Astronomy Fifth Test-Final Exam

Question 1

first theorized black hole

Answer 1

1916 by elder Swarzchild, applied einstein’s equations to a collapsing star, predicted it would collapse to a point- despite not knowing about white dwarfs or even stellar fusion

Question 2

photon sphere

Answer 2

where light waves orbit a black hole, similar to a planet going around the sun. about 1.5 swarzchild radii.

Question 3

lowest possible EV

Answer 3

fired directly at zenith (straight up). otherwise, curvature of space-time increases EV as you go out.

Question 4

exit cone

Answer 4

EV gets too great for an object (just light I guess) to escape the greater an angle you get from the zenith. The exit cone is where the EV is less than c, a cone around the zenith. Gets smaller and smaller the closer you get to a gravitationally huge object/the object gets bigger.

Question 5

star becoming black hole

Answer 5

collapses on itself at end of stellar life- collapses past event horizon, from the outside you never see it finish collapsing due to strong gravitational time dilation around the hole. asymptotically gets smaller. .001 sec to get to asymptotic point, 10^-2.

Question 6

singularity

Answer 6

A point where laws of physics become discontinuous- suddenly jump. In this case, super strong gravity causes this.

Question 7

swarzchild radius

Answer 7

distance of event horizon from center of black hole.

Question 8

event horizon

Answer 8

the “point of no return”, nothing inside it can be detected. inside it are things like other universes. everything with mass technically has an EV, but everyday objects have them very small.

Question 9

knowable properties of a black hole

Answer 9

measured from outside- mass, electric charge, angular momentum. Unknown if black holes have electric charge or angular momentum, which are quantizised.

Question 10

if sun became a black hole

Answer 10

we’d get cold but orbit would not change- gravity is about mass, not density

Question 11

jumping into a black hole

Answer 11

you’d get spaghettified, pulled apart into a long chain.

Question 12

black hole with angular momemtum

Answer 12

would twist space as it “rotates”, causing extreme frame dragging around it, curves around the photon sphere

Question 13

swarzchild radii for different masses

Answer 13

10^6 tons - (big hill/small mountain) - 13x10^-9 angstroms (cosmic black holes)
Earth mass - 1/3 inch
10 Ms - typical stellar black hole, 18.4 miles
10^8Ms - galactic black hole, orbit of mars

Question 14

cosmic black holes

Answer 14

tiny black holes formed after big bang, 10^6 tons, 13x10^-9 angstroms. would pass right through earth, almost non-existtant impact parameter. possibly detected through gamma emissions when hawking radiation evaporates them - NASA launched

Question 15

hawking radiation

Answer 15

a rare bridge between quantum mechanics and general relativity. at the event horizon, light can leave due to quantum tunneling, the quantum probability of transportation. this causes the black hole to lose mass, eventually evaporating in a gamma ray burst. Light leaks faster and faster the smaller the black hole. these gamma ray bursts supposedly detectable but none found

Question 16

stellar black holes

Answer 16

5-100 Ms- look for a binary star system, a red giant with a dark companion. the BH is bigger, died sooner, red giant has expanded past Lagrangian point and is being eaten by black hole. this stellar black hole would then have a ridiculous accretion disk that shoots X-rays and is easy to see.

Question 17

detecting stellar black hole

Answer 17

look for X-rays from accretion disk. scientists launched “uhuru” satellite from africa, not a nasa mission. need to be in space cause atmosphere blocks X-rays.

Question 18

galactic black hole

Answer 18

seen by doppler shift in orbiting material. must be at center of galaxy judging by how fast things orbit. one was measured at 10 million solar mass, now known to be 3 billion. really early and active ones are thought to be quasars. basically, find the smallest orbiting thing you can, the hole must be in there.

Question 19

how relativistic jets happen

Answer 19

frame dragging (warping of space by rotation of massive object) causes magnetic field lines to spiral out, jet corkscrews out.

Question 20

friend falling into BH with light

Answer 20

you’d never see the friend go in completely, time stops at event horizon. green flash becomes less frequent, gets redshifted. from his perspective, he’d blast right in

Question 21

Penrose diagrams

Answer 21

Universe 1, 2, singularity is lines on outside. learn this through diagram. Different universes out each side of black hole. Same black hole in both. Inner lines are EVs.

Question 22

kerr solution

Answer 22

a spinning black hole. Model 10Ms, non-zero angular momentum, charge not important. Has two event horizons, one of them round like normal and other one oblate. space between them is the ergosphere. Within the ergosphere, you are compelled to travel through space in a certain direction but not in time- reverses. Re-reverses within inner EH.

Question 23

white hole

Answer 23

the opposite of a black hole, puts out material. big bang singularity is possibly a white hole? At the other end of a penrose from the black hole. No white holes seen.

Question 24

negative space universe

Answer 24

sides of L penrose diagram. shrink something down from zero, repulsive gravity, put a - in equations and its the same basically. past singularities in penrose

Question 25

local group

Answer 25

Our group of galaxies, 50-60 members. Some clusters have billions of galaxies.

Question 26

“zoo” of galaxies

Answer 26

tons of different types (spiral, barred spiral, etc) all interacting with each other.

Question 27

galaxy interactions

Answer 27

galaxies gravitationally influence each other. sometimes some pass through others, leaving only a ring and the core. they pull each other apart, merge. 6 billion years from now, milky way will merge with andromeda.

