Astronomy Fifth Test-Final Exam Flashcards
first theorized black hole
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
photon sphere
where light waves orbit a black hole, similar to a planet going around the sun. about 1.5 swarzchild radii.
lowest possible EV
fired directly at zenith (straight up). otherwise, curvature of space-time increases EV as you go out.
exit cone
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.
star becoming black hole
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.
singularity
A point where laws of physics become discontinuous- suddenly jump. In this case, super strong gravity causes this.
swarzchild radius
distance of event horizon from center of black hole.
event horizon
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.
knowable properties of a black hole
measured from outside- mass, electric charge, angular momentum. Unknown if black holes have electric charge or angular momentum, which are quantizised.
if sun became a black hole
we’d get cold but orbit would not change- gravity is about mass, not density
jumping into a black hole
you’d get spaghettified, pulled apart into a long chain.
black hole with angular momemtum
would twist space as it “rotates”, causing extreme frame dragging around it, curves around the photon sphere
swarzchild radii for different masses
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
cosmic black holes
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
hawking radiation
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
stellar black holes
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.
detecting stellar black hole
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.
galactic black hole
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.
how relativistic jets happen
frame dragging (warping of space by rotation of massive object) causes magnetic field lines to spiral out, jet corkscrews out.
friend falling into BH with light
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
Penrose diagrams
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.
kerr solution
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.
white hole
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.
negative space universe
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
local group
Our group of galaxies, 50-60 members. Some clusters have billions of galaxies.
“zoo” of galaxies
tons of different types (spiral, barred spiral, etc) all interacting with each other.
galaxy interactions
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.
spiral arms of galaxies
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
shock front
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.
seeing our spiral arms
look for O+B stars out from earth in different lines of sight. position them, look for spiral-arm based shapes.
star formation and spiral arms
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.
galaxy differential rotation velocity
expect it to go down as you get outwards, due to less density of material/less gravity. instead it stays same…. why?
dark matter
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
hubble’s collection
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
Hubble’s law
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
hubble’s constant
73km/sec/mpsc
imagining expansion of universe
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.
lunar occultation method
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.
Discovery of quasar
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.
Maarten Shmidt discovery
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
quasars
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.
using quasars
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.
Classical cosmology questions
About universe: How big is it? How old is it? How did it originate? What is its future?
kinds of universe
Observed- seen by scientists. Observable- seeable , most relevant one. Physical- everything that exists
Size of observable universe
2x10^26m
Real question for how big
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?
Shapes of universe
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
other test of universe shape
Parallel lines: pos curvature curve toward each other, neg they curve away. Harder to see.
How to answer how old?
v <—- gal —–r—— gal.
Run expanding universe backwards- everything comes together
Math of age of universe
V = HR hubble's law R = HVTo common sense V = HVTo combine em 1 = HTo To = 1/H
expansion of universe
would expect it to be slowing down due to gravity. it’s not… it’s actually accelerating. Why? Dark energy.
Friedmann vs Steady state/einstein model
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.
Proof for friedman model
Hubble’s law (expansion of universe), CBR, quasars only in early.
CBR
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
discovery of CBR
- 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.
