dark energy: taking sides

Accelerators in the Universe March 2008Accelerators in the Universe March 2008Rocky Kolb University of ChicagoRocky Kolb University of ChicagoAccelerators in the Universe March 2008Accelerators in the Universe March 2008Rocky Kolb University of ChicagoRocky Kolb University of Chicago

Cold Dark Matter: (CDM) 25%

Dark Energy (): 70%

Stars:0.8%

H & He:gas 4%

Chemical Elements: (other than H & He)0.025%

Neutrinos: 0.17%

Radiation: 0.005%

CDMCDMCDMCDM

inflationary perturbations baryo/lepto genesis

Evidence for Dark EnergyEvidence for Dark EnergyEvidence for Dark EnergyEvidence for Dark Energy

Measuring the expansion history of the Universe

Friedmann equation (G GT) : Expansion rate H(z)

Equation of state parameter: w p wfor

if w w(a): 1

3 11 exp 3 1

w

a

daz w a

a

2 33 42 20 TOTAL

111 1 1 1

w

R wMH z H zz z z

Hubble constant curvature matter radiation dark energy

CMB LSS CMB H(z)

Evolution of Evolution of H(z)H(z) Is a Key Quantity Is a Key QuantityEvolution of Evolution of H(z)H(z) Is a Key Quantity Is a Key Quantity

• Age of the universe 0 1

z dzt z

z H z

Evolution of Evolution of H(z)H(z) Is a Key Quantity Is a Key QuantityEvolution of Evolution of H(z)H(z) Is a Key Quantity Is a Key Quantity

Many observables based on H(z) through coordinate distance r(z) 0

sin

( ) 1

sinh

z dzr z

H z

1Ld z r z z • Luminosity distance

Flux = (Luminosity / dL)

1A

r zd z

z

• Angular diameter distance

Physical size / dA

• Volume (number counts) N / V (z)

Robertson–Walker metric 2

2 2 2 2 221

drds dt a t r d

kr

2

21

r zdV drd

kr z

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supernova redshift z


• High-redshift supernova are about a half-magnitude fainter than expected in the Einstein-de Sitter model.

• Statements such as there is dark energy, or that the Universe is accelerating are interpretations of the observations.

* Similar data from Essence collaboration.

M

1. Find standard candle (SNe Ia)2. Observe magnitude & redshift 3. Assume a cosmological model

4. Compare observations & model

5. Fit needs cosmoillogical constant

or vacuum energy density

Astier et al. (2006)SNLS

Einstein–de Sitter model

Evidence ForEvidence ForDark EnergyDark EnergyEvidence ForEvidence ForDark EnergyDark Energy

• Assumes w (i.e., )

• Assumes priors on H,

spatial flatness, etc.

2.0

1.5

1.0

0.5

00 0.5 1.0

(The Unbearable Lightness of Nothing)

V 10–30 g cm!!!!!

V GV

length scale cm cm

mass scale eV eV

Cosmoillogical ConstantCosmoillogical ConstantCosmoillogical ConstantCosmoillogical Constant

All fields: harmonic oscillators with zero-point energy


3 2 2 3C

all particles all particles

d k k m dk k

4

419 4 28 112 4

43 4 12 48 4

13

: relax, it's field theory

10 GeV : 10 eV 10 eV

10 GeV : 10 eV 10 eV

10 GeV:

C

C Pl Pl

C SUSY SUSY

C

M M

M M

44 16 4Observed 10 eV 10 eV

The quantum vacuum has a Higgs potential

Higgs potential gives mass to quanta like quarks and electrons

e,W, Z, quarks … photon

Higgs potential: eV


The Higgs Bison at FermilabThe Higgs Bison at FermilabThe Higgs Bison at FermilabThe Higgs Bison at Fermilab

100 4 48 4

45 4 32 4

GUT: 10 eV SUSY: 10 eV

EWK: 10 eV CHIRAL: 10 eV

high-temperature

low-temperature

V


-16 4OBSERVED: 10 eV

V


Extra DimensionsExtra DimensionsExtra DimensionsExtra Dimensions

Illogical timing (cosmic coincidence):

