Dark Energy: Taking Sides

1. Dark Energy: Taking Sides Accelerators in the Universe March 2008 Rocky Kolb University of Chicago

2. Radiation: 0.005% Chemical Elements: (other than H & He)0.025% Neutrinos: 0.17% Stars: 0.8% LCDM H & He: gas 4% Cold Dark Matter: (CDM) 25% Dark Energy (L): 70% + inflationary perturbations + baryo/lepto genesis

3. Evidence for Dark Energy Measuring the expansion history of the Universe

4. Evolution of H(z) Is a Key Quantity Equation of state parameter: w= p / r (w = -1 for L) if w=w(a): Friedmann equation (G00= 8p GT00) : Expansion rate H(z) Hubble constant curvature matter radiation dark energy CMB LSS CMB H(z)

5. Evolution of H(z) Is a Key Quantity Robertson–Walker metric Many observables based on H(z) through coordinate distance r(z) • Luminosity distance Flux = (Luminosity / 4dL2) • Angular diameter distance a= Physical size / dA • Volume (number counts) N/V -1(z) • Age of the universe

6. Evidence for Dark Energy LCDM Einstein-de Sitter: spatially flat matter-dominated model (maximum theoretical bliss) confusing astronomical notation related to supernova brightness  brighter fainter  Astier et al. (2006)* SNLS supernova redshift z • High-redshift supernova are about a half-magnitude fainter • than expected in the Einstein-de Sitter model. • Statements such as there is dark energy, or that the Universe is • accelerating are interpretations of the observations. * Similar data from Essence collaboration.

7. 2.0 1.5 1.0 0.5 0 Evidence For Dark Energy Astier et al. (2006) SNLS • Find standard candle (SNe Ia) • Observe magnitude & redshift • Assume a cosmological model • Compare observations & model • Fit needs cosmoillogical constant • or vacuum energy density WL • Assumes w= -1 (i.e., L) • Assumes priors on H0, • spatial flatness, etc. 0 0.5 1.0 WM Einstein–de Sitter model

8. Cosmoillogical Constant rV 10–30g cm-3 !!!!! (The Unbearable Lightness of Nothing) • rV L = 8pGrV length scale 10-3cm10+29cm mass scale 10-4eV10-33eV

9. Cosmoillogical Constant All fields: harmonic oscillators with zero-point energy

10. Cosmoillogical Constant • The quantum vacuum has a Higgs potential • Higgs potential gives mass to quanta like quarks and electrons   e,W,Z, quarks … photon  Higgs potential: 246109eV

11. The Higgs Bison at Fermilab

12. Cosmoillogical Constant V(f) high- temperature low- temperature DV= L f

13. Cosmoillogical Constant Extra Dimensions

14. Cosmoillogical Constant We are here Illogical timing (cosmic coincidence):

15. Evidence for Dark Energy LCDM Einstein-de Sitter: spatially flat matter-dominated model (maximum theoretical bliss) Astier et al. (2006)* SNLS confusing astronomical notation related to supernova brightness supernova redshift z 1) Hubble diagram (SNe) The case for L: 5) Galaxy clusters 6) Age of the universe 7) Structure formation 2) Cosmic Subtraction 3) Baryon acoustic oscillations 4) Weak lensing * Similar data from Essence collaboration.

16. Evidence for Dark Energy dynamics lensing x-ray gas simulations cmb power spectrum WTOTAL=1 WM~ 0.3 WB ~ 0.04 CMB many methods CMB/BBN 1.0 - 0.3 = 0.7  0

17. How We “Know” There’s Dark Energy • Assume model cosmology: • General relativity • Homogeneity & isotropy (FLRW) • Energy (and pressure) content: =M+R++ • Input or integrate over cosmological parameters: H0, W0, etc. • Calculate observables dL(z), dA(z),H(z),  • Compare to observations • Model cosmology fits with L, but not without L • All evidence for dark energy is indirect: observed H(z) is not • described by H(z) calculated from the Einstein-de Sitter model • [spatially flat (from CMB) ; matter dominated (r = rM)] G00 = 8p G T00 H2=8 G /3 -k/a2

