1 / 36

Dark Energy: Extending Einstein

Dark Energy: Extending Einstein. Eric Linder University of California, Berkeley Lawrence Berkeley National Lab. From Data to Theory (and back). To compare observations and theory we need a statistical measure of goodness of fit.

kynthia
Download Presentation

Dark Energy: Extending Einstein

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dark Energy: Extending Einstein Eric Linder University of California, Berkeley Lawrence Berkeley National Lab

  2. From Data to Theory (and back) To compare observations and theory we need a statistical measure of goodness of fit. We need to compare the theory value, e.g. for distance-redshift, dlum = (1+z) 0z dz’ / H(z’; m,w(z’) ) to the data Dlumi. For example 2 or likelihood 2 = i,j[Dlumi- dlum(zi)] COV-1(i,j) [Dlumj- dlum(zj)]t L = exp(- 2/2) [Gaussian near max likelihood] We need 1) theory or robust parametrization w(z), 2) efficient method for estimating parameter errors given data characteristics.

  3. Fisher Matrix ( ) ) ( 2() COV(,w) COV(,w)2(w) F Fw Fw Fww C = F-1 = F = Fisher matrix gives lower limit for Gaussian likelihoods, quick and easy. Fij = d2 (- ln L) / dpi dpj = O(dO/dpi) COV-1 (dO/dpj) (pi) 1/(Fii)1/2 Example: O=dlum(z=0.1,0.2,…1), p=(m,w), COV=(d/d)d ij Fw=k(dOk/d)(dOk/dw)k-2 See: Tegmark et al. astro-ph/9805117 Dodelson, “Modern Cosmology” Also called information matrix. Add independent data sets, or priors, by adding matrices. e.g. Gaussian prior on m=0.280.03 via2 = (m-0.28)2/0.032

  4. Survival of the Fittest Fisher estimates give a N-dimension ellipsoid. Marginalize (integrate over the probability distribution) over parameters not of immediate interest by crossing out their row/column in F-1.Fixa parameter by crossing out row/column in F. 1 (68.3% probability enclosed) joint contours have d2=2.30 in 2-D (not d2=1). Read off 1 errors by projecting to axis and dividing by 1.52=2.30. Orientation of ellipse shows degree of covariance (degeneracy). Different types of observations can have different degeneracies (complementarity) and combine to give tight constraints.

  5. Model Independence We could check each theoretical model one by one against the data -- but there are 10x of them, each with their own parameters. We’d also like to predict / design results of different experiments. Want model independent approach.Remember H(z)=[m(1+z)3 + w exp{30z d ln(1+z) [1+w(z)]} ]1/2 Parametrize w(z). Keep close to the physics: both energy density and pressure enter the dynamics; directly related to kinetic/potential energy of scalar field.

  6. Model Independence • Simplest parametrization, with physical dynamics, • w(a)=w0+wa(1-a) • Recall a=(1+z)-1. • Virtues: • Model independent • Excellent approximation to exact field equation solutions • Robust against bias • Well behaved at high z • Problems: Cannot handle rapid transitions or oscillations. • N.B.: constant w lacks important physics; w(z)=w0+w1z is Taylor expansion about low z only - pathological at high z.

  7. Eigenmodes w0, wa makes for easiest, robust comparison. But sometimes want nonparametric form. Eigenmodes of w(z) give independent principal components(butdepend on model, experiment, and probe). Start with parameters of wi in z bins. Diagonalize Fisher matrix F=ETDE: D is diagonal, rows of E give eigenvectors. w(z) =  bi ei(z) Huterer & Starkman 2003 Huterer & Cooray 2005 Localized eigenmodes L=ETD1/2E

  8. Design an Experiment . Precision in measurement is not enough - one must beware degeneracies and systematics. Degeneracy: e.g. Aw0+Bwa=const Degeneracy: hypersurface, e.g. covariance with m p2 * or Systematic: floor to precision, e.g. calibration Systematic: offset error in data or model, e.g. evolution p1

  9. Mapping History Data over a range of redshifts can be effective at breaking degeneracies. Plus one gets leverage from a long baseline in expansion history.

  10. Controlling Systematics Controlling systematics is the name of the game. Finding more objects is not. Must understand the sources, instruments, and the theory interpretation. Forthcoming experiments may deliver 100,000s of objects. But uncertainties do not reduce by 1/N. Must choose cleanest probe, mature method, with multiple crosschecks.

  11. Complementarity • Complementarity of techniques (e.g. SN,WL,CMB,…) • improves precision • breaks degeneracies • immunizes against systematics

  12. Design an Experiment • How to design an experiment to explore dark energy? • Choose clear, robust, mature techniques • Rotate the contours thru choice of redshift span • Narrow the contours thru systematics control • Break degeneracies thru multiple probes

  13. Optimize an Experiment Optimization depends on the question asked. Recall that physics divided into 2 classes: thawing and freezing.

  14. Design an Experiment • How to design an experiment to explore dark energy? • Choose clear, robust, mature techniques • Rotate the contours thru choice of redshift span • Narrow the contours thru systematics control • Break degeneracies thru multiple probes With a strong experiment, we can even test the framework of physics.

  15. Acceleration = Curvature Height t´ t0 Time The Principle of Equivalence teaches that Acceleration = Gravity = Curvature Acceleration over time will get v=gh/c, so z = v/c = gh/c2 (gravitational redshift). But, tt0 parallel lines not parallel (curvature)!

