ISP 205, Section 3, Spring 2003, Prof. Stein
UNIT IV: THE UNIVERSE
A. Observations of the Universe
B. The Big Bang -- History of
C. Inflation -- Before the Big Bang
D. Fate of the Universe
E. Formation of Structure in
the Universe: Galaxies & Clusters of Galaxies
Science starts with questions. What are some generic Questions can we
ask about the Universe?
How can we get any information about the universe?
Need relics from the early universe that have survived to this day.
Are several relics from the earlier universe.
- Light from distant galaxies.
Light travels at a finite speed, 3x105 km/s, takes time to
reach us from objects far away.
See objects far away as they were long ago.
- Elements formed in first few minutes of the Universe
- Light from when universe became transparent = CMB
- Reading: Voyages, secs 4.1, 4.2, 4.6
- How do we see?
- Seeing activity
- An object must give off light, and our eye must receive the light,
in order to see the object.
- That light can be emitted by the object (e.g., a lightbulb or
- Or, light from another source can be reflected off the object
(a pen reflecting a lightbulb or the Moon reflecting sunlight)
- If there is no light, can't see an object
- What is light?
- Light is an Electromagnetic Wave.
- changing electric and magnetic force.
- wavelength (lambda) = distance between waves;
- period (P) = time between waves;
- frequency (f) = number of waves per unit time
- Demo: weighted rope
- Analogy: Buses
- [BE: 1747 (wave)]
- Mechanical Universe, Program 40: chapt 4 - light waves;
10 - Newton; 11 - Huygens; 12-15 - waves; 17 - lines of
force; 19 - oscillating charge; 40 - telescope
- Light is a Stream of Photons
- Higher Energy Photons = Shorter Wavelength Light
- ephoton = hf = hc/lambda
- Spectrum of Light
x-rays (< 20 nm) (high energy, short wavelength)
ultraviolet (20 nm - 0.4 microm)
visible yellow } lambda = 0.4-0.7 micrometers
infrared (1 microm - 1 mm)
microwave (1 mm - 1 cm)
radio (> 1 cm) (low energy, long wavelength)
[Examples: AST disk: radio 470, MW 471, visible 473, UV 474, XR 475]
- Doppler Shift
- Light from approaching source is shifted blueward, light from a
receding source is shifted redward.
- bus analogy
- water tank demo video
- Reading: Voyages, sec 25.5
- All but nearest galaxies are red shifted - moving away from us
- Universe is Expanding
- Hubble Law - The farther away a galaxy is the faster it is moving
away from us.
- measuring velocities: Doppler shift
- Redshift -
- measuring distances: Brightness
- Farther away appear fainter
- Need to know how bright really is - standard candle
- Brightest objects = supernova (exploding stars)
- Must calibrate brightness
- Expansion Activity
- Expansion Activity Results
- Hubble Law
Velocity = Hubble constant x Distance
- Must make sure have a representative sample of the universe.
- Stars are organized into galaxies.
Andromeda Galaxy (M31)
- Galaxies are organized into clusters of galaxies.
- Clusters of galaxies are organized into superclusters.
- Need to sample several superclusters to have a representative sample.
No center to universe
- Demo: blowing up balloon, stretching rubber band
- Space is expanding, not bound objects
- Neither we, nor Earth, nor the Sun or Solar
system, nor the Milky Way or any galaxy, nor the
clusters of galaxies are expanding. Only the distance
between clusters of galaxies and isolated galaxies is
Age of Universe
- V = D/t = H D or D = Vt = V/H
- H = 1/t
- Farther away, moving away faster, took same time to get there
- Implication: everything started from same place at same time
- t(age) = 1/H (must be careful of units)
- Example of converting units
- Original Hubble distances 10 x too small.
Found age ~ 2 billion years. Less than age of
oldest rocks on Earth.
- Current distance scale 10 x larger, age 13-15 billion years.
