A Review of Dark Energy
C. W. Kim
Korea Institute for Advanced Study
Johns Hopkins University
Cosmology
Birth, Evolution,
and
Fate of the Universe
Introduction
Discovery of Dark Energy
Dark Energy
Cosmological Constant
Quintessence (K-essence)
Extra Dimensions
Others
Summary
Three Major Miracles
1) Creation of the Universe
~ 14 billion years ago
2) Appearance of life on the Earth
~ 3.5 billion years ago
3) Why do we live now?
Anthropic Principle ?
Nancy Kerrigan:
Why me, Why now ?
Birth of Universe
 Genesis: 4004 B.C., October 26
(King James Version)
Even Newton believed it.
 Hindu-Buddhism : Eternal existence
 Modern Cosmology :
Born 14 B ys ago
How?
11-dimensional space-time and
Super strings were created from NOTHING.
“Nothing” fluctuates, decays, and is
unstable.
(Uncertainty Principle)
 Where ? Anywhere (our universe)
東洋의 十進法
즈믄천
일
분
리
모
사
一
分
厘
毛
絲
100
10-1
10-2
10-3
10-4
나눌 분
리리
털모
실사
104
일만 만
홀 忽
10-5
문득 홀
억 億
108
억억
미 微
10-6
작을 미
조 兆
1012
조조
섬 纖
10-7
가늘 섬
경 京
1016
서울 경, 클 경, 경 경
사 沙
10-8
모래 사
해 垓
1020
땅 가장자리 해
진 塵
10-9
자 秭
1024
만억억
양 壤
1028
부드러운 흙 양
애 埃
진의 억분의 일,
10-10 티끌 애.
-24
구 溝
1032
도랑 구
묘 渺
10-11
간 澗
1036
산골 물 간
아득할 묘, 10-32이란
해석도 있음
정 正
10-12
사막(아득할) 막
1040
바를 정
막 漠
재 載
1044
실을 재, 해 재
모호 摸湖
10-13
법 모, 호수 호
극 極
1048
지극할 극
준순 浚巡
10-14
물러갈 준, 순행할 순
수유 須臾
10-15
잠깐 수, 잠깐 유
순식 瞬息
10-16
눈깜짝할 순, 숨쉴 식
일 一
100
십 十
101
열십
백 百
102
온백
천 千
103
만 萬
티끌 진. 10-16 이란
해석도 있음
즉 10 이란 해석도 있음
항하사
恒河沙
1056
아승기
阿僧祇
1064 언덕 아, 중 승, 땅귀신
탄지 彈指
10-17
탄알 탄, 손가락 지,
손가락을 튀김
나유타
那由他
1072 어찌 나, 말미암을 유,
찰나 刹那
10-18
절 찰, 어찌 나,
ksana의 음역
육덕 六德
10-19
허공 虛空
10-20
청정 淸淨
10-21
기, 헤아릴수 없는 수
무량의 수
80
불가사의 10 or
不可思議 10120
무량수
無量數
갠지스 강의 무수한
모래라는 뜻
1088 or
10128
불가사의
불가사의의 억배
六元德 = 知, 仁, 聖,
義, 忠, 和
빌 허, 빌 공
맑을 청, 깨끗할 정
Big Bang Cosmology
Experimental Evidence: Expansion,CMB,
Element formation,….
Einstein’s Gravity (1915)
Cosmological Principle
Homogeneous and isotropic
(No center)
Equation of State
 Modified by Inflation (Flat Universe)
o 
 o (current density)
c (critical density)
c ~ 10 -29g/cm3
=1
Inflation
SHBBM :
 Horizon Problem
 Flatness Problem, Age Problem

  Problem,...
Inflation :
Expansion by more than 1030
10-35 sec 10-32 sec
all problems solved
Predictions :


O = 1 : Flat Universe

 created
Fate of the Universe
R
k = -1, 0 < 1
k = 0,
0 = 1
k = 1,
0 > 1
○
t = 0 t0= 15 Bys
t
Matter in The Universe
0=1
?
Dark
Matter
< 10 %
< 1 % (Visible)
Ordinary Matter ( proton, neutron, … )
Matter and Dark Energy
in The Universe
0=1
??
?
 X = 0.73
 m = 0.27
< 10 %
< 1 % (Visible)
Ordinary Matter : proton, neutron, neutrino…
Accelerating Universe
Accelerating Universe
R
k = -1, 0 < 1
k = 0, 0 = 1
k = 1, 0 > 1
t=0 t0=14b.y
t
X
1
SN Type Ia
0.73
BOOMERANG
Maxima
(CMB)
0.5
LSS
GL
0.0
0.27
0.5
1
matter
m = 0.27 ± 0.04
Rotation Curves
M
L
Hot Gas
m
8G
m 
3H2
Galaxy Halos
Gravitational Lensing
Large Scale structure
B = 0.044 ± 0.004
L = 0.005
 <
~ 0.015 (no degeneracy)
~

