1
Astrophysical Cosmology
Andy TaylorInstitute for Astronomy,
University of Edinburgh,
Royal Observatory Edinburgh
2
Lecture 1
3
4
The large-scale distribution of galaxies
5
Temperature Variations in the Cosmic Microwave Background
6
Properties of the Universe• Universe is expanding.• Components of the Universe are:
• Universe is 13.7 Billion years old.• Expansion is currently accelerating.
7
1920’s: The Great Debate
Are these nearby clouds of gas?
Or distant stellar systems (galaxies)
8
• In 1924 Edwin Hubble finds Cepheid Variable stars in M31.
• Cepheid intrinsic brightness correlate with variability (standard candle), so can measure their distance.
• Measured 3 million light years (1Mpc) to M31.
Eyenear far
brighter fainter
Edwin Hubble
1920’s: The Great Debate
9
The Expanding Universe• Between 1912 and 1920 Vesto Slipher finds most
galaxy’s spectra are redshifted.
Slipher is first to suggest the Universe is expanding !
Vesto Slipher
10
In 1929 Hubble also finds fainter galaxies are more redshifted. Infers that recession velocities increase with distance.
Hubble’s Law
HDV
DV
11
The Expanding Universe
1/tH HD,
Time Velocity x Distance
V
1. Grenade Model:
12
The Expanding Universe
1/tH HD,
Time Velocity x Distance
V
2. Scaling Model:
x(t) = R(t) x0
13
The Expanding Universe
1/tH HD,
Time Velocity x Distance
V
D
t
Now Hubble Time:tH=1/H
H=70 km/s/MpctH=14Gyrs
tH
0
14
In 1915 Albert Einstein showed that the geometry of spacetime is shaped by the mass-energy distribution.
General Theory of Relativityrequired to describe the evolution of spacetime.
Albert Einstein
Relativistic Cosmologies
15
• Cosmological Coordinates (t , x):• How do we lay down a global coordinate system? • In general we cannot.
• Can we lay down a local coordinate system?• Yes, can use Special Relativity locally, if we can cancel gravity.
• We can cancel gravity by free-falling (equivalence principle).
Relativistic Cosmologies
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Relativistic Cosmologies
• Equivalence Principle:
17
Relativistic Cosmologies
• Equivalence Principle:
18
Relativistic Cosmologies
• In free-fall, a Fundamental Observer locally measures the spacetime of Special Relativity. • Special Relativity Minkowski-space line element:
• So all Fundamental Observers will measure time changing at the same rate, dt.
• Universal cosmological time coordinate, t.
22222222 dzdydxdtcdcds
19
Relativistic Cosmologies
• How can we synchronize this Universal cosmological time coordinate, t, everywhere?
• With a Symmetry Principle:• On large-scales Universe seems isotropic (same in all directions, eg, Hubble expansion, galaxy distribution, CMB).
• Combine with Copernican Principle (we’re not in a special place).
20
Relativistic Cosmologies• Isotropy + Copernican Principle = homogeneity
(same in all places)
A
0
B
12
So 2=1=0.
So uniform density everywhere
21
Relativistic Cosmologies• Isotropy + homogeneity = Cosmological Principle
A
0
B
12
So 2=1=0.
So uniform density everywhere
22
Relativistic Cosmologies• With the Cosmological Principle, we have uniform density everywhere.
• Density will decrease with expansion, so = (t).
• So can synchronize all Fundamental Observers clocks at pre-set density, 0, and time, t0:
000 )( : tt
23
Relativistic Cosmologies• What is the line element (metric) of a relativistic cosmology?
• Locally Minkowski line element (Special Relativity):22222222 dzdydxdtcdcds
t
x
Worldline
Proper time: ddt – Universal time
dx/c
• Spacetime Diagram:
24
Lecture 2
25
Relativistic Cosmologies• A general line element (Pythagoras on curved surface):
• Minkowski metric tensor:
• We have Universal Cosmic Time of Special Relativity, t, so
),,,( 22 zyxctxdxdxgdc
1000
0100
0010
0001
g
metric ofpart Spatial23
23
2222
ddtcdc
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• What is spatial metric, 32?
• From Cosmological Principle (homogeneity + isotropy) spatial curvature must be constant everywhere.
• Only 3 possibilities:• Sphere – positive curvature• Saddle – negative curvature• Flat – zero curvature.
Relativistic Cosmologies
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• What is form of 32?
• Consider the metric on a 2-sphere of radius R, 2
2:
Relativistic Cosmologies
) sin( 222222 ddRd
d
d
d
R
28
Relativistic Cosmologies
)sin( 222222 ddRd
dr
d
d
R)sin( 222222 rddrRd
• The metric on a 2-sphere of radius R:
• Now re-label as r and as :
where r = (0,) is a dimensionless distance.
29
• Can generate other 2 models from the 2-sphere:
Relativistic Cosmologies
)(
)sinh(
)sin(
222222
222222
,
222222
drdrRd
rddrRd
rddrRd
Rr
iRRirr
k = +1
k = - 1
k = 0
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• General 3-metric for 3 curvatures:
Relativistic Cosmologies
k = +1
k = - 1
k = 0
1 ),sinh(
0 ,
1 ),sin(
)(
))(( 222222
kr
kr
kr
rS
drSdrRd
k
k
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• Different properties of triangles on curved surfaces:
Relativistic Cosmologies
k = 0
k = +1
r d
sin r d
r
r
d
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• Different properties of triangles on curved surfaces:
Relativistic Cosmologies
k = +1
k = - 1
k = 0
1 ),sinh(
0 ,
1 ),sin(
)(
))(( 222222
kr
kr
kr
rS
drSdrRd
k
k d
sin r d
sinh r d
r d
r
r
r
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• Finally add extra compact dimension:
• Promote a 2-sphere to a 3-sphere
• So metric of 3-sphere is
Relativistic Cosmologies
22222
2
222223
sin
),)((
dddd
drSdrRd k
2222 sin ddd
d
d
) sin)(()( 22222222 ddrSdrdrSdr kk
34
• The Robertson-Walker metric generalizes the Minkowski line element for symmetric cosmologies:
Relativistic Cosmologies
k = +1
k = - 1
k = 0
))(( 22222222
2222222
drSdrRdtcdc
drdrdtcdc
k
- The Robertson-Walker Metric
Invariant proper time
UniversalCosmic
timeScale factor
Co-moving radial
distance
Co-moving angulardistance
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• Alternative form of the Robertson-Walker metric:
Relativistic Cosmologies
k = +1
k = - 1
k = 02
1
22
2222
22222222
1
1sin
sin
1
))(sin(
ydy
yd
ry
ydy
dyRdtc
drdrRdtcdc
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• Alternative form of the Robertson-Walker metric:
Relativistic Cosmologies
k = +1
k = - 1
k = 02
1
22
2222
22222222
1
1
)(
1
))((
kydy
ydS
rSy
ydky
dyRdtc
drSdrRdtcdc
k
k
k
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• The Robertson-Walker models.
Relativistic Cosmologies
k = +1
k = - 1
k = 0
))(( 22222222 drSdrRdtcdc k
• k = +1: positive curvature everywhere, spatially closed, finite volume, unbounded.
• k = - 1: negative curvature everywhere, spatially open, infinite volume, unbounded.
• k = 0: flat space, spatially open, infinite volume, unbounded.
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• The Robertson-Walker models.
Relativistic Cosmologies
• We have defined the comoving radial distance, r, to be dimensionless.
• The current comoving angular distance is: d = R0Sk(r) (Mpc).
• The proper physical angular distance is: d(t) = R(t)Sk(r) (Mpc).
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Lecture 3
40
Relativistic Cosmologies
rtRtd )()(
• Superluminal expansion:The proper radial distance is
The proper recession velocity is:
?)()()( cdR
RrtRtdtv
What does this mean?Locally things are not moving (just Special Relativity).But distance (geometry) is changing. No superluminal information exchange.
41
Light Propagation
• How does light propagate through the expanding Universe?
• Let a photon travel from the pole (r=0) along a line of constant longitude (d=0,d=0).