Scale of the Universe
- if Milky Way is size of earth, then distance to horizon is 17 AU
(size of Uranus orbit)
Night sky is dark
- Olber's paradox
- If look out, line of sight should eventually hit a star
- Like looking through forest - eventually blocked by a tree
- Sky should be as bright as surface of star. Why isn't it?
- Universe has only existed for finite time
- Can't see far enough away to be blocked by star
- Universe is expanding
- Light from distant stars is red shifted to very low
- Reading: Voyages, secs 2.2, 2.3, 23.1, 23.2, 23.3
Is the expansion of the universe constant, slowing down or speeding
- Newton's Theory of Gravity
- Every object attracts every other object by force of gravity.
- A Force is a push or a pull.
- Gravity is a pull.
- Source of Gravity is Mass (amount of matter, not volume):
More Mass -> stronger gravity.
- Larger Distance -> weaker gravity.
Fgravity = G M m / D2
- Here Fgravity is the force of gravity between the two masses
M & m, D is the distance between the two masses and G is a number to
make units come out correctly.
- Gravity holds us on Earth
- Gravity holds Earth in orbit around Sun
- Gravity holds Sun in Milky Way
- Prediction: Attraction between all the galaxies and
clusters of galaxies in the universe should slow down the
expansion of the universe!
- Demo: paddle ball
- Einstein's Theory of Gravity
- Gravity is Geometry: Mass warps space-time. Warped
space-time controls how objects move.
- Everything travelling through the same space moves on the
Light is attracted by gravity: Bending of Light by the Sun
Video: Mechanical Universe, program 25, chapter 29
- Light has energy, no mass
Energy = Mass (E=mc2):
- TWO types of energy
- Kinetic Energy: energy of motion
move faster -> more energy -> more mass
- Potential Energy: (work needed to overcome a force)
Repulsive force -> must work to bring objects together ->
more energy when close together
Attractive force -> must work to separate objects ->
more energy when far apart
- Demo - spring
- Forces contribute to gravity
- Repulsive force = pressure -> more gravity
- Negative pressure -> less gravity
Enough negative pressure and the total mass energy
plus pressure can become negative and gravity changes
from an attractive to a repulsive force. Wierd!
Observation: Expansion of the Universe is accelerating
Conclusion: There must be something with negative
pressure that provides much of the mass of the universe at the
- Reading: Voyages, sec 28.4
- Matter is made of atoms.
- Atom - Different atom for each element.
- Nucleus orbited by Electrons.
- Nucleus - Composed of protons (+ charge) and neutrons (0 charge).
- Contains most of atoms mass.
- Electrons (- charge) - Orbit nucleus, attracted by electric force.
- Electric Force -
- Produced by charge,
- Opposite charges attract, like charges repel.
- Decreases with distance (like gravity)
- Element determined by number of protons in nucleus
- (H has 1 proton, He has 2 protons, C has 6 protons).
- [BE: 1764 (He atom)]
- Crude Model - atom = solar system
- If person = proton or neutron,
electron = cotton candy orbiting 100 km (60 mi) away (Flint).
- If nucleus = raisin, electron=400 m away, next atom 2.5-25
- If sun = raisin, Earth = 1 m away, nearest star 300 km away
- Early Universe was Hot and Dense
- Expansion -> cooling (example blowing on food)
- Expansion -> things farther apart (lower density)
- Going back in time, when universe was smaller, it was
hotter and denser than today.
- In beginning, universe so hot collisions destroyed nuclei
as fast as they formed.
- When universe cooled to about 50 times hotter than center
of Sun, nuclei could hold together
- protons + neutrons -> deuterium (H with extra neutron)
- deuterium + proton + neutron -> helium
- amount of deuterium depends on density in early universe
- Higher density -> less deuterium
- More gets converted to He
- Absorption of light from distant quasars by clouds of hydrogen gas
lying between the quasar and us at large redshift (z>2)
- Conclusion: Ordinary matter is only 4% of all the matter
in the universe.
