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High-redshift 21cm and redshift distortions

Explore the evolution from opaque to transparent universe & the impact of dark ages through 21cm observations & CMB temperature. Understand the information content compared to CMB and the complexities of observing baryons post-recombination. Learn about spin temperature, power spectrum, Boltzmann equation, redshift distortions, large-scale effects, and non-linear evolution at high redshifts.

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High-redshift 21cm and redshift distortions

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  1. High-redshift 21cm and redshift distortions Antony Lewis Institute of Astronomy, Cambridge http://cosmologist.info/ Work with: Anthony Challinor (IoA, DAMTP); astro-ph/0702600Richard Shaw (IoA), arXiv:0808.1724 Following work by Scott, Rees, Zaldarriaga, Loeb, Barkana, Bharadwaj, Naoz, Scoccimarro + many more …

  2. Evolution of the universe Opaque Easy Transparent Dark ages 30<z<1000 Hard Hu & White, Sci. Am., 290 44 (2004)

  3. CMB temperature

  4. Why the CMB temperature (and polarization) is great • Probes scalar, vector and tensor mode perturbations • The earliest possible observation (bar future neutrino anisotropy surveys etc…)- Includes super-horizon scales, probing the largest observable perturbations- Observable now Why it is bad - Only one sky, so cosmic variance limited on large scales - Diffusion damping and line-of-sight averaging: all information on small scales destroyed! (l>~2500)- Only a 2D surface (+reionization), no 3D information

  5. If only we could observe the CDM perturbations… - not erased by diffusion damping (if cold): power on all scales - full 3D distribution of perturbations What about the baryons? • fall into CDM potential wells; also power on small scales • full 3D distribution • but baryon pressure non-zero: very small scales still erased How does the information content compare with the CMB? CMB temperature, 1<l<~2000: - about 106 modes - can measure Pk to about 0.1% at l=2000 (k Mpc~ 0.1) Dark age baryons at one redshift, 1< l < 106: - about 1012 modes - measure Pk to about 0.0001% at l=106 (k Mpc ~ 100)

  6. What about different redshifts? • About 104 independent redshift shells at l=106 • - total of 1016 modes - measure Pk to an error of 10-8 at 0.05 Mpc scales e.g. running of spectral index: If ns = 0.96 maybe expect running ~ (1-ns)2 ~ 10-3Expected change in Pk ~ 10-3 per log k - measure running to 5 significant figures!? So worth thinking about… can we observe the baryons somehow?

  7. - after recombination, Hydrogen atoms in ground state and CMB photons have hν << Lyman-alpha frequency* high-frequency tail of CMB spectrum exponentially suppressed * essentially no Lyman-alpha interactions * atoms in ground state: no higher level transitions either • How can light interact with the baryons (mostly neutral H + He)? - Need transition at much lower energy* Essentially only candidate for hydrogen is the hyperfine spin-flip transition triplet singlet Credit: Sigurdson Define spin temperature Ts

  8. What can we observe? Spontaneous emission: n1 A10 photons per unit volume per unit proper time 1 h v = E21 Rate: A10 = 2.869x10-15 /s 0 Stimulated emission: net photons (n1 B10 – n0 B01)Iv Total net number of photons: In terms of spin temperature: Net emission or absorption if levels not in equilibrium with photon distribution - observe baryons in 21cm emission or absorption if Ts <> TCMB

  9. What determines the spin temperature? • Interaction with CMB photons: drives Ts towards TCMB • Collisions between atoms: drives Ts towards gas temperature Tg TCMB = 2.726K/a At recombination, Compton scattering makes Tg=TCMBLater, once few free electrons, gas cools: Tg ~ mv2/kB ~ 1/a2 Spin temperature driven below TCMB by collisions: - atoms have net absorption of 21cm CMB photons • (+Interaction with Lyman-alpha photons - not important during dark ages)

  10. What’s the power spectrum? Use Boltzmann equation for change in CMB due to 21cm absorption: Background: Perturbation: l >1 anisotropies in TCMB Fluctuation in density of H atoms,+ fluctuations in spin temperature Doppler shiftto gas rest frame CMB dipole seen by H atoms:more absorption in direction of gas motion relative to CMB + reionization re-scattering terms

  11. Solve Boltzmann equation in Newtonian gauge: Redshift distortions Main monopolesource Effect of localCMB anisotropy Sachs-Wolfe, Doppler and ISW change to redshift Tiny Reionization sources For k >> aH good approximation is (+re-scattering effects)

  12. 21cm does indeed track baryons when Ts < TCMB z=50 Kleban et al. hep-th/0703215 So can indirectly observe baryon power spectrum at 30< z < 100-1000 via 21cm

  13. Observable angular power spectrum: Integrate over window in frequency Small scales: 1/√N suppressionwithin window ‘White noise’from smaller scales Baryonpressuresupport baryon oscillations z=50

  14. What about large scales (Ha >~ k)? Narrow redshift window <~ 1% effect at l<50 Extra terms largely negligible

  15. New large scaleinformation?- potentials etccorrelated with CMB Dark ages~2500Mpc l ~ 10 14 000 Mpc z=30 Opaque ages ~ 300Mpc Comoving distance z~1000

  16. Non-linear evolution Small scales see build up of power from many larger scale modes - important But probably accurately modelled by 3rd order perturbation theory: On small scales non-linear effects many percent even at z ~ 50 + redshift distortions, see later.

