•  Luminosity of a star:
–  total energy output
–  independent of distance
•  Apparent Brightness of a star:
–  depends on the distance by the inverse-square
law of brightness.
–  measured quantity from photometry.
Flux-Luminosity Relationship:
Relates Apparent Brightness (Flux) and
Intrinsic Brightness (Luminosity) through the
Inverse Square Law of Brightness:
•  Color of a star depends on its Temperature
–  Red Stars are Cooler
–  Blue Stars are Hotter
•  Spectral Classification
–  Classify stars by their spectral lines
–  Spectral differences mostly due to Temperature
•  Spectral Sequence (Temperature Sequence)
•  O B A F G K M L T
The Spectral Sequence
O B A F G K M LT
Hotter
50,000K
Bluer
Cooler
2000K
Redder
Spectral Sequence is a Temperature Sequence
•  Types of Binary Stars
– Visual
– Spectroscopic
– Eclipsing
•  Only way to measure stellar masses:
–  Only ~150 stars
•  Radii are measured for very few stars.
Center of Mass
•  Two stars orbit about their center of mass:
a1
a2
M2
a
M1
•  Measure semi-major axis, a, from projected orbit
and the distance.
•  Relative positions give: M1 / M2 = a2 / a1
Measuring Masses
Newton’s Form of Kepler’s Third Law:
•  Measure Period, P, by following the orbit.
•  Measure semi-major axis, a, and mass
Ratio (M1/M2) from projected orbit.
Summary of Stellar Properties
•  Large range of Stellar Luminosities:
–  10-4 to 106 Lsun
•  Large range of Stellar Radii:
–  10-2 to 103 Rsun
•  Modest range of Stellar Temperatures:
–  3000 to >50,000 K
•  Wide Range of Stellar Masses:
–  0.1 to ~50 Msun
•  The Hertzsprung-Russell (H-R) Diagram
–  Plot of Luminosity vs. Temperature for stars.
•  Features:
–  Main Sequence (most stars)
–  Giant & Supergiant Branches
–  White Dwarfs
•  Luminosity Classification
Luminosity (Lsun)
H-R Diagram
Supergiants
106
104
102
Giants
1
10 -2
10 -4
40,000
White Dwarfs
20,000
10,000
Temperature (K)
5,000
2,500
Main Sequence
•  Most nearby stars (85%), including the Sun,
lie along a diagonal band called the
•  Main Sequence
•  Ranges of properties:
–  L=10-2 to 106 Lsun
–  T=3000 to >50,0000 K
–  R=0.1 to 10 Rsun
Giants & Supergiants
•  Two bands of stars brighter than Main
Sequence stars of the same Temperature.
–  Means they must be larger in radius.
•  Giants
R=10 -100 Rsun L=103 - 105 Lsun T<5000 K
•  Supergiants
R>103 Rsun L=105 - 106 Lsun T=3000 - 50,000 K
White Dwarfs
•  Stars on the lower left of the H-R Diagram
fainter than Main Sequence stars of the
same Temperature.
–  Means they must be smaller in radius.
–  L-R-T Relation predicts:
R ~ 0.01 Rsun (~ size of Earth!)
•  Main Sequence:
–  Strong correlation between Luminosity and
Temperature.
–  Holds for 85% of nearby stars including the sun
•  All other stars differ in size:
–  Giants & Supergiants:
Very large radius, but same masses as M-S stars
–  White Dwarfs:
Very compact stars: ~Rearth but with ~Msun!
Mass-Luminosity Relationship
•  For Main-Sequence stars:
In words:
“More massive M-S stars are more luminous.”
Not true of Giants, Supergiants, or White Dwarfs.
•  Observational Clues to Stellar Structure:
–  H-R Diagram
–  Mass-Luminosity Relationship
–  The Main Sequence is a sequence of Mass
•  Equation of State for Stellar Interiors
–  Perfect Gas Law
–  Pressure = density × temperature
•  Stars are held together by their self-gravity
•  Hydrostatic Equilibrium
–  Balance between Gravity & Pressure
•  Core-Envelope Structure of Stars
–  Hot, dense, compact core
–  cooler, low-density, extended envelope
•  Stars shine because they are hot.
–  need an energy source to stay hot.
•  Kelvin-Helmholtz Mechanism
–  Energy from slow Gravitational Contraction
–  Cannot work to power the present-day Sun
•  Nuclear Fusion Energy
–  Energy from Fusion of 4 1H into 1 4He
–  Dominant process in the present-day Sun
•  Energy generation in stars:
–  Nuclear Fusion in the core.
–  Controlled by a Hydrostatic “thermostat”.
•  Energy is transported to the surface by:
–  Radiation & Convection in normal stars
–  Conduction in white dwarf stars
•  With Hydrostatic Equilibrium, these
determine the detailed structure of a star.
