Dark Matter

ASTR 

115 

03/12/13 

“Why does a volcanic eruption sometimes create lightning Pictured above, the Sakurajima volcano in southern Japan was caught erupting 

in early January. Magma bubbles so hot they glow shoot away as liquid rock bursts through the Earth's surface from below. The above image 

is particularly notable, however, for the lightning bolts caught near the volcano's summit. Why lightning occurs even in common 

thunderstorms remains a topic of research, and the cause of volcanic lightning is even less clear. Surely, lightning bolts help quench areas of 

opposite but separated electric charges. One hypothesis holds that catapulting magma bubbles or volcanic ash are themselves electrically 

charged, and by their motion create these separated areas. Other volcanic lightning episodes may be facilitated by charge-inducing collisions 

in volcanic dust. Lightning is usually occurring somewhere on Earth, typically over 40 times each second.” 

http://apod.nasa.gov/apod/ap130311.html 

Tuesday, March 12, 13 

1

“Is our Milky Way Galaxy out to lunch Recent wide field images and analyses now indicate that our home galaxy is actually still in the 

process of devouring its closest satellite neighbor. This unfortunate neighbor, the Sagittarius Dwarf galaxy, is now seen to be part of a larger 

Sagittarius Tidal Stream, a loose filament of stars, gas, and possibly dark matter that entangles the Milky Way. An artist's depiction of the 

stream is shown above. Speculation also holds that the Sagittarius Dwarf was once pulled through the Milky Way disk very close to our 

Sun's current location. An important resulting realization is that galaxies contain a jumble of clumps and filaments of both dim and dark 

matter.” 



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SCHEDULE CHANGES 

•The extra credit movie is now on Wednesday at 5:45-7:30pm. 

•The extra credit poster session at Edmonds may be changed to this 

Thursday at 6:15pm in stead of next Tuesday. This is still up in the air, 

so I will update the webpage on Wednesday with any changes. 


3

WHAT ARE QUASARS 


4

If the center of a 

galaxy is unusually 

bright, we call it an 

active galactic 

nucleus. 

Quasars are the 

most luminous 

examples. 

Active nucleus in Galaxy M87 


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“Violent galaxy mergers can feed supermassive black holes. Theoretically, the result is intense emission from regions near the supermassive black holes, 

creating the some of the most luminous objects in the universe. Astronomers dub these Active Galactic Nuclei, or just AGN. But for decades only about 1 

percent of AGN seemed to be associated with galaxy mergers. New results from a premier sky survey by NASA's Swift satellite at hard (energetic) X-ray 

energies now solidly show a strong association of AGN with merging galaxies, though. The hard X-rays more readily penetrate dust and gas clouds in merging 

galaxies and reveal the presence of emission from the active black holes. In fact, these panels show the location (circled) of Swift X-ray detected supermassive 

black holes in a variety of merging galaxy systems. The optical images are from the Kitt Peak National Observatory in Arizona. At top center is NGC 7319 

and the compact galaxy group known as Stephan's Quintet.” 


6

The highly redshifted spectra of quasars indicate large distances. 

From brightness and distance we find that luminosities of some 

quasars are greater than 10 12 L Sun . 

Variability shows that all this energy comes from a region smaller 

than our solar system. 


7

THOUGHT QUESTION 

What can you conclude from the fact that quasars usually 

have very large redshifts 

A. They are generally very distant. 

B. They were more common early in time. 

C. Galaxy collisions might turn them on. 

D. Nearby galaxies might hold dead quasars. 


8


What can you conclude from the fact that quasars usually 

have very large redshifts 

A. They are generally very distant. 

B. They were more common early in time. 

C. Galaxy collisions might turn them on. 

D. Nearby galaxies might hold dead quasars. 

All of the above! 


9

Galaxies around 

quasars 

sometimes 

appear 

disturbed by 

collisions. 


10

Quasars powerfully radiate energy over a wide range of 

wavelengths, indicating that they contain matter with a wide 

range of temperatures. 


11

Radio galaxies contain active nuclei shooting out vast jets of 

plasma that emits radio waves coming from electrons that move 

at near light speed. 


12

The lobes of radio galaxies can extend over hundreds of 

thousands of light-years. 


13

An active galactic 

nucleus can shoot 

out blobs of plasma 

moving at nearly the 

speed of light. 

These ejection 

speeds suggests the 

presence of a black 

hole. 


14

Radio galaxies 

don’t appear as 

quasars because 

dusty gas clouds 

block our view 

of the accretion 

disk. 


