ASTR 1120 General Astronomy: Stars

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ASTR 1020 Introductory
Astronomy II Stars Galaxies
Week 14 (14 April) Galaxies, galaxy
clusters Determining distances the distance
ladder Dark matter, dark energy
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Announcements
Observing (last chance) Wed at
830 Brian Greene lecture Wed _at_
730 (Macky) Next Tuesday (21
April) Meet in Fiske New Mastering Astronomy
Homework posted Note Attendance in class
is manditory! (I use clickers to take
attendance)
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Galaxies
HST Hickson CG 44

• Spirals mostly in small groups of (3-10 galaxies)

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• Ellipticals - most often in dense clusters of
  galaxies (involve 100s to 1000s)
• Why? Chapter 21

HST Abell 1689
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Elliptical
Spiral
Irregular
Lenticular
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Hubble Tuning fork diagram
Spirals
Ellipticals
Barred spiral
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The Big Picture Universe is filled with network
of galaxies in groups and clusters
100 billion galaxies!
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Pattern of galaxies (3 million),15o portion of
sky
Brighter more galaxies
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What are the Magellanic Clouds?
Clicker Question

• Two nebulae in disk of Milky Way visible only in
  southern hemisphere
• Clouds of dust and gas in many places throughout
  the Milky Way galaxy
• Two small galaxies that orbit Milky Way
• Star-forming clouds in constellation Orion

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Our Local Group of galaxies
3 spirals Andromeda (M31) 3/2 MMW Milky Way 1
MMW Triangulum (M33) 1/5 MMW 2
irregulars LMC 1/8 MMW SMC 1/30 MMW 16
dwarfs
21 Galaxies
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Biggest is Andromeda (Sb - M31)

• Andromeda is 3 million light years away
  (or 30 MW diameters), has 1.5 mass of MW
• We see her as she was 3 million years ago, not as
  she is today! this is lookback time
• She may crash into MW in about 5 billion years!

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Andromeda (M31) UV
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Triangulum (M33)

• 1/5 mass of MW, spiral classified as Sc
• Several bright (pink) star forming regions

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Triangulum (M33) Visual
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Triangulum (M33)21 cm atomic H
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Large Small Magellanic Clouds
SMC
LMC
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LMC has 30 Doradus, home of SN 1987A
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How do we get distances to things far outside our
Galaxy?
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The Distance Ladder
Solar System (radar, etc.) gt the AU 1.5x1013
cm Parallax D 206,265 / p()
AU to 500 pc (0.002) in
the visual to 105 pc (10
micro-arcsec) at radio Clusters and Variable
stars (Cephids) to 10
Mpc (107 pc) Galactic rotation curves
(Tully-Fischer) Type Ia supernovae (CO white
dwarfs in close-binaries that accrete)
to gt 1 Gpc (109 pc) The recession of
galaxies (Hubble Expansion)
V 71 (km/s per Mpc) DMpc
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Mapping the Universe We need Distances to
Galaxies!
The problem
or

• Methods we are familiar with
• Radar and Stellar parallax

Only useful inside the SS
A few thousand ly
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New Methods Bootstrap our way

• Identify (and calibrate) objects that could serve
  as STANDARD CANDLES -- beyond direct
  measurement
• 1. Make some measure of an object which
  identifies its luminosity
• 2. Use this luminosity and measure apparent
  brightness to infer distance to it

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Main-Sequence Fitting
DISTANCE ESTIMATE 1

• Start with cluster A (upper) whose distance known
  via parallax
• Compare with other cluster B (lower)
• Get distance to B from brightness difference

A
B
Distances up to 200,000 light years
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Which cluster is closer?
Clicker Question

• Hyades
• Pleiades
• Not enough information to tell

A
B
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Main Sequence Fitting pinned to nearby Hyades
Cluster
Only 151 ly away
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Cepheid variable stars
DISTANCE ESTIMATE 2

• Instability strip -- region in H-R diagram with
  large, bright stars
• Outer regions of star are unstable and tend to
  pulsate
• Star expands and contracts, getting brighter and
  fainter

Reminder (Fig 15.14)
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Cepheid variable stars
DISTANCE ESTIMATE 2
Period - Luminosity relation
brighter Cepheids have longer periods
(Hummingbirds, Humans, and Elephants)
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Cepheid Period-Luminosity relation

