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Addressing the Dark Energy Crisis in Fundamental Physics Looking into the Crystal Ball Christopher Stubbs Department of Physics Department of Astronomy Harvard University 1 Outline Evidence for accelerating expansion = Dark Energy Consequence: a crisis for fundamental physics Tests of gravitation, and connections to Dark Energy - cosmic scales - solar system scales - laboratory scales Closing thoughts. 2 This is a remarkable time Our view of the Universe is shifting, yet again Sun-centered solar system Galactic structure Recognition of external galaxies General Relativity (originally with cosmological constant ) Discovery of Expansion of the Universe Big Bang + inflationary paradigm Dark Matter greatly outweighs all luminous matter Discovery that the rate of cosmic expansion is increasing: The “Accelerating” Universe Emergence of a Standard Cosmology Our geometrically flat Universe started in a hot big bang 13.7 billion yrs ago. It has been expanding ever since. The evolution of the Universe is increasingly dominated by the phenomenology of the vacuum, the “Dark Energy”. “Dark matter”: what is it? Ordinary matter is a minor component. Luminous matter comprises a very small fraction of the mass of the Universe. 4 3 Pieces of New Fundamental Physics Inflation: The early Universe grew by a factor of e60. Quantum fluctuations in this era seeded the large scale distribution of galaxies we see today. We don’t know what drove inflation, or how it ended. Dark Matter: The internal kinematics of galaxies like the Milky Way indicate more mass than we see in stars and gas. Furthermore, all measures of mass exceed the baryonic inventory. We don’t know what the dark matter is… axions, LSP…? Dark Energy: The Universe has recently entered a renewed phase of accelerating expansion. Something is driving repulsive gravity in the vacuum. We don’t understand the physics of dark energy. 5 It’s like living through a bad episode of Star Trek! Empty regions of space (vacuum) interact via a repulsive gravitational force. This effect will increasingly dominate, leading to all unbound galaxies eventually being unobservable At the interface between particle physics and gravity, we’re in a more sophisticated state of confusion than ever before… The accelerating Universe scenario is supported by multiple independent lines of evidence Lower bound on age, from stars Inventories of cosmic matter content Measurements of expansion history using supernovae Primordial element abundances “Baryon acoustic oscillations”: large scale galaxy dist. Cosmic Microwave Background provides strong confirmation 7 The Original Evidence for Accelerating Expansion, 1998 Schmidt et al, High-z SN Team Far away Distance to Supernova Nearby 0.01 0.1 Redshift = 8 1.0 Some notation…. There is a “critical density” that would eventually halt the current expansion, let’s call it crit. This quantity varies over cosmic time. crit 3H 2 , where H o =75 km/sec per Mpc 8 G Measure all densities in units of crit ~ 5 H atoms/m3 i 9 i crit And a cosmic sum rule… General Relativity and isotropy imply b dm curvature 1 But the relative proportions of these vary over cosmic time. 10 The data drive us to non-zero Why is this a crisis in fundamental physics? 11 Kowalski et al, ApJ 686, 749 (2008) The quantum mechanical vacuum is a seething turmoil… Lamb shift in Hydrogen (virtual QED process) Electron (g-2) (Hanneke et al, PRL 100, 1120801 (2008)) Casimir-Polder forces… (Lamoreaux, PRL 78, 5L (1997) & …) It’s confusing…. So let’s ask the theorists! 12 Dark Energy Theory 1010. Well, that can’t be right… 0. Through some profound but not yet understood mechanism, the vacuum energy must be cancelled to arrive at value of identically zero ummm... Supersymmetry uhhh ...Planck Mass 0.7, you say?? String landscapes….uhhhh No, wait! IT’S ANTHROPIC! 13 Two possible “natural” values Vacuum energy integrated up to Planck 120 scale 10 Cancellation via tooth fairy: 0.0000000000000000000000000000.... But it’s measured to be around 0.7! 