• The line element for a photon is a null geodesic (zero proper time):
0)( 222222 drtRdtcdc
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Light Propagation
• Equation of motion of a photon:
The comoving distance light travels.
t
tR
cdttr
drtRdtc
0
2222
)()(
)(
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Light Propagation
• Let’s assume R(t)=R0(t/t0):
1 ,1
'
''
)'(
')(
0
1
0
0
00
0
0
t
t
t
t
R
ct
dttR
ct
tR
cdttr
44
Causal structure
• Lets assume > 1:
tt
tctrtRtl
ttR
cttr
ttRtR
1
2
10
20
200
)()()(
11 )(
:)/()(
t
lFor t >> t1 , r is constant. This is called an Event Horizon.As t1 tends to 0, l(t) diverges, everywhere is causally connected.
l
R
t1
45
Causal structure
• Lets assume < 1:
cttrtRtl
tR
tctr
ttRtR
2)()()(
2)(
:)/()(
2/1
0
2/10
2/100
t
lAt early times all points are causally disconnected.The furthest that light can have travelled is called the Particle Horizon.
t=0
46
Cosmological Redshifts• Consider the emission and observation of light:
0
00
Rrd
r
tt
111
)(1
1
)(
1
0
rtRd
rr
ttt
t tRdt
47
Cosmological Redshifts• Consider the emission and observation of light:
0
00
Rrd
r
tt
111
)(1
1
)(
1
0
rtRd
rr
ttt
t tRdt
0
000
Rrd
r
ttt
1111
)(1
11
)(
11
00
rttRd
rr
ttttt
tt tRdt
A bit later:
48
Cosmological Redshifts• But the comoving position of an observers is a constant:
)()( 0
0
1
1
)()()()()(1
00
0
11
1
1
0
11
00
1
0
tR
t
tR
t
rtt
t tRdt
tt
t tRdt
t
t tRdt
tt
tt tRdt
t
t tRdt
Say the wavelength of light is = ct:
1
1
0
0
RR
0
1
1
0)1(R
R
v
vz so
49
Cosmological Redshifts• Can also understand as a series of small Doppler shifts:
R
Rt
R
RtH
c
V
v
v
1 Rv
d=ct
t=0 t=t
V=Hd=cHt
50
Decay of particle momentum• Every particle has a de Broglie wavelength:• So momentum (seen by FO’s) is redshifted too:
• Why? (“Hubble drag”, “expansion of space”?)
Rp
vp
/1
d=Rr, V=Hd
t=0 t=t
51
Lecture 4
52
The Dynamics of the ExpansionIn 1922 Russian physicist Alexandre Friedmann
predicted the expansion of the Universe
Rr
Rr
GMmrRmE
EPEKEnergyTotal
TOT
2)(2
1
...
Newtonian Derivation:
m
M=4(Rr)3/3
Birkhoff’s TheoremrRV
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The Dynamics of the ExpansionIn 1922 Russian physicist Alexandre Friedmann
predicted the expansion of the Universe
2
22
2
3
4
R
kcG
R
RH
Friedmann Equation:Rr
m
M=4(Rr)3/3
Birkhoff’s TheoremV
54
ctR
k
kcR
R
kcRG
R
1
0
3
8
22
2
222
• So a low-density model will evolve to an empty, flat expanding universe.
• There is a direct connection between density & geometry:
Geometry & Density
55
03
8
3
8
2
2
22
k
GH
RR
cGH
• So with the right balance between H and , we have a flat model.
• There is a direct connection between density & geometry:
Geometry & Density
56
• We can define a critical density for flat models and hence a density parameter which fixes the geometry.
Critical density & density parameter
k = +1 c >1
k = - 1 c
k = 0 c
2
2
3
88
3
H
GG
H
c
c
57
• How does evolve with time?
Critical density & density parameter
)()(1)(
3
81
3
8
22
2
22
2
2
2
22
tHtR
kct
HR
kc
H
G
R
kcGH
t
1
58
• What is present curvature length?
Critical density & density parameter
Mpch
kkH
cR 1
2/12/1
00
13000
1
Define a dimensionless Hubble parameter:
11100 Mpckms
Hh
59
• What is present density?
Critical density & density parameter
1SUN
211
32260
)M(1078.2
)(1088.1
Mpch
mkgh
Or 1 small galaxy per cubic Mpc.Or 1 proton per cubic meter.
60
The meaning of the expansion of space
)( 2222222 drdrdtcdc
22 11/' tcr
cV tttt
• Consider an expanding empty, spatially flat universe. c.f. a relativistic Grenade Model:
• Minkowski metric:
• Let v=Hr, H=1/t so v=r/t.
• Switch to comoving frame:
61
The meaning of the expansion of space
22
2'
22222
)(1' dr
drdtcdc
ctr
22
'1
ct
r• Rewrite in terms of t’ (comoving time):
• Hence in the comoving frame:
• but this is a k=-1 open model with R=ct!
• So what is curvature?
• And is space expanding ?
62
3/2
2
3
8
tR
GH
The matter dominated universe• Consider a universe with pressureless matter (dust, galaxies, or cold dark matter). • As Universe expands, density of matter decreases: =0(R/R0)-3.
• Consider a flat model: k=0, =1.R
t
63
Lecture 5
64
The matter dominated universe• The spatially flat, matter-dominated model is called the Einstein-de Sitter model.
R
t
Gyrs 3.93
2
3
2
00
00
3/2
Ht
tR
RH
tR
65
The matter dominated universe• Consider an open or closed, matter-dominated universe.• Define a conformal time, dcdt/R(t).
R
t
)])(()[( 2222222 drSdrdtRdc k
66
The matter dominated universe• Consider a closed, matter-dominated universe.• Define a conformal time, dcdt/R(t).
R
t
)sin()(
)cos1()(
2'
*
*
2
**
2
*
Rct
RR
R
R
R
R
R
R
2
300
* 3
4
c
RGR
67
The matter dominated universe• Consider an open or closed, matter-dominated universe.• Define a conformal time, dcdt/R(t).
R
t
)()(
)1()(
2'
*
*
2
**
2
*
k
k
SkRct
CkRR
R
Rk
R
R
R
R
2
300
* 3
4
c
RGR
68
The matter dominated universe
Expand forever
Eventual recollapse
“Big Bang’’ “Big Crunch’’
k = -1
k = +1
< 1
= 1
> 1
• So for matter-dominated models geometry/density=fate.
k = 0
69
The radiation dominated universe• As Universe expands, density of matter decreases:
m=0m(R/R0)-3.• Radiation energy density: r=0r(R/R0)-4.• At early enough times we have radiation-dominated Universe.
Log R
Log
m
rFor T(CMB)=2.73K, zeq=1000.
70
The radiation dominated universe• At early enough times we also have a flat model: k=0
240,
30,
2
2240,
30,
2
)/()/(3
8
1 ,0
)/()/(3
8
RRRRRG
R
R
kcRRRRRG
R
oom
oom
2/1240,
2 ,3
8tRRR
GR o
So Particle Horizon!
71
The radiation dominated universe• Timescales:
• Matter-dominated: R~t2/3
•Radiation dominated: R~t1/2
m
m
Gt
G
tH
6
1
3
8
3
2
Gt
G
tH
32
1
3
8
2
1
72
The radiation dominated universe• Spatial flatness at early times:
Recall:
How close to 1 can this be? At Planck time (t=10-43s)?
02
2
1)(
1)(t
tk
RH
kct
60
0
1011)( t
tt pl
t
1
73
Energy density and Pressure
• Thermodynamics and Special Relativity:
• So energy-density changes due to expansion.
0)/(3
)()(2
323
cpH
RpdcRd
pdVdE
2/33 cHpH
74
Energy density and Pressure
• For pressureless matter (CDM, dust, galaxies):
• Radiation pressure:
•Cf. electromagnetism.
3 0 Rp
)/(3 2cpH
2
4
3
1
4
cp
HR
• Conservation of energy:
75
Lecture 6
76
Pressure and Acceleration
• Time derivative of Friedmann equation:
•Acceleration equation for R:
)2(3
82
3
8
2
222
RRRG
RR
kcRG
dt
dR
dt
d
)/(3 2cpH
RcpG
R 2/33
4
77
Vacuum energy and acceleration
• Gravity responds to all energy. • What about energy of the vacuum?
• Two possibilities:1. Einstein’s cosmological constant.2. Zero-point energy of virtual particles.
78
Einstein’s Cosmological Constant
Einstein introduced constant to make Universe static.