1
SN Type Ia
0.73
BOOMERANG
Maxima
(CMB)
0.5
LSS
GL
0.0
0.27
0.5
1
matter
 t ~ 380,000 ys : T ~ 3000° K
i
1
 Universe then was
1000 in sizen
i
•
n
•
: proton
•
•
•: electron
•
•
: photon





•


•

•
•

•

•


•
•
•
•
•
•
•
•  Hydrogen Atom
•
First Light
Expansion of the
Universe (  103)
Microwaves
WMAP
“Best” Cosmological parameters :
Wilkinson Microwave Anisotropy Probe(WMAP) Observations :
Preliminary Result
Description
Total density
Symbol
tot
1.02
Value
+ uncertainty
0.02
-uncertainty
0.02
Age of universe (Gyr)
t0
13.7
0.2
0.2
Hubble constant
h
0.71
0.04
0.03
Equation of state of quintessence

< -0.78
95%CL
-
Dark energy density

0.73
0.04
0.04
Matter density
m
0.27
0.04
0.04
Baryon density
bh2
0.0224
0.0009
0.0009
Baryon density
b
0.044
0.004
0.004
Baryon density(m-3)
nb
0.25
0.1
0.01
Matter density
mh2
0.135
0.008
0.009
Light neutrino density
h2
<0.0076
95% CL
-
CMB temperature (K)a
Tcmb
2.725
0.002
0.002
CMB photon density (cm-3)b
n
410.4
0.9
0.9
Baryon-to-photon ratio

0.310-10
0.210-10
Baryon-to-matter ratio
bm-1
6.110-10
0.17
0.01
0.01
Age at decoupling (kyr)
tdec
379
8
7
Decoupling time interval (kyr)
tdec
118
3
2
zeq
3233
194
210
Redshift of matter-energy equality
-a From COBE (Mather, J. C. et al., 1999, ApJ 512, 511)
-b Derived from COBE (Mather, J. C. et al., 1999, ApJ, 512, 511)
CMB
Curvature of the Universe
o
Horizon size = 1
  0 = 1,
k= 0
( flat )
1°

(Observer)
  0 > 1,
k=+1
( Closed )

  0 < 1,
k=–1
( Open )

30 degree
1 degree
0.1 degree
The Power Spectrum
4/30/01 New York Times
(Latest BOOMERANG DATA)
6
Relative amplitude
Main tone
..
.
4
.
.
.
.
2
harmonics
.
.
. . . . .. .
0
7
1
0.5
0.3 0.25 0.2
Angular scale : degree
Boomerang determines the curvature
K=+1
K =0
K = -1

1
SN Type Ia
0.73
BOOMERANG
Maxima
(CMB)
0.5
LSS
GL
0.0
0.27
0.5
1
matter
Standard Candle (SN)
Direct measurement
dL =
1
1
(1 - q) Z2 + ……. ]
[Z+
2
H0
Hubble’s Law
Corrections
Deviation from Hubble’s Law can tell
values of q (deceleration parameter).
‥
R
–
q 
RH2
> 0 : deceleration
< 0 : acceleration
SN Type Ia (Standard candle)
Z = 0.2
m
Z = 0.5

m

Z = 1.0
m

Z = 1.7
m
  0
Chandra X-Ray Satellite
1 b years ago
3.5 b years ago
6.7 b years ago
Clusters seen by X-rays
Recent Independent Confirmation from
CHANDRA X-ray Obsevatory
 New Method to measure Dark Energy
(How fast the Universe is expanding)
 Observed 26 Clusters of galaxies at 1bly ~ 8bly
Clusters : Hundreds of Galaxies + Dark Matter
+ Hot Gases
Emit X-ray
d = f (z, q)
measured
 q (deceleration Parameter)
About 6 Billion years ago the Universe
began to accelerate.
“Consistent with SN Ia data”
Dark Energy
• Characteristic of space-time (?)
• Uniformly distributed (?)
Changes very slowly in time (?)
• No interference with formation of
structures (?)
•Acceleration of Expansion: Negative p
• E = mc2  gravity
• X ~ 10-29 g/㎤ ~ 0.73 x total density
Standard Matter (Including Energy)
Usual expansion: q
∝
p: total pressure
- (3 p +  ) <0
: total density
Matter and energy ( Including dark matter)
3p+>0
Satisfy:
Define :
p
W   (Equation of state func)
3p+>0 
w
> -
1
3
Universe with Standard Matter
and Energy with deceleration
Behavior of  as a function of W
pX
WX = —
X
Equation of State Func.
X ~ R-3(1+W)
pM ~ 0
Non-relativistic matter : WM = –—
M
 M ~ R-3
p
1
Relativistic matter : W = –— ~ —

