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Astronomy 1 > Introduction to Cosmology > Flashcards

Introduction to Cosmology Flashcards

1
Q

visible milky way

A

our galaxy seen ‘edge on’

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2
Q

how to determine distances to stars in the galaxy?

A

using distance indicators such as types of variable stars or the annual parallax of stars.

or by measuring apparent magnitudes of standard candles.

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3
Q

If we know luminosity and measure its flux from Earth, can estimate distance because…

A

the flux drops off as the inverse-square of the distance.

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4
Q

the distance modulus formula

A

expressing idea of distance indicators in terms of magnitudes.

relates apparent and absolute magnitude with a star or galaxy’s distance modulus.

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5
Q

distance modulus

A

a simple function of its distance in parsecs

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6
Q

parsec

A

parallax arc second

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7
Q

standard candle

A

a class of object assumed to have a predictable intrinsic luminosity

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8
Q

commonly used variable star distance indicators

A
  1. RR Lyrae stars
  2. Cepheid variable stars.
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9
Q

RR Lyrae stars

A

(A and F type giants which are pulsating. Often found in globular clusters)

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10
Q

Cepheid variable stars

A

F and G type supergiants

pulsate with a period around 1 to 50 days

absolute magnitude can be accurately estimated from their period.

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11
Q

extinction

A

absorption of starlight by interstellar dust grains which makes stars appear dimmer.

apparent density drop-off was due to extinction and was not a real effect.

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12
Q

how does a galaxy rotate?

A

not as a rigid body but differentially.

the angular speed of stars around the galactic centre depends on their distances from it.

Inner part rotates like a rigid body. (Think ice skaters holding hands)

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13
Q

Keplerian part

A

outer part of the disc

called keplerian since orbits approximately obey Kepler’s laws

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14
Q

rotation curve

A

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

rigid body rotation from x=0 (looks like /)
then kepler rotation after (——-)

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15
Q

estimation of the total mass of the galaxy interior to the Sun’s distance from the galactic centre

A

using Kepler’s 3rd law

GMgalP^2=4pi^2a^3

Mgal is mass of galaxy interior to a
P is suns orbital period

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16
Q

spiral structure of the galaxy

A

stars in the disc of the Milky Way are not uniformly distributed.

They lie along spiral arms wound tightly around the galactic bulge.

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17
Q

how can the spiral structure be mapped?

A

measuring the emission of neutral hydrogen.

Peak at 21cm line, if peak shifted, moving toward/away

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18
Q

the galactic halo

A

by plotting the rotation curve from radio observations, deduced that galactic disc appears to be embedded in roughly spherical halo of dark matter.

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19
Q

evidence for galactic halo

A

rotation curve does not drop off as rapidly as expected if only luminous stars in the disc were contributing to gravity.

equating grav force with cent forces, we get v is proportional to r^-1/2 so speed should fall of inversely with the square root of the distance.

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20
Q

dark matter

A

interacts gravitationally because it has mass but doesn’t electromagnetically.

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21
Q

density wave

A

spiral shaped wave pattern of high and low density regions.

cause gas to pile up in regions of higher density (like traffic jam)

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22
Q

what does the density wave theory predict?

A

inside edge of spiral arms are the most active star forming regions

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23
Q

without density wave

A

structure would be much more chaotic and disordered.

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24
Q

messier catalgue

A

contains many galaxies eg: andromeda and M31.

In M31, M stands for Messier and then galaxies are numbered.

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25
Q

Hubble identified three main types of normal galaxies. They are:

A
  1. Spirals
  2. Ellipticals
  3. Irregulars
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26
Q

classification of spiral galaxies

A

Sa-Sc

Sa has large central bulge and small, tightly spiral arms

Sc has a small central bulge and wide, open spiral arms

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27
Q

properties of spiral galaxies

A

diameters around 10-100kpc

mass of disc 10^11-10^12

spiral arms contain OB stars, dust and molecular clouds

disc rotates around the centre of the galaxy.