14 Why Dark Energy Constitutes A Crisis in Fundamental Physics Puzzle #1: why is so small? Puzzle #2: why is so large? Puzzle #3: what’s the underlying physics? Understanding the nature of the Dark Energy is arguably the most profound outstanding problem in contemporary physics. Are the properties of the Universe we see the result of some beautiful (but as yet not understood) underlying symmetry principles OR just an anthropic accident/selection effect? 15 Three philosophically distinct possibilities... A “classical” cosmological constant, as envisioned by Einstein, residing in the gravitational sector. A “Vacuum energy” effect, arising from quantum fluctuations in the vacuum, acting as a “source” term. Departure from GR on cosmological length scales. Regardless, it’s evidence of new fundamental physics! 16 The Next Step: Dark Energy’s Equation of State w= w= w= w= P = w 0, matter 1/3 ,radiation - 1, - N/3, topological defects c(1 z ) 3 3(1 w ) DL ( z ) (1 )(1 z ) (1 z ) dz H0 0 z • For a flat Universe, luminosity distance DL depends z, , w. • Evolution of Dark Energy density depends on w. • Any value of w other than -1 excludes cosmological constant • Any evolution in w excludes cosmological constant 17 Narayan et al, 2010 33 70 104 112 62 14 18 (1 w) 0.008 0.07(stat) 0.13(syst) Evolution of Precision, Convergence Kowalski et al, ApJ 686, 749 (2008) All observations to date are consistent with Dark Energy being the cosmological constant originally envisioned by Einstein: w = -1 What’s missing from this plot? 19 An analogy from the past… We’ve seen something like this before: ~ 1880’s - early 1900’s physics faced three profound experimental puzzles: 1.blackbody spectrum 2. discrete atomic spectra . . E4 E3 E2 E1 3. Photoelectric effect e- 20 But that’s not all… The challenge posed by the dark energy has shaken the reductionist and fundamentalist philosophy that has served us so well…. Physics has tried to determine a simple set of rules that govern the Universe, with the expectation that these rules and their associated parameters are both uniquely determined by some underlying principle. L=0 Vacuum energy melectro n mproton 21 The Anthropic Perspective An alternative to the unique fundamentalist approach is the claim that the most basic scientific observable is that we’re here, and that fact restricts the possible values of physical parameters. Proponents of the anthropic approach contend that the dark energy saturates the allowed upper bound that could give rise to life as we know it. All physical parameters (masses, charges, interaction strengths…) are essentially accidental, apart from the constraint imposed by an anthropic selection effect. 22 This is a vibrant ongoing debate Skeptics debate whether the anthropic approach is actually science, as opposed to philosophy. Is it falsifiable? I don’t know. So let’s return our attention to measurements we can make to better understand the dark energy. 23 Probing the Nature of Dark Energy • Higher precision supernova data • All-sky gravitational lensing • Mapping evolution of structure – abundance of galaxy clusters – large scale distribution of galaxies with • Dedicated spacecraft mission (JDEM) • Next-generation ground-based projects – especially surveys… 24 Dark Energy status… and 3 questions Measurements are “out of pace” with theoretical understanding. This is a Bad Thing. (Same as string theory, but with opposite sign.) Numerous next-generation projects are coming on line: e.g. PanSTARRS-1, SPT, BOSS, DES, LSST?, JDEM?… Current data favor w -1 (to 10%), and no evolution with cosmic time. 1. What if this is the real answer? cosmological observations? When do we quit with 2. If cosmology has thrown down this challenge to our understanding of fundamental physics, how long must we wait until it’s resolved? 3. What other experimental anomalies might shed light on the DE? Exotic Physics in the Dark Sector? OM DM DE Ordinary matter sweg g+w?+?? g + ? Dark matter g+w?+?? g + ?? Dark energy g + ?? Essentially everything we think we know about Dark Energy and Dark Matter comes from their gravitational influence on ordinary matter. 