• Henrietta Leavitt (1868-1921)
• Working at the Harvard College Observatory
  discovered the relation in 1912

• Died 3 years before Edwin Hubble made one of his
  most important discoveries using her results

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Cepheids variables as standard candles
DISTANCE ESTIMATE 2

• 1. Measure period of variability
• 2. From period-luminosity relation, infer the
  luminosity
• 3. Compare with apparent brightness and thus
  determine distance

Cepheid variable in M100 (HST) 65 Million light
years away!!
Distances up to 100 million ly
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Two Cepheid stars, Fred and Barney, have the same
apparent brightness. Fred has a period of 5 days,
and Barney of 10 days. Which is closer ?
Clicker Question

• Fred
• Barney

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Why A. Fred ?
Period-Luminosity Relation

• Fred has a shorter period and so must be less
  luminous (hummingbird)
• Less luminous but the same apparent brightness
  means that Fred is closer to us

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Tully-Fisher Relation
DISTANCE ESTIMATE 3

• Fast rotation speeds in spiral galaxies
• ? more mass in galaxy
• ? higher luminosity
• Measure rotation speeds to infer luminosity
• Need bright edge-on spirals, estimate tilt

Distances up to 10 billion ly
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Even brighter White dwarf supernovae
DISTANCE ESTIMATE 4

• Nearly the same amount of energy released every
  time.
• why?
• Standard explosion fusion of 1.4 solar
  masses of material

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Bright enough to be seen halfway across
observable universe
Useful for mapping the universe to the largest
distances
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Practical difficulty White dwarf SN

• Need to catch them within a day or two of the
  explosion
• About 1 per galaxy per century

• Need to monitor thousands of galaxies to catch a
  few per year ? galaxy clusters are useful

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Why are white dwarf supernovae useful for
distance measurements?
Reading Clicker Question

• They only go off in nearby galaxies so we can
  easily tell how far away they are.
• They last for a long time (months) so we have
  plenty of time to see them.
• Since many stars are in binary systems, they
  happen quite regularly so there are plenty to
  study.
• They all explode with the nearly the same
  brightness no matter if they are near or far.
• We can measure the amount of time it takes for
  the material they blow off into space to reach us
  and get a distance that way.

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Distance Ladder to measure universe
Different standard candles are useful for
different distances
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ASTR 1020 Introductory
Astronomy II Stars Galaxies
Week 14 (16April) The Hubble Expansion Mergers,
starburst, AGN, quasars, radio galaxies Galaxy
clusters and X-ray clusters Dark matter, dark
energy
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Andromeda found to be far outside Milky Way!

• Edwin Hubble in 1924 identified Cepheids in
  Andromeda (M33) ? showed they were far outside of
  Milky Way!
• Island Universes
• His first big discovery!
• But then he turned his attention to OTHER galaxies

Hubble using new 100 Hooker telescope at Mt.
Wilson (above LA)
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100 Hooker telescope at Mt Wilson
Begins new era in 1924
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Hubble showed universe appeared to be expanding!

• Vesto Slipher (1912) reported that most galaxies
  showed Doppler redshifts
• Edwin Hubble, using new 100 telescope, started
  busily measuring galaxy redshifts
• Hubble (1929) announced that redshifts of
  galaxies appear to increase with distance from us
• This was startling Suggested an EXPANDING
  UNIVERSE !

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How did Edwin Hubble get distances and redshifts?

• Distances He made an incorrect, but lucky
  standard candle assumption
• The brightest object in all galaxies was always
  the same luminosity
• Redshifts Looked at the spectra from these
  galaxies and match with expected spectra

REFERENCE
DISTANT GALAXY
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Hubbles Law
v Ho ? d
Velocity of Recession (Doppler Shift)
Hubbles Constant
Distance
(km/sec)
(km/sec/Mpc)
(Mpc)
Hubbles (1929) original
Scatter here from random velocities of nearby
galaxies, (unreliable distance estimates)
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velocity
Best current values for expansion Ho 71 /- 4
km/s/Mpc
distance
HUBBLES CONSTANT
Hubble (1929) plot extended only to 2 Mpc, Ho was
500!
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How fast is the Universe Expanding?