3
 R ~ R-4
Cosmological constant: W = - 1
0
 ~ R
= constant
W of Dark Energy
ρT
=
M
+
X
PT
=
PM
+
PX ≃ PX
(matter dominated)
Experiment :
X
⇒(
T
= 0.73
T
+ 3 PT ) = ( 1.37 X + 3 PX ) < 0
acceleration
WX < -0.46
Data shows :
WX < -0.78 (95% C.L.)
Significant acceleration!
Possible Candidates for X
 Vacuum Energy (Cosmological Constant, )
W = –1 : Satisfies all requirements
 = constant
 Quintessence (Rolling, Dynamic Scalar Field)
1 •2
—
 – V()
WQ = 2
1 •2
—
 + V()
: –1 < WQ < 1
2
Some changes in time
Clusters slightly on very large scale
 Topological defects
N
WT = – —
3
N=
1 for strings
2 for walls
Strings are ruled out!
*
 Graviton leak to Extra dimension
New models: Ads/CFT inspired , NonRiemannian, Bimetric, Time varying
cosmological constant, ….
 Break down of something?
(Friedmann Equation, Gravity, … )
Unexpected surprise?
Most attractive Interpretation as of now
 X =  : Vacuum Energy
Why so small?

Einstein’s Equation of Gravity
(1915)
R - 1 Rg - g = 8 GT
2
Space-time
matter
Cosmological Constant
(1917 )
  0  Negative Pressure
Vacuum energy
Basic Equation (Einstein)
.
T+V=E
1 R2 - 4 m R3 G - 1 R2 = - k2
3
R 6
2
1917, to make Universe Static.
.
 R
2
= 3 G m R2 + 1 R2 - k
8
3
~ 1
R
k = 0 : flat Universe
 > 0 : Repulsive force ! ( force = 1  R
3
 < 0 : Attractive force
Because of E = mc2 ,
 term acts as
mass with gravity
in Cosmology.
But ordinary matter does not feel the effect
since it is pulled by every direction equally.
  “only” affects expansion of the Universe.
New Cosmology (   0)
1917 : Introduced by Einstein to make a static
Universe
“Biggest Blunder in my Life !”
1917 : de Sitter
1922 : Friedman Cosmology (Standard)
1929 : Hubble : Hubble’s Law (d, z)
1931 : Lemaitre Cosmology (alternative to
Friedman Cosmology)

1980 : Guth : Inflation; o = 1, 
: Hoyle, Bondi, Gold : to solve an age crisis
: Solved by Sandage (1987) by a small Ho
(h0=0.42)
1997~98 : Perlmutter, Schmidt and Riess found
independently an indication of an accelerating
Universe (  0 ? ) using SN type Ia (z ~ 0.5)
1998 : BOOMERANG : o ~ 1 , m < 1    0
2001 : Established ?!
BOOMERANG, MAXIMA, COBE, … New
SN Type Ia data, Large scale structure Data,
CMB DATA
Observed Cosmological Constant
c =
2
3H0
8G
= 1.9  10-29 h2 g/cm3
~ 9.6  10-30 g/cm3

 = 0.73 c = 5.7  10-30 g/cm3
= 
No SDSS data
included
8G

for h = 0.71
 = 1.05  10-56 /cm2
~ 10-35 /sec2
~ 10-82 m 2
~ 10-84 (GeV)2
~ 10-122 Mp2
Smallest number known
in Science !
Hard to explain!

m

t
 was not important
now
Already,  > m (becoming de Sitter Universe)
Eventually  ~  = const : de Sitter Universe
Standard (k=0, =0) Cosmology
.
H  R : Hubble parameter
R
..
q  - RR
. 2 : deceleration
R
parameter