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28
Q

properties of elliptical galaxies

A

diameters 1-100kpc
masses 10^7-10^13 solar masses
spheroidal in shape
smooth brightness profile
little interstellar gas

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29
Q

properties of irregular galaxies

A

irregular in shape possibly due to recent collisions or mergers with other galaxies.

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30
Q

why are ellipticals old systems?

A

have little interstellar gas and dust and very little current star formation. (unlike spirals)

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31
Q

mass-to-light ratio in spirals and ellipticals

A

higher for ellipticals due to little current star formation.

ellipticals contain smaller proportion of young, massive stars

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32
Q

Hubble tuning fork diagram

A

NOT an evolutionary sequence
way of representing Hubble’s classification

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33
Q

active galaxies

A

galaxies whose luminosity is greater than that solely due to the stars they contain

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34
Q

cores of active galaxies

A

active galactic nuclei

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35
Q

three types of active galaxy

A
  1. radio galaxies
  2. seyfert galaxies
  3. quasars
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36
Q

properties of radio galaxies

A

elliptical or giant elliptical

ratio of radio to optical luminosity around 0.1-10

radio source shape double lobed or compact central, often with a jet

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37
Q

radio source spectrum radiation

A

usually synchrotron radiation.

indicates the presence of strong energy source and intense magnetic field

(synchrotron radiation is x-rays from electrons spiralling around magnetic field)

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38
Q

properties of seyfert galaxies

A

spiral with unusually luminous, blue nuclei

optical spectra show strong emission lines (narrow and broad)

broadening due to doppler motion of gases

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39
Q

properties of quasars

A

spectra contains strong emission lines. Balmer lines redshifted to longer wavelenghts (due to Hubble expansion - large distance=large recession velocity)

highly ionised emission lines on H, He, C, N, O, indicating very intense hot radiation field

vary in luminosity indicating compact emitting region

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40
Q

quasar absorption lines

A

light travelling through dust/gas

allows us to understand the rest of the universe

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41
Q

what powers quasars?

A

a supermassive black hole at its core

only explanation for such high luminosity produced in small volume

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42
Q

accretion disc

A

infalling matter forms accretion disc around a black hole

energy released by infalling matter produces two jets, producing beams of synchrotron radiation.

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43
Q

what do the large range of features exhibited by different AGN host galacies depend on?

A

several factors including:

evolutionary stage
orientation of galaxy
how obstructed the view of the galactic nucleus is

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44
Q

observations to support unified model of AGN

A

ALMA imaged accretion disc around supermassive black hole at centre of M77

Event horizon telescope imaged black hole in M87

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45
Q

spatial distribution of galaxies

A

not uniform - appear to be clustered

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46
Q

doppler shift of spectral lines from galaxies

A

z=λo-λe/λe

where o=observed and e=emitted

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47
Q

what did hubble find from plotting radial velocities of nearby galaxies against distance (from cepheid variables)

A

galaxies moving away from us and that their recession velocities were approximately proportional to their distance vrec=Hod

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48
Q

units of the hubble constant

A

kms^-1Mpc^-1

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49
Q

large uncertainties in Ho. Relative distances

A

Ho cancels V1/V2=d1/d2

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50
Q

red shift survey

A

accurate maps of the galaxy distribution on large scales using measured redshift to indicate separation

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51
Q

patterns in galaxy distributions that redshift surveys reveal

A

galaxy clusters
filaments (string/web-like structures)
voids (empty bits)

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52
Q

scales larger than around 30,000 kms^-1

A

universe begins to look uniform and homogeneous

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53
Q

unlike constellations, galaxy clusters are not ‘line of sight effects’, they are believed to have been…

A

formed together at the same epoch and are gravitationally bound together

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54
Q

epoch

A

common moment in time

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55
Q

peculiar velocities

A

specific velocities for local galaxies

vobs=H0d+vper

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56
Q

what causes peculiar velocities

A

gravitational interactions with other cluster members

more pronounced for galaxies that are close

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57
Q

vpec usually approx.