•  Main Sequence stars burn H into He in their
cores.
•  The Main Sequence is a Mass Sequence.
–  Lower M-S: p-p chain, radiative cores &
convective envelopes
–  Upper M-S: CNO cycle, convective cores &
radiative envelopes
•  Larger Mass = Shorter Lifetime
Putting Stars Together
•  Physics needed to describe how stars work:
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Law of Gravity
Equation of State (“gas law”)
Principle of Hydrostatic Equilibrium
Source of Energy (e.g., Nuclear Fusion)
Movement of Energy through star
Proton-Proton Chain:
3-step Fusion Chain
CNO Cycle:
Main Sequence Membership
•  For a star to be located on the Main
Sequence in the H-R diagram:
–  must fuse Hydrogen into Helium in its core.
–  must be in a state of Hydrostatic Equilibrium.
•  Relax either of these and the star can no
longer remain on the Main Sequence.
The Main Sequence is a Mass Sequence.
•  The location of a star along the M-S is
determined by its Mass.
–  Low-Mass Stars: Cooler & Fainter
–  High-Mass Stars: Hotter & Brighter
•  Follows from the Mass-Luminosity
Relation:
•  Luminosity ~ Mass3.5
Main Sequence Lifetime
•  How long a star can burn H to He depends
on:
–  Amount of H available = MASS
–  How Fast it burns H to He = LUMINOSITY
•  Lifetime = Mass ÷ Luminosity
•  Recall:
Mass-Luminosity Relationship:
•  Luminosity ~ Mass3.5
Main Sequence Lifetime
•  Therefore:
•  Lifetime ~ 1 / M2.5
•  The higher the mass, the shorter its life.
•  Examples:
Sun: ~ 10 Billion Years
30 Msun O-star: ~ 2 Million years
0.1 Msun M-star: ~ 3 Trillion years
Low Mass Stellar Evolution:
• Stages:
• Energy Source:
• Main Sequence
• Red Giant
• Horizontal Branch
• Asymptotic Giant
• White Dwarf
• H Burning Core
• H Burning Shell
• He Core + H Shell
• He Shell + H Shell
• None!
HighMass Stellar Evolution:
• Similar Stages as
Low Mass Stars
initially:
• Main Sequence
• Red Giant
• Horizontal Branch
• SuperGiant
• Additional Stages as
ever heavier elements are
used as fuel
• Ne, O, Si also can be
used
• Elements up to Iron (Fe)
• Fe not useful though,
because nucleus is too
tightly bound
Interior Structure of High Mass
Star towards the end of its life:
H Burning
Shell
He Burning
Shell
Core Radius: ~1 R
C Burning
Shell
Ne Burning
Shell
Inert
Fe-Ni
Core
O Burning
Shell
Si Burning
Shell
Envelope: ~ 5 AU
End of the Road
•  At the end of the Silicon Burning Day:
–  Star builds up an inert Fe core
–  Series of nested nuclear burning shells
•  Finally, the Fe core exceeds 1.2-2 Msun:
–  Fe core begins to contract & heat up.
–  This collapse is final & catastrophic
End of Life for a High Mass Star:
•  End of the Life of a Massive Star:
–  Burn H through Si in successive cores
–  Finally build a massive Iron core.
•  Iron core collapse & core bounce
•  Supernova Explosion:
–  Explosive envelope ejection
–  Main sources of heavy elements
Stellar Remnants:
•  White Dwarf:
–  Remnant of a star <8 Msun
–  Held up by Electron Degeneracy Pressure
–  Maximum Mass ~1.4 Msun
•  Neutron Star:
–  Remnant of a star < 18 Msun
–  Held up by Neutron Degeneracy Pressure
–  Pulsar = rapidly spinning neutron star
Using Clusters to Test Stellar
Evolution
•  Cluster H-R Diagrams give us a snapshot of
stellar evolution.
•  Observations of clusters with ages from a
few Million to 15 Billion years confirms
much of our picture of stellar evolution.
•  Remaining challenges are in small details,
but the big picture seems secure.
Using Clusters to Test Stellar
Evolution:
•  H-R Diagrams of Star Clusters
•  Ages from the Main-Sequence Turn-off
•  Open Clusters
–  Young clusters of few 1000 stars
–  Blue Main-Sequence stars & few giants
•  Globular Clusters
–  Old clusters of a few 100,000 stars
–  No blue Main-Sequence stars & many giants
The Structure and Evolution of
Galaxies
The Milky Way:
•  The Milky Way is our Galaxy
–  Diffuse band of light crossing the sky
–  Galileo: Milky Way consists of many faint stars
•  The Nature of the Milky Way
–  Philosophical Speculations: Wright & Kant
–  Star Counts: Herschels & Kapteyn
–  Globular Cluster Distribution: Shapley
The Milky Way and Other
Galaxies:
•  Disk & Spheroid Structure of the Galaxy
•  Pop I Stars:
–  Young, metal-rich, disk stars
–  Ordered, nearly circular orbits in the disk
•  Pop II Stars:
–  Old, metal-poor, spheroid stars
–  Disordered, elliptical orbits in all directions
•  Gives clues to the formation of the Galaxy.