:    = 2q
c
.
R2 - 8G R2 = 0
3
.
.
+(+p) 3 ( R) = 0
R
2
3H
c =
8G
1 x x
R
t = H 0  dx , x =
: R ~ t 2/3
0
Ro
Present :
H0 = h
1
, 0 = 1
2
1 1 x
2
t0 = H 0  dx = 3 H0
0
q0 =
100Km
Mpc sec
:
h = 0.65 (± 0.05)
 t0 = 10.25 By for h = 0.65
Age of the oldest globular clusters , white dwarf
13 ~ 15 By
Cosmology with k = 0 ,   0
.R 2
 =

8 G
 2
3 R =0
 = m +  ,  c =
8 G 2
3 R ,
3H2
8 G
 =  m +  = 1
m=
 =
R(t) =
  
3
8 G m
2( 3 t)
=
sech
3H2
2
8 G 
2( 3 t)
=
tanh
3H2
2
2
3
m,oRo3
sinh 3 ( 2  t )


q = 1 (1 -  ) = 1 ( m - 2 )
2
H2
2

 =
3H2
1
q = 0 when  m =  : Inflexion Point
2
Deceleration  Acceleration
1
2, 0 3
1
2
q = 2 (1 - 3 ) = 0 :  = H or Z =
 0 -1
Deceleration Parameter
q
1
2
q=
.
Deceleration
0
.
1
[ m - 2 ]
2
 = H2
t
Acceleration
.
- 1
2
-1
Z=1.7
(2001)
Z ~ 0.55
(1997 ~ 2000)
t0
– 0.48
t
Miracle
m
~

~
R(t)
~

3
t)
2
3
tanh 2 (
t)
2
2
sinh 3 ( 3  t )
2
sech 2 (
 3 ~ 10 -17/sec
t o ~ 10 17 sec
  t ~ O (1) : Miracle !
3
Anthropic Principle ?
 =  m +  =
1

 =1
m

3.4 By
(z =1.7)
10 By t o = 14 By
(z ~ 0.23)
t
Quintessence
The Fifth element (Greek philosophy)
Air, Earth, Fire, Water and Quintessence
Perfect Substance
Quintessence as Dark Energy: Caldwell, Dave, Steinhardt
(1998)
Quantum Field() with a very long wavelength
(about the size the visible Universe)
In the context of Super string theory
L=
    ν  - V( )
→  + 3H  + V’() = 0

1
2
g
μν
P =
ρ =
P
W =
=
ρ
1 2
  V (  )
2
:
1 2
  V (  )
2
1
2
1
2
2
 - V()
2
 + V()
 1  W 
-1
V( ) : Extremely long wavelength and period
1 2
 << │V( )│ → W  ~ -1
2
Few - Examples
: V 0 e  t
Pure exponential
Tracker
: M 4t /  , M 4 e
Modified exponential
: V

( ) e
 /
 

Tracker field
  Rad> Mat: mimics radiation

 Rad
 Mat > Rad :  mimics matter
(due to Hubble damping)

Mat
Some mechanism locks  to be
nearly a constant.
(Kinetic – energy – driven
quintessence k-essence)
t0
t
Cosmological Constant

Λ
Quintessence
 -1 ≤
W = -1
W Q
≤1
Recent epoch

W ≠ W (t)
W
Q= W Q(t)

W ≠ W  (x)
W
Q= W Q(x)

ripple in CMB

Universe accelerates forever
  could decay into new
forms of matter and
radiation → Universe with
new structures
Future:


Large – Aperture Synoptic Survey Telescope (LSST)


Deep Extragalactic Evolutionary Probe (DEEP)
Super Nova Acceleration Project (SNAP) ~ 2000
Planck Satellite (2007)

Can they really distinguish?
Extra- Dimensions
4
5
9
Calabi-Yau Extra-dimensions
Extra-dimensions
 Not seen because
Super String : 11 dim.
6 ~ 7 dimensions are
 curled up in a small world (~10-33 cm).
 may not be curled up but we live on our 3-D world.
Everything is confined to our 3-D world (Brane),
except gravitons (g).
Gravitons:
M
Virtual
real
String theory
g
M'
Gravitational waves from SN, collision
of galaxies, …
e, , q, ….
:
graviton
:
Tied to a brane
Leaks out
Changes in Gravity
Zero - Point Energy of Reheating Potential
V()
Inflation