15

CHARACTERISTICS OF 

ACTIVE GALAXIES 

•Their luminosities can be enormous (>10 12 L Sun ). 

•Their luminosities can rapidly vary (come from a space 

smaller than solar system). 

•They emit energy over a wide range of wavelengths (contain 

matter with a wide temperature range). 

•Some galaxies drive jets of plasma at near light speed. 


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WHAT IS THE POWER SOURCE FOR QUASARS 

AND OTHER ACTIVE GALACTIC NUCLEI 


17

Accretion of gas onto a supermassive black hole appears to be 

the only way to explain all the properties of quasars. 


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ENERGY FROM A BLACK HOLE 

•Gravitational potential energy of matter falling into black hole turns 

into kinetic energy. 

•Friction in an accretion disk turns kinetic energy into thermal energy 

(heat). 

•Heat produces thermal radiation (photons). 

•This process can convert 10 to 40% of 

E = mc 2 into radiation. 


19

Jets are thought to come from twisting of magnetic field in the 

inner part of accretion disk. 


20

DO SUPERMASSIVE BLACK 

HOLES REALLY EXIST 


21

Orbits of stars at 

center of Milky 

Way stars 

indicate a black 

hole with mass of 

4 million M Sun . 


22

The orbital speed and distance of gas orbiting the center of 

Galaxy M87 indicate a black hole with mass of 3 billion M Sun . 


23

BLACK HOLES IN GALAXIES 

•Many nearby galaxies—perhaps all of them—have 

supermassive black holes at their centers. 

•These black holes seem to be dormant active galactic nuclei. 

•All galaxies may have passed through a quasar-like stage earlier 

in time. 


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GALAXIES AND BLACK 

HOLES 

•The mass of a 

galaxy’s central 

black hole is 

closely related to 

the mass of its 

bulge. 


25

GALAXIES AND BLACK 

HOLES 

•The development of 

the central black 

hole must be 

somehow related to 

galaxy evolution. 


26

HOW DO QUASARS LET US 

STUDY GAS BETWEEN THE 

GALAXIES 


27

Gas clouds between a quasar and Earth absorb some of the 

quasar’s light. 

We can learn about protogalactic clouds by studying the 

absorption lines they produce in quasar spectra. 


28

CHAPTER 22 

DARK MATTER, DARK ENERGY, 

AND THE FATE OF THE UNIVERSE 


29

WHAT DO WE MEAN BY DARK 

MATTER AND DARK ENERGY 


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UNSEEN INFLUENCES 

Dark Matter: An undetected form of mass that emits little or no light, 

but whose existence we infer from its gravitational influence 

Dark Energy: An unknown form of energy that seems to be the 

source of a repulsive force causing the expansion of the universe to 

accelerate 


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CONTENTS OF UNIVERSE 

•“Ordinary” matter: ~ 4.4% 

•Ordinary matter inside stars: ~ 0.6% 

•Ordinary matter outside stars: ~ 3.8% 

•Dark matter: ~ 23% 

•Dark energy ~ 73% 


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WHAT IS THE EVIDENCE FOR DARK 

MATTER IN GALAXIES 


33

We measure the 

mass of the solar 

system using the 

orbits of planets: 

• orbital period 

• average distance 

For circles: 

• orbital velocity 

• orbital radius 


34

Rotation curve 

A plot of orbital 

velocity versus 

orbital radius 

The solar system’s 

rotation curve 

declines because 

the Sun has almost 

all the mass. 


35

Who has the 

largest orbital 

velocity 

A, B, or C 


36

Who has the 

largest orbital 

velocity 

A, B, or C 

Answer: C 


37

The rotation 

curve of a 

merry-go-round 

rises with 

radius. 


38

The rotation 

curve of the 

Milky Way stays 

flat with distance. 

Mass must be 

more spread out 

than in the solar 

system. 


39

Mass in the Milky 

Way is spread 

out over a larger 

region than its 

stars. 

Most of the Milky 

Way’s mass 

seems to be 

dark matter! 


40

Mass within the Sun’s 

orbit: 

1.0 × 10 11 M Sun 

Total mass: 

~10 12 M Sun 


41

The visible 

portion of a 

galaxy lies 

deep in the 

heart of a 

large halo of 

dark matter. 


42

We can 

measure the 

rotation 

curves of 

other spiral 

galaxies using 

the Doppler 

shift of the 

21-cm line of 

atomic 

hydrogen. 


43

Spiral galaxies all tend to have flat rotation curves, indicating 

large amounts of dark matter. 


44

Broadening of 

spectral lines in 

elliptical galaxies tells 

us how fast the stars 

are orbiting. 