M R


We are here

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3) Baryon acoustic oscillations4) Weak lensing

1) Hubble diagram (SNe)2) Cosmic Subtraction

The case for :5) Galaxy clusters6) Age of the universe7) Structure formation

* Similar data from Essence collaboration.

cmb

dynamics x-ray gaslensing

simulations

power spectrum

TOTAL M B

CMB many methods CMB/BBN


How We “Know” There’s Dark EnergyHow We “Know” There’s Dark EnergyHow We “Know” There’s Dark EnergyHow We “Know” There’s Dark Energy• Assume model cosmology:

– General relativity– Homogeneity & isotropy (FLRW)

– Energy (and pressure) content: M R +

– Input or integrate over cosmological parameters: H, , etc.

• Calculate observables dL(z), dA(z), H(z),

• Compare to observations

• Model cosmology fits with , but not without

• All evidence for dark energy is indirect : observed H(z) is not described by H(z) calculated from the Einstein-de Sitter model [spatially flat (from CMB) ; matter dominated (M)]

G = 8 G T00

H2 G/ k/a2

Taking Sides!Taking Sides!Taking Sides!Taking Sides!• Can’t hide from the data – CDM too good to ignore

– SNe– Subtraction: – Baryon acoustic oscillations– Galaxy clusters– Weak lensing– …

H(z) not given by

Einstein–de Sitter

G(FLRW) G T(matter)

• Modify left-hand side of Einstein equations (G)

1. Beyond Einstein (non-GR: f (R), branes, etc.)

2. (Just) Einstein (back reaction of inhomogeneities)

• Modify right-hand side of Einstein equations (T)

1. Constant (“just” a cosmoillogical constant )

2. Not constant (dynamics driven by scalar field: M eV)

Anthropic/LandscapeAnthropic/LandscapeAnthropic/LandscapeAnthropic/Landscape

• Many sources of vacuum energy

• String theory has many (?) vacua

• Some of them correspond to cancellations that yield a small

• Although exponentially uncommon, they are preferred because …

• More common values of results in an inhospitable universe

The Cosmological Constant and String Theory are made for each other…

Cosmological constant (): A result in search of a theory

String theory (or M-theory): A theory in search of a result

QuintessenceQuintessenceQuintessenceQuintessence

• Many possible contributions. • Why then is total so small?• Perhaps unknown dynamics sets global vacuum energy equal to zero……but we’re not there yet!

V()

0

Requires m eV

QuintessenceQuintessenceQuintessenceQuintessence

• Not every scalar potential works– scaling, tracking, stalking potentials

• Couplings to matter (dark or otherwise)– mavens (mass varying neutrinos)– chameleon (dark energy is sensitive to environment)– changing fundamental constants (, etc.)

• k-essence– phantom, ghost condensate, Born-Infeld, …, Chaplygin gas

• Episodic dark energy (acceleration happens)

Punctuated Vacuum DominationPunctuated Vacuum DominationPunctuated Vacuum DominationPunctuated Vacuum Domination

Dodelson, Kaplinghat, Stewart

dark energy cosmic destiny

( ) 1 sinV e

Modifying the Left-Hand SideModifying the Left-Hand SideModifying the Left-Hand SideModifying the Left-Hand Side• Braneworld modifies Friedmann equation

• Gravitational force law modified at large distance

• Tired gravitons

• Gravity repulsive at distance R Gpc

• n = 1 KK graviton mode very light, m (Gpc)

• Einstein & Hilbert got it wrong f (R)

• Backreaction of inhomogeneities

Five-dimensional at cosmic distances

Deffayet, Dvali& Gabadadze

Gravitons metastable - leak into bulkGregory, Rubakov & Sibiryakov;

Dvali, Gabadadze & Porrati

Kogan, Mouslopoulos,Papazoglou, Ross & Santiago

Csaki, Erlich, Hollowood & Terning

Räsänen; Kolb, Matarrese, Notari & Riotto;Notari; Kolb, Matarrese & Riotto

Binetruy, Deffayet, Langlois

1 4 416S G d x g R R Carroll, Duvvuri, Turner, Trodden

Acceleration from InhomogeneitiesAcceleration from InhomogeneitiesAcceleration from InhomogeneitiesAcceleration from InhomogeneitiesHomogeneous model Inhomogeneous model

3

h

h h

h h h

a V

H a a

3

i

i i

i i i

x

a V

H a a

h i x

We think not!