33
82
22
R
kcGH
79
Einstein’s Cosmological Constant• Problem goes back to Newton (1670’s).
• Einstein’s 1917 solution:
rg rG4 4 22 G
0g G4
4
2 G
80
Einstein’s Cosmological Constant
Einstein called this:“My greatest blunder.”
• But this is not stable to expansion/contraction.
81
British physicist Paul Dirac predicted antiparticles. Werner Heisenberg’s Uncertainty Principle:
Vacuum is filled with virtual particles. Observable (Casmir Effect) for electromagnetism.
Zero-point vacuum energy
+ -+-
vnEn
2
1
82
The Vacuum Energy Problem
• So Quantum Physics predicts vacuum energy.• But summation diverges.• If we cut summation at Planck energy it
predicts an energy 10120 times too big. Density of Universe = 10 atoms/m3
Density predicted = 1 million x mass of the Universe/m3
• Perhaps the most inaccurate prediction in science? Or is it right?
83
Vacuum energy• Vacuum energy is a constant everywhere: V ~ R0
• Thermodynamics: Consider a piston:
2
33232
cp
RdpRdcRcd
VV
VVV
The equation of state of the vacuum.
84
Vacuum energy and acceleration• Effect of negative pressure on acceleration:
• So vacuum energy leads to acceleration.
VVV cpR 2)/3( 2
Ht
V
eRR
HRR
H
0
2 const
R
t• Eddington: is the cause of the expansion.
85
General equation of State• In general should include all contributions to energy-density.
20,0,
4,
3,
20
02
22
)1(
/)( ,3
8
aaaH
RtRaR
kcGH
mVoomV
Log R
Log
m
r
V
86
General equation of State• In general must solve F.E. numerically.• Geometry is still governed by total density:
0 1 2 m
1V
0
-1
OPEN
CLOSEDFLAT
87
General equation of State• In general must solve F.E. numerically.• But in general no geometry-fate relation:
0 1 2 m
1V
0
-1
OPEN
CLOSEDFLAT
RECOLLAPSE
R t
EXPAND FOREVER
R t
88
General equation of State
0 1 2 m
1V
0
-1
OPEN
CLOSEDFLAT
RECOLLAPSE
R t
EXPAND FOREVER
R t
R t
No singularity
89
Age and size of Universe• Evolution of redshift:
where
Age of the universe:
)()1(
)()1(
20
0
zHzR
RR
dt
dz
tR
Rz
20,0,
4,
3,
20
2 )1)(1()1()1()( zzzHzH mVoomV
0 )()1()(
zHz
dzzt z=0
z=infinity
90
Age and size of Universe• Usually evaluate t0 numerically, but approximately:
3.0
00 )1(3.07.0
3
2 VmHt
m
1
H0t0
2/3
0
v=0
v+m=1
91
Age and size of Universe• Comoving distance-redshift relation: dr=cdt/R=cdz/R0H(z).
m=1 m=0
1
00
2/1
00
)1()1(2
)1(12
zzH
crR
zH
crR
r(z)
z
w = 0
w = -1 v=1, R0r=cz/H0
92
Lecture 7
93
Age and size of Universe• Comoving distance-redshift relation: dr=cdt/R=cdz/R0H(z).
instein de Sitterm=1 de SitterV=1
zH
crR
zH
crR
00
2/1
00 )1(1
2
r(z)
z
m = 1
v=1
94
Observational Cosmology• Size and Volume:
• Start from line element:
•Angular sizes:
•Volumes:
22222222 )()( drSdrtRdtcdc k
k = 0
k = +1
r d
sin r d
r
r
d dzrSzRdl k )()(
drdzrSzRzdV k )]([)()( 23
95
Observational Cosmology• Angular diameter distance:
• Einstein-de Sitter universe de Sitter universe
)]([)()( zrSzRzD kA
2/1
0
)1(1)1(
2)(
z
zH
czDA
DA(z)
0 1 z
r=2c/H0
)1()(
0 zH
czzDA
dS
EdS
96
Observational Cosmology• Angular size:
• Einstein-de Sitter universe de Sitter universe
)()(
zD
dlzd
A
2/1
0
)1(1)1(
2)(
z
zH
czDA
d(z)
0 1 z
)1()(
0 zH
czzDA
1/zz EdS
dS
97
Observational Cosmology• Luminosity and flux density:
• Euclidean space:
• Curved, expanding space:•L=E/t~(1+z)-2
•Lv=dL/dv d/dv0=(1+z)d/dv•v=(1+z)v0
24
)(
r
vLS v
v
2220
220
00
)1(41
)1()(4
])1[()(
zSR
LS
z
dvS
zzrSR
vzLvS
k
TOTvTOT
k
vv
98
Observational Cosmology• Surface brightness, Iv:
dIS vv
33/3 )1(
])1[()/(
1
1
vz
vzBTvn
ev
I vkThv
v
d
So the high-redshift objects are heavily dimmed by expansion.
99
Observational Cosmology• Luminosity distance:
• Einstein-de Sitter:
• de Sitter:
)]([)1()( ,)(4 02
zrSRzzDzD
LS kL
L
TOTTOT
)1()(
)1(1)1(2
)(
0
2/1
0
zH
czzD
zH
zczD
L
L
DL(z)
0 1 z
dS
EdS
100
Observational Cosmology• Magnitude-redshift relation:
• The K-correction: redshift shifts frequency & passbands.
)(10
)( log5 zK
pc
zDMm L
][
])1[()1(log5.2)(
vL
vzLzzK
v
v
Lv
dv v(1+z)dv
101
Observational Cosmology• Galaxy Counts: Number of galaxies on sky as function of flux, N(>S).
•Euclidean Model: Consider n galaxies per Mpc3 with same luminosity, L, in a sphere of radius D.
2/3
2/33
2/1
)()(
SSnVSN
SDV
SD
102
Observational Cosmology• Olbers Paradox:
The Sky brightness:
which diverges as S goes to zero.
Too many sources as D increases, due to increase in volume.
02/1
0
2/3
0
)(1
SdSS
SdS
SdNdS
AI
103
Lecture 8
104
Observational Cosmology• Olbers Paradox: Why is the night sky dark?
which diverges as S goes to zero.
Too many sources as D increases, due to increase in volume.
02/1
0
2/3
0
)(1
SdSS
SdS
SdNdS
AI
D
105
Observational Cosmology• Relativistic Galaxy Count Model: =1, k=0 Einstein-de Sitter model:
Flux density: Lv~v
Number counts. z<<1 z>>1
30
2/1
)(3
))1(1(2
zrRA
V
zr
)1(22/112
0
)1())1(1()1()(4
zz
zrR
LS v
v
V(z)
0 1 z
EuclideanEdS
32
2/33
0)(
)(
HcVSN
SzVSN
106
Observational Cosmology• Relativistic Galaxy Count Model:
Log N(>S)
S
Euclidean
z>>1 sources
S-3/2
Counts converge due to finite volume/age /distance at high-z.So solves Olber’s paradox.
107
Distances and age of the Universe
• Cosmological Distances: c/H0 = 3000h-1Mpc.• Cosmological Time: 1/H0 = 14 Gyrs.
•Recall our solution for age of Universe:
So if we know m, V and H0, we can get t0. Or if we know H0 and t0 we can get m and V.
3.0
0
0
0
)1(3.07.03
2
)()1(
VmH
zHz
dzt
108
Distances and age of the Universe
• Estimating the age of the Universe, t0:
• Nuclear Cosmo-chronology:Natural clock of radioactive decay, ~10Gyrs.
• Heavy elements ejected from supernova into ISM:Thorium (232Th) => Lead (208Pb) 20 GyrsUranium (235U) => Lead (207Pb) 1 GyrUranium (238U) => Lead (206Pb) 6.5 Gyrs
109
Distances and age of the Universe
• Estimating the age of the Universe:
• No new nuclei produced after solar system forms, just nuclear decay:
• But don’t know D0, so how to measure D?
P0
D0
P D)1(
)1(/
/0
t
t
eP
eP
PD
110
Distances and age of the Universe• Estimating the age of the Universe:
• Take ratio with a stable isotope of D, S.