  17. Also lensing Modified Bessel function Unlensed Lensed Wigner functions Lensing potential power spectrum Lewis, Challinor: astro-ph/0601594 c.f. Madel & Zaldarriaga: astro-ph/0512218

  18. like convolution with deflection angle power spectrum;generally small effect as 21cm spectrum quite smooth Cl(z=50,z=50) Cl(z=50,z=52) Lots of information in 3-D (Zahn & Zaldarriaga 2006)

  19. Observational prospects No time soon… - (1+z)21cm wavelengths: ~ 10 meters for z=50- atmosphere opaque for z>~ 70: go to the moon?- fluctuations in ionosphere: phase errors: go to moon?- interferences with terrestrial radio: far side of the moon?- foregrounds: large! use signal decorrelation with frequency But: large wavelength -> crude reflectors OK See e.g. Carilli et al: astro-ph/0702070, Peterson et al: astro-ph/0606104 Current 21cm: LOFAR, PAST, MWA: study reionization at z <20SKA: still being planned, z< 25

  20. Things you could do with precision dark age 21cm • High-precision on small-scale primordial power spectrum(ns, running, features [wide range of k], etc.)e.g. Loeb & Zaldarriaga: astro-ph/0312134,Kleban et al. hep-th/0703215 • Varying alpha: A10 ~ α13(21cm frequency changed: different background and perturbations)Khatri & Wandelt: astro-ph/0701752 • Isocurvature modes(direct signature of baryons; distinguish CDM/baryon isocurvature)Barkana & Loeb: astro-ph/0502083 • CDM particle decays and annihilations(changes temperature evolution)Shchekinov & Vasiliev: astro-ph/0604231, Valdes et al: astro-ph/0701301 • Primordial non-Gaussianity(measure bispectrum etc of 21cm: limited by non-linear evolution)Cooray: astro-ph/0610257, Pillepich et al: astro-ph/0611126 • Lots of other things: e.g. cosmic strings, warm dark matter, neutrino masses, early dark energy/modified gravity….

  21. Back to reality – after the dark ages? • First stars and other objects form • Lyman-alpha and ionizing radiation present:Wouthuysen-Field (WF) effect: - Lyman-alpha couples Ts to Tg- Photoheating makes gas hot at late times so signal in emissionIonizing radiation: - ionized regions have little hydrogen – regions with no 21cm signalBoth highly non-linear: very complicated physics • Lower redshift, so less long wavelengths:- much easier to observe! GMRT (z<10), MWA, LOFAR (z<20), SKA (z<25)…. • Discrete sources: lensing, galaxy counts (~109 in SKA), etc.

  22. How do we get cosmology from this mess? Would like to measure dark-matter power on nearly-linear scales. Want to observe potentials not baryons 1. Do gravitational lensing: measure source shears - probes line-of-sight transverse potential gradients (independently of what the sources are) 2. Measure the velocities induced by falling into potentials - probe line-of-sight velocity, depends on line-of-sight potential gradients Redshift distortions

  23. A closer look at redshift distortions Real space Redshift space y(z) y x x

  24. Density perturbed too. In redshift-space see + Both linear (+higher) effects – same order of magnitude. Note correlated. More power in line-of-sight direction -> distinguish velocity effect

  25. n Linear-theory Redshift-space distance: Actual distance: Define 3D redshift-space co-ordinate Transform using Jacobian: Redshift-space perturbation Fourier transformed:

  26. Linear power spectrum: Messy astrophysics Depends only on velocities -> potentials (n.k)4 component can be used for cosmology Barkana & Loeb astro-ph/0409572

  27. Is linear-theory good enough? Radial displacement ~ 0.1-1 MPc RMS velocity 10-4-10-3 Megaparsec scale Redshift space Real Space Looks like big non-linear effect! M(x + dx) ≠ M(x) + M’(x) dx BUT: bulk displacements unobservable. Need more detailed calculation.

  28. Non-linear redshift distortions Shaw & Lewis, 2008 also Scoccimarro 2004 Assume all fields Gaussian. Power spectrum from: Exact non-linear result (for Gaussian fields on flat sky):

  29. Significant effectDepending on angle Small scale power boosted by superposition of lots oflarge-scale modes z=10

  30. More important at lower redshift.Not negligible even at high z. Comparable to non-linear evolution z=10

  31. Similar effect on angular power spectrum: (A ~ velocity covariance), u= (x-z,y-z’) (sharp z=z’ window, z=10)

  32. What does this mean for component separation? Angular dependence now more complicated – all μ powers, and not clean. Assuming gives wrong answer… z=10 Need more sophisticated separation methods to measure small scales.

  33. Also complicates non-Gaussianity detectionRedshift-distortion bispectrum • Mapping redshift space -> real space nonlinear, so non-Gaussian Linear-theory source Just lots of Gaussian integrals (approximating sources as Gaussian).. Zero if all k orthogonal to line of sight. Can do exactly, or leading terms are: Also Scoccimarro et al 1998, Hivon et al 1995 Not attempted numerics as yet

  34. Conclusions • Huge amount of information in dark age perturbation spectrum- could constrain early universe parameters to many significant figures • Dark age baryon perturbations in principle observable at 30<z< 500 up to l<107 via observations of CMB at (1+z)21cm wavelengths. • Dark ages very challenging to observe (e.g. far side of the moon) • Easier to observe at lower z, but complicated astrophysics • Redshift-distortions probe matter density – ideally measure cosmology separately from astrophysics by using angular dependence • BUT: non-linear effects important on small scales - more sophisticated non-linear separation methods may be required

  35. Correlation function z=10

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