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Lensing of the CMB. Antony Lewis Institute of Astronomy, Cambridge http://cosmologist.info/. Talk based on recent review – this is recommended reading and fills in missing details and references :. Physics Reports review: astro-ph/0601594.

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Lensing of the CMB


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    1. Lensing of the CMB Antony Lewis Institute of Astronomy, Cambridge http://cosmologist.info/

    2. Talk based on recent review – this is recommended reading and fills in missing details and references : Physics Reports review: astro-ph/0601594

    3. Recent papers of interest (since review):- Cosmological Information from Lensed CMB Power Spectra; Smith et al. astro-ph/0607315 • For introductory material on unlensed CMB, esp. polarization see Wayne Hu’s pages athttp://background.uchicago.edu/~whu/Anthony Challinor’s CMB introduction: astro-ph/0403344

    4. Outline • Review of unlensed CMB • Lensing order of magnitudes • Lensed power spectrum • CMB polarization • Non-Gaussianity • Cluster lensing • Moving lenses • Reconstructing the potential • Cosmological parameters

    5. Evolution of the universe Opaque Transparent Hu & White, Sci. Am., 290 44 (2004)

    6. Observation as a function of frequencyBlack body spectrum observed by COBE Residuals Mather et al 1994 - close to thermal equilibrium: temperature today of 2.726K ( ~ 3000K at z ~ 1000 because ν ~ (1+z))

    7. (almost) uniform 2.726K blackbody Dipole (local motion) O(10-5) perturbations (+galaxy) Observations: the microwave sky today Source: NASA/WMAP Science Team

    8. Can we predict the primordial perturbations? • Maybe.. Inflation make >1030 times bigger Quantum Mechanics“waves in a box” calculationvacuum state, etc… After inflation Huge size, amplitude ~ 10-5

    9. Perturbation evolution photon/baryon plasma + dark matter, neutrinos Characteristic scales: sound wave travel distance; diffusion damping length

    10. Observed ΔT as function of angle on the sky

    11. CMB power spectrum Cl • Use spherical harmonics Ylm Observe: Theory prediction - variance (average over all possible sky realizations)- statistical isotropy implies independent of m

    12. CMB temperature power spectrumPrimordial perturbations + later physics diffusion damping acoustic oscillations primordial powerspectrum finite thickness Hu & White, Sci. Am., 290 44 (2004)

    13. Perturbations O(10-5) Calculation of theoretical perturbation evolution Simple linearized equations are very accurate (except small scales)Fourier modes evolve independently: simple to calculate accurately Physics Ingredients • Thomson scattering (non-relativistic electron-photon scattering) - tightly coupled before recombination: ‘tight-coupling’ approximation (baryons follow electrons because of very strong e-m coupling) • Background recombination physics • Linearized General Relativity • Boltzmann equation (how angular distribution function evolves with scattering) linearized GR + Boltzmann equations Initial conditions + cosmological parameters Cl

    14. Sources of CMB anisotropy Sachs Wolfe: Potential wells at last scattering cause redshifting as photons climb out Photon density perturbations: Over-densities of photons look hotter Doppler: Velocity of photon/baryons at last scattering gives Doppler shiftIntegrated Sachs Wolfe: Evolution of potential along photon line of sight: net red- or blue-shift as photon climbs in an out of varying potential wellsOthers: Photon quadupole/polarization at last scattering, second-order effects, etc.

    15. Temperature anisotropy data: WMAP 3-year + smaller scales BOOMERANG Hinshaw et al + many more coming up e.g. Planck (2008)

    16. What can we learn from the CMB? • Initial conditionsWhat types of perturbations, power spectra, distribution function (Gaussian?); => learn about inflation or alternatives.(distribution of ΔT; power as function of scale; polarization and correlation) • What and how much stuffMatter densities (Ωb, Ωcdm);; neutrino mass(details of peak shapes, amount of small scale damping) • Geometry and topologyglobal curvature ΩK of universe; topology(angular size of perturbations; repeated patterns in the sky) • EvolutionExpansion rate as function of time; reionization- Hubble constant H0 ; dark energy evolution w = pressure/density(angular size of perturbations; l < 50 large scale power; polarization) • AstrophysicsS-Z effect (clusters), foregrounds, etc.

