11
TheThe Darkness Darkness of the Universe:of the Universe:
Mapping Expansion and GrowthMapping Expansion and Growth
Eric Linder Lawrence Berkeley National Laboratory
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Discovery! AccelerationDiscovery! Acceleration
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Exploring Dark EnergyExploring Dark Energy
First Principles of Cosmology E.V. Linder (Addison-Wesley 1997)
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Fundamental PhysicsFundamental Physics
Astrophysics Cosmology Field Theory
a(t) Equation of state w(z) V()
V ( ( a(t) ) )SN
CMB
LSS
Map the expansion history of the universe
The subtle slowing and growth of scales with time – a(t) – map out the cosmic history like tree rings map out the Earth’s climate history.
STScI
55
Standard CandlesStandard Candles
Brightness tells us distance away (lookback time)
Redshift measured tells us expansion factor (average distance between galaxies)
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Type Ia SupernovaeType Ia Supernovae
• Exploding star, briefly as bright as an entire galaxy• Characterized by no Hydrogen, but with Silicon• Gains mass from companion until undergoes thermonuclear runaway
Standard explosion from nuclear physics
Insensitive to initial conditions: “Stellar amnesia”Höflich, Gerardy, Linder, & Marion 2003
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Standardized CandleStandardized Candle
Redshift tells us the expansion factor a
Time after explosion
Bri
ghtn
ess
Brightness tells us distance away (lookback time t)
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Standardized CandleStandardized Candle
Wang et al. 2003, ApJ 590, 944
Color vs. Magnitude -- “HR” diagram
CMAGICNew method:
• Physics based
• Less dispersion (4% in distance?)
• Less sensitive to systematics from dust extinction
99
Images
Spectra
Redshift & SN Properties
data analysis physics
Nature ofDark Energy
Each supernova is “sending” us a rich stream of information about itself.
What makes SN measurement special?What makes SN measurement special? Control of systematic uncertaintiesControl of systematic uncertainties
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
1010
~2000 SNe Ia
10 billion years
Hubble DiagramHubble Diagram
redshift z
0.2 0.4 0.6 0.8 1.0
b
rig
htn
ess
(expansion)
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Nearby Supernova Factory
Understanding SupernovaeUnderstanding Supernovae
Cleanly understood astrophysics leads to cosmology
Supernova Properties Astrophysics
G. Aldering (LBL)
1212
Looking Back 10 Billion YearsLooking Back 10 Billion Years
STScI
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Looking Back 10 Billion YearsLooking Back 10 Billion Years
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
STScI
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Looking Back 10 Billion YearsLooking Back 10 Billion Years
To see the most distant supernovae, we must observe from space.
A Hubble Deep Field has scanned 1/25 millionth of the sky.
This is like meeting 10 people and trying to understand the complexity of the entire population of the US!
STScI
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Dark Energy – The Next GenerationDark Energy – The Next Generation
SNAP: Supernova/Acceleration Probe
Dedicated dark energy probe
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Design a Space MissionDesign a Space Mission
colorful
wide
GOODS
HDF
9000 the Hubble Deep Field
plus 1/2 Million HDF
deepdeep• Redshifts z=0-1.7 • Exploring the last 10 billion years • 70% of the age of the universe
Both optical and infrared wavelengths to see thru dust.
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Controlling SystematicsControlling Systematics
Same SN, Different z Cosmology Same z, Different SN Systematics Control
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Our ToolsOur Tools
Expansion rate of the universe a(t)
ds2 = dt2+a2(t)[dr2/(1-kr2)+r2d2]
Einstein equation (å/a)2 = H2 = (8/3) m + H2(z) = (8/3) m + C exp{dlna [1+w(z)]}
Growth rate of density fluctuations g(z) = (m/m)/a
€
′ ′ g + [5 + 12
d ln H 2
d ln a ] ′ g a−1 + [3 + 12
d ln H 2
d ln a − 32 G Ωm (a)] ga−2 = S(a)
Poisson equation 2(a)=4Ga2 m= 4Gm(0) g(a)
1919
Cosmic Background RadiationCosmic Background Radiation
Hot and cold spots simultaneously the smallest and largest objects in the universe: single quantum fluctuations in early universe, spanning the universe at the time of decoupling.