Reheating
..
 + 3H. + 3V’() = 0
V ( ) : Harmonic Oscillator
1
H (t) : ∼
t
⇒ Power – law suppressed damped H.O.
Boundary Conditions: Reheating starts at tr∼10 34sec
14
Energy Scale ∼ 10 GeV
Quantum Mechanical Solution: Zero-point energy
ρ
29
3(∼
(t
=
t
)
∼
10
g/cm
0
0
ρΛ! )
What Next ?
 Need more accurate value of X
(To a few % accuracy)
 Crucial to know
if W(t,z) or W = constant
and space dependence
 CMB Experiments (Boomerang, WMAP,
Maxima, Plank, …)
May not reach to ~ 0.05
 SDSS (Galaxy, Cluster counts), …
 Super Nova / Acceleration Probe : Satellite
4 years to construct, 3 year mission
Expect to see ~2000 SN (z = 0.2 ~ 1.7)
May not be enough to see w(t)
So far, Supernova Cosmology Project (Perlmutter)
High – Z Supernova Search (Schmidt)
HST
(Riess)
~ 100 SN (6 at z > 1.25)
z
Weller and Albrecht 2001
Accelerating Universe
de Sitter universe
R
k = -1, 0 < 1
k = 0, 0 = 1
k = 1, 0 > 1
t=0 t0=15b.y
t
W ( t ) will eventually determine the fate!
Comments on Vacuum Energy
• Maybe the most fundamentally mysterious
thing in basic Science.
• Every attempt to calculate it has
gotten an absurd answer.
• Basically, people don’t have a clue
as to how to solve the problem.
• Vacuum energy would be NO.1 on my
list of things to figure out.
• Right now, not only for cosmology but for
elementary particle theory, this is the bone
in our throat.
Summary
 Dark Energy
?
Vacuum Energy
Quintessence
Extra dimension
Changes in Gravity
 Vacuum Energy : Creation ?, Why so small?
 Acceleration: No galaxies seen in the sky!

Why do we live now?
Life on Earth begins when
matter density = dark energy density
Most difficult problem in 21st
Century
Galileo’s first Telescope
1.33 m,
X 14
1.2 meter Mt. Palomar Telescope
E. Hubble discovers the expanding Universe
Hubble Space Telescope
Keck 10 meter
Telescopes
Chandra X-Ray Satellite
C
h
a
n
• Riess, 1998
° Perlmutter, 1999
M = 0.3, = 0.7
m= 0.3
= 0.7
m = 0.3
= 0
 =1
 = 0
Hubble Diagram (can tell q values)
Cosmological Constant() : Vacuum(Dark) Energy
R - 1 Rg -  g  = 8GT (1915)
2
Gravitational Constant (1917)
Acceleration
‥
R
4

R = 3 RG m + 3 
Decrease
Expansion Rate
Increase
curvature
•
( R )2 =  k 2 + 8 G + 1 
m
R
3
R
3
Decrease
Constant
Static Universe
0 = – (curvature, matter) + (Vacuum Energy)
“Biggest blunder in my life!”
SN Type Ia
Light Curves :
Type I
TypeII
Type Ia : Produces a lot of Fe
Lack of H lines, Abundant in early Universe
Standard Candles : distance measurements
( Not quite but correctable : colors and
temperatures)
Spectroscopy of various Bands : I, J, K, ...
A Hint that the Universe is accelerating lately
(for small z (0.4~1)) was discovered
independently by Perlmutter and Riess (1997~8)
Acceleration is fairly recent !
SN Type Ia 1997ff (Riess) with z=1.7
shows no acceleration !
How to calculate the age
R
Present Rate(H )

•
°
Einstein theory
H , density
°
Now (t )
°
time(t)
14 By
20 By
Age (t ) ~
°
1
H
°
1
1 + density
Ω o= 1
Ωo > 1
Ωo < 1
Inflation  Flat Universe
o = 1
3x3x3
Inflation and Horizon Problem
SHBB Model
1 cm
Causally connected
(Same
temperature)
. .B
A
10 -24 cm 10 25
tinf=10 -35 sec
.A
.
10 28 cm
B
10 3
10 23 cm
t de = 10 5 y
.A
 B
t 0 = now
.
How to Cure the Problems
in SHBB Cosmology
Inflation !
10 -24 cm
> 10 30
10 6cm
Horizon problem
solved !
10 25
(10
-35
10
-34
10 25 cm
sec)
Predictions
.A