• Measuring Ho is hard v Ho x d
• Ho v /
  d
• Nearby galaxies random motions through space
  similar to expansion velocity. Doppler shifts
  we measure are not purely from the expanding
  universe
• Far away galaxies large expansion velocities,
  but hard to measure distances

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Hubble Space Telescope was designed to settle this

• High resolution images to find faint Cepheid
  variable stars in very distant galaxies

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Use Hubbles Law itself to estimate vast distances
DISTANCE ESTIMATE 5

• Measure velocity, then D v / Ho
• Example using Ho 71 km/sec/Mpc,
• and finding that v 710 km/sec
• D (710 km/sec) / (71 km/sec/Mpc) 10 Mpc
• ? 33 million light years

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Another reason to measure Ho

• The Hubble constant also provides the age of the
  universe!
• How?
• Imagine the expanding universe going backwards in
  time
• Expanding universe suggests that in the past
  everything was much closer together
• A single infinitely dense origin of all space,
  matter, energy
• The Big Bang

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Your friend leaves your house. She later calls
you on her cell phone, saying that shes been
driving at 60 mph (miles per hour) directly away
from you the whole time and is now 60 miles away.
Without looking at your watch, can you tell how
long has she been gone?
Clicker Question

• A. Yes, 1 minute
• B. Yes, 30 minutes
• C. Yes, 60 minutes
• Yes, 120 minutes
• No, not enough information to tell

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The Age of the Universe

• IF the universe has been expanding at the same
  speed always
• Distance velocity ? time ? time
  distance/velocity
• Hubbles Law v Ho ? D ? Ho
  velocity/distance
• Time (Age) 1 / Ho
• For 71 km/sec/Mpc Age 13.7 billion years
• For larger Ho, shorter time
• For smaller Ho, longer time

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Is this anywhere near correct?

• Age of the solar system 4.6 billion years
• Age of the oldest star clusters 13 billion
  years
• General agreement, but well revisit the
  assumption of constant expansion soon

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No matter which direction we look, we see
galaxies moving away from us. Therefore, we must
be at the center of the expansion.
Clicker Question

• True
• False

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the Expanding Universe

• NOT like an explosion of galaxies THROUGH space
  from a center place
• The space BETWEEN galaxies is expanding, carrying
  the galaxies way from each other
• Why dont galaxies themselves expand? Gravity!

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Balloon analogy for expanding universe

• On an expanding balloon, no galaxy is at the
  center of expansion no edge
• Expansion happens into a higher dimension (2-D
  surface into a 3-D space)

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If we were alive 5 billion years ago, would we
measure a different Hubble Constant?
Clicker Question

• Yes, it would be higher
• Yes, it would be lower
• No change

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Balloon analogy for expanding universe
REVIEW

• On an expanding balloon, no galaxy is at the
  center of expansion no edge
• Expansion happens into a higher dimension (2-D
  surface into a 3-D space)
• Is our 3-D space expanding through a 4th
  dimension?

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Number of Fuzzier Distance Estimators

• A. Apparent brightness of (resolved) red and
  blue supergiants
• B. Size and brightness of ionization nebulae or
  starbirth regions
• C. Intercompare distances so deduced for
  specific galaxies (overlapping rungs in distance
  ladder)

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Measuring big distances to galaxies

• STANDARD CANDLES -- important ones in
  distance ladder, or chain
• 1. Main-sequence fitting
• 2. Cepheid variables
• 3. Tully-Fisher relation
• 4. White dwarf supernovae

Brightness Luminosity / (Distance)2

• 5. Hubble Expansion of the cosmos

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Making ellipticals

• Higher density much faster star formation uses
  up all the gas
• Nothing left to make a disk
• or
• Lower spin
• Gas used up before angular momentum took over
• Now we see a sphere of old stars

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Or now a different story.

• Spiral galaxy collisions destroy disks, leave
  behind elliptical
• Burst of star formation uses up all the gas
• Leftovers train wreck
• Ellipticals more common in dense galaxy clusters
  (centers of clusters contain central dominant
  galaxies)
• So what?

NGC 4038/39 Antennae
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Colliding Galaxies NGC 4676
Mice with HST Advanced Camera for Surveys
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Stephans Quintet in HST detail
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A mature exampleElliptical shape but with dust
lanes?
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It may happen to us in future!
Andromeda (M31) in future
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Messages From Galaxy Interactions

• In dense clusters, galaxy collisions (grazing or
  even head-on) must have been common
• With successive passages, spiral galaxies can
  tumble together to form a big elliptical
• Vastly increased star birth from shocking the gas
  and dust (starburst galaxies coming up next!)
• Start rapid feeding of supermassive black hole
  lurking at center of most galaxies (quasars
  coming up soon!)

