Lecture 16

Question 1

what reflects mass perturbations

Answer 1

CMB temp fluctuations

Question 2

what happens in a dense region of the CMB

Answer 2

gravity pulls mass in, the region grows, attracts more mass, so on

this is an instability, CMB mass perturbations grow by gravitational instability

Question 3

hw do density perturbations grow specifically

Answer 3

gravity pulls matter in, expansion pull sit apart
matter in less dense regions expands but matter right around the perturbations is pulled in
eventually growing

Question 4

what happens when density perturbations get huge

Answer 4

the size of the density perturbations become larger than the average density itself, the perturbation goes non linear and grows very rapidly

Question 5

what shape are perturbations usually

Answer 5

ellipsoids

Question 6

how does an ellipsoidal perturbation collapse

Answer 6

collapses in each direction, fastest along the shortest axis as closest to centre

Question 7

what are the stages of ellipsoidal collapse and how is the speed of this determined

Answer 7

plane/sheet
cigar/filament
node

speed depends on size of initial perturbation
large mass - fast
small - slow

Question 8

what stops gravity from crushing an ellipsoid into a point

Answer 8

velocity dispersion

Question 9

what is velocity dispersion

Answer 9

random velocities of particles that keep it suspended against gravity

Question 10

what is a dark matter halo

Answer 10

an object whose dark matter is suspended against gravity

Question 11

what is the size of a dark matter halo known as

Answer 11

virial radius

Question 12

for a dark matter halo to work, what are the conditions

Answer 12

dark matter has to be collision less and frictionless