0

  0 = 1 + 10 -N : N very large

Flat Universe
t = 10 5 ys
.
B
X = 0.73 ± 0.04
 CMB
0 = 1 ,
0 and B
m = 0.27

X
 X = 0.73
(Nature unknown)
 SN Type Ia can tell us about m / X
Nature of X
Also some cross - checking
of CMB, LSS, SN
Cosmological Constant() : Vacuum(Dark) Energy
R - 1 Rg -  g  = 8GT (1915)
2
Gravitational Constant (1917)
Acceleration
‥
R
4

R = 3 RG m + 3 
Decrease
Expansion Rate
Increase
curvature
•
( R )2 =  k 2 + 8 G + 1 
m
R
3
R
3
Decrease
Constant
Static Universe
0 = – (curvature, matter) + (Vacuum Energy)
“Biggest blunder in my life!”
Phase Transition
T > Tc

T = Tc

T < Tc

Creation of a Universe
(Phase transition of vacuum)
Ground state
or vacuum
Uncertainty
Principle
New Vacuum
(New ground state)
Quantum
Fluctuation
Energy created
by the change
of nature
of vacuum
A Brief History of the Universe
• t = 0 : Big Bang (No space – time before)
From singular Point ? ( ρ > 0, P > 0 )
How? (why) : Quantum Fluctuation of
Vacuum
(nothing → something)*
Born as 11- dimensional Universe ?
(string theory)
• t ~ 10-43 sec : T ~ 1033 oK (~ 1019 GeV)
( Planck time )
Unified force → Gravity + others
Homogeneous. 11 dim. → 4 (=3+1) dim.
• t ~ 10-35 sec : T ~ 1030 oK (~1016 GeV)*
strong force + others split
Inflation : 10-35 sec - 10-32 sec
Universe expands by > 1030 times
Present Univ. was 1cm.
*
(δρ∕ρ) Created (homogeneous until then)
(observed now in CMBR)
nB ≠ nB created → We exist.
• t ~ 10-10 sec : T ~ 1018 ºK (=250GeV)
Weak and E/M interactions separated
(W,Zº became massive)
• t ~ 10-4 sec : T ~ 1012 ºK (~MeV)
p, n, π, … appear : Before this
only leptons, quarks
• t ~ 1 sec : T ~ 1010 ºK (~MeV)
Neutrinos decoupled
340
( nν ~ cm3 now)
• t ~ 3 min : T ~ 109 ºK (0.1 MeV)
Nucleosynthesis : d, He, Li, formed
• t ~ 3,000 years :
ρR = ρM
• t ~ 380,000 years : T = 0.26eV ( ~3000 ºK)
H atoms form.
Photons decoupled :
δρ*
First light !!! Carrying ρ
now nγ ~ 400/cm3, microwaves
R(t) ~ 10-3 :
dH = 1023 cm
R(t0)
Our Univ. was 1025 cm
→ horizon problem.
• t >~ 1,000,000 years :
Stars, Galaxies, Quasars, Black Holes
Supernovae,….
• t ~ (14 - 4.5) billion years (9.5 b years)
Solar System formed from S.N. remnants
• t ~ (14 - 3) = 11 b years
Life begins : Major Miracle
• t ~ 14 b years : Now
T ~ 2.735 ºk
Present Universe
1) Universe is expanding (Astronomy)
km
1920’s : Hubble
V = Ho  L : H o= 71
Mpcsec
(Isotropic)
2) “Visible” Size * ~ 1028 cm (~14 billion l.y.)*
Age ~ 14 billion years old
Uncertain due to H o
3) Visible Universe contains
~100 billion (=1011) galaxies
Each galaxy contains ~100 billion stars*
~
10-30 g/cm3
Q  0,
~ 1 proton/m3
L0
4) 2.70 K Microwave Background
Radiation (Atomic Physics)
N ~ 1090 : n  400 / cm3
o
T  2.735 K*
T 
Uniform, isotropic to 10-5 
T
Universe was once 3000o K hot.
5) 4He, d, …… Abundance (Nuclear Physics)
Nucleosynthesis
M(4He)
~ 0.24
M(p,n)
Universe was once 109 OK hot.
6) Baryon number (Particle phyics)
Baryogenesis
NB ~
1080
-10
n
B ~ 10
: =
n
No B = 0
nB
-19
n  ~ 10
Universe was once 1028 oK hot.
7) Relic Neutrinos (particle physics)
N ~ 330 / cm3 : Hard to detect
No confirmation, yet
8) Evidence ↔ Big Bang !!
Inflation  Flat Universe
o = 1
3x3x3
Timeline of the Universe