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Starburst Galaxies
M82 - visible
Chandra X-ray

• Milky Way forms about 1 new star per year
• Starburst galaxies form 100s of stars per year

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M82 Starburst Result of interaction with M81
NGC3077
M82 - M81 - in visual
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M82 Starburst interaction
NGC 3077
M 81
M 82
M82 - M81 - in 21 cm HI (radio)
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Vigorous star birth The Antennae
HST detail NGC 4038/39
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Starburst galaxies emit most of their light at
infrared wavlengths

• Star formation heats dust to very hot
  temperatures
• Hot dust glows strongly in the infrared
• Much evidence for galactic fountains and giant
  supernova-driven galactic winds
• Usually triggered by galaxy collisions or close
  passages of another galaxy

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Active Galactic Nuclei Another Type of Galactic
Fireworks

• Galaxies with strange stuff going on in their
  centers
• Some galaxies at high redshift (large lookback
  times) have extremely active centers
• More than 1000 times the light of the entire
  Milky Way combined from a point source at the
  center!!

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Quasars

• Quasi-Stellar Radio Source
• Nuclei so bright (at nearly all wavelengths) that
  the rest of the galaxy is not easily seen
• First discovered as radio sources - then found to
  have very high redshifts!

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Sources of the radiation from bright nuclei in
active galaxies

• Thermal radiation from a massive star cluster
• Emission lines from hot gas
• 21 cm from hydrogen gas
• H-alpha from hydrogen gas
• Synchrotron radiation from a black hole

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Synchrotron

• Synchrotron light is bright at both radio and
  X-ray wavelengths (far ends of the spectrum)

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Whatever is powering these QSOs must be very
small!!

• Some quasars can double their brightness within a
  few hours.
• Therefore they cannot be larger than a few
  light-hours across (solar system size)
• Why? Think about the time it takes light from the
  front of the object to get to us compared to the
  light from the back.

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Quasar Central Engines

• How do quasars emit so much light in so little
  space?
•  
• They are powered by accretion disks around
  supermassive black holes
•  
• In some quasars, huge jets of material are shot
  out at the poles. These jets are strong radio
  sources.

JET
DISK
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Central Engine -- artists conception

• Accretion disk around super-massive black hole
• Inner parts of disk may or may not be obscured by
  dust
• If bright nucleus is visible, looks like a
  quasar, if not, then its a radio galaxy

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M87
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M 87 Elliptical-galaxy In Virgo
cluster Active Galactic Nucleus
(AGN) Syncrotron jet from super-massive black
hole central
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Prototypical radio galaxy
Giant elliptical galaxy NGC 5128 with dust
lane (from spiral galaxy?) Centaurus A
radio source (color lobes)
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Cygnus A radio jets
400,000 ly
Jet as fine thread, big lobes at end, central hot
spot
VLA
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Radio tails many shapes
NGC 1265 100K ly
3C 31 2 M light years
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M87 elliptical with jet
800 km/s 60 ly away

• Active galactic nucleus beams out very narrow jet
• Accretion disk shows gas orbiting a 2.7 billion
  solar mass black hole first real proof !

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Another example of central beaming engine
radio
active nucleus - HST

• 400 light year wide disk of material in core of
  elliptical galaxy with radio jets looks like a
  supermassive black hole at work!

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The Cosmological Principle
The universe looks about the same no matter where
you are within it

• Matter is evenly distributed on very large scales
  in the universe
• No center no edges
• Not proven but consistent with all observations
  to date

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Wilkinson Microwave Anisotropy Probe
1) Matter is evenly distributed on very large
scales in the universe

• WMAP showed that the universe is, for the most
  part, isotropic (physically equal in all
  directions)
• Variations in above image are at the .001 level!!

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How can the universe have no edge?
2) Universe has no center no edges

• Space, like the surface of the Earth, can curve
• If it curves enough, space can join back on
  itself no edge!
• Or, its just INFINITE!

