1. STSci Mar 7th 2007 1/46 Could Dark Energy be novel matter or modified gravity? Rachel Bean Cornell University

2. STSci Mar 7th 2007 2/46 Modern (20th century) Cosmology ObservationsTheory Hubble Einstein

3. STSci Mar 7th 2007 3/46 Philosophy As we know, There are known knowns. There are things we know we know. We also know There are known unknowns. That is to say We know there are some things We do not know. But there are also unknown unknowns, The ones we don't know We don't know. —Feb. 12, 2002, Department of Defense news briefing Modern (20th century) Cosmology

4. STSci Mar 7th 2007 4/46 Overview The `known knowns’ –kinematical acceleration The ‘known unknowns’ –theoretical approaches to dark energy Can we know the known unknowns? –Observational tests for dark energy

5. STSci Mar 7th 2007 5/46 Expansion today Observe a cosmological redshift expansion rate and acceleration Luminosity distance F = L/4  d L 2 a(t) t today Hubble factor Deceleration parameter dLdL

6. STSci Mar 7th 2007 6/46 Hubble’s law: true to even larger distances Riess 1996 Hubble 1929 3 million Distance (lightyears) 0 500 1000 Velocity (km/sec) 0 0 6 million Observations of distant Supernovae Distance (millions of lightyears) 300 600 900 1200 1500 0 0 Velocity (km/sec)

7. STSci Mar 7th 2007 7/46 Two alternative explanations for the expanding universe In this model, the universe will always look the same, and had no beginning The universe is expanding but new matter is being created all the time (from nothing!). Universe is expanding and is always changing, matter becomes less dense & cooler Universe had a hot and incredibly small beginning: “The Hot Big Bang” Steady State Model vs.Big-Bang Model

8. STSci Mar 7th 2007 8/46 Which theory was correct? Pigeons?Primordial radiation? What was causing it? Penzias & Wilson in NJ in 1964 observed electromagnetic radiation from all directions in the sky, all at the same temperature, 2.7 Kelvin.

9. STSci Mar 7th 2007 9/46 Evidence of deceleration followed by recent acceleration z evolution of luminosity distance of Supernovae in HST/Goods survey z Riess et al 2004

10. STSci Mar 7th 2007 10/46 Overview The `known knowns’ –kinematical acceleration The ‘known unknowns’ –theoretical approaches to dark energy Can we know the known unknowns? –Observational tests for dark energy

11. STSci Mar 7th 2007 11/46 Observations have driven key theoretical developments But also create many unanswered questions in themselves! What happened right at the beginning? What is dark matter? Can we detect the dark matter in ground based experiments? What is dark energy? An expanding universe Cosmic microwave background Galaxy outer rotation velocities An accelerating universe General relativity Applied to the cosmos Hot Big BangDark matter Dark energy Hubble 1929 1 Distance (Mpc) 0 500 1000 Velocity (km/sec) 0 0 2

12. STSci Mar 7th 2007 12/46 Einstein’s description of the cosmos On the largest scales the universe is controlled by gravity. Our best description of gravity is GR, as formulated by Einstein: G  = 8  G T  c4c4 Einstein’s Tensor: Evolution of space and time Stress-Energy Tensor: Evolution of matter density and pressure Matter tells space how to bend/expand Space tells matter how to move Matter makes the universe expand

13. STSci Mar 7th 2007 13/46 Friedmann-Lemaitre-Robertson-Walker Universe The CMB shows that the universe is homogeneous and isotropic FLRW applies Einstein’s equations to this simplified case –Described by single number, the size of the universe, a(t) Acceleration possible only if dominant matter has negative pressure, w<-1/3 Friedmann

14. STSci Mar 7th 2007 14/46 Evidence of deceleration followed by recent acceleration z evolution of luminosity distance of Supernovae in HST/Goods survey z Riess et al 2004

15. STSci Mar 7th 2007 15/46 Einstein’s `Biggest Blunder’? Prior to Hubble’s observations, scientists believed the universe was static Einstein added in a fudge factor to his equations, called the “cosmological constant” to enable a static universe. Later when Hubble’s discovery of expansion was made, Einstein is said to have called the cosmological constant “my biggest blunder”.