These galaxies also 

have dark matter. 


45


What would you conclude about a galaxy whose 

rotational velocity rises steadily with distance beyond 

the visible part of its disk 

A. Its mass is concentrated at the center. 

B. It rotates like the solar system. 

C. It’s especially rich in dark matter. 

D. It’s just like the Milky Way. 


46


What would you conclude about a galaxy whose 

rotational velocity rises steadily with distance beyond 

the visible part of its disk 

A. Its mass is concentrated at the center. 

B. It rotates like the solar system. 

C. It’s especially rich in dark matter. 

D. It’s just like the Milky Way. 


47

WHAT IS THE EVIDENCE FOR DARK 

MATTER IN CLUSTERS OF GALAXIES 


48

We can measure 

the velocities of 

galaxies in a 

cluster from their 

Doppler shifts. 


49

The mass we find 

from galaxy 

motions in a 

cluster is about 50 

times larger than 

the mass in stars! 


50

Clusters contain large 

amounts of X rayemitting 

hot gas. 

Temperature of hot gas 

(particle motions) tells 

us cluster mass: 

85% dark matter 

13% hot gas 

2% stars 


51

Gravitational lensing, the bending of light rays by gravity, can 

also tell us a cluster’s mass. 


52


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All three methods of measuring cluster mass indicate similar 

amounts of dark matter in galaxy clusters. 


54


What kind of measurement does not tell us the mass of a 

cluster of galaxies 

A. measuring velocities of cluster galaxies 

B. measuring the total mass of cluster’s stars 

C. measuring the temperature of its hot gas 

D. measuring distorted images of background galaxies 


55


What kind of measurement does not tell us the mass of a 

cluster of galaxies 

A. measuring velocities of cluster galaxies 

B. measuring the total mass of cluster’s stars 

C. measuring the temperature of its hot gas 

D. measuring distorted images of background galaxies 


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DOES DARK MATTER 

REALLY EXIST 

Insert TCP 6e Figure 22.11 

unannotated 


57

OUR OPTIONS 

1. Dark matter really exists, and we are observing the effects of 

its gravitational attraction. 

2. Something is wrong with our understanding of gravity, 

causing us to mistakenly infer the existence of dark matter. 


58

OUR OPTIONS 

1. Dark matter really exists, and we are observing the effects of 

its gravitational attraction. 

2. Something is wrong with our understanding of gravity, 

causing us to mistakenly infer the existence of dark matter. 

Because gravity is so well tested, most astronomers prefer 

option #1. 


59

Some observations of the universe are very difficult to 

explain without dark matter. 


60

WHAT MIGHT DARK MATTER 

BE MADE OF 


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HOW DARK IS DARK MATTER 

… NOT AS BRIGHT AS A STAR. 


62

TWO BASIC OPTIONS 

•Ordinary Dark Matter (MACHOS) 

•Massive Compact Halo Objects: 

dead or failed stars in halos of galaxies 

•Extraordinary Dark Matter (WIMPS) 

•Weakly Interacting Massive Particles: 

mysterious neutrino-like particles 


63

TWO BASIC OPTIONS 

•Ordinary Dark Matter (MACHOS) 

•Massive Compact Halo Objects: 

dead or failed stars in halos of galaxies 

•Extraordinary Dark Matter (WIMPS) 

•Weakly Interacting Massive Particles: 

mysterious neutrino-like particles 

The 

best 

bet 


64

MACHOs 

occasionally make 

other stars appear 

brighter through 

lensing… 

… but there are 

not enough 

lensing events to 

explain all the 

dark matter. 


65

WHY BELIEVE IN WIMPS 

•There’s not enough ordinary matter. 

•WIMPs could be left over from Big Bang. 

•Models involving WIMPs explain how galaxy formation works. 


66

WHAT IS THE ROLE OF DARK MATTER 

IN GALAXY FORMATION 


67

Gravity of dark matter is what caused protogalactic clouds to 

contract early in time. 


68


WIMPs can’t 

collapse to the 

center because 

they don’t 

radiate away 

their orbital 

energy. 


69

Dark matter is still 

pulling things 

together. 

After correcting 

for Hubble’s law, 

we can see that 

galaxies are 

flowing toward 

the densest 

regions of space. 


70

WHAT ARE THE LARGEST STRUCTURES 

IN THE UNIVERSE 


71

Maps of galaxy positions reveal extremely large structures: 

superclusters and voids. 