87
Distance (in an expanding universe)

• Say it takes 400 million years for light to get
  from galaxy A to us in Milky Way
• Yet during travel in spacetime, both A and MW
  have changed positions by expansion
• Thus distance is a fuzzy concept LOOKBACK
  TIME is more accurate

TIME
A
MW
DISTANCE
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Since the universe is expanding, light traveling
through the universe feels the stretch as it
travels

• Cosmological Redshift

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Redshift often expressed in the form of z
Dont need to memorize!

• Present day z 0
• Furthest galaxy z 7-10
• Cosmic Microwave Background z 1089
• Big Bang z 8

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What does the expansion of the universe most
accurately mean?
Clicker Question

• Galaxies are moving apart through space
• Space itself is expanding
• Everything is expanding, including the earth, our
  bodies, etc
• The Milky Way is at the center of the universe
  and all other galaxies are expanding away from us.

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Chapter 21 Galaxy Evolution

• Observing galaxies at different redshifts
  (lookback times)
• ?
• Allows us to assemble a sequence of galaxies
  showing birth and evolution
• ?
• Check via computer models of gas, gravity and
  star formation

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The Hubble Deep Field
Galaxies to z4!
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Making of a spiral galaxy

• Start with a fairly uniform cloud of hydrogen
• Gravitational collapse forms protogalactic clouds
• First stars are born in this spheroid (such stars
  are billions of years old ? fossil record)

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Small variant in spiral making

• Several smaller protogalactic clouds may have
  merged to form a single large galaxy
• May explain slight variations in stellar ages in
  the MW

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Forming a disk with spiral

• As more material collapses, angular momentum
  spins it into a disk
• Stars now formed in dense spiral arms disk
  stars are younger!

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Or now a different story.

• Spiral galaxy collisions destroy disks, leave
  behind elliptical
• Burst of star formation uses up all the gas
• Leftovers train wreck
• Ellipticals more common in dense galaxy clusters
• So what?

NGC 4038/39 Antennae
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Why are collisions between galaxies more likely
than between stars within a galaxy?
Clicker Question

• Galaxies are much larger than stars
• Galaxies travel through space much faster than
  stars
• Relative to their sizes, galaxies are closer
  together than stars
• Galaxies have higher redshifts than stars

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What is meant by dark energy?
Reading Clicker Question

• The energy associated with dark matter through
  Emc2
• Whatever it is that may be causing the expansion
  of the universe to accelerate.
• Any unknown force that opposes gravity
• Highly energetic particles that are believed to
  constitute dark matter
• The total energy in the universe after the Big
  Bang but before the first stars

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Quasars
REVIEW

• Quasi-Stellar Radio Source
• Nuclei so bright (at nearly all wavelengths) that
  the rest of the galaxy is not easily seen
• First discovered as radio sources - then found to
  have very high redshifts!

100
Do ALL galaxies have supermassive black holes?

• probably YES!
• Part of normal galaxy formation?
• More quasars seen in the distant (early) universe
  than now
• Black holes gradually grow, but can run out of
  available fuel and become nearly invisible (like
  in our Milky Way)

101
Somehow, the rest of the galaxy knows about the
SMBH during formation!!
102
Resurrected by galaxy collisions?

• Many galaxies with bright nuclei show signs of
  being disturbed
• Collisions funnel material down into the black
  hole lurking at the core
• Expect more such collisions in denser early
  universe
• This may help explain why fewer quasars today

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Quasars reveal Protogalactic Clouds

• Looking for gas between the galaxies
• Cold, invisible, too dim even at 21 cm
• But quasars provide the way to detect them!

Simulation of universe
104
Use quasars as bright beacons see absorption
lines from intergalactic gas
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Quasar spectra
Redshifted from emission lines Many
absorption lines (forest)
Lyman Alpha Forest
106
Now on to Case for Dark Matter Chapter 22

• gt 90 of mass of universe is dark matter
  (invisible, missing matter)
• Detectable ONLY via its gravitational forces on
  luminous matter (gas and stars)
• Note -- this dark matter is NOT the same as black
  holes, brown/black dwarfs, or dust

107
Spiral galaxy ROTATION CURVES

• Discovered by Vera Rubin in the 1970s
• Highly controversial until many rotation curves
  confirmed