A

300kms^-1

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58
Q

superclusters

A

galaxy clusters are themselves clustered and the large scale structures they form are superclusters

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59
Q

local group

A

milky way is part of a small cluster of about 30 galaxies

roughly disc-shaped and about 2 Mpc in diameter

dynamics dominated by milky way and andromeda

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60
Q

nearest galaxies to milky way

A

large and small magellanic clouds

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61
Q

properties of galaxy group/small cluster

A

scale around 1Mpc
between 10-100 galaxies
examples: local group,fornax cluster

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62
Q

properties of rich clusters

A

scale up to 10Mpc
around 1000 galaxies
examples: virgo and coma clusters

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63
Q

properties of superclusters

A

scale approx 50-100Mpc
many thousand galaxies
examples: local supercluster

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64
Q

where are elliptical galaxies preferentially found?

A

in the cores of rich clusters

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65
Q

morphological segregation

A

elliptical and spiral galaxies are found in different locations in clusters

elliptical near centre; spirals on outside

thought to be the consequence of the galaxy formation process

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66
Q

galaxy formation process

A

believed that spirals existed briefly in galaxy clusters shortly after clusters formed but their discs could not survive the strong gravitational tidal forces in the cores of clusters

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67
Q

if vpec=300kms^-1, H0=71kms^-1, for a galaxy d>100Mpc, vpec is less than 5% of the cosmic expansion velocity. Therefore

A

Hubble’s law will hold to within a few % as long as not in immediate neighbourhood where vpec affect measurements

cannot rely on Hubble;s law to measure distance to nearby galaxies

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68
Q

what to use if cannot use Hubble’s law

A

distance indicators that are independent of redshift

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69
Q

estimating value of H0

A

distance indicators combined with measured recession velocities of more distant galaxies (where Hubble’s law holds)

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70
Q

apparent magnitude of standard candle equation

A

mobs=Mabs+5logr+25 (Mpc) or -5 in pc

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71
Q

standard candle

A

small spread in absolute magnitude
luminous at large distances

72
Q

examples of standard candles in common use

A
  1. Sc spiral galaxies
  2. brightest cluster elliptical galaxies
  3. type 1a supernovae
73
Q

primary distance indicators

A

can be calibrated from theory or from distances measured within immediate neighbourhood. eg: cepheid variables, annual stellar parallax

74
Q

secondary distance indicators

A

must be calibrated using a sample of galaxies beyond the local group whose distances have been determined by other methods

eg: type 1a supernovae, tully-fisher relation

75
Q

cosmological distance ladder

A

shows how different overlapping measuring techniques allow us to measure distances out to Gpc scales

76
Q

distance indicators - can get an excellent indication of luminosity by

A

using other directly measurable quantity that is correlated with absolute magnitude eg: cepheid variables

77
Q

cepheid variables - period-luminosity relation

A

a linear relationship exists between mean absolute magnitude and the log of the pulsation period.

78
Q

errors with cepheid variables

A
  1. statistical scatter (Mabs at given period not always straight line)
  2. systematic errors - due to extinction (light absorbed slightly by gas/dust so stars appear dimmer)

cepeids often in spiral arms - lots of star formation/dust

79
Q

Reasons why Hubbles original data gave Ho value of around 500kms^-1

A
  1. Only measured velocities out to around 1000kms^-1 within which peculiar velocities dominate
  2. Grossly underestimated distances to calibrating galaxies due to wrong absolute magnitude for cepheid variables, making wrong correction for extinction and misclassifying objects as cepheids when they were not
80
Q

Reason for disagreement over H0=50 or 100 kms^-1Mpc^-1 in 1980s

A

Disputes over distance to the Virgo galaxy cluster

81
Q

To determine H0, need to combine primary band secondary distance indicators. Why?

