leung center for cosmology and particle astrophysics ...€¦ · 1 the dark energy puzzle pisin...

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The Dark Energy PuzzleThe Dark Energy PuzzlePisin Chen

Leung Center for Cosmology and Particle Astrophysics&

Department of Physics and Graduate Institute of AstrophysicsNational Taiwan University

IPMU seminar, Nov. 19, 2008First LeCosPA

Mini-Workshop on Drak

Energy and Vacuum Structure

December 2-4, 2008

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•

Vacuum energy/cosmological constant•

Destiny of the Universe

•

Related to Dark Matter, Inflation, Neutrino Mass?•

Connections to SUSY/Superstrings/Extra dimensions?

•

Signal of new gravitational physics?•

Hole in the Universe?

•

Crack in the Cosmic Egg?•

Narcissistic Principle?

Dark Energy is a profound mystery because it touches so many other

important puzzles

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What We KnowWhat We Know 10 Years On10 Years On

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Discovery! Discovery! –– 19981998

Hi z Supernova TeamSupernova Cosmology Project

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The Discovery DataThe Discovery Data

Perlmutter et al, 1999 Riess et al, 1998

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Carl Sagan:Carl Sagan:

Extraordinary Extraordinary Claims Claims RequireRequire

Extraordinary Extraordinary EvidenceEvidence

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Did Did Science Science jump the gun?jump the gun?

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…… in any case, the in any case, the extraordinary evidence is extraordinary evidence is

now in placenow in place

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CMB Provides An Independent Line of

Evidence

WMAP

BOOMERanG DASI

Michael S Turner 11

Curve = concordance cosmology Ω0 = 1.005 ±

0.006ΩM = 0.28 ±

0.015

only consistent ifΩΛ-like = 0.72 ±

0.015

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1000 1000 SNeSNe from: from: the original teams + the original teams +

SNLS, ESSENCE, SDSS, SNLS, ESSENCE, SDSS, CSP, CSP, ……

More data, stronger signalMore data, stronger signal

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SDSSSDSS--II Supernova SurveyII Supernova Survey

~500 Well studied SNe

Ia, suitable for framing

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Evidence for past acceleration: Evidence for past acceleration: Important reality checkImportant reality check

HST ACS Sample of high-z SNe: A. Riess et al, Ap.J

607, 665 (2004)

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Consistent Age for UniverseConsistent Age for Universe

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Consistent with all observations:

ΩΛ

= 0.76 ±

0.02

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Dark Energy:ΩDE = 0.76 ±

0.02

w = -0.94 ±

0.1 (±

0.1 sys)

Where We Are Today

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Allow w to vary:

w = w0

+ wa

(1-a)

ΩDE = 0.76 ±

0.02w0

= -1 ±

0.2wa

~ 0 ±

1Possible variation is not well constrained

No Reason to Believe w is Constant

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What we understandWhat we understand

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•

Smooth, very elastic, non-particulate (medium) dark energy w/in GR, important at late times

•

Simple physics model: quantum vacuum energy (current null hypothesis)

•

Dynamical models: rolling scalar field•

Physical parameter that describes and discriminates

between models: w = p/ρ

< -1/3

•

Look out for new gravitational physics•

Framework for treating GR alternatives (Hu et al)

•

Other effects: Long range forces, …

Theory has provided strong guidance & interpretation

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Some Lessons•

Two problems?: smallness of cosmological constant and cause of cosmic acceleration

•

One model ruled out (LTB) and one in trouble (flat DGP)

•

“GR Exceptionalism”: hard to modify enough to accommodate cosmic acceleration and satisfy other constraints (importance of dynamical tests)

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Two Big Dark QuestionsTwo Big Dark Questions (worthy of Swedish Gold)

Is Dark Energy consistent with a Quantum Vacuum Energy?

Does General Relativity Self Consistently Describe Cosmic

Acceleration?

Yes, at the 10 to 20% level

Not well tested

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Smallness of Dark Energy• Combination of recent data from WMAP3 + SDSSdetermines

w = −

0.94 ±

0.09

for dark energy (DE) equation of state

p = wρ. • SNLS gives

w = −1.023 ±

0.090 (stat) ±

0.54(sys).

Why much smaller than standard model scale?

DE likely a bone fide

CC:

w = −

1.

• Observations

~ 10-15

!ρDE

1/4

MSM

If DE never changes in space and time, then it must be associated with fundamental properties of spacetime.

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Another Hierarchy in PhysicsAnother Hierarchy in Physics

• Gravity

is much weaker, or Planck scale (1019 GeV),much larger, than that of SM gauge interactions:

MPlMSM

Two well-known solutions:ADD : large (but flat) extra dimensions RS : warped geometry in x-d

~ 1016

.