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Time in billions of years 

0.5 2.2 5.9 8.6 13.7 

13 35 70 93 140 

Size of expanding box in millions of light-years 

Models show that gravity of dark matter pulls mass into denser 

regions—the universe grows lumpier with time. 


73

Models show that gravity of dark matter pulls mass into denser 

regions—universe grows lumpier with time. 


74

Structures in galaxy maps look very similar to the ones found in 

models in which dark matter is WIMPs. 


75

“The latest map of the cosmos again indicates that dark matter and dark energy dominate our universe. The Sloan Digital Sky Survey 

(SDSS) is on its way to measuring the distances to over one million galaxies. Galaxies first identified on 2D images, like the one shown 

above on the right, have their distances measured to create the 3D map. The SDSS currently reports 3D information for over 200,000 

galaxies, now rivaling the 3D galaxy-count of the Two-Degree Field sky map. The latest SDSS map, shown above on the left, could only 

show the galaxy distribution it does if the universe was composed and evolved a certain way. After trying to match many candidate universes 

to it, the Cinderella universe that best fits the above map has 5% atoms, 25% dark matter, and 70% dark energy. Such a universe was 

previously postulated because its rapid recent expansion can explain why distant supernovas are so dim, and its early evolution can explain 

the spot distribution on the very distant cosmic microwave background.” 



76


“What has created this huge empty volume in the universe No one is yet sure, and even the extent of the estimated billion-light year void 

is being researched. The void is not a hole in space like a black hole, but rather a vast region of the universe that appears to be mostly 

devoid of normal matter and even dark matter. The void is still thought to contain dark energy, though, and is clearly traversable by light. 

The void's existence is being postulated following scientific curiosity about how unusually cold spots came to appear on WMAP's map of 

cosmic microwave background (CMB) radiation. One possibility was that this CMB region was not actually very cold but light from the 

spot somehow became more cosmologically redshifted than normal along the way. Other voids in the universe are known to exist, but this 

void appears to have an unusually large gravitational effect, and so might possibly be the largest in our entire visible universe. 

Investigating this, a recent study found an unusually low number of cosmic radio sources between Earth and the CMB cold spot, which 

led to the inference of this giant void. An artist's depiction of the huge cosmic void is shown above.” 


77

WILL THE UNIVERSE CONTINUE 

EXPANDING FOREVER 


78

Does the 

universe have 

enough kinetic 

energy to escape 

its own 

gravitational pull 


79

The fate of 

the universe 

depends on 

the amount of 

dark matter. 


80

Since the amount of dark 

matter is ~25% of the 

critical density, we expect 

the expansion of the 

universe to overcome its 

gravitational pull. 


81

In fact, the 

expansion appears 

to be speeding up! 

Dark 

energy 


82

Estimated age depends on the amount of both dark matter 

and dark energy. 


83


Suppose that the universe has more dark matter than we 

think there is today. How would this change the age 

we estimate from the expansion rate 

A. The estimated age would be larger. 

B. The estimated age would be the same. 

C. The estimated age would be smaller. 


84


Suppose that the universe has more dark matter than we 

think there is today. How would this change the age 

we estimate from the expansion rate 

A. The estimated age would be larger. 

B. The estimated age would be the same. 

C. The estimated age would be smaller. 


85

IS THE EXPANSION OF THE 

UNIVERSE ACCELERATING 


86


The brightness of distant white dwarf supernovae tells us how 

much the universe has expanded since they exploded. 


87

An accelerating universe best fits the supernova data. 


88


“How will our universe end Recent 

speculation now includes a pervasive growing 

field of mysterious repulsive phantom energy 

that rips virtually everything apart. Although 

the universe started with a Big Bang, analysis 

of cosmological measurements allows a 

possibility that it will end with a Big Rip. As 

soon as few billion years from now, the 

controversial scenario holds, dark energy will 

grow to such a magnitude that our own Galaxy 

will no longer be able to hold itself together. 

After that, stars, planets, and then even atoms 

might not be able to withstand the expansive 

internal force. Previously, speculation on the 

ultimate fate of the universe centered on either 

a re-collapsing Big Crunch or a Big Freeze. 

Although the universe's fate is still a puzzle, 

piecing it together will likely follow from an 

increased understanding of the nature of dark 

matter and dark energy.” 