?h iH H

(Buchert & Ellis)

Acceleration from InhomogeneitiesAcceleration from InhomogeneitiesAcceleration from InhomogeneitiesAcceleration from Inhomogeneities• No dark energy

• No acceleration in the normal sense (no individual fluid element accelerates)

Dark EnergyDark EnergyDark EnergyDark Energy• Many theoretical approaches (none compelling i.m.o.!)

• Perhaps nothing more can be done by theorists …

"Nothing more can be done by the theorists. In this matter it is only you, the astronomers, who can perform a simply invaluable service to theoretical physics."

Einstein in August 1913 to Berlin astronomer Erwin Freundlich encouraging him to mount an expedition to measure the deflection of light by the sun.

Dark EnergyDark EnergyDark EnergyDark Energy

Dark EnergyDark EnergyDark EnergyDark Energy• Many theoretical approaches (none compelling i.m.o.!)

• Perhaps nothing more can be done by theorists …

• Observables: dL(z) , dA(z) , distances, …

• … which (in FRW at least) all depend on H(z)

• Might as well assume FRW and measure(?) H(z)

• (Measure what you can as well as you can)

• Dark energy enters through (z), parameterized by w(z)

• Can’t measure a function; parameterize w(z)

0

0

1

1

a

a

w a w w a

zw z w w

z

Dark EnergyDark EnergyDark EnergyDark Energy

: w ; wa

Quintessence: w / ; wa model dependent

Modified gravity: w can be less than ; wa unconstrained

Back reactions: ?????

0

0

1

1

a

a

w a w w a

zw z w w

z

DETF* Experimental Strategy: DETF* Experimental Strategy: DETF* Experimental Strategy: DETF* Experimental Strategy:

• Determine as well as possible whether the accelerating expansion is consistent with being due to a cosmological constant. (Is w ?)

• If the acceleration is not due to a cosmological constant, probe the underlying dynamics by measuring as well as possible the time evolution of the dark energy. (Determine w(a).)

• Search for a possible failure of general relativity through

comparison of the effect of dark energy on cosmic expansion with the effect of dark energy on the growth of cosmological structures like galaxies or galaxy clusters. (Hard to quantify.)

* Dark Energy Task Force

H(z)

dL(z) dA(z) V(z)

baryonosc.

stronglensing

weaklensing

SNIa clusters clusters

Alcock-Paczynski

Growth of structure

clustersweak

lensing P(k,z)

Observational ProgramObservational ProgramObservational ProgramObservational Program

Test gravity

solarsystem

millimeterscale

accelerators P(k,z)

2 4 0k k kH G

source?

stronglensing

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3) Baryon acoustic oscillations4) Weak lensing

1) Hubble diagram (SNe)2) Cosmic Subtraction

The case for :5) Galaxy clusters6) Age of the universe7) Structure formation

Supernova Type IaSupernova Type IaSupernova Type IaSupernova Type Ia

• Measure redshift and intensity as function of time (light curve)

• Systematics (dust, evolution, intrinsic luminosity dispersion, etc.)

• A lot of information per supernova

• Well developed and practiced

• Present procedure:

– Discover SNe by wide-area survey on 4m-class telescopes

– Follow up spectroscopy (lots of time on 8m-class telescopes)

• The future:

– Better systematics (space-based?)

– Huge statistics (say with LSST) with photometric redshifts

• Each overdense region is an overpressure that launches a spherical sound wave

• Wave travels outward at c / 3

• Photons decouple, travel to us and observable as CMB acoustic peaks

• Sound speed plummets, wave stalls

• Total distance traveled 150 Mpc imprinted on power spectrum

WMAP

SDSS

Baryon Acoustic OscillationsBaryon Acoustic OscillationsBaryon Acoustic OscillationsBaryon Acoustic Oscillations

Mpc

dA

Mpc Hz

Baryon Acoustic OscillationsBaryon Acoustic OscillationsBaryon Acoustic OscillationsBaryon Acoustic Oscillations• Acoustic oscillation scale depends on Mh and Bh

(set by CMB acoustic oscillations)

• It is a small effect (B h ¿ M h)

• Dark energy enters through dA and H

• Virtues

– Pure geometry.