• Plot D/S versus P/S
D0/S
)1( /0 teS
P
S
D
S
D
D/S
P/S
Slope (et/-1)
Meterorites: SS = 4.57(+/-0.04) Gyrs+ Nuclear theory: MW = 9.5 Gyrs
111
Surface Temperature
Log
(L
umin
osit
y) Red GiantBranch
Main Sequence
Turnoff
•Age from Stellar evolution: t~M/L
Distances and age of the Universe
GC = 13-17 Gyrs.
Recall for EdS t0=9.3Gyrs!
112
• Local distance:
Distances and age of the Universe
• Use Cepheid Variables (cf Hubbles measurement to M31).• Mass M=3 – 9Msun
• Moving onto Red Giant Branch Luminosity~(Period)1.3
L~ 1/D2
D~1/L1/2
Need to know DLMC=51kpc +/- 6%.From parallax, or SN1987a.
Surface Temperature
Red GiantBranch
113
• Larger distances:
Distances and age of the Universe
• Use supernova Hubble diagram.• SN Ia, Ib, II.• SNIa standard candles.
• Nuclear detonation of WD.
• HST Key programme:H0 = 72+/-8kms-1.
(error mainly distance to LMC)
White dwarf
Red Giant
114
• So now know H0 and t0 so now know H0t0=0.96.• What can we infer about m and V?
• This implies that if V=0, m=0….• Or V=2.3m! Implies vacuum domination…• And if flat (k=0) V=0.7, m=0.3.
m
1
H0t0
2/3
0
v=0
v+m=1
Distances and age of the Universe
115
• Can measure m and V from luminosity distances to standard candles – the supernova Hubble diagram.
Cosmological Geometry
DL(z)
0 1 z
dS
EdS
z
VmL zzH
c
zH
dzzzD
0
2
0
4/)22()(
)1()(
116
• The supernova Type Ia are fainter than expected given their redshift velocity.
Type 1a supernova Hubble Law
Faintness= - log DL Decelerating
Universe
AcceleratingUniverse
Redshift
Cosmological Geometry
117
• Can measure m and V from luminosity distances to standard candles – the supernova Hubble diagram.
Cosmological Geometry
118
Lecture 9
119
The thermal history of the Universe• Recall that as Universe expands: m=0m(R/R0)-3 r=0r(R/R0)-4 = v0 (R/R0)0
• At early enough times we have radiation-dominated Universe.
Log R
Log
m
rFor T(CMB) = 2.73K today.
v
120
The thermal history of the Universe• Also expect a neutrino background, v=0.68 (see later).
Log R
Log
m
rFor T(CMB) = 2.73K today.
v
23
4
102.4
h
aT
r
r
vr
473.2
2
0,
0,
0,
0,
40,
30,
)(900,231
1)1(
)1(
KT
mr
m
r
meq
r
m
r
m
hz
z
z
121
The thermal history of the Universe• How far back do we think we can go to in time?• To the Quantum Gravity Limit:
• Quantum Mechanics: de Broglie:• General Relativity: Schwartzschild:
sc
Gt
mc
G
GeVG
cm
pl
pl
pl
455
353
19
10
10
10
2
2
22
c
Gmmcv
c
S
dB
Log dB
S
Log mmpl
Quantum Classical
122
Thermal backgrounds• If expansion rate < interaction rate we have thermal equilibrium.• Shall also assume a we have perfect gas.
• Occupation number for relativistic quantum states is:
2242 cpcm f(x)
x=(-)/kT
+ fermions– bosons 1
1)(
/])([ kTpe
pf
fermions
bosons
e-(-)/kT – Boltzmann
1/2
1
123
Thermal backgrounds
• The Chemical Potential, :
• A change of energy when change in number of particles.• As in equilibrium, expect total energy does not change:
dNpdVTdSdE
0
124
Thermal backgrounds
• The particle number density:
• N(p) is the density of discrete quantum states in a box of volume V with momentum p:
)()(1
pfpdNV
n
kp
g = degeneracy factor (eg spin states)
ky
k
kx
dk
kdV
gkdN 33)2(
)(
125
Thermal backgrounds
• The number density of relativistic quantum particles:
g = degeneracy factor (eg spin states)
ky
k
kx
dk
0 /)(
2
3
3
3
3
1
4
)2(
)()2(
kTpe
dppg
pfpdg
n
126
Thermal backgrounds• The ultra-relativistic limit: p>>mc, kT>>mc2 (bosons)
• The non-relativistic limit: kT<<mc2 (Boltzmann)
3
0 2/
2
3
)3(
1
4
)2(
c
kTg
e
dppgn
kTpc
kTmckTcpmc emkT
gdpepg
n /2/3
20
/)2/(23
22
24
)2(
Log n
Log kT
Threshold mc2
T3
T3/2e-mc2/kT
Particle production
Particle annihilation
127
Thermal backgrounds
• Proton-antiproton production and annihilation:
• mp = 103 MeV so for T > 1013 K there is a thermal background of protons and antiprotons.
• But when T<1013K annihilation to photons.
• Should annihilate to zero, but in fact p/p=10-9! (or else we wouldn’t be here.)
• So there must have been a Matter-Antimatter Asymmetry!!
p
p
128
Thermal backgrounds
• The energy density of relativistic quantum particles (bosons):
g = degeneracy factor (eg spin states)
ky
k
kx
dk 433
2
0 /)(
2
3
0 3
3
32
)(30
)(1
4
)2(
)()()2(
kTc
g
pe
dppg
ppfpdg
cu
kTp
129
Thermal backgrounds• The entropy, S, of relativistic quantum particles:
•The entropy is an extensive quantity (like E & V):
V
S
V
S
V
E
V
E
E1 V1 S1
E2
V2 S2
+
E=E1+E2
V=V1+V2 S=S1+S2
pV
TS
V
E
pdVdTT
STdV
V
STdT
T
EdV
V
E
pdVTVTdSTVdE
),(),(so
Hence
T
cp
V
Ss
3/
130
Thermal backgrounds• So in the ultra-relativistic case:
3
4
3
3
43
1
TT
s
up
Tu
Tn
• So
• But (entropy is a conserved quantity).
• Usual to quote ratios e.g. baryon density nB/s=10-9.
0s
ns
131
Lecture 10
132
Thermal backgrounds• Given these simple scalings with T for bosons, what is the scaling for n, u and s for fermions when kT >> mc2 ?
• Formally expand:
• So occupation numbers:
1
2
1
1
1
12
xxx eee
)2/(2)()( TfTfTf BBF
133
Thermal backgrounds• Given these simple scalings with T for bosons, what is the scaling for n, u and s for fermions when kT >> mc2 ?
• For kT>>mc2:
• Number densities:
3
3
Tgn
Tgn
BB
FF
)(4
3
2
21)(
)2/(2)()(
3
Tng
g
Tng
g
TnTng
gTn
BB
F
BB
F
BBB
FF
134
Thermal backgrounds• Given these simple scalings with T for bosons, what is the scaling for n, u and s for fermions when kT >> mc2 ?
• For kT>>mc2:
• Energy densities:
4
4
Tgu
Tgu
BB
FF
)(8
7
2
21)(
)2/(2)()(
4
Tug
g
Tug
g
TuTug
gTu
BB
F
BB
F
BBB
FF
135
Thermal backgrounds• Given these simple scalings with T for bosons, what is the scaling for n, u and s for fermions when kT >> mc2 ?
• For kT>>mc2:
• Energy densities:
4
42
3/ , ,
Tgu
upTguTc
pus
BB
FF
)(8
7
2
21
3
4)(
)3/11)(2/(2)3/11)(()(
42
2
Tsg
g
Tc
Tu
g
g
TuTuTcg
gTs
BB
F
B
B
F
BBB
FF
136
Thermal backgrounds• Given these simple scalings with T for bosons, what is the scaling for n, u and s for fermions when kT >> mc2 ?
• Define an effective number of relativistic particles:
• So energy of all relativistic particles is:
)(8
7)( ),(
8
7)( ),(
4
3)( Ts
g
gTsTu
g
gTuTn
g
gTn B
B
FFB
B
FFB
B
FF
Fermions
FBosons
B ggg8
7*
43
* )()(30
kTc
guTotal
137
Thermal backgrounds• The effective number of relativistic particles will change with time as kT<mc2 and particles become non-relativistic.
Fermions
FBosons
B ggg8
7*
g*
100
10
1 103 10-1 104 T (GeV)
• For high-T g*=100. If supersymmetric, g*=200.