26 There are Numerous Reasons to Test the Foundations of Gravity The Dark Energy sits right along the intellectual fault line between quantum mechanics and gravity. Numerous speculative scenarios invoke modifications to General Relativity to account for the accelerating expansion. Growing interest in exotic couplings in the dark sector, involving some combination of dark matter and dark energy. The evidence for dark matter comes from assuming normal law of gravity. This might be inappropriate. 27 Tests of Gravitation Tests of the Equivalence Principle Do all forms of mass-energy experience same free fall in a gravitational field? Are there new long-range forces? Does the growth of cosmic structure behave as expected? Measuring Geffective as a function of environment Tests of Inverse Square Law Do interactions in lowdensity regions look the same as in high-density regions? F ~ 1/r2 ? Is there any evidence for extra dimensions? 28 But… there is history here 1960s Claim of GW detection Positrons didn’t fall down 1970s Rotation curves imply missing force problem* 1980s Claim of 5th force, a Yukawa coupling to Baryon # Gyroscope L vs. R anomaly MOND claims to fit rot. curves 1990s “Pioneer” anomaly Claims of grav. Shielding Claim of accelerating Universe* MOND claims to fit rot. curves 29 2000s Antigravity for v/c > 1/sqrt(3) MOND claims to fit rot. curves The first anomalous gravity Pioneer anomaly experiment? … … Testing gravity on the cosmic scale Testing gravity in the solar system Testing gravity in the laboratory 30 Millenium simulation team, Raul Angulo m=0.25 =0.75 514 Mpc on each side 31 Opportunities for Cosmic Tests of Gravity Isotropy of Cosmic Expansion - Test SN Hubble diagram in different directions. DE uniformity? Kinematics of galaxies – Test CDM paradigm, comparing with N-body simulations – Halo streams as tracers of Galactic gravitational potential Cosmological parameters m + + k =? 1 – w = P/ = -1, or w(z)? Evolution of Cosmic Large Scale Structure – compare gravitational potentials sensed by light, matter, and dark matter – probe the transition from granular to uniform Overconstrained systems – H0, luminosity distances, lensing, geometry, time delays, mass distr. 32 The “Chameleon” Idea A new long-range interaction, repulsive between like objects, takes effect on cosmic scales. But it’s “screened” in some way near mass concentrations. One fashionable mechanism is to introduce a term that depends on the local scalar curvature, f(R). This, in turn, depends on both the environment (e.g. a void vs. a cluster) as well as properties of the object under consideration. 33 Comparison of galaxy-galaxy lensing, galaxy clustering, and velocities Reyes et al, Nature 464,7286 (2010) also Lombriser, Slosar, Seljak and Hu, arXiv:1003:3009 (2010) 34 Comparing galaxy kinematics, in clusters and in voids Hui, Nicolis & Stubbs, PRD 80, 104002, (2009) NGC 5907 (Credit: R. Jay Gabany) 35 Is the accelerating expansion the same in different directions? A. Diercks’ PhD thesis, UCSB,1999 Cook & Lynden-Bell, MNRAS 401, 1409 (2009), and references therein 36 The PanSTARRS Survey 1.4 Gpix camera 3.3 degree FOV 37 1.8 meter telescope PanSTARRS supernovae: > 300 in 6 weeks 38 Testing gravity on the cosmic scale Testing gravity in the solar system (with some slides from Tom Murphy, UCSD) Testing gravity in the laboratory 39 Lunar Retroreflector Arrays Corner cubes Apollo 11 retroreflector array Apollo 14 retroreflector array Apollo 15 retroreflector array 40 Apache Point Observatory Lunar Laser-ranging Operation Mapping the lunar orbit to 1 mm 41 APOLLO: Reaching 1 mm 3.5 meter telescope 1 arcsec median seeing Avalanche Photodiode Array (lenslet gives full fill factor) = 30 m 42 -Courtesy of Lincoln Lab Record-setting photon detection rates Reflector APOLLO max APOLLO max APOLLO max photons/run photons/5-min photons/shot (5 min avg) APOLLO max photons/shot (15 sec avg) Apollo 11 4497 (26) 5395 (65) 0.90 1.4 Apollo 14 7606 (36) 9125 (69) 1.52 