11

TheThe Darkness Darkness of the Universe:of the Universe:

The Heart of DarknessThe Heart of Darkness

Eric Linder Lawrence Berkeley National Laboratory

22

New FrontiersNew Frontiers

Beyond Einstein: What happens when gravity is no longer an attractive force?

Scientific American

Discovery (SCP,HiZ 1998): 70% of the universe acts this way! Fundamentally new physics.

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

33

The world is w(z)The world is w(z)

Don’t care if it’s braneworld, cardassian, vacuum metamorphosis, chaplygin, etc.

Simple, robust parametrization

w(a)=w0+wa(1-a)

Braneworld [DDG] vs. (w0,wa)=(-0.78,0.32)

Vacuum metamorph vs. (w0,wa)=(-1,-3)

Also agree on m(z) to 0.01 mag out to z=2

44

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)

55

Revealing PhysicsRevealing Physics

• Time variation w(z) is a critical clue to fundamental physics. • Modifications of the expansion history = w(z).• But need an underlying theory - ? beyond Einstein gravity?

Growth history and expansion history work together.

w0=-0.78 wa=0.32

cf. Lue, Scoccimarro, Starkman Phys. Rev. D69 (2004) 124015 for braneworld perturbations

66

Testing the FrameworkTesting the Framework

Extensions to gravitation

E.g. scalar-tensor theories: f/2-();;-VTake linear coupling to Ricci scalar R: f/ = F R Allow nonminimal coupling

F=1/(8G)+ 2

R-boost (note R0 in radiation dominated epoch) gives large basin of attraction: solves fine tuning yet w ≈ -1. [Matarrese,Baccigalupi,Perrotta 2004]

But growth of mass fluctuations altered: S0 since G 1/F.

77

Scalar-Tensor GravityScalar-Tensor Gravity

Consider a general linear coupling

R/(8G) F() R

Today, have F = 1/(8G) and Jordan-Brans-Dicke parameter

JBD = F/F2

Can treat as coupled scalar-tensor theory in physical = Jordan frame

Or as separated scalar+tensor theory in spin = Einstein frame

88

Scalar-Tensor GravityScalar-Tensor Gravity

In terms of H2, following Baccigalupi, Matarrese, & Perrotta

(3/8G) H2 = V + (1/2)F2(q-1)(q+5) +3H2 [F-1/(8G)]

Can show that effective Equation of State

1+w ~ 1/JBD

w′ = dw/d ln a ~ 1/JBD

Note JBD > 40000 (or is it?) so “extended quintessence” has attractor solution to GR, and acts like cosmological constant.

However, has non-vanishing anisotropic stress, so may affect structure growth.

99

Lambda, Quintessence, or Not?Lambda, Quintessence, or Not?

Many models asymptote to w=-1, making distinction from difficult. Can models cross w=-1? (Yes, if w<-1 exists.)

All models match CMB power spectrum for CDM

1010

Dark Dark EntropyEntropy

Holographic principle relates entropy to horizon area (Susskind 1994; Fischler & Susskind 1995)

Conjecture: use horizon area, i.e. entropy, as physical basis.

Key quantity is not energy in volume, but entropy on horizon. No physical dark energy, rather dark entropy S~H-2.

Invert to solve for expansion dynamics H2 (derived Friedmann equation).

Anything interesting? Linder hep-th/0410017

1111

Cosmic DynamicsCosmic Dynamics

H

H

w

wtot

Dark entropy always accelerates: w=-1/3...-1

Dynamical behaviors can be nonmonotonic!

Hubble diagram, growth history can lie within 0.02 mag, 15% in growth of CDM.

Acceleration motivated by physical principle (not ad hoc addition to Friedmann equation). First steps look intriguing.

Benchmark -- model clearly distinct from . No limit in phase space where becomes , unlike field theory models.

1212

Dark Energy SurprisesDark Energy Surprises

There is still much theoretical work needed!