138
Time and Temperature• At early times radiation and matter are strongly coupled and thermalized to temperature, T, of radiation.
• Recall: in a radiation-dominated universe,
and:
• Hence:
rGt
32
3
• Note also that T=2.73K(1+z), so z~T/(1 K).
seconds108.1
2
102/1
*
K
Tgt
22/1*
4* so , TgtTgr
139
A Thermal History of the Universe
• With time now related to temperature, and hence energy, we can map out the thermal history of the Universe.
140Inflation?
Dark Matter formed?
Proton-antiproton annihilation
e- e+ annihilation
• z ~ T/(1K)• E=kT ~ T/(3x103)eV
= 1010K
= 1013K
= 1015K
= 1028K
= 1032K
141
Freeze-out and Relic Particles• Electrons - Positrons annihilation:
•For first 3 seconds we have
• Then T drops and energy in becomes too low, so electrons and positrons annihilate.
• Stops when annihilation rate drops below expansion rate.
ee
e+
e-
142
Freeze-out and Relic Particles• Need the Boltzmann Equation to describe reactions:
2v3 nHnn
Rate of changeDilution by expansion
exp~1/H~R2
Loss due to annhilationint~1/(<v>n)~R3• Timescales:
log
log R
exp > int thermal equilibrium
exp < int Particle Freeze-out/
Decoupling
143
Freeze-out and Relic Particles• Creation, annihilation and freeze-out of particle relics:
kTmceT /2/3 2
Log n
log kT
T3
T3
Pair production
Pair annihilation
Freeze-out
144
Freeze-out and Relic Particles• Electrons-Positrons annihilation and neutrino decoupling:
• What happens to the energy released by ?
• At early times only have photons, neutrinos and e-e+ pairs in equilibrium.
• At T = 5x109K (3 seconds) e-e+ pairs annihilate.
ee
vv
ee
As weak force decoupled at T=1010K.
145
Freeze-out and Relic Particles• So radiation is boosted above neutrino temperature by neutrino decay.
• Before nv~n, after nv < n and Tv < T.
• But recall entropy is conserved
where
0s
3*Tgs
Bosons Fermions
FB ggg8
7*
146
Lecture 11
147
Freeze-out and Relic Particles• How much is photon temperature boosted ?
• Entropy: ge=2 g=2
• Neutrino Temperature:
)(4
11
)(1
)()(
87
vs
s
eess
before
beforeg
gg
beforeafter
ee
3*Tgs
KKTTv 95.1)73/2(11
43/1
148
Freeze-out and Relic Particles• How much is radiation energy boosted ?
• Neutrino energy:
• Enhances r by factor of 1.68 for 3 neutrino species:
68.1
)3( 2
cuu vr
uuuv 227.011
4
8
73/4
log
log R
v
Neutrinos annihilate
149
Relic massive neutrinos• Can we put cosmological constraints on the mass of neutrinos ?
• 1960’s: Particle physics models with mv = 0.
• 1970’s: Particle physics models with massive neutrinos.
• 1990’s: Non-zero mass detected (Superkamiokande, and Sudbury Neutrino Observatory (SNO) confirms Solar model).
150
Relic massive neutrinos• Can we put cosmological constraints on the mass of neutrinos ?
• Number-density of cosmological neutrinos:
• Mass-density:
speciescmv
KTnvvn BF
// 113
)95.1|()(3
43
G
Hnm v
vvv
8
3 2
151
Relic massive neutrinos• Can we put cosmological constraints on the mass of neutrinos ?
• Density-parameter of cosmological neutrinos:
• Re-arrange:
• Compare with lab: mve<15eV, mv<0.17MeV, mv<24MeV. m2=mi
2-mj2=[7x10-3 eV]2.
21
1 7.03.036.4
3
1
hN
eVmm vvN
ivv
v
i
v
i
N
ivv m
eVh 125.93
1
152
Big-Bang Nucleosynthesis (BBN)• As R tends to zero and T increases, eventually reach nuclear burning temperatures.• 1940’s: George Gamow suggests nuclear reactions in early
Universe led to Helium.
• Prediction of a radiation background (CMB)• Predicted 25% helium by mass, as found in stars.
• 1960’s: Details worked out by Hoyle, Burbidge and Fowler.
• First need free protons and neutrons to form.
153
Big-Bang Nucleosynthesis (BBN)• So first need proton & neutron freeze-out.• Recall:
• Below T<1013K (Mp~Mn ~ 103 MeV):
• Annihilation leaves a residual p/p~n/n~10-9.
• Protons and neutrons undergo Weak Interactions:
nn
pp
vpen
vnep
154
Big-Bang Nucleosynthesis (BBN)
• Assume equilibrium and low energy (kT<<mc2) limit:
• Ratio of neutrons to protons at temp T (m=mn-mp=1.3MeV):
kTmceTn /2/3 2
TKkTmc
p
n een
n /105.1/ 102
Log n
log kT
T3
Pair productionPair
annihilation
Freeze-out
155
Big-Bang Nucleosynthesis (BBN)
• Annihilations stop when p & n freeze-out occurs:
Reaction expansion time > timeint~1/vn exp~1/H
• So ratio is frozen in at Tfreezeout
freezeoutfreezeout TKkTmc
p
n een
n /105.1/ 102
Log n
log kT
T3
Pair productionPair
annihilation
Freeze-out
156
Big-Bang Nucleosynthesis (BBN)
• What is the observed neutron-proton ratio?
• Most He is in the form of 4He:
• He fraction by mass is:
• Observe Y=0.25 for stars.
• So np/nn=2/Y-1=7, or:
nppn
n
nnmnn
mnY
/1
2
)(
)2/4(
p
n
n
p
14.07
1
p
n
n
n
157
Big-Bang Nucleosynthesis (BBN)
• At what time, then, does neutron freeze-out happen?
• Need to know weak interation rates: <v>weak
• This was calculated by Enrico Fermi in 1930’s.
• Find
• So expected neutron-proton ratio is:
34.0/105.1 10
freezeoutTK
p
n en
n
KnT freezeout10104.1)(
158
Big-Bang Nucleosynthesis (BBN)
• Expect nn/np=0.34.
• But we said from observed stellar abundances.
• Close, but a bit big.
• But: 1. We have assumed kTfreezeout>>mec2
but really kTfreezeout~mec2
2. Neutrons decay. n=887 +/- 2 seconds for free neutrons. Need to be locked away
in a few seconds:25.0 14.034.0 Y
n
n
p
n
14.0p
n
n
n
159
Big-Bang Nucleosynthesis (BBN)
• The onset of Nuclear Reactions.
• At the same time nuclear reactions become important.
•Neutrons get locked up in Deuteron via the strong interaction
• Happens at deuteron binding energy kT~2.2MeV
• Dominant when T(D formation)=8x108K, or at a time t=3 minutes.
Dpnn
p
D
n
pD
160
Big-Bang Nucleosynthesis (BBN)• The formation of Helium.
• 4He is preferred over H or D on thermodynamic grounds. Binding energies: E(He)= 7 MeV
E(D) = 1.1 MeV• After Deuteron forms:•
•Then
pTDD
nHeDD
3
nHeDT
pHeHeD
HepT
HeDD
4
43
4
4
T gets too low and reactionsstop at Li & Be.
BBN starts at 1010K, t=1s.Ends at 109K, t=3mins.
161
Lecture 12
162
Big-Bang Nucleosynthesis (BBN)• Summary of BBN:• T=1013K, t~0.1s: Neutron & proton annihilation (p/p~n/n~10-9).
• T=1010K, t=1s: Neutron freeze-out (nn/np=0.34.) Neutrons decay (nn/np=0.14.) Nuclear reactions start.
• T=109K, t=3mins: Formation of Helium. Peak of D formation. End of nuclear reactions H, D, 3He, 4He, 7Li, 7Be
Dpn
163
Big-Bang Nucleosynthesis (BBN)• The number of neutrino generations:• The ratio of neutrons to protons is:
• Depends weakly on B: higher baryon density means closer packed, so n locked up in nucleons (D) faster.
• Depends on Nv: More neutrinos, more r (g*), so Hubble rate increases, neutron freezeout happens sooner, so more n.
• Cosmological constraint that Nv<4.
• In 1990’s LEP at CERN sets Nv=3.