Snapshot of universe at 380,000 years old, when 1/1100 size now
Planck satellite (2007)
WMAP/ NASA
2020
ComplementarityComplementarity
SN+CMB have excellent complementarity, equal to a prior (M)0.01. Frieman, Huterer, Linder, & Turner 2003
SN+CMB can detect time variation w´ at 99% cl (e.g. SUGRA).
Supernovae tightly constrain dark energy models… And play well with others.
=w
a/2
Present value of “negativity”
Tim
e v
aria
tio
n
2121
Deceleration and AccelerationDeceleration and Acceleration
CMB power spectrum measures n-1 and inflation.
Nonzero ISW measures breakdown of matter domination: at early times (radiation) and late times (dark energy).
Large scales (low l) not precisely measurable due to cosmic variance. So look for better way to probe decay of gravitational potentials.
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Gravitational LensingGravitational Lensing
Gravity bends light… - we can detect dark matter through its gravity, - objects are magnified and distorted, - we can view “CAT scans” of growth of structure
2323
Gravitational LensingGravitational Lensing
“Galaxy wallpaper” Lensing by (dark) matter along the line of sight
N. Kaiser
2424
Gravitational LensingGravitational Lensing
Lensing measures the mass of clusters of galaxies.
By looking at lensing of sources at different distances (times), we measure the growth of mass.
Clusters grow by swallowing more and more galaxies, more mass.
Acceleration - stretching space - shuts off growth, by keeping galaxies apart.
So by measuring the growth history, lensing can detect the level of acceleration, the amount of dark energy.
2525
Weak Lensing - ShearWeak Lensing - Shear
More area, less source density, shallower sources, e.g Ground
Less area, more source density, deeper sources, e.g. Space
Large scales Small scales
Err
or
in s
hea
r es
tim
atio
n
statistics only!
Unique suitability of space for weak lensing: ◊ Control of systematics -- Small, stable, isotropic PSF; accurate photo-z
◊ Deep survey, area just grows with time, access to nonlinear mass spectrum (high l)
adapted from C. Vale
systematics
2626
Weak Lensing - CosmographyWeak Lensing - Cosmography
Identify foreground structures, cross-correlate with background slices at various redshifts.
• Removes some systematics:
- Uncorrected PSF shapes average to zero when cross-correlated with foreground- Non-linear power spectrum form irrelevant so information from all scales is useful
• But requires very accurate photometric redshifts
Jain and Taylor 2003, Bernstein and Jain 2004, Zheng, Hui, & Stebbins 2004, Hu and Jain 2004
2727
Supernovae + Weak LensingSupernovae + Weak Lensing
• Comprehensive: no external priors required!
• Independent test of flatness to 1-2%
• Complementary: w0 to 5%, w′ to 0.11 (with systematics)
• Flexible: if systematics allow, can cover 10000 deg2
√Bernstein, Huterer, Linder, & Takada
2828
Linear Structure: Baryon OscillationsLinear Structure: Baryon Oscillations
The same primordial imprints in the photon field show up in matter density fluctuations.
Eisenstein 2002
Galaxy cluster size
Hubble horizon today
Ma
tte
r P
ow
er
Sp
ec
tru
m
Since the photons and baryons are tightly coupled until z<1100, there are baryon “acoustic oscillations”, submerged amid dark matter.
2929
Structure Growth: LinearStructure Growth: Linear
Baryon oscillations:
- Standard ruler: we know the sound horizon by measuring the CMB; we measure the “wiggle” scale
geometric distance
- Just like CMB – simple, linear physics
- But, only works while mass perturbations linear, so need to look on very large scales, at z=1-2
- Require large, deep, accurate galaxy redshift surveys (millions of galaxies, thousand(s) of square degrees)
- Possibly KAOS+SNAP or SNAP H survey
- Complementary with SN if dark energy dynamic
3030
3131
3232
Exploring the UnknownExploring the Unknown
Complementary probes give crosschecks, synergy, reduced influence of systematics, robust answers.
Space observatory gives multiwavelength and high redshift measurements, high resolution and lower systematics.
This gives us the ability to test the framework.
Next: The Darkness of the Universe 4: The Heart of Darkness
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