A
  1. H0 estimates require both accurate distances and recession velocities
  2. Primary distance indicators only extend to around 20Mpc
  3. At 20Mpc observed radial velocities of galaxies still seriously affected by peculiar motions
82
Q

Cosmological distance ladder

A

Combination of two or more primary and secondary distance steps

83
Q

Secondary distance indicator: type Ia supernovae

A

All have ~same mass so very similar peak luminosities

84
Q

When do type Ia supernovae occur

A

When white dwarf accreted sufficient matter from binary companion to push itself over mass limit, causing a thermonuclear explosion

85
Q

By plotting SNIa light curve we can determine

A

Apparent magnitude at maximum light

86
Q

Why are SNIa good standard candles

A

Hubble diagram is linear

(Plot of maximum apparent magnitude against log of recession velocity)
If Max constant then Max-5log10H0+25 constant

87
Q

Tully fisher relation for spiral galaxies

A

Linear relationship between absolute magnitude and log of recession velocity
(Rotation velocity usually velocity in flat part of rotation curve)

88
Q

Why is Tully fisher relation a secondary distance indicator

A

Requires to be calibrated using set of nearby galaxies whose distance has been determined through primary distance indicators

89
Q

Tully fisher equivalent for elliptical galaxies

A

Relation between intrinsic diameter of galaxy and range in velocity of central stars

Problematic because no suitable large elliptical galaxies within local group

90
Q

Reasons for being unable to calibrate all secondary distance indicators before HST

A
  1. Lack of elliptical galaxies in local group
  2. Lack of local group spirals suitable to calibrate Tully fisher relation
  3. Lack of local SNIa to calibrate Hubble diagram
91
Q

After launch of HST, cepheids became directly observable within nearby clusters. This allowed

A

Direct calibapration of secondary distance indicators and provided link to more distant clusters where hubbles law assumed to hold within few percent

92
Q

Taking one big jump

A

Fewer steps = fewer errors

93
Q

HST allowed us to miss out

A

Local group rung on distance ladder and get to H0 in only two steps

94
Q

Olber’s paradox

A

Why is sky dark at night
Star in every line of sight if universe is infinite

95
Q

Solution to Olber’s paradox

A
  1. Stars have finite lifetimes
  2. Speed of light finite so only stars within a finite distance can be observed
  3. Universe has a finite age
96
Q

Hot Big Bang model

A

Standard model for origin and evolution of Universe

Universe began between 10 and 20 billion years ago and has been expanding ever since

97
Q

Cosmological principle

A

Assumption that universe is homogeneous and isotropic

98
Q

Universe homogenous

A

Looks the same no matter where you are in it

99
Q

Universe isotropic

A

Universe looks same no matter what direction you look in

100
Q

What scale does cosmological principle hold

A

Scales larger than ~30000kms^-1

Obviously not going to hold on small scales

101
Q

Universe can be described by size of

A

Dimensionless number that we call the cosmic scale factor R(t)

Measures characteristic size of universe at time t

102
Q

What does expansion look like (analogy)

A

Axis gets bigger and system gets bigger but individual coordinates the same

103
Q

Proper distance r(t)

A

The actual separation measured in Mpc

104
Q

Co-moving separation

A

Separation expressed in coordinate system that expands along with the background space

not changed by expansion of universe
(Think ggow and NY on globe regardless of size)

105
Q

R0

A

Present day value for scale factor

106
Q

Can give another interpretation of redshift of light from distance object in terms of

A

Amount by which the Universe has expanded since light from object was emitted

107
Q

What are cosmological redshifts due to

A

Result of stretching of wavelength of object’s light as it propagates through expanding space

not due to motions of distant objects

108
Q

Proper velocity

A

Rate of change of proper distance

109
Q

Hubbles law expressed using proper velocity

A

v=dr/dt=d/dt(Rs) = Rdot . s = R dot / R x (Rs) = R dot/ R r

110
Q

Hubbles constant is not constant in

A

Time but constant in space at any given time

111
Q

Condition defining Big Bang

A

R(t) —>0 at t=0

112
Q

Estimating time elapsed since Big Bang

A

Assuming constant expansion rate H(t)=H0 for all t

V=Hor=distance/time=r/t

Ie t=H0^-1
t in years for H0 in km/s/Mpc

113
Q

Including gravity in Hubble time

A

Give age smaller than Hubble time as gravity will slow down expansion

114
Q

Friedmann’s equation

A

Semi derive equation for the evolution of R(t) using only Newtonian concepts

115
Q

Derivation of friedmann’s equation basis

A

Galaxy mass m, proper distance r from centre of sphere containing many galaxies
Gravitationally attracted by other galaxies within sphere, force equivalent to that of point mass of sphere
Uniform density