108
Nearly all galaxies show these same rotation
curves

• Flat rotation curve of a galaxy reveals
• High speeds far from luminous center
• indicates large amounts of matter in the outer
  regions
• Dark Matter

109
Individual galaxies have huge amounts of dark
matter

• Rotation curves motions of stars in the galaxy
• Reveal that dark matter extends beyond visible
  part of the galaxy, mass is 10x stars and gas

110
Galaxy Clusters revealdark matter in three ways

• 1 Galaxy velocities too large to be explained
  by gravity of visible galaxies
• Expected 100 km/sec for a typical cluster, found
  1000 km/sec!
• Discovered in 1930s by Fritz Zwicky (they didnt
  believe him, either)

111
2 Hot x-ray emitting gas in cluster

• Gas between galaxies is also moving because of
  gravity of dark matter gets very hot
• 1000 km/sec ? 100 million K emits X-rays!

112
Two galaxy clusters are studied. Cluster A has
typical velocities for its galaxies of 300
km/sec, Cluster B has 1000 km/sec. Which is most
likely?
Clicker Question

• Cluster A has more galaxies than cluster B
• Cluster A is more massive than cluster B
• Gas between galaxies in cluster A will have lower
  temperature than gas in cluster B
• Cluster B galaxies are more likely to be spirals

113

• C. Lower velocities in Cluster A mean that there
  is less mass overall in that cluster. This
  probably means fewer galaxies. Less mass also
  means a cooler intracluster gas temperature

114
3 Gravitational Lenses

• Dark ( luminous) matter warps space
• acts like a lens and distorts and magnifies the
  view of more distant galaxies
• Lens properties reveal how much mass is contained
  (in total, both luminous and dark) in the cluster

115
Gravitational lensing how it works
116
Compared to a low-mass cluster, how will the
lensed images of background galaxy that has been
lensed by a high-mass cluster look?
Clicker Question

• The images will be closer together
• The images will be further apart
• There will be no change in the position of the
  images

117
Gravitational lensing can make a variety of shapes
118
Single galaxies can act as lenses too!
119
How much dark matter overall?

• All cluster methods generally agree (yay!)
• Overall, about 10 times as much dark matter as
  normal matter in the universe
• Note Our solar system has much more light matter
  than dark matter here! (DM probably immeasurable.)

120
What is dark matter?

• Two leading contenders
• Possibility 1 MACHOs
• MAssive Compact Halo Objects
• This IS stuff weve studied already very faint,
  normal things baryonic matter (atoms, protons,
  neutrons)
• Brown dwarfs, black holes, black dwarfs, cold
  neutron stars, etc
• Could be floating through galaxy halos unnoticed

121
MACHO Searches

• Use gravitational lensing
• When a MACHO passes directly in front of a star,
  that star suddenly brightens!
• Focusing effect of a compact massive object

122
MACHO hunt results

• MACHOs have been reliably detected since 1997 by
  looking at the LMC
• One team looked at nearly 12 million stars (over
  6 years) and discovered 13-17 MACHOs
• Not nearly enough to account for all the
  missing mass

123
Possibility 2 WIMPs
Weakly Interacting Massive Particles

• Non-baryonic matter? subatomic particles
• Neutrinos? Probably not. They move too fast and
  cant be collected into stable galaxy halos
• Other particles???
• Leftover material from the Big Bang
• Slower particles Cold Dark Matter

Unknown particles!!
124
Which of the following is not an acceptable model
for the fate of the universe?
Reading Clicker Question

• A recollapsing universe
• A steady-state universe
• A coasting universe
• A critical universe
• An accelerating universe

125
What is dark matter?
REVIEW
Possibility 1 MACHOs

• MAssive Compact Halo Objects
• This IS stuff weve studied already very faint,
  normal things baryonic matter
• atoms, protons, neutrons
• Brown dwarfs, black holes, black dwarfs, cold
  neutron stars, etc
• Could be floating through galaxy halos unnoticed
• MACHO searches dont detect enough mass to
  explain all dark matters as MACHOs

126
Possibility 2 WIMPs
Weakly Interacting Massive Particles

• Non-baryonic matter? subatomic particles
• Neutrinos? Probably not. They move too fast and
  cant be collected into stable galaxy halos
• Other particles???
• Leftover material from the Big Bang
• Slower particles Cold Dark Matter

Unknown particles!!
127
Both our past and our future depend on dark matter

• Past Birth of galaxies and clusters
• Dark matter provided the first tugs to assemble
  galaxies and clusters out of protogalactic clouds
• Future Fate of the universe
• Is there enough matter in the universe (both
  light and dark) to reverse the expansion and pull
  the universe back together again?

