# Presentation: 748777

**     ****The**** Darkness ****	****  ****of the Universe:**
***The Heart of Darkness***

**Eric Linder **
**Lawrence Berkeley National Laboratory**

## New 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**

## The world is w(z)

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

**Simple, robust parametrization**
**    ****w(a)=w****0****+w****a****(1-a)**
**Braneworld**** ****[DDG]**** ****vs.**** ****(w****0****,w****a****)=(-0.78,0.32)**
**Vacuum metamorph**** ****vs.**** ****(w****0****,w****a****)=(-1,-3)**
**Also agree on m(z) to 0.01 mag out to z=2**

## Our Tools

**Expansion rate of the universe**** a(t) **
**ds****2 ****= ********dt****2****+a****2****(t)[dr****2****/(1-kr****2****)+r****2****d********2****]**
**   ****Einstein equation***** ***
**(å/a)****2**** = H****2**** = (8********/3) ********m**** + ********H****2****(z)**
**	****	****         ****= (8********/3) ********m**** + C exp{********dlna [1+****w(z)****]}**
**Growth rate of density fluctuations***** *****g(z)**** ****= (********m****/********m****)/a **

**Poisson equation**** ********2********(a)****=4********Ga****2 ********m****= 4********G********m****(0) g(a)**

## Revealing 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. **

**w****0****=-0.78 w****a****=0.32**

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

## Testing the Framework

**Extensions to gravitation**
**E.g. scalar-tensor theories: ****f/2********-********(********)********;************;********-V****	****	****Take linear coupling to Ricci scalar R: ****f/******** = F R**** ****	****Allow nonminimal coupling **
**	****	****F=1/(8********G)+ ********2**
**R-boost (note R********0 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: S********0 since G ******** 1/F.**

## Scalar-Tensor Gravity

**Consider a general linear coupling **
**	****	****	****R/(8********G) ******** F(**********) R**
**Today, have ****	****F = 1/(8********G) ****	****	****	****	****and ****Jordan-Brans-Dicke**** parameter**** **
**	****	****	********JBD**** = F/F**********2**
**Can treat as coupled scalar-tensor theory in physical = Jordan frame **
**Or as separated scalar+tensor theory in ****	****	****spin = Einstein frame**

## Scalar-Tensor Gravity

**In terms of ********H****2****, ****following**** ****Baccigalupi, Matarrese, & Perrotta**
**(3/8********G) ********H****2**** ****= V + (1/2)F**********2****(q-1)(q+5) +3H****2 ****[F-1/(8********G)]**
**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. **

## 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**

## Dark Entropy

**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 ****H****2**** (derived Friedmann equation). **
**Anything interesting? ****	****	****Linder hep-th/0410017**

## Cosmic Dynamics

**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. **

## Dark 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!**** **

## Heart of Darkness

**Is dark energy dark – only interacts gravitationally?**
**Self interaction:**** ****pseudoscalar quintessence **
**Coupling to matter: ****Chaplygin gas**** **
**Leads to 5****th**** 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 quintessence**
**Can clump on subhorizon scales **
**Can “turn on” from nonlinear structure formation?!**
**Higher dimension gravity:**** ****Scalaron quintessence**
**Can be written in terms of scalar-tensor and w****eff**

- Sandvik et al. 2003

**The horror! The horror!**

## Dark 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**

## 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?**** **

## Hunting Dark Energy

***w****** ́***** is the 1****st**** step for fundamental physics, on or  beyond ********.**
***w****** ́=dw(z)/d*****ln*****a|*****z=1*****=w******a******/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. **

## Beyond Dark Energy

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

## Present Day Inflation

**Map the expansion history precisely and see the transition from acceleration to**** ****deceleration.**** **

## Density History of the Universe

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

## Frontiers

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

## Frontiers

**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?”**

## The 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?**

## Frontiers 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!**