16. STSci Mar 7th 2007 16/46 Current conclusions Observations from supernovae, cosmic microwave background and large scale structure all give a remarkably consistent picture However, this picture is dumbfounding since we do not understand 96% of it! w=-1w~0

17. STSci Mar 7th 2007 17/46 The key dark energy questions How do we modify Einstein’s Field Equations to explain acceleration? - Non-minimal couplings to gravity? –Higher dimensional gravity? –Effects of anisotropy and inhomogeneity Adjustment to gravity? -An ‘exotic’, dynamical matter component “Quintessence”? – ‘Unified Dark Matter’? Adjustment to matter? Cosmological constant “  ” ? -“Vacuum energy” left over from early phase transitions? -Holographic? -Anthropic?

18. STSci Mar 7th 2007 18/46 Why so small? UV divergences are the source of a dark energy fine-tuning problem The cut off scale would have to be way below the scales currently in agreement with QFT (Casimir effect, Lamb shift)  - The problems with it = ? a) QFT = ∞ ? b) regularized at the Planck scale = 10 76 GeV 4 ? c) regularized at the QCD scale = 10 -3 GeV4 ? d) 0 until SUSY breaking then = 1 GeV 4 ? e) all of the above= 10 -47 GeV 4 ? f) none of the above = 10 -47 GeV 4 ? g) none of the above = 0 ? Why now? Coincidence problem –Any later    still negligible, we would infer a pure matter universe – Any earlier  chronically affects structure formation; we wouldn’t be here Inevitably led to anthropic arguments –At most basic predict   /  m <125 Transition to dark energy domination e+e+ e-e-

19. STSci Mar 7th 2007 19/46 Scalar  x,t  - spin 0 particle (e.g Higgs) Accelerative expansion when potential dominates Scaling potentials – Evolve as dominant background matter – Need corrections to create eternal or transient acceleration Tracker potentials –Insensitive to initial conditions Wetterich 1988, Ferreira & Joyce 1998 Kinetic    Potential V(  Tackling the fine-tuning problem Ratra & Peebles 1988 Wang, Steinhardt, Zlatev 1999 Scaling potentials Tracker potentials..

20. STSci Mar 7th 2007 20/46 w(z) evolution with an oscillatory potential W tot Dodelson, Kaplinghat, Stewart 2000 Tackling the coincidence problem: are we special? We’re not special: universe sees periodic epochs of acceleration We are special: the key is our proximity to the matter/ radiation equality –Non-minimal coupling to matter (Amendola 2000, Bean & Magueijo 2001) –k-essence : A dynamical push after z eq with non-trivial kinetic Lagrangian term (Armendariz-Picon, et al 2000) –the coincidence is a result of a coupling to the neutrino (Fardon et al 2003), ghostlike behavior (de la Macorra et al 2006) V~M 4 e - (1+Asin ) Dodelson, Kaplinghat, Stewart 2000 w(z) evolution with a non-minimal coupling to dark matter log(a) w tot Bean & Magueijo 2001

21. STSci Mar 7th 2007 21/46 Bean, Hansen, Melchiorri 2001 Early constraints on dark energy density Dark energy vs baryon density BBN constraints Tackling the  problems: implications for BBN Scaling potentials can predict significant dark energy at earlier times –treating  Q as  N rel Ferreira & Joyce 1998, Bean, Hansen, Melchiorri 2001 Bounds on Helium mass fraction, Y He –Y He =0.24815 ± 0.00033 ±0.0006 (sys) Stegiman 2005 Relative Deuterium abundance D/H –D/H=(2/58+0.14-0.13).10 -5 Steigman(2005) –But collated more recent value D/H = 2.6 ±0.4).10 -5 Kirkman et al (2003) Abundance limits conservatively correspond to  N rel <0.2 –This translates into  Q (MeV)<0.05 (2  )