128
Formation of Structure

• In the beginning
• Density distribution mostly smooth but very small
  ripples exist in density
• Gravity pulls together dark matter in slightly
  denser regions to form dark halos
• Light matter radiates energy and sinks to the
  middle to form galaxies

129
WMAP

• WMAP showed that space was relatively isotropic
  (physically similar) but different at the .001
  level

130
If the Universe was mostly smooth, how did those
lumps turn into galaxies?

• Simulations show that gravity of dark matter
  pulls mass into denser regions universe grows
  lumpier with time
• Those lumps are galaxy clusters

131
Formation Animations
132

• Observations of galaxy positions reveal extremely
  large structures clusters, superclusters,
  walls, voids

133
vs
Computer simulations
Real data

• Agreement is generally pretty good!
• Despite the fact that we dont know what the CDM
  is!

134
Lessons from Imaginary Universes

• Cold (Slow) dark matter works better than hot
  (fast) dark matter
• Neutrinos are too fast structure would be
  smeared out
• What is slow and dark enough? We dont know yet!
• Particle experiments under way..

135
Dark Matter and the Fate of the Universe

• Expansion begins with the Big Bang (well talk
  about this next week)
• At that point, everything in the universe is
  flung apart at outrageous speeds!
• Several different models for Past and Future
  depending upon the amount of dark matter

136

• Some say the world will end in fire
• Some say with ice
• From what Ive tasted of desire
• I hold with those who favor fire
• But if I had to perish twice
• I think I know enough of hate
• To say that for destruction ice
• Is also great
• And would suffice
• -- Robert Frost (1874-1963)

National Poet Laureate
137
Predictions of General Theory of Relativity

• Einstein in 1917 realized GTR predicted universes
  in motion, but preferred steady state added
  cosmological constant (CC) as repulsive force
  in space-time to counteract attractive force of
  gravity (A fudge factor!)
• Willem de Sitter (A, Dutch, 1917) solves GTR
  equations with no CC and low density of matter
  showed universe must expand
• Alexander Friedmann (M, Russian, 1920) solves GTR
  with no CC but any density of matter universes
  can expand forever, or collapse again, depending
  on mean matter density
• Georges Lemaitre (P, Belgian, 1927) rediscovers
  Friedmann solutions, told Hubble (observing
  redshifts since 1924) that cosmic expansion
  suggests more distant galaxies should have
  greater redshifts (Hubble publishes V Hod in
  1929)
• Einstein visited Hubble in 1932, said CC was the
  greatest blunder of his career

138
Very important diagram

• Average distance between galaxies
• 1 / expansion factor
• 1 / (1 Z)
• NOW is fixed in time (Z0)
• Hubble constant NOW sets how fast universe is
  expanding NOW

OPEN
SIZE
FLAT
CLOSED
NOW
TIME
Big Bang when distance zero Z
infinity
139
Cannonball Analogy
140
The expansion rate of the universe is not
necessarily constant for all time

• Just like our cannonball, GRAVITY should SLOW
  expansion rate ? deceleration
• Different models for different amounts of dark
  matter
• Lets ignore accelerating for now

141
Since gravity is what pulls everything back in,
there must be a magic number

• Just the right amount of mass (in our current
  universe) to pull everything back together in an
  infinite amount of time
• Just like our exact escape velocity for the
  cannonball
• We call this exact amount of matter (spread out
  over the observable universe), the CRITICAL
  DENSITY
• 10-29 grams/cm3 a few atoms in a closet

142
Critical Universe

• Density of matter critical density
• Will expand forever, but just barely

143
Recollapsing Universe

• Dark matter density is greater than critical
  density
• Expansion will stop in the future, will collapse
  back in
• Big Crunch
• Oscillations?

144
Coasting Universe

• The universe has always expanded at the same rate
  (no deceleration due to gravity!)
• The age of the Universe 1/Ho

145
Which model predicts the largest age for the
universe today?
Clicker Question

• A. Recollapsing
• (closed)
• B. Critical
• (flat)
• C. Coasting
• (open)