14.03
)(163.02.0
04.02/2
v
BkTmc
Decayp
n Nhef
n
nFreezeout
164
Big-Bang Nucleosynthesis (BBN)• Testing BBN:• The abundance of elements is sensitive to density of baryons:
• is number density of baryons per unit entropy.
• This gives us the matter-antimatter difference of p/p~10-9
Agreement between BBN theory and observation is a spectacular confirmation of the Big-Bang model !
10103
n
nn pn
10-11 10-10 10-9
165
Big-Bang Nucleosynthesis (BBN)• Using BBN to weigh the baryons:• The abundance of elements is sensitive to the density of baryons.
• The photon density scales as T-3, so:
• This yields
• But:
So most of the matter in the Universe cannot be made of Baryons !
103
28 10373.2
)(1074.2
K
Th
n
nnB
pn
2
7.02 04.0 002.002.0 h
BBh
Bm 3.0
166
Recombination of the Universe
• Energy-density of the Universe:
Log R
Log
m
r
Plasma Era:, p, e- in a plasma
Thomson scattering (g-e)
zeq~104
T=103Kzrec~103
Bound atoms (H)and free photons
Recombination (a misnomer)
167
Recombination of the Universe
• Ionization of a plasma:• Assume thermal equilibrium.
• Use Saha equation for ionization fraction, x:HH
Hx
kTen
mkT
x
x /3
2/32
)2(
)2(
1
=13.6 eV – H binding energy.
168
Recombination of the Universe
• Ionization of a plasma:• But equilibrium rapidly ceases to be valid. Interactions are too fast, and photons cannot escape.
• Escape bottleneck with 2-photon emission:
• This means the ionization fraction is higher than predicted by the Saha equation.
H H+E= >>
H+E= <
2s
1s
0
169
Recombination of the Universe
• The surface of last scattering:• In plasma photons random walk (Thomson scattering off electrons).
• After recombination photons travel freely and atoms form.• The last scattering surface forms a photosphere (like sun), The Cosmic Microwave Background.
IonizedPlasma
z
z=1100
z=80
170
The Cosmic Microwave Background• The CMB spectrum:• The CMB was discovered by accident in 1965 by Arno Penzias and Bob Wilson, two researchers at Bell Labs, New Jersey. • This confirmed the Big-Bang model, and ruled out the competing Steady-State model of Hoyle.
• They received the 1978 Nobel Prize for Physics.
171
The Cosmic Microwave Background• The CMB spectrum:• The Big Bang model predicts a thermal black-body spectrum (thermalized early on and adiabatic expansion).
• The observed CMB is an almost perfect BB spectrum:
KT 002.0725.2
• Accuracy limited by reference BB source.
• CMB contributes to 1% of TV noise.
172
The Cosmic Microwave Background
1/1/ 11)( 0
DkThvkThv eevn
• The CMB dipole:• The CMB dipole is due to our motion through universe.
• Doppler Shift: v=Dv0, D=1+v.r/rc - dipole.
• Same as temperature shift: T=(1+v.r/rc)T0
173
The Cosmic Microwave Background
• The CMB dipole:• This gives us the absolute motion of the Earth (measured by George Smoot in 1977):
VEarth = 371+/- 1 km/s, (l,b) = (264o,48o)
assuming no intrinsic dipole.
• What is its origin?• Not due to rotation of sun around galaxy (wrong direction). v=300km/s, (l,b)=(90o,0o).• Motion of the Local Group? • Implies VLG=600km/s (l,b)=(270o,30o).
174
Lecture 13
175
The Cosmic Microwave Background
• The CMB dipole:• What is its origin of the dipole?
• Motion of the Local Group.• VLG=600km/s (l,b)=(270o,30o).
• Motion due to gravitational attraction of large-scale structure:
• LG is falling into the Virgo Supercluster (~10Mpcs away)
• Which is being pulled by the Hercules Supercluster (the Great Attractor, ~150Mpc away).
176
Dark Matter• Recall from globular cluster ages, supernova and BBN:
• So we infer most of the matter in the Universe is non-baryonic.
• How secure is the density parameter measurement?
• If it’s wrong and lower, could all just be baryons.
04.03.0 Bm
177
Dark Matter
Lm L
M
• Mass-to-light ratio of galaxies:
• We can expect M/L=F(M)
Comets:
Low-Mass stars:
Galaxy stars: Sun
Sun
Sun
Sun
Sun
Sun
L
M
L
M
L
M
L
M
L
M
L
M
101
10
1012
M/L
M
M -3
luminosity densityfrom galaxy surveys
178
Dark Matter• In blue star-light:
• So we find:
This is way above the M/L=10 we see in stars.
• So not enough luminous baryons in stars.
• In fact not enough baryons in stars to makeB=0.04, so there must be baryonic dark matter too.
3211
38
1078.2
10)7.00.2(
MpcMh
MpchL
Sunmcrit
SunL
Sun
Sunm
Blue L
Mh
L
M
7.03.0100300
179
Dark Matter• Dark Matter in Galaxy Halos:
• In 1970’s Vera Rubin found galaxies rotate
like solid spheres, not Keplerian.
• For V=const, need M(<r)~r, so
• Density profile of Isothermal Sphere.• Yields dark matter = 5 x stellar mass
23
1
rr
M
r
rGMV
)(2
r
V
Keplerian 1/r1/2
180
• In 1933 Fritz Zwicky found the Doppler motion of galaxies in the Coma cluster were moving too fast to be gravitationally bound.
• First detection of dark matter.
• Assume hydrostatic equilibrium:
• So need 10 – 100 x stellar mass.
Dark Matter in Galaxy Clusters
Zwicky (1898-1974)
22
11)(Vr
Prr
rGMF
Coma
Velocity dispersion
181
Dark Matter in Galaxy Clusters
rd
d
rd
Td
m
rkT
r
rGM gas
p ln
ln
ln
ln)()(
Coma
1.07.0
09.001.02/3
h
MMM
MM
M
M
DMstarsgas
starsgas
Tot
B
• X-ray emission from galaxy clusters. • Hot gas emits X-rays. • Assume hydrostatic equilibrium.• Equate gravitational and thermal potentials:
• Get both total mass, and baryonic (gas) mass.
• So MDM=10MB
182
Dark Matter in Galaxy Clusters
)( EE M
• Gravitational lensing by clusters of galaxies.
• Use giant arcs around clusters
to measure projected mass.
• Strongest distortion at the
Einstein radius:
• Independent of state of cluster (equilibrium).
• Find again MTot =10 – 100 Mstars
Abell 2218
183
Dark Matter and m
3.0
1078.2
1010311
3314
m
Suncrit
Sunclusterclusterm
MpcM
MpcMnM
• So independent methods show in galaxy clusters:
MDM ~ 10 Mgas ~ 100 Mstars
• Can estimate mass-density of Universe from clusters:
184
Large-scale structure in the Universe
• The distribution of matter in the Universe is not uniform.
• There exists galaxies, stars, planets, complex life etc.
• Where does all this structure come from?
• Is there a fossil remnant from when it was formed?
• How do we reconcile this structure with the Cosmological Principle & Friedmann model ?
185
The large-scale distribution of galaxies
6 billion light years
The 2-degree Field Galaxy Redshift Survey (2dFGRS)
186
The large-scale distribution of galaxies
The 2-degree Field Galaxy Redshift Survey (2dFGRS)
187
Large-scale structure in the Universe• The matter density perturbation:
• Fourier decomposition:
)()(
rr
r
(r)
k
iker k.r )(
L
kx=n/L, n=1,2…
188
Large-scale structure in the Universe• The statistical properties:
– The Ergodic Theorem:
Volume averages are equal to ensemble averages.
• Moments of the density field:
• Define the power spectrum:
rdV
31
kk
k
ikerd
Vrrd
V
22
3232 1
)( 1
,0 k.r
)/2(ln2
)()(
)(
2
2
2
32
2
kRkd
dkPkk
kP k
189
Large-scale structure in the Universe• The statistical properties:• The correlation function:
• So correlation function is the Fourier transform of the power spectrum, P(k).
• For point processes, correlation function is the excess probability of finding a point at 2 given a point at 1: k
rikk
k
rxikk
k
xikk eeexd
Vrxxxd
Vrxxr .2
'
).('
.33 1
)()( 1
)()()(
212 ))(1()1|2( dVdVrndP
1
2r
190
Lecture 14
191
Large-scale structure in the Universe• The Matter Power Spectrum:• So 2-point statistics can be found from P(k).