116
Q

Kinetic energy of galaxy

A

KE=1/2m r dot ^2 = 1/2 Rdot ^2 s^2

117
Q

Potential energy of galaxy

A

PE= -GMm/r = -4/3piR^2s^2Gpm

118
Q

Total energy of galaxy

A

Constant
1/2ms^2[R dot^2 - 8piG p R^2/3 = constant

Simplifies to R dot ^2 / R^2 -8piGp/3= -k/R^2 (k constant)

119
Q

What does Freidmann’s equation describe

A

How gravity slows the rate of expansion of the Universe

120
Q

Second equation for evolution of scale factor

A

Using general relativity

121
Q

For normal matter ( density and mean pressure > or =0)

A

Cannot have a static universe as that would require R double dot=0

(Common belief at time was static universe)

122
Q

How did Einstein fix his steady universe problem

A

Introducing an extra constant lambda - cosmological constant (think of as integration constant)

123
Q

Positive value of lambda

A

Behaves like ‘anti gravity’
Repulsion force that overcomes attraction of gravity on very large scales

124
Q

Evidence for cosmological constant >0

A

Type Ia SN, CMBR, pattern of galaxy clustering

Cosmological constant currently - dark energy

125
Q

k=1

A

Universe is closed with positive curvature
PE>KE
Celestial sphere, globes

BOUNDED: expands then recollapses

126
Q

k=-1

A

Universe open with negative curvature (Pringle, KE>PE, expanding forever
UNBOUNDED: expands indefinitely

127
Q

k=0

A

Universe flat with zero curvature
KE=PE
JUST UNBOUNDED: slows to R dot =0 as R approaches infinity

128
Q

Analytic solution of Friedmann’s equation is straightforward when

A

Cosmological constant =0 and case of flat universe k=0

129
Q

I’d assume universe is matter dominated and mass is conserved

A

(dR/dt)^2=A/R
A is constants which do not depend on time

130
Q

Solution to Freidmann’s equation

A

R(t)=at^2/3

131
Q

Critical density

A

When k=0, rearrange Friedmann for p
Density required to just close the universe

132
Q

p>pcrit

A

Universe recollapses

133
Q

p<pcrit

A

Universe expands indefinitely

134
Q

omega (t)

A

Dimensionless parameter = p(t)/pcrit(t)

135
Q

Omega >1

A

Universe closed

136
Q

Omega<1

A

Universe open

137
Q

Omega =1

A

Universe flat

138
Q

Methods of estimating matter density of universe

A

Visible stars in Milky Way
Galaxy rotation curves
Galaxy clusters
Gravitational lending
Hubble diagram and standard candles
Large scale structure

139
Q

Matter density - visible stars in Milky Way

A

Assume all stars in galaxy are one solar mass
Count up all the stars and divide by volume

Not very practical, what about elliptical galaxies

140
Q

Matter density - galaxy rotation curves

A

Measuring rotation velocity of clouds of neural hydrogen gas within disc of spiral galaxies as a function of their radial distance from centre, can deduce amount of mass inside that radius

Observed velocity >expected - dark matter

141
Q

Matter density - galaxy clusters

A

Assuming galaxy cluster is virialised (steady state)
Virial theorem (balance o PE and KE)
2KE+PE=0

Can rearrange for virial mass estimate using <v^2> (3D mean square peculiar velocity)

142
Q

Matter density- gravitational lending

A

General relativity predicts light deflected in strong gravitational field

143
Q

Weak lensing

A

Light from distant galaxies is distorted by passage through an intervening cluster

Amount of distortion allows cluster mass density to be estimated

144
Q

Microlensing

A

Light from stars in LMC and bulge of Milky Way distorted by dark matter crossing line of sight , temporary rise in brightness of background stars

Shape of microlensed star’s light curve allows one to lace constraints on mass of lensing object e