  16. Finding Our Way in the Dark Dark energy is a completely unknown animal. A new theory or a new component? Track record: Inner solar system motions General Relativity Outer solar system motions Neptune Galaxy rotation curves Dark Matter

  17. Expansion History Observations that map out expansion history a(t), or w(a), tell us about the fundamental physics of dark energy. Alterations to Friedmann framework  w(a) Suppose we admit our ignorance: H2 = (8/3) m + H2(a) Effective equation of state: w(a) = -1 - (1/3) dln (H2) / dln a Modifications of the expansion history are equivalent to time variation w(a).Period. gravitational extensions or high energy physics

  18. Expansion History For modifications H2, define an effective scalar field with V = (3MP2/8) H2 + (MP2H02/16) [ d H2/d ln a] K = - (MP2H02/16) [ d H2/d ln a] Example:H2 = A(m)n w = -1+n Example:H2 = (8/3) [g(m) - m] w= -1 + (g-1)/[ g/m - 1 ]

  19. The world is w(z) Don’t care if it’s braneworld, cardassian, vacuum metamorphosis, chaplygin, etc. Simple, robust parametrization w(a)=w0+wa(1-a) Braneworld[DDG]vs.(w0,wa)=(-0.78,0.32) Vacuum metamorphvs.(w0,wa)=(-1,-3) Also agree on m(z) to 0.01 mag out to z=2

  20. Hidden Dimensions Extra dimensions have been used for unification in physics since the 1920s. Large extra dimensions -- braneworlds -- can be tested astronomically. Spacetime is warped by e-y as one moves a distance y off a brane. Think of the spacetime properties as an index of refraction: such a spatial gradient n localizes light (and the rest of physics). Electro-magnetism? Gravity?

  21. Warped Gravity Like localized light in a fiber optic, gravity will eventually leak off into hidden dimensions. Or think of a tuning fork: it radiates sound in all directions, but the waves are stronger if localized. On large (cosmological distances) there may be leaking gravity. The cosmic expansion would appear slower over these distances, i.e. accelerating today!

  22. DGP Braneworld Dynamics rc = MPl2/(2M53) rc = (2H0rc)-2 More than 3 from flatness SNAP could determinercto (rc)=0.003 Fairbairn & Goobar astro-ph/0511029

  23. Gravity Beyond 4D z=3 z=2 =1/2 z=1 =1 (BW) DGP Braneworld,and H mods,obey freezer dynamics in w-w Can reproduce expansion or growth with quintessence, but not both.

  24. 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. Linder 2004, Phys. Rev. D 70, 023511 cf. Lue, Scoccimarro, Starkman Phys. Rev. D 69 (2004) 124015 for braneworld perturbations

  25. Growth History Growth rate of density fluctuationsg(a)= (m/m)/a

  26. Physics of Growth Growthg(a)=(/)/adepends purely on the expansion history H(z) -- andgravity theory. 0 Expansion effects via w(z), but separate effects of gravity on growth. g(a) = exp { 0ad ln a [m(a)-1] } Growth index= 0.55+0.05[1+w(z=1)] Works to 0.05 – 0.2%!

  27. Growth and Expansion With  as free fit parameter, we can test framework, and the origin of dark energy. Keep expansion history as w(z), growth deviation from expansion by . Paradigm: To reveal the origin of dark energy, measure w, w, and . e.g. use SN+WL.

  28. Going Nonlinear Previousfit functions were only good to ~10% -- for . New technique is good to 1.5%, for general dark energy. Linder & White 2005 PRD 72, 061304(R) Efficient generation of grid of dark energy cosmologies.

  29. Gravity’s Bias Neglecting modified gravity will bias the cosmology unless gravity is properly accounted for (e.g. ). Huterer & Linder 2006

  30. Going Beyond Einstein To test Einstein gravity, we need growth and expansion. To test dark energy and GR, we need superb data. 9 parameter cosmology fit. Testing GR via growth index  degrades w0, wa by 15-25%.

  31. Fitting Beyond Einstein How well can we fit gravity? WL+SN+CMB can determine  to 8%. N.B. it’s important to include other effects on large scale structure such as m.

  32. Dark Energy Surprises • Dark energy is… • Dark • Smooth on cluster scales • Accelerating Maybe not completely! Clumpy in horizon? Maybe not forever! It’s not quite so simple! There is still much theoretical work needed!

  33. Heart of Darkness • Is dark energy dark – only interacts gravitationally? • Self interaction:pseudoscalar quintessence • Coupling to matter: Chaplygin gas • Leads to 5th 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 weff The horror! The horror! Sandvik et al. 2003

  34. Theory and Data Pinpointing Physics Is it?dynamics via w Checking Geometryallowing curvature Testing GRnew gravity Thanks to Gary Bernstein, Dragan Huterer, Masahiro Takada for key contributions

  35. Complementarity When you have a mystery ailment,you want a diagnosis with blood tests, EKG, MRI,... Cosmic acceleration is so revolutionary we needthecrosschecks, synergy, reduced influence of systematics,robust answersofcomplementary probes. SNAP space missiongives infrared and high redshift measurements, high resolution and lower systematics. SNAPwide field telescopegives multiple probes (e.g. SN Ia, Weak Lensing, Clusters, Strong Lensing, SN II) and rich astronomical resources.

  36. Frontiers of the Universe 1919 1998 Breakthrough of the Year 2003 Let’s find out! Cosmology holds the key to new physics in the next decade.

More Related