Dark energy is…• Dark• Smooth on cluster scales• Accelerating

Maybe not completely! Clumpy in horizon? Maybe not forever!

It’s not quite so simple!

1313

Heart of DarknessHeart of Darkness

Is dark energy dark – only interacts gravitationally?

Self interaction: pseudoscalar quintessence

Coupling to matter: Chaplygin gas Leads to 5th force: limited by lab tests Unify dark energy with dark matter? Distorts matter power spectrum: ruled out unless within 10-5 of

Coupling to gravitation: Scalar-tensor theories = Extended quintessenceCan clump on subhorizon scales Can “turn on” from nonlinear structure formation?!

Higher dimension gravity: Scalaron quintessenceCan be written in terms of scalar-tensor and weff

Sandvik et al. 2003

The

horror!

The

horror!

1414

Dark Energy DreamingDark Energy Dreaming

Direct detection? (Dark energy in solar system = 3 hours of sunlight).

Variations of fundamental constants: lab and accelerator and universe.

CMB: cosmic variance, 0.003% of present age (2 cells)

Direct acceleration? Redshift drift (Sandage 1959; Linder 1991,1997)

dz=10-8 over 100 years

1515

What if Our Eyes Saw Dark (Energy)?What if Our Eyes Saw Dark (Energy)?

The night sky is dark in photons, implying a finite past (Big Bang).

The sky is bright in (dark energy density dominates), implying an infinite future.

Will this turn out to be as significant a discovery as the Big Bang?

1616

Hunting Dark EnergyHunting Dark Energy

w´ is the 1st step for fundamental physics, on or beyond .

w´=dw(z)/dlna|z=1=wa/2 goes a long way toward w(z).

w(z) is a very general “language” for the underlying physics.

In our hunt for the dark energy, the data decides how to go on beyond .

Require precision + complementarity + systematics control.

Expansion history and growth history (e.g. SN+WL) work well together.

1717

Beyond Dark EnergyBeyond Dark Energy

Determine not only dark energy parameters, but test the cosmology framework – alternative gravitation, higher dimensions, etc.

1818

Present Day InflationPresent Day Inflation

Map the expansion history precisely and see the transition from acceleration to

deceleration.

1919

Density History of the UniverseDensity History of the Universe

Map the density history precisely, back to the matter dominated epoch.

2020

FrontiersFrontiers

With every new discovery in physics, there is less of a dividing line between Particle Physics and Astrophysics.

Inflation:

Particle accelerators GUT scale physics

Early universe Origin of matter, gravitational waves

Dark Energy: vacuum energy, quantum fields, extra dimensions

acceleration of the universe fate of the universe

Dark Matter: LHC, direct detection galaxy clusters, gravit’l lensing

2121

FrontiersFrontiers

With every new discovery in physics, there is less of a dividing line between Particle Physics and Astrophysics.

Inflation:

Particle accelerators GUT scale physics

Early universe Origin of matter, gravitational waves

Dark Energy: vacuum energy, quantum fields, extra dimensions

acceleration of the universe fate of the universe

Dark Matter: LHC, direct detection galaxy clusters, gravit’l lensing

“SUSY in the sky”

“Tevatron Planck-otron”

“Quantum Cosmology?”

2222

The Next PhysicsThe Next Physics

The Standard Model gives us commanding knowledge about physics -- 5% of the universe (or 50% of its age).

What is dark energy?Will the universe expansion accelerate forever?

Does the vacuum decay? Phase transitions?How many dimensions are there?

How are quantum physics and gravity unified? What is the fate of the universe?

That 5% contains two fundamental forces and 57 elementary particles.

What will we learn from the dark sector?!

How can we not seek to find out?

2323

Frontiers of the UniverseFrontiers of the Universe

Breakthrough of the Year

1919

Cosmology holds the key to new physics in the next decade.

1998

2003 Let’s find out!