CP and Distance Ladder

Question 1

What is the current picture of the universe?

Answer 1

it is expanding
it is aging
there is a relic of a hot dense early era

Question 2

What time in the early universe did the atomic nuclei form?

Answer 2

3 mins

Question 3

When did the first stars and galaxies form?

Answer 3

18Myrs

Question 4

What is the cosmological principle?

Answer 4

On average, the universe is the same everywhere and what we see from our position is what anyone else anywhere in the universe would also see

it allows us to use Newtonian mechanics and simple thermodynamics

Question 5

What does the CP imply about the universe?

Answer 5

it is isotropic: looks the same in all directions
it is homogeneous: looks the same in all places (galaxy distribution)

(they are independent of one another)

Question 6

What evidence is there of isotropy?

Answer 6

the CMB (temperature looks the same in all directions at T = 2.7K)

Question 7

What evidence is there of homogeneity?

Answer 7

Redshift surveys of galaxies show that density is constant

When looking outwards, fewer galaxies are seen because they are harder to be seen not due to their lack of existence so separate regions of universe need to be observed (so CP still holds)

Question 8

How are galaxies identified?

Answer 8

Using spectra (measure lamda observed) which also gives redshift

Question 9

What is used as a proxy for distance?

Answer 9

redshift

Question 10

If z = 1, is the object moving faster than c?

Answer 10

No as the redshifting is doing work against gravity so v is not greater than c
(for higher z don’t use v, only z)

Question 11

What do galaxies show?

Answer 11

Absorption lines (dips in spectra) from atmosphere of stars and some show emission lines (lines of increased intensity)

Question 12

How are galaxies found?

Answer 12

Using colour (especially young galaxies)
Red, green, blue, infrared filters (on brightness vs wavelength graph)

young galaxies emit UV light

Question 13

What is the Lyman limit?

Answer 13

Where all photons gets ionised by neutral hydrogen and become unobservable as non of these photons escape to us

at 91.2nm

ground state to infinity transition of hydrogen

Question 14

What is the photo-ionisation potential?

Answer 14

13.6eV

Question 15

How are cosmological models tested?

Answer 15

By comparing how the redshift and distance relate to each other

(as redshift and distance are not the same)

Question 16

What is bootstrapping?

Answer 16

Measuring real distances to galaxies indirectly

Question 17

What is the only direct method of determining distances?

Answer 17

parallax: a star at a distance of 1 parsec from the Sun has a parallax of 1 arcsec

due to effects of atmosphere, parallax is found using satellites

only measures out to a few kpc currently

Question 18

What is parallax used to do?

Answer 18

Calibrate “secondary” methods on the distance ladder

Question 19

What do bluer stars indicate?

Answer 19

Hotter stars
(red stars are colder)

Question 20

What is main sequence fitting?

Answer 20

Luminosity (found using parallax) of stellar clusters are compared to main sequence of distant clusters

Can’t apply to distant stars as they begin to merge at a distance

Question 21

What are cepheids?

Answer 21

Evolved massive stars which pulsate regularly (due to gravity) on the instability strip

period is proportional to luminosity

Question 22

Why is it is difficult to use cepheids?

Answer 22

Cepheids need to be well established to be used

Stars have different metallicities which changes period-luminosity relations

Dust causes objects to appear redder and fainter

Only seen out to 10Mpc (methods become less accurate beyond this)

accuracy of distance +- 10%

Question 23

How are cepheids (standard candle) used?

Answer 23

Their luminosity is found using period and flux and distance can be found

(they are partially calibrated from parallax, mostly from main sequence cluster fitting)

Question 24

What are Type I and Type II Cepheids?

Answer 24

Type I are stars with normal metallicity (in our galaxy)

Type II are stars with lower metallicity

Question 25

What is a standard candle?

Answer 25

Any source whose intrinsic luminosity can be measured and takes measured flux to get distance

Question 26

What is used to find the distance to distant sources?

Answer 26

Supernovae Type Ia as they are very bright at about z = 2
(in a binary system as mass is transferred from primary to secondary star, critical mass is reached and system is destroyed causing bang and a lot of light emitted)

Question 27

How are Supernovae Ia found?

Answer 27

Host galaxy is imaged
Then host galaxy and supernova
Then final shot of host galaxy is taken

The brightness varies with time so both the rise and fall need to be observed due to variation (this standard candle is less accurate)

Question 28

What is measured to find Supernovae Ia?

Answer 28

The luminosity vs time from peak luminosity

the duration is the width of the peak as luminosity is proportional to duration, distance can be found

Both sides of peak need to be observed (rise time is only a few days)

Question 29

How are Supernovae Ia distinguished from other supernovae curves?

Answer 29

Spectrum proves it and gives the redshift

Peak also indicates them

Supernovae show that universe is accelerating

Question 30

What are the pros and cons of using Supernovae Ia?

Answer 30

evolutionary differences (e.g metallicity) have small effect

Not in dusty regions so dust does not obscure them

Sufficient nearby SN Ia to check properties

Show what’s happening in late universe (last few billion years)

Shows as far as we can accurately see