Monitoring programmes have checked brightness of millions of LMC

145
Q

Matter density - Hubble diagram of standard candles

A

Nearby objects: relation between apparent magnitude and log redshift is linear

Distant objects: relation begins to curv, amount of curve tells us about curvature of the Universe

146
Q

Matter density - large scale structure

A

Patterns in galaxy redshift surveys can be used to place limits on omega0
Higher matter density = stronger pattern of galaxy clustering
Studying patterns of galaxy peculiar velocities can also be used

147
Q

Baryonic matter

A

Think of as everyday matter
Interacts electromagnetically

148
Q

Conclusive evidence for existence of dark matter

A

Estimates of matter density from visible stars are a factor of 100 smaller than estimates from galaxy clusters, large scale motions and gravitational lensing

149
Q

Dark matter

A

Simply matter that cannot be seen through telescope
Can be baryonic or non baryonic

150
Q

Baryonic dark matter candidates

A

Gas clumps in galaxy halos and clusters
MACHOs such as brown dwarfs and undetected white dwarfs (unlikely after HST)

151
Q

Non baryonic dark matter candidates

A

WIMPS (weakly interacting massive particles) such as massive neutrinos, exotic particles or primordial black holes

152
Q

Hot dark matter

A

If non baryonic dark matter was moving relativistically at the time of decoupling from baryonic matter eg neutrinos

153
Q

Cold dark matter

A

If non baryonic dark matter was moving non-relativistically at decoupling

154
Q

How does CMBR provide support for cosmological principle

A

CMBR is isotropic to better than on part in 10^4

155
Q

Early universe: free electrons scattered photons so much that

A

Universe was effectively opaque (think fog)

Think going through crowded room and getting scattered, cannot walk in a straight line, have to move past people

156
Q

Matter and radiation in early universe

A

Coupled since photon interacted so strongly with free eclectics

157
Q

At what temp could free protons and electrons combine to form neutral hydrogen

A

3000K

158
Q

Why did early universe fog clear

A

Neutral hydrogen formed which was much less effective at scattering photons

Photons can now propagate freely

159
Q

Epoch of recombination

A

Matter and radiation decoupled

160
Q

CMBR consists of

A

Photons that were emitted at the epoch of recombination and have travelled towards us ever since

161
Q

Typical energy of a black body photon of temperature T

A

Given by Etyp=kT

162
Q

Etypical value for 3000K and why it is a problem

A

Approx 0.26eV but know ionisation energy of hydrogen is 13.6eV

Going backwards from 13.6eV gives T=15800K

163
Q

How can you explain the difference in Etypical

A

Black body photons have a distribution of energies

There’s a long tail of photons with energy E>kT

164
Q

Energy density of matter in Universe

A

u=pmatterc^2

165
Q

Universe is currently matter dominated

A

umatter»uradiation

166
Q

Epoch of matter radiation equality

A

n= urad/umat

Epoch at which mean energy densities of matter and radiation are equal

167
Q

Big Bang model and standard model valid from when

A

10^-40 seconds after Big Bang

168
Q

Quark hadron transition

A

Quark soup condenses
Universe cooled enough to form stable hadrons
Quarks no longer exist as free particles

169
Q

Primordial nucleosynthesis

A

Cooled sufficiently to allow protons and neutrons to combine together and form stable light nuclei

170
Q

Omega B

A

=pB/pcrit = baryon density/critical density

171
Q

Dipole anisotropy

A

CMBR not perfectly smooth
Not believed to be intrinsic to CMBR but instead due to our peculiar motion which causes a Doppler shift of radiation that varies with direction

172
Q

Tiny variations in temperature of CMBR indicate

A

Universe was not completely smooth when CMBR was emitted

173
Q

Hot dark matter smooths out clustering on small scales so

A

In models with hot dark matter, we expect to see large structures forming first and then later fragment

174
Q

Models where dark matter is cold

A

Structures form on both small and large scales from the outset

175
Q

Concordance model

A

5% of matter and energy in Universe today from baryons
Rest consists of dark matter (25%) and dark energy (70%)

Astronomy 1 (6 decks)

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