22. STSci Mar 7th 2007 22/46 ‘Unified’ dark matter/ dark energy – Clustering at early times like CDM, w~0, c s 2 ~0 – Accelerating expansion at late times like , w <0 Phenomenology: Chaplygin gases –an adiabatic fluid, parameters w 0,  Strings interpretation? Born-Infeld action is of this form with   e.g. Gibbons astro-ph/0204008 ) w lg(a) Tackling the dark matter and dark energy problems as one Bean and Dore PRD 68 2003 Evolution of equation of state for Chaplygin Gas

23. STSci Mar 7th 2007 23/46 Quintessential inflation (e.g. Copeland et al 2000, Binetruy, Deffayet,Langlois 2001) – Brane world scenario –  2 term increases the damping of  as rolls down potential at early (inflationary) times –inflation possible with V (  ) usually too steep to produce slow-roll Curvature on the brane (Dvali,Gabadadze Porrati 2001) –Gravity 5D (Minkowski) on large scales l>l c ~H 0 -1 i.e. only visible at late times –Although 4D on small scales not Einstein gravity –Potential implications for solar system tests as well as horizon scales Large scale modifications to GR –Modify action so triggered at large scales R~H 0 2 –Potential implications for solar system tests as well as horizon scales Modifications to gravity rather than matter

24. STSci Mar 7th 2007 24/46 Overview The `known knowns’ –kinematical acceleration The ‘known unknowns’ –theoretical approaches to dark energy Can we know the known unknowns? –Observational tests for dark energy

25. STSci Mar 7th 2007 25/46 Dark energy perturbations: an important discriminator? Natural extension to looking for w≠-1,dw/dz≠0 from  (a) –Include constraints on  (a) sensitive to c s 2 = dP/d  To distinguish between theories … – only effecting the background , alterations to FRW cosmology) – with negligible clustering c s 2 = 1 (minimally coupled quintessence) – that could contribute to structure formation (non-minimally coupled DE, k- essence) To test if dark matter and dark energy are intertwined? – unified dark matter? From a theorist’s perspective, to decipher the dark energy action – probing the dark energy external and self-interactions (or lack of) bound up in an effective potential From an observational perspective, to check that a prior assuming no perturbations is fair –Does it effect the combination of perturbation independent (SN) and potentially dependent (CMB/LSS/WL) observations?

26. STSci Mar 7th 2007 26/46 Linking theory and observations SN 1a HST Legacy, Essence, DES, SNAP Baryon Oscillations SDSS Alcock-Paczynski test CMB WMAP CMB/ Globular cluster Tests probing background evolution only Late time probes of w(z) – Luminosity distance vs. z – Angular diameter distance vs. z Probes of w eff – Angular diameter distance to last scattering – Age of the universe

27. STSci Mar 7th 2007 27/46 Linking theory and observations Galaxy /cluster surveys, SZ and X-rays from ICM SDSS, ACT, APEX, DES, SPT CMB and cross correlation WMAP, PLANCK, with SNAP, LSST, SDSS Tests probing perturbations and background Weak lensing CFHTLS, SNAP, DES, LSST Late time probes of w(z) – Luminosity distance vs. z – Angular diameter distance vs. z Probes of w eff – Angular diameter distance to last scattering – Age of the universe Late time probes of w(z) and c s 2 (z) –Comoving volume * no. density vs. z – Shear convergence –Late time ISW

28. STSci Mar 7th 2007 28/46 Early time probes of  Q (z) –Early expansion history sensitivity to relativistic species BBN/ CMB WMAP Tests probing early behavior of dark energy Linking theory and observations Late time probes of w(z) – Luminosity distance vs. z – Angular diameter distance vs. z Probes of w eff – Angular diameter distance to last scattering – Age of the universe Late time probes of w(z) and c s 2 (z) –Comoving volume * no. density vs. z – Shear convergence –Late time ISW

