Section 4

Question 1

matter density

Answer 1

ρ = Nstars x M☉ / volume of milky way

Question 2

The outer part of the disc is known as

Answer 2

the Keplerian part

Question 3

A rotation curve is

Answer 3

a plot of rotational speed as a function of distance from the centre of the galactic disc

Question 4

Kepler’s Third Law

Answer 4

G Mgal P^2 = 4 π^2 a^3

Question 5

the galactic disc appears to be embedded

Answer 5

in a roughly spherical halo of dark matter

Question 6

Virial theorem to galaxy clusters

Answer 6

assume galaxy cluster is virialsed

2K + U = 0

KE = 1/2M<v>^2 = 3/2 Mσ(vr)^2</v>

PE = -GM^2/Re = -1/2 Σ i, j≠i Gmimj/|r͢j-r͢i|

M = <v>^2 Re / G</v>

<v>^2 = 3σ
</v>

Question 7

Gravitational Lensing

Answer 7

GR predicts that light will be deflected in strong gravitational fields

Question 8

Nucleosynthesis

Answer 8

considers the lightest elements minutes after the Big Bang.

constrains the density of baryons

Question 9

CMBR

Answer 9

studying the pattern of temperature variations in the CMBR, the relic radiation from the Big Bang

Question 10

Evidence from nucleosynthesis suggests

Answer 10

that a substantial fraction of dark matter in the universe is non-baryonic

Question 11

Baryonic

Answer 11

MACHOs
Low surface brightness galaxies
Gas clumps in galaxy halos and clusters

Question 12

Non-Baryonic

Answer 12

WIMPs
primordial black holes

Question 13

Hot dark matter

Answer 13

moving relativistically

Question 14

Cold dark matter

Answer 14

moving non-relativistically

Question 15

Fg = Fc

Answer 15

GMm/r^2 = mv^2/r^2

Question 16

if r &laquo_space;a near galactic centre then

Answer 16

ρ(r«a) ~ ρo = constant mass density

implying a rigid-body rotation

Question 17

ρ =

Answer 17

M/V

V = 4/3 π r^3

Question 18

evidence for dark matter

Answer 18

galaxy rotation curves, peculiar velocities of galaxies in clusters and gravitational lensing effects

Question 19

Dark matter interacts mainly

Answer 19

gravitationalyl

Question 20

cold dark matter vs hot dark matter

Answer 20

cold dark matter enhances the formation of structure opposed to hot dark matter

Question 21

how does the virial theorem lead to dark matter existence

Answer 21

if astronomers can measure the radial velocity dispersion and the effective radius of the cluster, they can obtain an estimate of the amount of gravitating matter in the cluster. Comparing this to the mass that must be present to explain its luminosity yields a large discrepancy -> dark matter.

Question 22

proper distance

Answer 22

distance measured with a ruler

Question 23

luminosity distance

Answer 23

the distance to the object implied by the observed flux

Question 24

the angular diameter distance

Answer 24

is the distance implied by the small angle approximation

Question 25

the practical limit of the distance we can observe is set by the

Answer 25

CMBR. Beyond the surface of last scattering the universe becomes opaque to radiation because it is a plasma, and free electrons scatter photons efficiently.