1. 1 1 The Darkness of the Universe: The Darkness of the Universe: The Heart of Darkness Eric Linder Lawrence Berkeley National Laboratory

2. 2 2 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

3. 3 3 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

4. 4 4 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)

5. 5 5 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

6. 6 6 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.

7. 7 7 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

8. 8 8 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.

9. 9 9 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

10. 10 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

11. 11 Cosmic Dynamics H HH w w tot 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.

12. 12 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!

13. 13 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!

14. 14 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

15. 15 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?

16. 16 Hunting Dark Energy w´ is the 1 st step for fundamental physics, on or beyond . w´=dw(z)/dlna| 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.

17. 17 Beyond Dark Energy Determine not only dark energy parameters, but test the cosmology framework – alternative gravitation, higher dimensions, etc.

18. 18 Present Day Inflation Map the expansion history precisely and see the transition from acceleration to deceleration.

19. 19 Density History of the Universe Map the density history precisely, back to the matter dominated epoch.

20. 20 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

21. 21 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?”

22. 22 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?

23. 23 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!