89

NOW -- TO IMPORTANT 

MATTERS 

HOW COULD ASTRONOMICAL EVENTS 

KILL ALL LIFE ON EARTH 

(NOTE: APPROPRIATE FOR ALL AGES, AND NOT SOMETHING TO 

LOSE ANY SLEEP OVER) 


90

IN NO PARTICULAR ORDER (IGNORING 

HUMANITIES OWN ATTEMPTS TO DESTROY 

LIFE ON EARTH) 

•Killer asteroid 

•Volcanoes 

•Gamma-ray burst 

•Supernova explosion 

•Super solar flares 

•Earth’s magnetic field flipping 

•Sun’s temperature increases to the point that Earth is no longer in the 

habitable zone. 

•Galactic collision 


91

KILLER ASTEROIDS 

•Small impacts happen 

almost daily. 

•Impacts large enough to 

cause mass extinctions 

happen many millions of 

years apart. 

•Bigger than 10km, and 

all humans at least will 

die. 

In about 1.7 million years, Giliese 710 is predicted to pass within 1.1 light 

years of our Sun. 


92

SUPER VOLCANOS 

•A super volcano eruption is considered to be ten times more likely in 

the (near) future than a killer asteroid impact. 

•Yellowstone erupts approximately every 600,000 years. 

•Last eruption: 640,000 years ago... 

•Yellowstone is a 34 mile by 45 mile active volcanic caldera. 

•Volcanic ash could block most of the sunlight across the entire 

planet for years. (volcanic winter) 

•Could even trigger a runaway greenhouse effect. 


93

NEARBY SUPERNOVA 

EXPLOSION 

•While theorists disagree on the exact number, typically they say that if 

a supernova occurred within 30 light-years of us (assuming the jet is 

not pointed at us) then mass extinctions could occur here on Earth. 

•The closest star expected to go supernova any time soon is 500 light 

years away. At that distance, even if the jet was pointed at us it is 

unlikely to do more than perhaps cause some technology issues 

(those sensitive satellites). 


94

GAMMA-RAY BURST 

•To see a gamma-ray burst, the jet has to be pointed directly at us. 

•At about 1000 lightyears it would be as bright as our Sun (in visible 

light). 

•Even at that distance they could cook the atmosphere, creating nitrous 

oxides that would destroy the ozone layer. Without the ozone layer, it 

would only be a matter of time until we died from the radiation of 

our own Sun. 

•If it was closer, the effects could kill us even faster. 

•BUT, there are no known progenitors of gamma-ray bursts in our part 

of the galaxy. 


95

SUPER SOLAR FLARES 

•Some tentative evidence that they can happen on sun-like stars. (They 

briefly brightened by a factor of 20.) 

•But stars tend to become less active as they age. 

•The flare would also have to be aimed at Earth. 

•Within a few hours the super solar flare could fry the side of Earth 

facing the sun, and destroy the ozone layer (so even on the night-side 

of the planet you would eventually die). 


96

EARTH’S MAGNETIC FIELD 

FLIPPING 

•Obviously, this will not kill all life on earth, but it could kill humanity, or 

at least our current civilization. 

•On average it flips every 250,000 years (but it could occur on 

timescales of 100’s of years). 

•It’s been 780,000 years since the last reversal. 

•For some length of time during the flip, the Earth has no magnetic 

field (days/months/years/decades we have no idea). 

•During this time we would have no protection from the solar 

wind. 


97

GALACTIC COLLISION 

•Andromeda is heading toward the Milky way at about 140 km/s (87 

miles/sec) 

•Impact expected in about 3 billion years. 

•May or may not cause difficulties here in our solar system. 

•Close encounters with other stars could modify the orbits of all or 

some (Jupiter) of the planets. 

•Don’t worry though, we’ll be dead by then :-) 


98

SUN HEATING UP 

•Regardless of all the other factors, this one WILL kill us. (Unless we 

can move to another planet, or physically move our planet to a 

different orbit). 

•As the sun heats up over time, it changes the climate here on Earth. 

•Eventually as the habitable zone moves past the Earth, we will 

experience the same sort of runaway greenhouse effect that Venus 

experienced. 

•optimist view: 3 billion years from now 

•pessimist view: ~1 billion years from now (includes water in the 

model) 


99

OTHER THINGS 

(INCLUDING THE LEAST LIKELY ASTRONOMY) 

•Global nuclear warfare ---> kills most life on earth. 

•Global epidemic ---> could kill all humans on earth. 

•Global climate change (Global warming)---> early runaway 

greenhouse effect could kill all life on earth. 

•Mercury’s orbit gets disturbed by Jupiter (1% chance) and hits the 

Earth (very unlikely) ---> destroys the planet. 

•The solar system has a very close encounter with a star/planet/black 

hole ---> could destroy the planet. 

•AI’s or aliens kill us all (hey, it could happen) ---> destroys humans. 


100

Dark Matter

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