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What is the Dark Energy?

David Spergel

Princeton University

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One of the most challenging problems in

Physics Several cosmological observations

demonstrated that the expansion of the universe is accelerating

What is causing this acceleration?

How can we learn more about this acceleration, the Dark Energy it implies, and the questions it raises?

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Outline A brief summary on the contents of the universe

Evidence for the acceleration and the implied Dark Energy Supernovae type Ia observations (SNe Ia) Cosmic Microwave Background Radiation (CMB) Large-scale structure (LSS) (clusters of galaxies)

What is the Dark Energy?

Future Measurements

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Contents of the universe (from current observations)

Baryons (4%)

Dark matter (23%)

Dark energy: 73%

Massive neutrinos: 0.1%

Spatial curvature: very close to 0

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A note on cosmological parameters

The properties of the standard cosmological model are expressed in terms of various cosmological parameters, for example: H0 is the Hubble expansion parameter today

is the fraction of the matter energy density in the critical density(G=c=1 units)

is the fraction of the Dark Energy density (here a cosmological constant) in the critical density

cMM ρρ /≡Ω

πρ

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3 2Hc ≡

cρρ /ΛΛ ≡Ω

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Evidence for cosmic acceleration: Supernovae type Ia

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Evidence for cosmic acceleration: Supernovae

type Ia

Standard candles Their intrinsic luminosity is know Their apparent luminosity can be measured The ratio of the two can provide the

luminosity-distance (dL) of the supernova The red shift z can be measured

independently from spectroscopy Finally, one can obtain dL (z) or equivalently

the magnitude(z) and draw a Hubble diagram

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Evidence for cosmic acceleration: Supernovae type Ia

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Evidence from Cosmic Microwave Background

Radiation (CMB)

CMB is an almost isotropic relic radiation of T=2.725±0.002 K

CMB is a strong pillar of the Big Bang cosmology

It is a powerful tool to use in order to constrain several cosmological parameters

The CMB power spectrum is sensitive to several cosmological parameters

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This is how the Wilkinson Microwave Anisotropy Probe

(WMAP) sees the CMB

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ADIABATIC DENSITY FLUCTUATIONS

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ISOCURVATURE ENTROPY FLUCTUATIONS

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Determining Basic Parameters

Baryon Density

Ωbh2 = 0.015,0.017..0.031

also measured through D/H

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Determining Basic Parameters

Matter Density

Ωmh2 = 0.16,..,0.33

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Determining Basic Parameters

Angular Diameter Distance

w = -1.8,..,-0.2

When combined with measurement of matter density constrains data to a line in Ωm-w space

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Simple Model Fits CMB data

Readhead et al. astro/ph 0402359

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Evolution from Initial Conditions IWMAP team assembled

DA leave Princeton

WMAP completes 2 year of observations!

WMAP at Cape

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Evidence from large-scale structure in the universe

(clusters of galaxies)

Counting clusters of galaxies can infer the matter energy density in the universe

The matter energy density found is usually around ~0.3 the critical density

CMB best fit model has a total energy density of ~1, so another ~0.7 is required but with a different EOS

The same ~0.7 with a the same different EOS is required from combining supernovae data and CMB constraints

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Cosmiccomplementarit

y:Supernovae,

CMB,and Clusters

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What is Dark Energy ?What is Dark Energy ?

“ ‘Most embarrassing observation in physics’ – that’s the only quick thing I can say about dark energy that’s also true.”

Edward Witten

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What is the Dark Energy?

Cosmological Constant Failure of General Relativity Quintessence Novel Property of Matter

Simon Dedeo astro-ph/0411283

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Why is the total value measured from cosmology so small compared to quantum field theory calculations of vacuum energy? From cosmology: 0.7 critical density ~ 10-48 GeV4

From QFT estimation at the Electro-Weak (EW) scales: (100 GeV)4

At EW scales ~56 orders difference, at Planck scales ~120 orders

Is it a fantastic cancellation of a puzzling smallness?

Why did it become dominant during the “present” epoch of cosmic evolution? Any earlier, would have prevented structures to form in the universe (cosmic coincidence)

COSMOLOGICAL CONSTANT??

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Anthropic Solution?

Not useful to discuss creation science in any of its forms….

Dorothy… we are not in Kansas anymore …

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Quintessence Introduced mostly to address

the “why now?” problem Potential determines dark

energy properties (w, sound speed)

Scaling models (Wetterich; Peebles & Ratra)

V() = exp

Most of the tracker models predicted w > -0.7

ρ

matter

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Zlatev and Steinhardt (1999)

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Current Constraints

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Seljak et al. 2004

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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Looking for Quintessence Deviations from w = -1

BUT HOW BIG? Clustering of dark energy Variations in coupling constants (e.g., )

FF/MPL

Current limits constrain < 10-6

If dark energy properties are time dependent, so are other basic physical parameters

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Big Bang Cosmology

Homogeneous, isotropic universe

(flat universe)

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Rulers and Standard Candles

Luminosity Distance

Angular Diameter Distance

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Flat M.D. Universe

D = 1500 Mpc for z > 0.5

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Volume

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Techniques

Measure H(z) Luminosity Distance (Supernova) Angular diameter distance

Growth rate of structure

.

Checks Einstein equations to first order in perturbation theory

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What if GR is wrong? Friedman equation (measured through

distance) and Growth rate equation are probing different parts of the theory

For any distance measurement, there exists a w(z) that will fit it. However, the theory can not fit growth rate of structure

Upcoming measurements can distinguish Dvali et al. DGP from GR (Ishak, Spergel, Upadye 2005)

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Growth Rate of Structure Galaxy Surveys

Need to measure bias Non-linear dynamics Gravitational Lensing Halo Models Bias is a function of galaxy

properties, scale, etc….

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A powerful cosmological probe of Dark Energy:

Gravitational Lensing

Abell 2218: A Galaxy Cluster Lens, Andrew Fruchter et al.

(HST)

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The binding of light

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Gravitational Lensing by clusters of galaxies

From MPA lensing group

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Weak Gravitational Lensing

Distortion of background images by foreground matter

Unlensed LensedCredit: SNAP WL group

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Gravitational Lensing

Advantage: directly measures mass

Disadvantages Technically more difficult Only measures projected mass-

distribution

Tereno et al. 2004

Refregier et al. 2002

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Baryon Oscillations

C()

C()

CMB

Galaxy Survey

Baryon oscillation scale

1o

photo-z slices

Selection

function

Limber Equation

(weaker effect)

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Baryon Oscillations as a Standard Ruler

In a redshift survey, we can measure correlations along and across the line of sight.

Yields H(z) and DA(z)!

[Alcock-Paczynski Effect]

Observer

r = (c/H)zr = DA

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Large Galaxy Redshift Surveys

By performing large spectroscopic surveys, we can measure the acoustic oscillation standard ruler at a range of redshifts.

Higher harmonics are at k~0.2h Mpc-1 (=30 Mpc). Measuring 1% bandpowers in the peaks and troughs requires

about 1 Gpc3 of survey volume with number density ~10-3 galaxy Mpc-3. ~1 million galaxies!

SDSS Luminous Red Galaxy Survey has done this at z=0.3! A number of studies of using this effect

Blake & Glazebrook (2003), Hu & Haiman (2003), Linder (2003), Amendola et al. (2004)

Seo & Eisenstein (2003), ApJ 598, 720 [source of next few figures]

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Conclusions Cosmology provides lots of evidence for

physics beyond the standard model. Upcoming observations can test ideas

about the nature of the dark energy.