29. STSci Mar 7th 2007 29/46 Linking theory and observations Late time probes of w(z) – Luminosity distance vs. z – Angular diameter distance vs. z Probes of w eff – Angular diameter distance to last scattering – Age of the universe Late time probes of w(z) and c s 2 (z) –Comoving volume * no. density vs. z – Shear convergence –Late time ISW Early time probes of  Q (z) –Early expansion history sensitivity to relativistic species Alternate probes of non-minimal couplings between dark energy and R/ matter or deviations from Einstein gravity – Equivalence principle tests – Deviation of solar system orbits – Varying alpha tests Tests probing general deviations in GR or 4D existence

30. STSci Mar 7th 2007 30/46 Evolution of H(z) is the primary observable In a flat universe, many measures based on the comoving distance Luminosity distance Angular diameter distance Comoving volume element Age of universe r(z) = ∫ 0 z dz’ / H(z’) d L (z) = r(z) (1+z) d A (z) = r(z) / (1+z) dV/dzdΩ(z) = r 2 (z) / H(z) t(z) = ∫ z ∞ dz/[(1+z)H(z)]

31. STSci Mar 7th 2007 31/46 Acoustic baryon oscillations Compares tranverse + radial scale Observe the ‘sound wave’ generated at last scattering surface –500 million light years across –Expect correlations in the large scale structure on this scale Systematics do not mimic features in correlation (Seo and Eisenstein 2003) –Dust extinction, –galaxy bias, –redshift distortion –non-linear corrections w mh2mh2 mh2mh2 Distance to z=0.35 (Mpc) Eisenstein et al 2004 SDSS ~48000 galaxies with z~0.35

32. STSci Mar 7th 2007 32/46 Kinematical constraints on constant w and w(a) Davis et al 2007 w0w0 wawa w0w0

33. STSci Mar 7th 2007 33/46 Davis et al 2007 Kinematical constraints on DGP background

34. STSci Mar 7th 2007 34/46 Kinematical constraints on Chaplygin gas as dark energy Davis et al 2007 w0w0 w0w0

35. STSci Mar 7th 2007 35/46 f(R) gravity has difficulties fitting observations Bean et al 2006

36. STSci Mar 7th 2007 36/46 Ansatz for H(z), d l (z) or w(z) w(z) applies well to scalar fields as well as many extensions to gravity Linder 2003 –Taylor expansions robust for low-z Do parameterizations relate to microphysical properties (w=p/  and  c s 2 =  p/  ) or just an effective description? –Need to have multi pronged observational approach Reconstructing dark energy : a cautionary note But, parameterizations can mislead Need to consider additional parameter dependencies (Curvature, neutrino mass) Maor et al 2002 Reconstructing dynamic evolution w=-0.7+0.8z with constant w Constant w fit w>-1 fit

37. STSci Mar 7th 2007 37/46 Beyond  CDM: Dark Energy Robust  m v (eV) kk w CMB + SN + LSS w + massive neutrinos w CMB + SN + LSS w + curvature Spergel et al 2006

38. STSci Mar 7th 2007 38/46 Should also leverage evolution on different spatial scales From Max Tegmark for SDSS

39. STSci Mar 7th 2007 39/46 Dark energy domination suppresses growth in gravitational potential wells,     Ga 2  Late time Integrated Sach’s Wolfe effect (ISW) in CMB photons results - Net blue shifting of photons as they traverse gravitational potential well of baryonic and dark matter on way. ISW important at large scales Dark energy clustering counters suppression due to accelerative expansion –Decreases ISW signature  x  x(t) CDM dominated or clustering DE  dominated or Non clustering DE CMB spectra for DE models incl/excl perturbations w<-1 w>-1 without with without with Hu 1998, Bean & Dore PRD 69 2003 Lewis & Weller 2003 ISW: Dark energy signature in CMB photons