– Systematic effects should be small.

• Problems:

– Amplitude small, require large scales, huge volumes

– Photometric redshifts?

– Nonlinear effects at small z, cleaner at large z Dark energy not expected to be important at large z

Baryon Acoustic OscillationsBaryon Acoustic OscillationsBaryon Acoustic OscillationsBaryon Acoustic Oscillations

Weak LensingWeak LensingWeak LensingWeak Lensing

dark energyaffects growth

rate of M

4 LS

OS

DGM

b D

dark energyaffects geometricdistance factors

observedeflection

angle

DOS

DLS

b

Weak LensingWeak LensingWeak LensingWeak Lensing

• Dominant source is PSF of atmosphere and telescope

• Errors in photometric redshifts

Space vs. Ground:• Space: no atmosphere PSF

• Space: Near IR for photo-z’s

• Ground: larger aperture

• Ground: less expensive

The signal from any single galaxy is very small,but there are a lot of galaxies! Require photo-z’s?

• Current projects– 100’s of sq. degs. deep multicolor data– 1000’s of sq. degs. shallow 2-color data

• DES (2010)– 1000’s of sq. degs. deep multicolor data

• LSST (2013)– full hemisphere, very deep 6 colors

Systematic errors: The Landscape:

Galaxy ClustersGalaxy ClustersGalaxy ClustersGalaxy Clusters

Cluster redshift surveys measure• cluster mass, redshift, and spatial clustering

Sensitivity to dark energy• volume-redshift relation• angular-diameter distance–redshift relation• growth rate of structure• amplitude of clustering

Problems:• cluster selection must be well understood• proxy for mass?• need photo-z’s

CMB WMAP 2/3 WMAP 5 yr

Planck Planck 4yr

Clusters AMI

SZA

APEX

AMIBA

SPT

ACT

DES

SNe

Pan-STARRS

DES LSST

JDEMSDSS CFHTLS

CSP

ATLAS

SKAFMOS WFMOS

SDSS

Lensing CFHTLS

ATLAS KIDS

DES, VISTA

JDEM

LSST SKA

Pan-STARRS

SUBARU

20202008

ESSENCE

XCS

DUNE

2010

BAO LSST

SDSSDLS

LAMOST

Hyper suprime

DES, VISTA,VIRUS

Pan-STARRSHyper suprime JDEM

What’s AheadWhat’s AheadWhat’s AheadWhat’s Ahead2015

Roger Davies

Conclusions: Two Sides to the StoryConclusions: Two Sides to the StoryConclusions: Two Sides to the StoryConclusions: Two Sides to the Story

1. Right-Hand Side: Dark energy• “constant” vacuum energy, i.e., • time varying vacuum energy, i.e., quintessence

2. Left-Hand Side • Modification of GR • Standard cosmological model (FLRW) not applicable

The expansion history of the universe is not described by the Einstein-de Sitter model.

Explanations:

1. Well established: Supernova Ia2. Circumstantial: subtraction, age, structure formation, …3. Emergent techniques: baryon acoustic oscillations, clusters, weak lensing

Phenomenology:1. Measure evolution of expansion rate: is w ?2. Order of magnitude improvement feasible

Accelerators in the Universe March 2008Accelerators in the Universe March 2008Rocky Kolb University of ChicagoRocky Kolb University of ChicagoAccelerators in the Universe March 2008Accelerators in the Universe March 2008Rocky Kolb University of ChicagoRocky Kolb University of Chicago

dark energy: taking sides

Documents

theres dark energy

energy cosmoillogical

dark energymeasuring

dark energyhow

vacuum energy densityastier

universeevolution of

cosmological model

magnitude redshift