Question 28

spiral arms of galaxies

Answer 28

not caused by frame dragging, and companions are not an automatic ticket to them. They are compression wave shock fronts, not at speed of sound due to gravity influence. They circle the galaxy, the shock front picks up more material and the higher grav holds onto this material for a while, sustaining them. they are started by companion galaxies

Question 29

shock front

Answer 29

Past the speed of sound, stuff builds up ahead of object- think bullet > sound or bubble that forms in front of supersonic jet. This is the process that sustains shock fronts.

Question 30

seeing our spiral arms

Answer 30

look for O+B stars out from earth in different lines of sight. position them, look for spiral-arm based shapes.

Question 31

star formation and spiral arms

Answer 31

interstellar gas+dust needs to contract 10x in radius and have 1000x great density. The shock front does this, collapses clouds of gas so star formation happens. O+B stars formed from big clouds are born and die in the arms, others manage to escape.

Question 32

galaxy differential rotation velocity

Answer 32

expect it to go down as you get outwards, due to less density of material/less gravity. instead it stays same…. why?

Question 33

dark matter

Answer 33

invisible stuff causing anomalous rotation of galaxies. thought to only interact with other stuff gravitationally, is outside “particle zoo.” 80% of matter is dark matter. Other ideas for it are black holes and cold gas

Question 34

hubble’s collection

Answer 34

Edwin Hubble collected a huge survey of redshift data from tons of different galaxies, got their distance with cepheid variables. discovered that the further away they are, the more the galaxies were redshifted… thus UNIVERSE IS EXPANDING

Question 35

Hubble’s law

Answer 35

Vrec is correlated with distance. Involves the constant ratio between these two, bubble’s constant. Can be used to predict distance, measure redshift get distance

Question 36

hubble’s constant

Answer 36

73km/sec/mpsc

Question 37

imagining expansion of universe

Answer 37

A balloon with polka dots. When the balloon is blown up, polka dots move away from each other. Closer ones separate slower, farther ones separate faster. no center because surface of balloon has no center.

Question 38

lunar occultation method

Answer 38

a single radio telescope doesn’t have the resolution to tell you where in the sky something is coming from. However, the position of the moon is very well known. Watch for moon occulting a radio source (its signal dropping) to know exact position.

Question 39

Discovery of quasar

Answer 39

One radio source blinked out instead of fading (big, non-point sources fade more gradually) This means it’s a point source, but what point source is so bright. Quasar quasi-stellar radio source.

Question 40

Maarten Shmidt discovery

Answer 40

Quasars were very mysterious for a while- their spectral lines made no sense. Schmidt however calculated that they do, they’re just SUPER redshifted. Meaning- they’re super far away, from early universe. Fastest quasar found moving away from us a 96%c

Question 41

quasars

Answer 41

from “quasi-stellar radio source”. A point source of enormously energetic radio emissions that are super redshifted, so they must be far away. They have presented problems. In the past, wtf spectral lines; now, how can they have so much energy? They’re so far away but so bright!
They are thought to be accretion disks around galactic black holes- spiral arms have been observed around some.

Question 42

using quasars

Answer 42

they show us early universe (thought they only existed then) and everything in between. Watch the light of a quasar get fuddled with by material in between- calculate dist+mass of galaxy by bending of quasar light.

Question 43

Classical cosmology questions

Answer 43

About universe: How big is it? How old is it? How did it originate? What is its future?

Question 44

kinds of universe

Answer 44

Observed- seen by scientists. Observable- seeable , most relevant one. Physical- everything that exists

Question 45

Size of observable universe

Answer 45

2x10^26m

Question 46

Real question for how big

Answer 46

because of expansion, this question is really how old. How can it be infinite? Can you go forever in straight line? How can it be finite but bounded?

Question 47

Shapes of universe

Answer 47

Maybe a sphere, which is boundless but finite, you can go in a straight line forever. Testable by measuring a triangle. Measure a triangle IRL by looking out and watching shape. Can’t get good enough data. No answer to how big/what shape ATM

Question 48

other test of universe shape

Answer 48

Parallel lines: pos curvature curve toward each other, neg they curve away. Harder to see.

Question 49

How to answer how old?

Answer 49

v <—- gal —–r—— gal.

Run expanding universe backwards- everything comes together

Question 50

Math of age of universe

Answer 50

V = HR hubble's law
R = HVTo common sense
V = HVTo combine em
1 = HTo
To = 1/H

Question 51

expansion of universe

Answer 51

would expect it to be slowing down due to gravity. it’s not… it’s actually accelerating. Why? Dark energy.

Question 52

Friedmann vs Steady state/einstein model

Answer 52

Einstein disliked the idea that the universe changes and that it came out of a singularity/white hole. Added a “cosmological constant” to the equations to keep size of universe constant. Friedmann disagreed, came up with correct model of universe expanding out from one point. Einstein later calls this his biggest mistake.

Question 53

Proof for friedman model

Answer 53

Hubble’s law (expansion of universe), CBR, quasars only in early.

Question 54

CBR

Answer 54

Cosmic background radiation. light left over from big bang. Up until 380,000 years after it, free electrons absorbed all light, but at that point they cooled down enough to join hydrogen atoms. This light has redshifted today to become blackbody radiation at 2.7K in the microwave range. looks roughly the same in every direction

Question 55

discovery of CBR

Answer 55

1964. Predicted by Robert Dicke, discovered by Robert Wilson and Arno Penzias. Thought it was bird crap. COBE in 1989 saw it at predicted brightness. Can use it to build map of universe.