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)

66

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

77

Standardized CandleStandardized Candle

Redshift tells us the expansion factor a

Time after explosion

Bri

ghtn

ess

Brightness tells us distance away (lookback time t)

88

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)

1111

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

1313

Looking Back 10 Billion YearsLooking Back 10 Billion Years

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

STScI

1414

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

1515

Dark Energy – The Next GenerationDark Energy – The Next Generation

SNAP: Supernova/Acceleration Probe

Dedicated dark energy probe

1616

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.

1717

Controlling SystematicsControlling Systematics

Same SN, Different z Cosmology Same z, Different SN Systematics Control

1818

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.

2222

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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