2.0 Apollo 15 15730 (26) 18875 (67) 3.15 4.5 750 (11) 900 (31) 0.15 0.24 Lunokhod 2 APOLLO has greatly surpassed previous records (relative to pre-APOLLO record) max rates for French and Texas stations about 0.1 and 0.02, respectively APOLLO can operate at full moon other stations can’t (except during eclipse), though EP signal is max at full moon! Often a majority of APOLLO returns are multiple-photon events record is 12 photons in one shot (out of 12 functioning APD elements) APD array (many buckets) is crucial 43 LLR through the decades Station Dish Seeing Rate (/min) MLRS, USA 0.8 m 3” 1 OCA, France 1.5 m 1-7” 4 APOLLO, USA 3.5 m 1” 600 44 Equivalence Principle Signal, Illustrated moon close moon far 45 Graphic excerpt from San Diego Union Tribune LLR is a precision probe of fundamental gravity physics With one millimeter range uncertainty: Weak EP a/a Strong EP =4-3- Gravitomagnetism (dG/dt)/G Geodetic precession Long range forces LLR currently provides the best limit on all but WEP 46 10-14 3×10-5 10-4 10-13 yr -1 3×10-4 10-11 × the strength of gravity The Strong Equivalence Principle Earth’s energy of assembly amounts to 4.610-10 of its total mass-energy The ratio of gravitational to inertial mass for this self energy is The resulting range signal is then Currently is limited by LLR to be ≤4.510-4 47 The Inverse Square Law r mM V(r) G (1 e ) r 48 Monitoring the Strength of Gravity, G • If G changes with time, Kepler’s law is broken • Range signal (semi-major axis) and period (phase) no longer run in lock-step • The rate of phase slippage grows linearly in time • The phase offset grows quadratically in time, so sensitivity to G-dot grows rapidly as data accumulate • LLR sensitivity now limits change to ≤10-12/yr variation • Less than 1% change over age of Universe 49 Compact Extra Dimensions? Gravity might not be so feeble, intrinsically. Perhaps most of the gravitational force is confined to compact extra dimensions… 50 Dark Energy + Extra Dimensions require both G and w to vary, for broad classes of theories If compact extra dimensions exist, accelerating expansion causes their size to vary. This happens independent of the scale, even down to the to Planck length! The strength of gravity (which is feeble because it “leaks” into these extra dimensions) therefore changes Steinhardt and Wesley show (arXiv:1003:2815) that the joint constraints on G(t) and w(t) do better than either does alone. 51 Steinhardt and Wesley (arXiv:1003:2815) CRF= conformal Ricci-flat metrics NEC= null energy condition 52 Steinhardt and Wesley (arXiv:1003:2815) 53 Testing gravity on the cosmic scale Testing gravity in the solar system Testing gravity in the laboratory (with slides from Eric Adelberger, Univ. Of Washington) 54 Compact Extra Dimensions? Gravity might not be so feeble, intrinsically. Perhaps most of the gravitational force is confined to compact extra dimensions… 55 The Planck length is the “natural scale” for the size of these compact dimensions Planck hG -35 1.6 10 meters 3 c Ouch. This is one reason why testing String Theory is so tough. Planck length is 1020 times smaller than a proton This corresponds to an energy of 1019 Gev, or ~1015 times the LHC String Theory = Ouch15 56 Does dark energy define a new fundamental length scale in physics? a second “Planck length”? 57 the 42-hole inverse-square law pendulum 58 PhD project of Dan Kapner, at Univ. of Washington 59 Mary Levin photo 95% confidence upper limits on Inverse Square Law violation r mM V(r) G (1 e ) r size of any compact dimension < 44 m D.J. Kapner et al., PRL 98, 021101 (2007) E.G. Adelberger et al., PRL 98, 13104 (2007) 60 Summary The discovery of the Dark Energy poses a profound challenge to our understanding of fundamental physics. It’s difficult to understand =0.7. So far, the data are consistent with the cosmological constant scenario: w=-1, w(a)=0. We are in a situation analogous to the era before quantum mechanics. Testing gravity on all accessible scales and energies is a fishing expedition, but we should be unapologetic about this. 61