    17. CMB summary • Time from big bang to last scattering (~300Mpc comoving; ~300 000 years) – determines physical size of largest overdensity (or underdensity) • Distance of last scattering from us(~14Gpc comoving; 14 Gyr)- determines angular size seen by us • Damping scale (angular size ~ arcminutes) - determines smallest fluctuations (smooth on small scales) • Thickness of last scattering (~Hubble time, 100Mpc)- determines line of sight averaging- determines amount of polarization (see later) • Other parameters - determine amplitude and scale dependence of perturbations

    18. Lensing of the CMB Last scattering surface Inhomogeneous universe - photons deflected Observer

    19. Not to scale!All distances are comoving largest overdensity ~200/14000 ~ degree Ionized plasma - opaque Neutral gas - transparent Recombination ~200Mpc 14 000 Mpc ~100Mpc Good approximation: CMB is single source plane at ~14 000 Mpc

    20. Zeroth-order CMB • CMB uniform blackbody at ~2.7 K(+dipole due to local motion) 1st order effects • Linear perturbations at last scattering, zeroth-order light propagation; zeroth-order last scattering, first order redshifting during propagation (ISW)- usual unlensed CMB anisotropy calculation • First order time delay, uniform CMB- last scattering displaced, but temperature at recombination the same - no observable effect

    21. 1st order effects contd. • First order CMB lensing: zeroth-order last scattering (uniform CMB ~ 2.7K), first order transverse displacement in light propagation A B Number of photons before lensing---------------------------------------------Number of photons after lensing A2 ---- B2 Solid angle before lensing ----------------------------------- Solid angle after lensing = = Conservation of surface brightness: number of photons per solid angle unchanged uniform CMB lenses to uniform CMB – so no observable effect

    22. 2nd order effects • Second order perturbations at last scattering, zeroth order light propagation-tiny ~(10-5)2 corrections to linear unlensed CMB result • First order last scattering (~10-5 anisotropies), first order transverse light displacement- this is what we call CMB lensing • First order last scattering (~10-5 anisotropies), first order time delay- delay ~1MPc, small compared to thickness of last scattering- coherent over large scales: very small observable effect • Others e.g. Rees Sciama: second (+ higher) order reshiftingSZ: second (+higher) order scattering, etc…. Hu, Cooray: astro-ph/0008001

    23. CMB lensing order of magnitudes (set c=1) Ψ β Newtonian argument: β = 2 Ψ General Relativity: β = 4 Ψ (β << 1) Potentials linear and approx Gaussian: Ψ~ 2 x 10-5 β ~ 10-4 Characteristic size from peak of matter power spectrum ~ 300Mpc Comoving distance to last scattering surface ~ 14000 Mpc total deflection ~ 501/2 x 10-4 pass through ~50 lumps ~ 2 arcminutes assume uncorrelated (neglects angular factors, correlation, etc.)

    24. So why does it matter? • 2arcmin: ell ~ 3000- on small scales CMB is very smooth so lensing dominates the linear signal • Deflection angles coherent over 300/(14000/2) ~ 2°- comparable to CMB scales- expect 2arcmin/60arcmin ~ 3% effect on main CMB acoustic peaks NOT because of growth of matter density perturbations!

    25. Comparison with galaxy lensing • Single source plane at known distance(given cosmological parameters) • Statistics of sources on source plane well understood- can calculate power spectrum; Gaussian linear perturbations- magnification and shear information equally useful - usually discuss in terms of deflection angle; - magnification analysis of galaxies much more difficult • Hot and cold spots are large, smooth on small scales- ‘strong’ and ‘weak’ lensing can be treated the same way: infinite magnification of smooth surface is still a smooth surface • Source plane very distant, large linear lenses- lensing by under- and over-densities; • Full sky observations- may need to account for spherical geometry for accurate results

    26. Lensed temperature depends on deflection angle Newtonian potential co-moving distance to last scattering Lensing Potential Deflection angle on sky given in terms of angular gradient of lensing potential c.f. introductory lectures

    27. Power spectrum of the lensing potential • Expand Newtonian potential in 3D harmonics with power spectrum Angular correlation function of lensing potential:

    28. jl are spherical Bessel functions Use Orthogonality of spherical harmonics (integral over k) then gives Then take spherical transform using Gives final general result

    29. Deflection angle power spectrum On small scales (Limber approx) Deflection angle power ~ Non-linear Linear Deflections O(10-3), but coherent on degree scales  important! Computed with CAMB: http://camb.info

    30. Lensing potential and deflection angles LensPix sky simulation code: http://cosmologist.info/lenspix

    31. Note: can only observe lensed field • Any bulk deflection is unobservable – degenerate with corresponding change in unlensed CMB:e.g. rotation of full sky translation in flat sky approximation • Observations sensitive to differences of deflection angles

    32. Correlation with the CMB temperature very small except on largest scales

    33. Calculating the lensed CMB power spectrum • Approximations and assumptions: - Lensing potential uncorrelated to temperature - Gaussian lensing potential and temperature - Statistical isotropy • Simplifying optional approximations - flat sky - series expansion to lowest relevant order

    34. Unlensed temperature field in flay sky approximation • Fourier transforms: • Statistical isotropy: where So Similarly for the lensing potential (also assumed Gaussian and statistically isotropic)

    35. Lensed field: series expansion approximation (BEWARE: this is not a very good approximation! See later) Using Fourier transforms, write gradients as Then lensed harmonics then given by

    36. Lensed field still statistically isotropic: with Alternatively written as where (RMS deflection ~ 2.7 arcmin) Second term is a convolution with the deflection angle power spectrum - smoothes out acoustic peaks- transfers power from large scales into the damping tail

    37. Lensing effect on CMB temperature power spectrum

    38. Small scale, large l limit: - unlensed CMB has very little power due to silk damping: - Proportional to the deflection angle power spectrum and the (scale independent) power in the gradient of the temperature

    39. Accurate calculation -lensed correlation function • Do not perform series expansion Lensed correlation function: Assume uncorrelated To calculate expectation value use

    40. where Have defined: small correction from transverse differences - variance of the difference of deflection angles

    41. So lensed correlation function is Expand exponential using Integrate over angles gives final result: Note exponential: non-perturbative in lensing potential

    42. Power spectrum and correlation function related by used Bessel functions defined by Can be generalized to fully spherical calculation: see review, astro-ph/0601594 However flat sky accurate to <~ 1% on the lensed power spectrum

    43. Series expansion in deflection angle? Only a good approximation when: - deflection angle much smaller than wavelength of temperature perturbation - OR, very small scales where temperature is close to a gradient CMB lensing is a very specific physical second order effect; not accurately contained in 2nd order expansion – differs by significant 3rd and higher order terms Series expansion only good on large and very small scales

    44. Other specific non-linear effects • Thermal Sunyaev-ZeldovichInverse Compton scattering from hot gas: frequency dependent signal • Kinetic Sunyaev-Zeldovich (kSZ)Doppler from bulk motion of clusters; patchy reionization;(almost) frequency independent signal • Ostriker-Vishniac (OV)same as kSZ but for early linear bulk motion • Rees-SciamaIntegrated Sachs-Wolfe from evolving non-linear potentials: frequency independent

    45. Summary so far • Deflection angles of ~ 3 arcminutes, but correlated on degree scales • Lensing convolves TT with deflection angle power spectrum - Acoustic peaks slightly blurred - Power transferred to small scales large scales small scales

    46. Lensing important at 500<l<3000Dominated by SZ on small scales

    47. Thomson Scattering Polarization W Hu

    48. CMB Polarization Generated during last scattering (and reionization) by Thomson scattering of anisotropic photon distribution Hu astro-ph/9706147

    49. Observed Stokes’ Parameters - - Q U Q → -Q, U → -U under 90 degree rotation Q → U, U → -Q under 45 degree rotation Measure E field perpendicular to observation direction nIntensity matrix defined as Linear polarization + Intensity + circular polarization CMB only linearly polarized. In some fixed basis

    50. Alternative complex representation e.g. Define complex vectors And complex polarization Under a rotation of the basis vectors - spin 2 field all just like the shear in galaxy lensing