- Question: what is the rest of the matter?
- Reading: Voyages, sec 28.4
- The universe is expanding ->
the universe was hotter and denser in the past than now.
- example: blowing on hand
- Young universe was OPAQUE.
- When universe was about 1/1000 its present size, electrons could not
stay attached to protons as H atoms
- temperature = 3000K (half that of the surface of the Sun)
- photons had high energy
- atoms and electrons moved fast and collided hard
- every time an electron became attached to a proton to make a hydrogen
atom it got zapped by a photon or another electron with enough
energy to rip them apart again.
- Before this time electrons and protons were not attached to each
other, but moved around by independently themselves.
- Photons are easily scattered by such free electrons (the direction
they move is changed).
- Photons could not travel far -> universe was opaque
- example: fog bank
- Photons move freely through the universe since it became transparent.
- Universe is filled with these photons
- They come at us from all directions
- Radiation is almost isotropic -- photons from all directions have
nearly the same energy
- Spectrum of photon energy is that of thermal equilibrium (blackbody)
- Equilibrium = in balance, everything at same temperature
- Temperature of universe is now 2.725 K
- Universe was once hot an dense enough for photons and
matter to be in thermal equilibrium
- Universe is nearly isotropic and homogeneous
- If universe is so isotropic and homogeneous, how did
the stars, galaxies and clusters of galaxies -- the nonhomogeous
structure of the universe -- form
- The Earth is moving through space, why don't we see the Doppler
shift of the CMB photons.
- Answer: early instruments were not sensitive enough to detect
variations of 1:1000.
- Newer, more sensitive instruments do see Doppler effect.
- Solar system is moving at 300 km/s through universe.
- Milky Way galaxy is moving at 600 km/s through universe.
- Now see fluctuations of dT/T=10-5, the seeds of present
- The fluctuation spectrum
- Size of fluctuations at time universe became transparent is just
right to produce the galaxies and clusters of galaxies we see today.
- Space is flat (no large scale curvature, only locally)
- Universe is expanding at its escape velocity
- Escape velocity = speed needed to just escape pull of gravity
- Normal matter is only 4% of that needed to make gravity strong
enough so expansion velocity = escape velocity. What is the rest?
- There is some (unkown) kind of dark matter that contributes about
30% of the matter (gravity) in the universe.
- From motions of stars in galaxies and galaxies in clusters of
galaxies, we know there is 7-8 times more gravity than can be
produced by ordinary matter.
- From bending of light from distant quasars around galaxies and
clusters of galaxies, called gravitational lensing, we know there
is 7-8 times more gravity than can be produced by ordinary matter.
- 2/3 of the gravity in the universe must be produced by something
strange that provides the mass to generate the gravity but has a
negative pressure to make the expansion of the universe accelerate.
- Composition of the Universe:
- 4% ordinary matter (protons, neutrons, electrons, neutrinos)
- 30% cold dark matter (not ordinary, but no pressure)
- 66% something with negative energy
- 1. Steady State Model
- Problem: Earth can't be older than universe.
- expansion age originally determined by Hubble
was 2 billion years,
- age of oldest rocks on earth is 3.5 billion years.
- Solution: Universe has existed forever. Average properties
of the universe do not change.
- Galaxies move apart, new hydrogen atoms form to fill space,
condense into new galaxies.
- Developed theory of heavy element formation in stars.
- Disproved, but was productive
- Prediction: universe does not change
- Quasars seen only long ago (far away).
- Cosmic Microwave Background radiation ->
universe was once hot and dense
- 2. Big Bang Model
- Expansion -> cooling.
- In the past, matter and radiation were hotter and denser.
- Era of Equilibrium
- Very hot. All nuclear reactions very fast.
- Photons have enormous energy, destroy nuclei.
Too hot for nuclei.
- Era of Primordial Nucleosynthesis, 3 min < t < 30 min:
- 109 > T > 107 K.