40. STSci Mar 7th 2007 40/46 Beyond  CDM: Dark Energy Sensitive to assumptions about clustering properties of Dark Energy Clustering dark energy c s 2 =1 w≠-1If fluctuations in DE negligible WMAP WMAP+SDSS WMAP WMAP+2dF WMAP WMAP+SN (HST/GOODS) WMAP WMAP+SN (SNLS) WMAP WMAP+SDSS WMAP WMAP+2dF WMAP WMAP+SN (HST/GOODS) WMAP WMAP+SN (SNLS) mm mm mm mm w w w w w w w w mm mm mm mm Spergel et al 2006

41. STSci Mar 7th 2007 41/46 Degeneracies & cosmic variance prevent constraints on clustering itself –Large scale anisotropies also altered by spectral tilt, running in the tilt and tensor modes Dark energy clustering will be factor in combining future high precision CMB with supernova data. Avoid degeneracies by cross correlating ISW with other observables …. – galaxy number counts – Radio source counts – Weak lensing of galaxies or CMB …. ISW: Perturbations and CMB & LSS inferences Bean & Dore 2003, Lewis & Weller 2003 ‘Constraints’ on w and c s 2 from WMAP

42. STSci Mar 7th 2007 42/46 ISW intimately related to matter distribution Cross-correlation of CMB ISW with LSS. e.g. NVSS radio source survey (Boughn & Crittenden 2003 Nolta et al 2003, Scranton et al 2003) WMAP+SDSS LRG +SDSS QSO +NVSS 6 sigma detection (Scranton et al in progress) Current observations cannot distinguish dark energy features (Bean and Dore PRD 69 2003) Future large scale surveys which are deep, ~z=2, such as LSST might well be able to (if w≠ -1) (Hu and Scranton 2004)  ( deg) C NT (cnts  k )   contours Nolta et al. 2003 Cross correlation of radio source number counts and WMAP ISW 0 5 10 15 20 0 10 30 50 40   Likelihood 01 12 18 0 1 ISW: CMB cross correlation with LSS Nolta et al 2003

43. STSci Mar 7th 2007 43/46 Weak lensing: recent constraints from CFHT Hoekstra et al 2005, Semboloni et al 2005 -1.5 -0.5 0. w -1.5 -0.5 0. w mm 0.4 0.2 0.6 0.8 mm 0.4 0.2 0.6 0.8 CFHT Legacy Survey Wide Field Survey (22sq deg) CFHT Legacy Survey Wide Field +Deep Surveys (2x 1 deg) Constraints on CDM density and dark energy equation of state from Weak lensing -2.0 Spergel et al 2006 mm 88

44. STSci Mar 7th 2007 44/46 Weak lensing tomography :prospects SNAP and LSST offer exciting prospects for WL –e.g. SNAP measuring 100 million galaxies over 300 sqdeg Tomography => bias independent z evolution of DE –Ratios of growth factor (perturbation) dependent observables at different z give growth factor (perturbation) independent measurement of w, w’ Possibly apply technique to probe dark energy clustering ? Understanding theoretical and observational systematics key –effect of non-linearities in power spectrum –Accurately reconstructing anisotropic point spread function – z-distribution of background sources and foreground halo – inherent ellipticities … –Use of higher order moments to reduce these … SNAP collaboration Aldering et al 2004 Deep survey Wide survey SNAP SN1a w 0.0 -0.5 -1.5 -2.0 0.0 0.2 0.40.60.8 Wide survey+ non-Gaussian info MM Prospective constraints on w from the SNAP SN1a + WL measurements

45. STSci Mar 7th 2007 45/46 Acoustic baryon oscillations: prospects 30,000 sq degree survey with ~54 galaxies per arcminute 2 Redshift slices between z=0.2 and 3 Competitive predictions with WL tomography and future large supernovae surveys Cross correlation of weak lensing and BAO ? 2000 SN1a

46. STSci Mar 7th 2007 46/46 Linking Dark Energy Observations and Theory Einstein on Theory: “If an idea does not appear absurd at first then there is no hope for it” Einstein on Observation: "Joy in looking and comprehending is nature's most beautiful gift.”