ISP 205, Section 3, Fall 1999, Prof. Stein

UNIT IV: THE UNIVERSE

OUTLINE


 * A. Our Galaxy, The Milky Way
 * B. Galaxies, Normal & Peculiar
 * C. Observations of the Universe
 * D. Evolution of the Universe
 * E. Formation of the Universe
 * F. Fate of the Universe

A. The MILKY WAY, OUR GALAXY

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
M(R)=V2R/G

5. Formation Models

Must explain the structure and ages of components

B. GALAXIES

1. Classification by Shape (Morphology)

Galaxy images
a. Spirals (~15%)
   
i. Disk
Young stars, gas and dust
Rotates
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. None close.
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
Evidence: rotation speed of gas and stars near centers of galaxies.

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).

Virgo Cluster Coma Cluster

4. Formation

Clues:
different orbits of old and young stars
current interactions with satellite galaxies
1. Theory 1: Top Down
Largest mass clouds collapse first -> clusters of galaxies.
Fragment into galaxies.
Stars formed during contraction have elongated, spherically distributed orbits.
As gas contracts, rotates, forms disk. Stars formed from the disk have disk-shaped orbits.
2. Theory 2: Bottom up
Globular cluster mass clouds collapse first.
Gravitationally cluster into larger and larger hierarchies to form galaxies and clusters of galaxies.
Tests
Observations indicate may need both theories
Galaxy mergers common
Cannibal galaxy
Spirals probably form by collapse of radiatively cooling gas cloud.
Ellipticals probably form by mergers of spirals
  Simulation of Merger
Ancient Galaxies from Hubble
HST DEEP 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 on them.

C. OBSERVATIONS of the UNIVERSE

1. Fair sample

Universe is uniform and isotropic
When look out in space are looking back in time, takes time for light to travel from far away to us

2. Expansion of Universe

All but nearest galaxies are red shifted - moving away from us (Doppler shift)
Universe is Expanding
Hubble Law

Velocity = Hubble constant x Distance

Determining the Hubble constant
measuring velocities: Doppler shift
measuring distances:
geometry - parallax
brightness -
cluster main sequences
Cepheid variables
Cepheid Variables in M100 from HST
globular clusters
HII regions
brightest galaxies in clusters
Redshift -
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)
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 15-20 billion years.
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 expanding.

Scale of the Universe
if Milky Way is size of earth, then distance to horizon is 17 AU (size of Uranus orbit)

3. 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 energy

D. EVOLUTION of the UNIVERSE

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
Test: Quasars seen only long ago (far away).

2. Big Bang Model

Expansion -> cooling.
In the past, matter and radiation were hotter and denser.
Scenario
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 < million years:
Too cold for fusion, too hot for atoms.
Free electrons, protons, helium nuclei and photons.
Universe opaque.
Recombination, Decoupling, t = million years:
T = 3000 K
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.
Predictions:
photons from when universe was hot
helium and deuterium from primordial nucleosynthesis

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 very uniform.
Cobe Results

d. Determine our motion through universe
(ii) Helium and Deuterium

E. FORMATION of the UNIVERSE

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. Just is.

F. FATE of the UNIVERSE

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 recontracting?
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 common
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)

Links to other Galaxy and Cosmology resources
 * Wendy Freedman - The Expansion Rate and Size of the Universe (Sci.Amer.)
 * mpeg movies of interacting galaxies
 * Hubble Space Telescope DEEP Field: Interactive Mode
 * Collision of Milky Way and Andromeda
 * Galaxies from Univ. Alabama
 * Cosmic Background Radiation
 * Index of Astronomy Pictures of the Day
 * The Big Bang by Mike Guidry
 * Introduction to Cosmology
 * Cosmology: a Research Briefing

This page will be updated continually throughout the course.
Updated: 1999.09.08 (Wednesday) 17:36:05 EDT
This page has been accessed times.


Visions of the Universe
Bob Stein's home page, email: steinr@pilot.msu.edu