- Cool enough for nuclei (photons have too little
energy to destroy nuclei),
- Fuse protons and neutrons -> deuterium and helium.
- Scale of universe = 10-9 to 10-7 present
- Era of Radiation, 30 min < t < 3-4 hundred thousand years:
- Too cold for fusion, too hot for atoms.
- Free electrons, protons, helium nuclei and photons.
- Universe opaque.
- Recombination, Decoupling, t = 3-4 hundred thousand years:
- T = 3000 K
- size=1/1000 present, redshift = 1000
- Cool enough for atoms. Electrons and nuclei combine.
- Universe becomes transparent.
- Matter and radiation no longer in thermal
equilibrium with each other
- Scale of universe = 10-3 present
- Era of galaxies, t > million years:
- Clouds of hydrogen and helium gas contract
to form galaxies.
- photons from when universe was hot
- helium and deuterium from primordial nucleosynthesis
See Fig P17, page 12.
January 1, midnight: Big Bang
January 1, 2 hrs after midnight: Universe becomes transparent, CMB
September 10: Formation of the Sun and solar system
December 31, early evening: First humans
- 3. Tests: relics of the big bang
- (i) 3 K Background Radiation - photons reaching us from when universe became transparent
- a. Existence of 3 K background radiation means universe
was once hot.
- b. Present temperature allows us to calculate temperature
of early universe.
- c. Uniformity of background radiation shows universe was
- d. Determine our motion through universe
- (ii) Helium and Deuterium
- 1. Problems with the Big Bang
- a. Why Hot?
- b. Why so uniform - 3 K radiation?
- c. Why nearly flat?
- d. Origin of irregularities that become stars and galaxies?
- 2. Solution -- Inflation
- Very early universe (Equilibrium Era) expanded enormously
- smooths out fluctuations, flattens space-time.
- 3. Singularity?
- Infinite density, singularity, at beginning of universe
implies gravity so strong quantum effects important
- Classically: either existed forever, or else began in singularity
- Quantum mechanics: possible for space-time to be finite,
but have no boundary, no singularity
- Example: Vertex of cone (origin of time) vs. N pole on sphere
(not a special location)
- Time just like spatial coordinates, if beginning of time
rounded like sphere then not a special point
- Implication: universe self-contained, Not created or destroyed.
- 1. Expansion velocity vs. Escape velocity
- Is universe expanding fast enough to continue forever, or will
gravitational attraction of all matter for each other slow it
down enough to bring expansion to halt and start universe
- How determine Fate of universe?
- Measure Expansion Velocity
- Determine Hubble Constant, H
Recent Hubble Space Telescope Results
- Measure Escape Velocity (strength of gravity)
- a. measuring mass density
- galaxies -> expansion velocity =
10 x escape velocity
- clusters of galaxies -> expansion velocity
= 3 x escape velocity
- b. measuring the deceleration due to gravity
number density of galaxies as a function of
expansion velocity -> expansion velocity = escape velocity
- c. deuterium abundance depends on density of matter
-> expansion velocity = 3 x escape velocity
- 2. Geometry of the Universe
- a. If expansion velocity > escape velocity
- Universe will expand forever
- Universe is infinite
- Universe has saddle like geometry
- b. If expansion velocity = escape velocity
- Universe will expand forever
- Universe is infinite
- Universe has flat geometry
- c. If expansion velocity < escape velocity
- Universe will stop expanding, and contract
- Universe is finite, but has no boundary
- Universe has sphere like geometry
- 3. A Detailed Prediction of Fate of the Universe under 2.(a) or 2.(b) Geometry
- a. 100-4 billion years: Radiation-Dominated Era
- assumes the Big Bang Formation Scenario as above
- Ends at Recombination
- b. 106-14 billion years: Stelliferous Era
- Galaxies (groups of stars) are the building blocks of the Universe
- Most star formation occurs when galaxies collide, which is
- We are now at 1010.2
- Ends when no new stars form
- c. 1015-37 billion years: Degenerate Era
- all that's left is remnants
from stars are black holes, white dwarfs, neutron stars,
planets and failed stars
from galaxies with super-massive black holes, only the black hole
- Ends when the protons decay and destroy everyything with protons
- d. 1038-100 billion years: Black Hole Era
- only black holes, as they have no protons
- Ends when the black holes evaporate via Hawking radiation
- e. 10100+ billion years: Dark Era
- photons (low-energy), electrons, postitrons, neutrinos
- never ends
- [American Scientist, May-June 1997, pp 223-225]
- [Laughlin et al., Reviews of Modern Physics, April 1997]
"To doubt everything or to believe everything are two equally convenient solutions:
both dispense with the necessity of reflection."