18. Taking Sides! • Can’t hide from the data – LCDM too good to ignore • SNe • Subtraction: 1.0 - 0.3 = 0.7 • Baryon acoustic oscillations • Galaxy clusters • Weak lensing • … H(z) not given by Einstein–de Sitter G00 (FLRW)8GT00(matter) • Modify right-hand side of Einstein equations (DT00) • Constant (“just” a cosmoillogical constant L) • Not constant(dynamics driven by scalar field: M ~ 10-33eV) • Modify left-hand side of Einstein equations (DG00) • Beyond Einstein (non-GR: f (R), branes, etc.) • (Just) Einstein (back reaction of inhomogeneities)

19. Anthropic/Landscape • Many sources of vacuum energy • String theory has many (10500?) vacua • Some of them correspond to cancellations that yield a small L • Although exponentially uncommon, they are preferred because … • More common values of L results in an inhospitable universe The Cosmological Constant and String Theory are made for each other… Cosmological constant (L): A result in search of a theory String theory (or M-theory): A theory in search of a result

20. Quintessence • Many possible contributions. • Why then is total so small? • Perhaps unknown dynamics sets global • vacuum energy equal to zero……but we’re not there yet! V(f) Requires mf 10-33eV L 0 f

21. Quintessence • Not every scalar potential works • scaling, tracking, stalking potentials • Couplings to matter (dark or otherwise) • mavens (mass varying neutrinos) • chameleon (dark energy is sensitive to environment) • changing fundamental constants (a, etc.) • k-essence • phantom, ghost condensate, Born-Infeld, …, Chaplygin gas • Episodic dark energy (acceleration happens)

22. Punctuated Vacuum Domination Dodelson, Kaplinghat, Stewart dark energy  cosmic destiny

23. Modifying the Left-Hand Side • Braneworld modifies Friedmann equation • Gravitational force law modified at large distance • Tired gravitons • Gravity repulsive at distance R Gpc • n = 1 KK graviton mode very light, m  (Gpc)-1 • Einstein & Hilbert got it wrong f (R) • Backreaction of inhomogeneities Binetruy, Deffayet, Langlois Deffayet, Dvali & Gabadadze Five-dimensional at cosmic distances Gregory, Rubakov & Sibiryakov; Dvali, Gabadadze & Porrati Gravitons metastable - leak into bulk Csaki, Erlich, Hollowood & Terning Kogan, Mouslopoulos, Papazoglou, Ross & Santiago Carroll, Duvvuri, Turner, Trodden Räsänen; Kolb, Matarrese, Notari & Riotto; Notari; Kolb, Matarrese & Riotto

24. Acceleration from Inhomogeneities Homogeneous model Inhomogeneous model We think not! (Buchert & Ellis)

25. Acceleration from Inhomogeneities • No dark energy • No acceleration in the normal sense • (no individual fluid element accelerates)

26. Dark Energy • Many theoretical approaches (none compelling i.m.o.!) • Perhaps nothing more can be done by theorists …

27. Dark Energy Einstein in August 1913 to Berlin astronomer Erwin Freundlich encouraging him to mount an expedition to measure the deflection of light by the sun. "Nothing more can be done by the theorists. In this matter it is only you, the astronomers, who can perform a simply invaluable service to theoretical physics."

28. Dark Energy • Many theoretical approaches (none compelling i.m.o.!) • Perhaps nothing more can be done by theorists … • Observables: dL(z) , dA(z) , distances, … • … which (in FRW at least) all depend on H(z) • Might as well assume FRW and measure(?) H(z) • (Measure what you can as well as you can) • Dark energy enters through rL(z), parameterized by w(z) • Can’t measure a function; parameterize w(z)

29. Dark Energy L: w0= -1 ; wa= 0 Quintessence: -1 w0 -1/3 ; wa= model dependent Modified gravity: w0 can be less than -1; wa unconstrained Back reactions: ?????