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BRANE WORLD OF STRING BRANE WORLD OF STRING THEORYTHEORY

WE LIVE ON A 3WE LIVE ON A 3DD BRANE IN AN 11BRANE IN AN 11DD SPACESPACEBRANES COLLIDE CREATING BIG BANGSBRANES COLLIDE CREATING BIG BANGS

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A Numerical CoincidenceA Numerical Coincidence•

A remarkable numerical coincidence,

•

Perhaps not accidental but implies a deeper connection:

•

Caution: Unlike the 1st

hierarchy that links 4 fundamental

interaction strengths, DE must be a secondary, derived quantity.

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Analogy in Atomic PhysicsAnalogy in Atomic Physics

•

Bohr atom•

Fundamental energy scale in Schrödinger equation: me

•

Ground state energy suppressed by 2 powers of fine structure constant

•

Dark energy•

Fundamental energy scale in quantum gravity: MPl

•

Dark energy suppressed by 2 powers of “gravity fine structure constant”

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Randall-Sundrum Warped Geometry

Y=0Y=π

Visible brane Hidden brane

dxμdxνGravity lives in the bulk while gauge interactions live on the brane.

30(a ∞) a

CasimirCasimir EnergyEnergy

ρcasimir

=

ρvac

(

| |)

− ρvac

(a

→∞) ∞ a-4

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Vacuum Energy Problem Solved Vacuum Energy Problem Solved by by SupersymmetrySupersymmetry or ?or ?

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Strategy for the Smallness of Dark Energy

Supersymmetry Brane World

BUT SUSY only on the brane

= 0ρvac :boson fermion

+

ρvac(4)

~ (mn

- mn-1

)2

δmn

2

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Casimir Energy under SUSY-Breaking

Putting all these together, we find

PC, Nucl. Phys. Proc. Suppl.173, 137 (2007).PC and J-A. Gu, Mod. Phys. Lett. A22, 1995 (2007); arXiv:0712.2441

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Known Probes of Dark EnergyKnown Probes of Dark Energy•

Supernovae: Geometric

•

BAO: Geometric + simple physics•

Weak Lensing: Geometric + dynamic

•

Clusters: Dynamics + geometric•

Evolution of large-scale structure (dynamic)–

Must reproduce LCDM

–

Growth factor/red-shift space distortions (new kid on the block: Guzzo

and Selljack)

•

CMB and other precision data that pin down cosmological parameters (provide priors)

Michael S Turner 35

ΛCDM Scenario Well Formulated and Tested: Test Bed for Dark Energy

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•

BAO: SDSS/2dF, WiggleZ, FMOS, BOSS HETDEX, WFMOS, PAU

•

CL: SZA, SPT, DES, ACT, 400d, eROSITA•

SNe: JDEM, DES

•

WL: PS, JDEM, EUCLID, DES, PanSTARRS•

CMB et al: WMAP/ACT/SPT/Planck –

cosmological

degeneracies

make many other observations valuable

On the way to few % in w0

, 10% in wa

, significant tests of underlying gravity theory …

and deeper understanding of

dark energy

Impressive Array of Dark Energy Impressive Array of Dark Energy Projects on the HorizonProjects on the Horizon

Michael S Turner 37

Four Four ““Stage IVStage IV”” FuturesFutures

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1. w = -1 & theory breakthrough

Percent level measurements of w and wa

and LSS consistent with ΛCDM

+Theoretical understanding of small vacuum energy

Problem Solved for Cosmologists and Particle Physicists

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2. “w = -1” & theory breakthrough

Percent level measurements of w and wa and LSS consistent with ΛCDM

+New compelling theoretical prediction for time variation of w and/or wa

–

just beyond the reach of Stage IV

=Problem Solved for Cosmologists,

Particle Physicists think about Stage V

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3. “w ≠ -1” or wa ≠ 0

Detection of signature that DE is not vacuum energy

Potential implications for both particle physics and cosmology

With or without theoretical breakthroughboth Cosmologists, Particle Physicists think about what to measure in Stage V

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4. w = -1 & no theory breakthrough

Percent level measurements of w and wa

and LSS consistent with ΛCDM

+No theoretical understanding of small vacuum

energy

Problem solved for cosmologists, but not for particle physicists

“Time out”: take a break and think hard about what to do next

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SummarySummary

The quest to understand The quest to understand DE is a grand scientific DE is a grand scientific

adventure adventure –– and we are all and we are all lucky to be involved!lucky to be involved!

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