• What is the form of P(k)?
• For simplicity let’s assume for now it’s a power-law:
where A is an amplitude and n is the spectral index.
2
32
2)(
)(
n
n
Akk
AkkP log 2(k)
log k
192
Large-scale structure in the Universe• The Potential Power Spectrum:• Can we put limits on spectral index, n?
• Consider the potential field, .
• So far we have assumed <<1 (so metric is Freidmann).
• Poisson equation:
• So
20
02
02
4
4
4
kG
Gk
G
kk
kk
12
42
)(
)(
n
nk
kk
kkP
193
Large-scale structure in the Universe• The Potential Power Spectrum:• Can we put limits on spectral index, n?
• To keep homogeneity, need n less than or equal to 1. • To avoid black holes, need n greater or equal to 1.
• So must have n=1, with 2=const~10-10 (from CMB).
• n=1 is scale invariant (fractal) in the potential field.
12
42
)(
)(
n
nk
kk
kkP
log 2(k)
log kn<1
n>1
n=1
194
Structure in the Universe
• Where did this structure come from ?
• In 1946 Russian physicist Evgenii Lifshitz suggested small variations in density in the Early Universe grow due to gravitational instability.
density
position
195
Dynamics of structure formation
r
t
r
)sin(
)cos1(
Bt
Ar
• Consider the gravitational collapse of a sphere:• Assume Einstein-de Sitter (m=1, pm=0).
• Behaves like a mini-universe, so
196
Dynamics of structure formation
• Linear theory growth:
• 0th order:
- expansion of universe.
• 1st order:
r
t
)1()sin(
)1()cos1(2
2013
61
21212
21
BBt
AAr
232
find we Since GMBAr
GMr
)(6
2
3/2
0 taB
tAr
3/23/26
20
11
6
2 B
t
B
tAr
r0
197
Dynamics of structure formation• Linear growth of over-densities:
• Density ~ 1/Vol:
• Linear growth in E-dS:
r
t)1()1(
1
31
03/1
0
3
00
rrr
r
r
)(6
20
33/2
taB
t
3/23/26
20
11
6
2 B
t
B
tArr
r0
position
198
Dynamics of structure formation• Nonlinear growth of over-densities:
• Linear growth:• Turnaround:• Collapse:• Virialization:
69.1 ,177 , ,r ,2
0,r ,2
55.5 , ,
23
max21
linrKV
Bt
a
r
virialization
r
t
r0
Linear turnaround collapse
199
Formation of a galaxy cluster
200
Dark Matter and the Power Spectrum• Since ~ a on all linear scales, the matter power spectrum preserves its shape in the linear regime.
• Linear regime, <<1, valid at early times and on large scales. • If n>-3 initial shape will be preserved on large scales.
• Power spectra is shaped by dark matter, so leaves imprint.
r
log 2(k)
log k
201
Dark Matter and the Power Spectrum• Have already seen we need non-baryonic dark matter• (B=0.04 < m=0.3, from BBN and clusters, SN, ages…).
• But what can the dark matter be?• Massive Neutrinos?
Now know to have mass, so possibly.
• Black holes? Also now know to exist at centre of all galaxies. But
if too large, disrupt galactic disk & lens stars in LMC and galactic bugle (MACHO and OGLE surveys). Too small and will over-produce Hawking radiation emission.
• A frozen-out particle relic from the early universe: Weakly Interacting Massive Particles (WIMPS).
202
• Must be weakly interacting to avoid detection so far.• A promising idea in particle physics is Supersymmetry:
• The lightest supersymmetric particle (the neutralino = gravitino+photino+zino) could be detected in 2007 at Europe’s CERN Large Hadron Collider (LHC).
What is Dark Matter ?
Matter Particles (fermions) Force Particles (boson)
electron photon
selectronphotino
203
Dark Matter and the Power Spectrum• It is convenient to divide dark matter candidates into 3 types: 1. Hot Dark Matter (HDM):
Relativistic at freeze-out (e.g. neutrinos), kT>>mc2.
2. Warm Dark Matter (WDM):Some momentum at freeze-out, kT~mc2.
3. Cold Dark Matter (CDM):No momentum at freeze-out, kT<<mc2.
eVmnn HDMHDM ,
keVmnn WDMWDM 101 ,
MeVmmemn /DMh2
m
2eV 10GeV
HDM CDM
204
Dark Matter and the Power Spectrum• The matter Transfer Functions: Dark matter affects the matter power spectrum of density perturbations.• HDM: Free-streaming and damping: HDM freezes-out relativistically.
log k2
log k
n=1
>>ctct
HDM escapes
HDM trapped
~a2
kH~1/Ldamp, Ldamp=Damping scale
205
Lecture 15
206
Dark Matter and the Power Spectrum• HDM: Free-streaming and damping:
• HDM freezes-out relativistically, v~c, so can free-stream out of density perturbations in matter-dominated regime.
• So if HDM, expect no structure (galaxies) on small-scales today!.• This rules out an HDM-dominated universe.
log k2
log k
~a2
MpchneutrinoL
hzzzDmckTctL
vdamp
meqeqmeqHdamp
12
22/12
)(16)(
)(900,23 ,)()(
207
Dark Matter and the Power Spectrum• Baryons + photons: Baryon Oscillations and Silk damping.
• t<trec: Baryon-photon plasma
log k2
log k
n=1
>>ctct
Photons & baryonstrapped in plasma
- No collapse
Photons & baryons trapped
~a2
kH~1/DH, DH=Horizon scale
208
Dark Matter and the Power Spectrum• Baryons + photons: Baryon Oscillations and Silk damping.
• t>trec: Baryon and photons free. Baryons oscillate.
log k2
log k
n=1
>>ctct
Photons free-streamcarrying baryons(Silk damping).
Baryons oscillate.
Photons & baryons trapped
~a2
kH~1/DH, DH=Horizon scale
209
Dark Matter and the Power Spectrum• CDM + photons: The Meszaros Effect.
• Recall at early times >>m
log k2
log k
No growth
n=1
>>ctct
Radiation escapes
Radiation trapped
~a4
kH~1/DH, DH=Horizon scale
210
Dark Matter and the Power Spectrum
• CDM + photons: The Meszaros Effect.• After matter-radiation equality, all scales grow the same.
• Produces a break in the matter power spectrum at comoving horizon scale at zeq=23,900mh2.
Predicts hierarchical sequence of structure formation (smallest first).
log k2
log k
n=-3~a2
kH~1/DH, DH=Horizon scale
~a2
MpchzrRzD meqHeqH12
0 )(16)()(
211
Dark Matter and the Power Spectrum• Transfer Functions:
• Can quantify all this with the transfer function, T(k):
log k2
log k
CDM
BaryonicHDM
)()0( 222 zTz kkk
212
Observations: 2dFGRS Power-Spectrum
• No large oscillations or damping.
• Rules out a pure baryonic or pure HDM universe.
• Smooth power – expected for CDM-dominated universe.
• Detection of baryon oscillations – trace baryons.
213
Cosmological Parameters from 2dFGRS
Likelihood contours from the shape of the power spectrum:
Break scale: Matter density: mh = 0.19 ± 0.02
Baryon oscillations: Baryon fraction
= 0.18 ± 0.06 (if n = 1)
So m=0.27 (h/0.7) -1
B=0.04 (h/0.7) -1
214
Observations: 2dFGRS Power-Spectrum • Information about the amplitude of the power spectrum is confused, as we are looking a galaxies, not matter.
• We usually assume a linear relation between matter and density perturbations:
mattergalaxies b b-bias parameter
galaxiesmatter
So amplitude of galaxy clustering mixes primordial power and process of galaxy formation.
215
Structure Formation in a CDM Universe
216
Cosmological Inflation
• Standard Model of Cosmology explains a lot (expansion, BBN, CMB, evolution of structure) but does not explain:
• Origin of the Expansion: Why is the Universe expanding
at t=0?
• Flatness Problem: Why is ~1?
R
t
t
1
217
Cosmological Inflation
• Standard Model of Cosmology explains a lot (expansion, BBN, CMB, evolution of structure) but does not explain:
• Horizon Problem: Why is the CMB so uniform over large angles, when the causal horizon is 1 degree?