- Henri Poincare' (1854-1912)
- 1. Classification by Shape (Morphology)
- a. Spirals (~15%)
- i. Disk
- Young stars, gas and dust
- ii. Spherical component - Halo and Nucleus
- Old stars, no gas or dust
- S0 galaxies - unusual variety of spiral
- i. Disk - Old stars, no gas or dust
- ii. Spherical component - Nucleus and Halo
- b. Ellipticals (~70%)
- Spherical component - Nucleus and Halo
- Old stars, no gas or dust
- No disk. Some rotate.
- c. Irregular galaxies - like Milky Way's Magellanic Clouds
- d. Unusual Galaxies
- look like one of the 3 types above, but disturbed
- Chain or Merging #39 from HST DEEP
- 2. Classification by Spectra
- a. Normal galaxies (~95%)
- stellar spectrum - absorption lines in continuous spectrum
- b. Active galaxies (~5%)
- non-thermal emission
- i. Radio Galaxies
- Excess radio emission.
- Synchotron radiation from very fast electrons
moving through a magnetic field
- Double lobed large emission regions.
Often connected to galaxy by jets.
- ii. Quasars
- Broad emission lines. Large redshift -> large distance.
- Bright yet far away -> very luminous. Source of energy?
- Brightness varies in time of hours -> small size
(= solar system)
- Quasars occur in galaxies
A gallery of quasar images
- c. Center of Milky Way
- Radio emission
- Broad emission lines -> high speed (orbital or thermal) ->
high mass (= 10^6 Msun in size of solar system)
- d. Model
- Need large amount of energy
- Only source is Gravitational Potential Energy of
supermassive Black Hole
- Gas falls in, gets very hot, emits energetic photons,
Rotates rapidly, collimates jets
rotation speed of gas and stars near centers of
- 3. Large Scale Distribution
- Clusters of galaxies.
- Spirals found mostly in field, small groups, and
outer regions of clusters.
- Ellipticals and S0 dominate in centers of rich clusters.
- Galaxies distributed as on surfaces of intersecting bubbles.
- Most galaxies on arcs where bubble surfaces intersect,
- Next most galaxies on bubble surfaces (sheets),
- Fewest galaxies in interior of bubbles (voids).
- 4. Formation
- Gravitational Instability
- Slight excess matter -> slight excess gravity
- Matter falls into gravitational well, increases gravity
- More matter pulled in by more gravity
- Magnitude of density variations grows
- different orbits of old and young stars
- current interactions with satellite galaxies
- super-clusters of galaxies just forming now
- oldest observed galaxies seem to be smaller than now
- Bottom up Formation
- Largest initial irregularities in density of matter on small
scales, so small masses of matter (size of globular
clusters) collapse first
- smaller masses are stopped from collapsing by pressure
- Need DARK MATTER to make large enough variation in mass
and hence gravity
- Ordinary matter pulled in by mass of dark matter
- Progressively larger masses merge (pulled together by
gravity) and collapse to form new larger bound systems -
galaxies, clusters of galaxies and super-clusters
- Gas which accretes smoothly settles in a rotating
disk-like structure and is slowly transformed into stars ->
disks of spiral galaxies
- Violent mergers convert disks into spheroidal
distributions and produce large bursts of star formation ->
central bulges of spiral galaxies and elliptical galaxies
- Galaxy mergers common
- Spirals form by collapse of radiatively cooling gas cloud.