30. DETF* Experimental Strategy: • Determine as well as possible whether the accelerating expansion is consistent with being due to a cosmological constant. (Is w= -1?) • If the acceleration is not due to a cosmological constant, probe the underlying dynamics by measuring as well as possible the time evolution of the dark energy. (Determine w(a).) • Search for a possible failure of general relativity through comparison of the effect of dark energy on cosmic expansion with the effect of dark energy on the growth of cosmological structures like galaxies or galaxy clusters. (Hard to quantify.) * Dark Energy Task Force

31. Observational Program strong lensing source?

32. Evidence for Dark Energy LCDM Einstein-de Sitter: spatially flat matter-dominated model (maximum theoretical bliss) Astier et al. (2006) SNLS confusing astronomical notation related to supernova brightness supernova redshift z 1) Hubble diagram (SNe) The case for L: 5) Galaxy clusters 6) Age of the universe 7) Structure formation 2) Cosmic Subtraction 3) Baryon acoustic oscillations 4) Weak lensing

33. Supernova Type Ia • Measure redshift and intensity as function of time (light curve) • Systematics (dust, evolution, intrinsic luminosity dispersion, etc.) • A lot of information per supernova • Well developed and practiced • Present procedure: • Discover SNe by wide-area survey on 4m-class telescopes • Follow up spectroscopy (lots of time on 8m-class telescopes) • The future: • Better systematics (space-based?) • Huge statistics (say with LSST) with photometric redshifts

34. Baryon Acoustic Oscillations • Each overdense region is an • overpressure that launches a • spherical sound wave • Wave travels outward at c/ 3 • Photons decouple, travel to us • and observable as CMB • acoustic peaks • Sound speed plummets, • wave stalls • Total distance traveled 150 Mpc • imprinted on power spectrum WMAP SDSS

35. Baryon Acoustic Oscillations 150Mpc=dAdq 150Mpc=H-1d z • Acoustic oscillation scale depends on Mh2 and Bh2 • (set by CMB acoustic oscillations) • It is a small effect (B h2¿M h2) • Dark energy enters through dA and H

36. Baryon Acoustic Oscillations • Virtues • Pure geometry. • Systematic effects should be small. • Problems: • Amplitude small, require large scales, huge volumes • Photometric redshifts? • Nonlinear effects at small z, cleaner at large z~ 2-3 • Dark energy not expected to be important at large z

37. Weak Lensing observe deflection angle dark energy affects geometric distance factors b dq DLS DOS dark energy affects growth rate of M

38. Weak Lensing The signal from any single galaxy is very small, but there are a lot of galaxies! Require photo-z’s? Systematic errors: The Landscape: • Dominant source is PSF of • atmosphere and telescope • Errors in photometric redshifts Space vs. Ground: • Space: no atmosphere PSF • Space: Near IR for photo-z’s • Ground: larger aperture • Ground: less expensive • Current projects • 100’s of sq. degs. deep multicolor data • 1000’s of sq. degs. shallow 2-color data • DES (2010) • 1000’s of sq. degs. deep multicolor data • LSST (2013) • full hemisphere, very deep 6 colors

39. Galaxy Clusters • Cluster redshift surveys measure • cluster mass, redshift, and spatial clustering • Sensitivity to dark energy • volume-redshift relation • angular-diameter distance–redshift relation • growth rate of structure • amplitude of clustering • Problems: • cluster selection must be well understood • proxy for mass? • need photo-z’s

40. What’s Ahead 2020 2008 2010 2015 Lensing CFHTLS SUBARU DUNE LSST SKA DES, VISTA Hyper suprime DLS SDSS ATLAS KIDS JDEM Pan-STARRS BAO FMOS DES, VISTA,VIRUS WFMOS LSST SKA LAMOST Hyper suprime SDSS ATLAS JDEM Pan-STARRS SNe CSP ESSENCE DES LSST SDSS CFHTLS Pan-STARRS JDEM Clusters AMI APEX SPT DES XCS SZA AMIBA ACT CMB WMAP 2/3 WMAP 5 yr Planck Planck 4yr Roger Davies

41. Conclusions: Two Sides to the Story The expansion history of the universe is not described by the Einstein-de Sitter model. • Well established: Supernova Ia • Circumstantial: subtraction, age, structure formation, … • Emergent techniques: baryon acoustic oscillations, clusters, weak lensing Explanations: • Right-Hand Side: Dark energy • “constant” vacuum energy, i.e.,L • time varying vacuum energy, i.e., quintessence • Left-Hand Side • Modification of GR • Standard cosmological model (FLRW) not applicable • Phenomenology: • Measure evolution of expansion rate: is w = -1? • Order of magnitude improvement feasible

42. Dark Energy: Taking Sides Accelerators in the Universe March 2008 Rocky Kolb University of Chicago