Week 11

Question 1

cepheids

Answer 1

• stars which brighten and dim periodically

- tight relation between period of oscillation and absolute luminosity

Question 2

Ladder step 4: Nearby galaxies

Answer 2

Measuring cepheids period and observed brightness gives us distance

Question 3

Ladder step 1: our solar system

Answer 3

• Radar ranging, i.e. sending a signal and seeing how long it takes to return
• d = 1/2 * (time-lag * Velocity)

Question 4

Ladder step 2: nearby stars

Answer 4

parallax - d(parsecs)=1/angle(arcseconds)

only in our galaxy

Question 5

Ladder step 3: distant stars

Answer 5

• Main sequence fit idea

- Current limit: Nearest galaxies [Need to identify individual stars within star clusters]

Question 6

Ladder step 5: distant galaxies

Answer 6

Rotation rate of galaxy and observed brightness gives

us distance

Question 7

What did Edwin Hubble show?

Answer 7

distance and velocity are linked

“The farther away a galaxy is, the faster it is moving away from us.” i.e universe expansion

Question 8

Hubbles law

Answer 8

v = H × d
v - recession velocity (Doppler Shift) (km/sec)
H - hubbles constant (km/sec/Mpc)
d - distance (Mpc)

Question 9

Redshift gives us?

Answer 9

distance

Question 10

Star clusters

Answer 10

• Distribution not uniform in space
• stars and galaxies preferentially found near other stars
• Galaxy clustering induced by the gravitational attraction

Question 11

Void size

Answer 11

grows overtime as matter is pulled away from underdense regions

Question 12

Galaxy cluster mass

Answer 12

increases over time

Question 13

Why don’t galaxies themselves expand?

Answer 13

• Gravity

- only the space between them is moving

Question 14

Centre of the universe

Answer 14

• no galaxy is at the “center”;
• no galaxy at the edge
• Every galaxy sees every other galaxy moving away

Question 15

How is light affected by universe expansion?

Answer 15

• light wave is being stretched as it travels through expanding universe
• cosmological redshift

Question 16

Why does hubbles law produce an age of the universe that is slightly off?

Answer 16

assumed constant expansion

Question 17

Lookback time

Answer 17

• how long light has taken to get to us today

- used as distance in an expanding universe requires time aswell

Question 18

the big stretch

Answer 18

The fabric of space BETWEEN galaxies
is STRETCHING, carrying the galaxies
away from one another

Question 19

universe growth and acceleration

Answer 19

-universe expansion is acclelerating
-but Size can grow with no acceleration (constant velocity
expansion)

Question 20

What could change the expansion rate of the universe?

Answer 20

• Attractive force = deceleration Gravity pulls mass together
• Repulsive force = acceleration Something pushes spacetime apart, speeds expansion

Question 21

Naive expectation is that the expansion is slowing down: WHY?

Answer 21

Gravity

Question 22

Instead of decelerating, why is the universe is accelerating?

Answer 22

Dark Energy counteracts gravity

Question 23

Until what point was universe expansion decelerating?

Answer 23

5 billion years ago

Question 24

primary constituent of the Universe

Answer 24

dark energy (70%), dark matter (25%), ordinary matter (5%)

Question 25

Density of dark energy

Answer 25

remains constant even though the Universe

expands: DE grows with time!

Question 26

event horizon

Answer 26

a region in spacetime beyond which events cannot affect an outside observer (can’t see)

Question 27

Future light cone

Answer 27

Larger light cone at future times