Other Galaxies:
•  Three basic types of Galaxies:
–  Spirals
•  Disk and spheroid component
•  Rotation of disk allows measurement of galaxy mass
–  Ellipticals
–  Irregulars
•  Differ in terms of
–  Relative Gas content
–  Star Formation History
–  Internal Motions
•  Galaxies tend to group into Clusters
–  Groups, clusters, and superclusters
–  Galaxies can collide and merge
•  Some galaxies have “active” nuclei
–  Powered by large Black holes in the center
The Structure and Evolution of
the Universe
Special Relativity:
•  Postulates of Special Relativity:
–  The laws of physics are the same for all
uniformly moving observers.
–  The speed of light is the same for all observers.
•  Consequences:
–  Different observers measure different times,
lengths, and masses.
–  Only spacetime is observer independent.
General Relativity:
•  General Relativity:
–  Modern Theory of Gravitation
–  Matter tells spacetime how to curve.
–  Curved spacetime tells matter how to move.
•  Tests of General Relativity:
–  Perihelion Precession of Mercury
–  Bending of Starlight near the Sun
–  The Binary Pulsar (Gravitational Waves)
Expansion of the Universe:
•  Hubble’s Law:
–  Galaxies are receding from us.
–  Recession velocity gets larger with distance.
•  Hubble Constant:
–  Rate of expansion of the Universe.
•  Cosmological Redshift:
–  Redshift distances
–  Redshift maps of the Universe.
Cosmology:
•  Cosmological Principle:
–  The Universe is Homogeneous and Isotropic on
Large Scales.
–  No special places or directions.
•  General Relativity predicts an expanding
universe.
•  Cosmological Constant
Big Bang:
•  Big Bang Model of the Universe
–  Starts in a hot, dense state
–  Universe expands and cools
•  Expansion and Redshift
•  Critical Density
–  Geometry of the Universe
•  Hubble Time = Maximum age of the
Universe
Evidence for the Big Bang:
•  Fundamental Tests of the Big Bang
•  Primordial Nucleosynthesis
–  Primordial Deuterium & Helium
–  Primordial light elements (Li, B, Be)
•  Cosmic Background Radiation
–  Relic blackbody radiation from Big Bang
–  Temperature: T = 2.726 K
First 3 Minutes in the History of
the Universe:
•  Physics of the Early Universe
–  Informed by experimental & theoretical physics
•  The Cosmic Timeline:
–  Observations go back to t~3 minutes
–  Reasonably firm physics back to t~10-6 sec
–  Speculative back before t~10-12 sec
–  Present theories stop at t~10-43 sec
Critical Density
•  All galaxies attract each other via gravity.
–  Gravitational attraction slows the expansion.
•  How it behaves depends on the density:
–  High Density: Expansion slows, stops, &
reverses.
–  Low Density: Keeps expanding forever.
•  Dividing Line = “Critical Density”
Density Parameter: Ω
•  Ω>1: High Density “Closed” Universe
•  Ω=1: Critical Density “Open” Universe
•  Ω<1: Low Density “Open” Universe
End of Universe:
•  The Fate of the Universe depends on the
density of matter.
•  Closed Universe:
–  Enough matter to stop the expansion
–  Collapses in a “Big Crunch”
•  Open Universe:
–  Expands forever
–  Ends in a cold, disordered state
–  Dark Energy seems to make this outcome more
likely
Physics
•  Gravity
–  Newton & Relativity
–  Dynamics (how things move)
•  Radiation
–  Photons
–  Electromagnetic spectrum
•  Thermodynamics
–  Pressure, density, temperature
–  Degenerate electron/neutron gas
•  Thermonuclear fusion
Connect to stuff here at home
•  Gravity
–  Things fall!
–  Things move!
•  Radiation
–  How we see
–  How we manipulate “light”
•  X-rays
•  Radio
•  microwaves
Connect to stuff here at home
•  Thermodynamics
–  How refrigerators work
–  The “pressure” you feel on your ears at the
bottom of a pool
•  Thermonuclear fusion
–  Boom!
Three Questions:
1) What is it?
–  Describe it: how bright, far, energetic, etc.
2) How does it work?
–  Underlying Physics (testable theories)
3) How does it evolve?
–  How does it form, develop & end its
existence?
Scientific process
•  Answer questions
–  Look for inconsistencies
–  Alter ideas
–  Look for things not understood
•  New things to investigate
–  Dark matter
–  Dark energy
–  New forms of matter and energy