ISP 205, Section 3, Spring 2003, Prof. Stein
 
UNIT IV: THE UNIVERSE
 
OUTLINE

* Prologue: Questions
* A. Observations of the Universe
* B. The Big Bang -- History of the Universe
* C. Inflation -- Before the Big Bang
* D. Fate of the Universe
* E. Formation of Structure in the Universe: Galaxies & Clusters of Galaxies

QUESTIONS to ask about the UNIVERSE

Science starts with questions. What are some generic Questions can we ask about the Universe?

A. OBSERVATIONS of 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.
  1. 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.
  2. Elements formed in first few minutes of the Universe
  3. Light from when universe became transparent = CMB

LIGHT

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 the Sun)
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)
                  blue      \
                  green     |
   visible        yellow    }    lambda = 0.4-0.7 micrometers
                  orange    |
                  red       /
   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

UNIVERSE is EXPANDING

Reading: Voyages, sec 25.5
All but nearest galaxies are red shifted - moving away from us (Doppler shift)
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
brightness -
Expansion Activity
Expansion Activity Results

Hubble Law


 
Velocity = Hubble constant x Distance

 

Fair sample

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.

Coma

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 expanding.

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?
  1. Universe has only existed for finite time
  2. Can't see far enough away to be blocked by star
  3. Universe is expanding
  4. Light from distant stars is red shifted to very low energy

GRAVITY

Reading: Voyages, secs 2.2, 2.3, 23.1, 23.2, 23.3

Is the expansion of the universe constant, slowing down or speeding up?

  1. 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

  2. Einstein's Theory of Gravity

    Gravity is Geometry: Mass warps space-time. Warped space-time controls how objects move.
    Analogy: marbles

    Predictions:
    Everything travelling through the same space moves on the same path.
    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):

    Energy
    TWO types of energy
    1. Kinetic Energy: energy of motion
      move faster -> more energy -> more mass
    2. 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 (speeding up)

Conclusion: There must be something with negative pressure that provides much of the mass of the universe at the moment

RELIC ELEMENTS

Reading: Voyages, sec 28.4
Atoms
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 mi away.
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
Prediction:
amount of deuterium depends on density in early universe
Higher density -> less deuterium
More gets converted to He

Observation:
D/H=(3+-0.2)x10-5
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?

RELIC LIGHT

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

Conclusions:

Questions:
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 structure
  Large Scale

  Small Scale

The fluctuation spectrum

Conclusions:
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

B. 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
Tests:
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.
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 < 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.
Predictions:
photons from when universe was hot
helium and deuterium from primordial nucleosynthesis

Timeline:

See Fig P17, page 12.
January 1, midnight: Big Bang
January 1, 2 hrs after midnight: Universe becomes transparent, CMB radiation emitted
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 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)

A. 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 Coma Cluster

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

  movie

Clues:
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
Tests
Galaxy mergers common
Cannibal galaxy
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
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.

B. 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

Links to other Galaxy and Cosmology resources

* NASA Introduction to Cosmology
* University of Oregon cosmology course notes
* Cosmic Background Radiation
* 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.
Updated: 2004.01.11 (Sunday) 23:28:33 EST
This page has been accessed times.

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