• Structure Problem: What is the origin of the structure?
218
0/33
4 2 cpRG
R
t
lt
l
0R
• Lets tackle the horizon problem first.• Recall for R~t1/2 we have a particle horizon:
• But if R~t, >1, can causally connect universe:
• More generally
• This happens when:
which we get from Vacuum Energy, 2cp VV
Cosmological Inflation
219
We are here !
In 1980 Alan Guth proposed that the Early Universe had undergone acceleration, driven by vacuum energy.
He called this Cosmological Inflation.
Inflates a small, uniform causal patch to the size of observable Universe.
Explains why Universe (& CMB) looks so uniform.
Cosmological Inflation
220
• Expansion Problem:• Vacuum energy leads to acceleration of the Early Universe• This powers the expansion (recall Eddington).
Ht
V
eRR
HRR
H
0
2 const
R
t
• Need Inflation to end to start BB phase.
Cosmological Inflation
Inflation Big-Bang (radiation-dominated)
221
03
8
3
8
2
2
22
k
GH
RR
kcGH
V
V
• So Inflation predict =m+v=1 to high accuracy.• Compare with SN & galaxy clustering results.
• The Flatness Problem: Recall that if we expand a model with curvature, it looks locally flat:
Cosmological Inflation
222
• How much Inflation do we need?• Usually assume Inflation happens at GUT era, EGUT~1015GeV.• So how large is the current horizon at the GUT era?
• But causal horizon at GUT era is just dGUT=ctGUT=3x10-27m.
• So need to stretch GUT horizon by factor aInfl=1029 ~ e60
mMpcGUTd
GeV
GeV
E
Ez
ztodaydGUTd
Mpchtodayd
H
CMB
GUTGUT
GUTHH
H
224
2713
15
1
1010)(
10105.2
101
),1/()()(
6000)(
Cosmological Inflation
223
Lecture 16
Lecture Notes, PowerPoint notes,Tutorial Problems and Solutions are now available at:
http://www.roe.ac.uk/~ant/Teaching/Astro%20Cosmo/index.html
224
Dynamics of Inflation We need a dynamical process to switch off inflation. Simplest models are based on scalar fields (spin-0), , the inflaton
(e.g. -mesons, Higgs bosons). No idea what is…
Must obey energy equation: E2 = p2c2 + m2c4
Quantize to get Klein-Gordon equation:
Assume is uniform and add
expansion term, 3H:
Equation of a particle in a
potential V=(m2c4-2).
V
)( 24222 cmc
ddVV' ),('3 VH
tiEip ,
225
Dynamics of Inflation Evolution of a scalar field, , in a potential V().
Energy of scalar field:
Assume Slow Roll:
So like a vacuum energy with pressure
Drives acceleration of expansion.
)(2
1 2 V
V
.
2
constV
V
2cp
226
Dynamics of Inflation The End of Inflation. Eventually the inflaton will reach the bottom of the
potential and oscillate.
No longer slow-rolling.
The Inflaton can then decay into other particles and radiation, re-heating Universe for radiation-domination.
V
V2
227
Dynamics of Inflation
The Origin of Structure in Inflation. The evolution of the inflaton has quantum fluctuations
Fluctuations due to Hawking
Radiation (c.f. Black Holes).
So the universe expands at different rates, leading to density perturbations:
quantumclassical V
2
H
tHtH
R
R ,333
228
Dynamics of Inflation The Origin of Structure in Inflation. During Slow-Roll equation of motion simplifies:
The induced density field is:
The density power spectrum is
Exponential expansion generates a fractal in the potential field: so spectral index is n=1.
H
VVH
3
' ,'3 V
'V
'
V
'2
93
3/23
VV
HH
2
32
')(
V
VkP
229
• Structure created by freezing in quantum fluctuations during an inflationary epoch. Leaves imprint in structure.
Inflationary Epoch
Big Bang Epoch
t
r
Quantum fluctuationsof massless virtual particles
(inflaton, gravitational waves)excited by expansion.
Frozen-in structure & gravitational waves
Gravitational wave spectrum:PGW(k) ~ 2~ V
A Gravitational Wave Background from Inflation
230
The Multiverse in Inflation Inflation can happen lots of times, producing a Multiverse.
So somewhere life will appear.
231
Observerz =1000 z = infinity
Plasma
Recombination
T=2.73K
The Cosmic Microwave Background
232
Observerz =1000 z = infinity
Plasma
Recombination
T=2.73K
The Cosmic Microwave Background
233
Observerz =1000 z = infinity
Plasma
Recombination
T=2.73K
The Cosmic Microwave Background
Causal horizon~1o
234
• Recall in 1930’s George Gamow predicts an afterglow from the Big Bang.
• Detected in 1969 by Arno Penzias & Robert Wilson (who won Nobel Prize).
• Formed at reionization at z = 1100.
• In 1960’s Sachs and Wolfe predicted there should be anisotropies due to potential perturbations.
Anisotropies in the CMB
235
• Anisotropies first detected in 1992 by the COBE satellite.
• On large angular scales T/T=10-5
• Detail images from the Wilkinson Microwave Anisotropy Probe (WMAP) in 2003.
• First year data.• Third year data expected any
day now.
Anisotropies in the CMB
236
Spherical Harmonic Analysis of the CMB
• Expand fluctuations in CMB temperature field in spherical harmonics on the celestial sphere:
m
mm YT
),( ),T(
237
• The squared harmonic modes of the temperature can be averaged to give a power spectrum:
where
TTm CT 2
)/2(2
)1(2
T
TCTT
Spherical Harmonic Analysis of the CMB
238
T on super-horizon scales (>1o)
• Sachs-Wolfe Effect: Gravitational redshift due to photons climbing out of potential wells,
3
1
T
T Newtonian potential
239
T on sub-horizon scales (<1o)
• Acoustic Oscillations: Baryons oscillate in and out of dark matter potential wells,
)cos(3
4
kAT
T
ct
240
T on sub-horizon scales (<1o)
• Photon Diffusion: Photons random walk out of potential wells,
)2/exp( 22sT
T
241
The CMB Temperature Power Spectrum
• Sachs-Wolfe Effect
• Acoustic Peaks
• Diffusion Damping
Wavenumber
02
)1(T
C
242
CMB Temperature Power Spectrum seen by WMAP (2003)
T/T)2
• Universe is spatially flat (m+v=1).• Potential amplitude: =10-5
• Spectral slope n~1
243
Geometry of the Universe from the CMB Temperature Power Spectrum
• The peak of the CMB power spectrum is given by the horizon size at recombination.• Hence it is a fixed “ruler”.
deg8.1
)(181)1(2
)(
2/1
2/122/1
00
mH
mmzH MpchzH
cdrRzD
Matter only universe:
244
Dark Matter and Vacuum Energy• The CMB acoustic peak
measure spatial curvature of Universe.
• Combine with supernova, galaxy clusters, galaxy clustering.
• Likelihood contours for m- V plane.
• So four independent methods converge on one model.
3.0,7.0 mV
245
Putting it all together
The “Standard Model” of Cosmology: • CMB acoustic peaks: m+V=1
• Supernova: 2V-m=1.1
• CMB + SN: V=0.7
• Galaxy clusters and galaxy clustering: m=0.3
• BBN and CMB: B=0.04• CMB Sachs-Wolfe effect: =10-5
• CMB + galaxy clustering: n~1• HST key programme and CMB: h=0.7• Age of Universe: 13.7+/- 0.2 Gyrs
246
Structure in the Universe with CDM
Amplitude of Structure,
247
Open Questions and Speculations
• Need to explain this strange Universe.• What is vacuum energy (dark energy)?• Why is V ~ (1 eV)4 ~ m??
• New particle physics, change gravity?• What is the Cold Dark Matter?
• CDM – neutralino in LHC?•Did inflation happen?
• Detect gravity wave background?• How did galaxies form?
•Watch them form at high-z?
248
A Multiverse? Inflation and superstring theory both predict a multiverse.
Find ~10120 universes…
Life will appear only in those Universes with small vacuum energy.
249
• Speculative ideas like the Ekpyrotic Universe try to unify Dark Energy and Inflation.
Parallel Universe Cosmologies
Time
Distance between galaxies
Our Universe Another Universe
250
The End