- Ellipticals probably form by mergers of spirals
Simulation of Merger
- Most stars are in large galaxies like the Milky Way
- smaller structures merge to form large galaxies
- gas in super large galaxies would not have had time
to cool yet to form stars
- Ancient Galaxies from Hubble
Field is a sample of galaxies back to 1 billion years after
the big bang from HST. Section of sky about as big as the period
after this sentence.
Interactive HST DEEP Field
Same galaxies back to 1 billion years after the big bang from HST
with information on individual galaxies available by clicking
- What do other nearby galaxies look like?
- Mostly they come in 3 types:
- Irregulars - Magellanic Clouds [BE 3052,3055]
- Spirals - Andromeda [Be 2723, 2727]
- Ellipticals - companions to Andromeda
- Note: color of stars, dust, globular clusters
- Which type is the Milky Way?
- 1. Recognition of the Nature of the Milky Way
- a. Band of Light across the sky - Thin Disk of Stars [BE 2659]
- b. Star distribution - sun near center
"Kapteyn Universe" of 1922
- c. Globular Cluster distribution
- BE 1963,1965,1966,1967,1968]
- spherical distribution, diameter 105 LY, centered off to one side
- Sun =3x104 LY from center of Galaxy
- d. Interstellar dust - obscures distant stars
- e. Island Universe vs. One of Many Galaxies
- 2. Components of the Milky Way
- a. Disk
- Stars of 0-10 billion-years old, includes Open (galactic) clusters.
[Visible Light BE 2659]
- Clouds of dust. (IR shows dust BE 2669)
- Clouds of gas. [Radio: neutral hydrogen BE 2661, cold H2 BE 2664]
- Orbits nearly circular, lie in common plane.[BE 2686]
- (Similar to planets in solar system.)
- Spiral Arms
- Overhead view [BE 2650-2652], "Exploded" Structure
- Apparent in
- massive, young, blue, main sequence stars
(not intermediate age stars like the Sun),
- Massive stars have such short lives,
don't have time to move from their birth place.
- H gas distribution - radio observations
- Birthplace of Stars
- Density Wave Theory
- spiral arms are compression waves that move through galaxy gathering stars and clouds.
- "Disk in a Dishpan" demo showing disk stars move like a fluid
- Rotation curve shows the speed of gas depends on place in disk.
- b. Spherical Component
- i. Halo
- Old (10-18 Billion years) Stars, mostly red giants and red MS stars.
- Globular Clusters. (Little gas or dust.) [Andromeda: BE 2726]
- Orbits elongated ellipses, randomly oriented.
(Similar to comets in solar system.)
- ii. Nuclear Bulge (flattened spheroid)
- Bulge at center of Galaxy [in direction of Sagittarius: BE 2686].
- Old and young stars.
- Hot Gas and Dust.
- iii. Center of Galaxy
- High velocity clouds of hot gas
- 5x105 MSun within 10 AU. Black hole?
- [BE 2585,2589,2602]
- c. Companion Galaxies
Magellanic Clouds, dwarf spheroidals
- Diagram of Milky Way
- 4. Mass of the Milky Way
- Kepler's Law M=D3 / P2
- Rotation curve
- 5. Formation Models
- Must explain the structure and ages of components
Links to other Galaxy and Cosmology resources
NASA Introduction to Cosmology
University of Oregon cosmology course notes
History of the Universe
The Big Bang by Mike Guidry
Cosmology: a Research Briefing
Hubble Space Telescope DEEP Field: Interactive Mode
Collision of Milky Way and Andromeda
Galaxies from Univ. Alabama
Index of Astronomy Pictures of the Day
This page will be updated continually throughout the course.
2004.01.11 (Sunday) 23:28:33 EST
This page has been accessed
Visions of the Universe
Bob Stein's home page,