Presentation on theme: "High-redshift 21cm and redshift distortions Antony Lewis Institute of Astronomy, Cambridge Anthony Challinor (IoA, DAMTP); astro-ph/0702600."— Presentation transcript:
High-redshift 21cm and redshift distortions Antony Lewis Institute of Astronomy, Cambridge Anthony Challinor (IoA, DAMTP); astro-ph/ Richard Shaw (IoA), arXiv: Following work by Scott, Rees, Zaldarriaga, Loeb, Barkana, Bharadwaj, Naoz, Scoccimarro + many more … Work with:
Hu & White, Sci. Am., (2004) Evolution of the universe Opaque Transparent Easy Hard Dark ages 30
Why the CMB temperature (and polarization) is great Why it is bad - 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 - 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
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
About 10 4 independent redshift shells at l= total of modes - measure P k to an error of at 0.05 Mpc scales What about different redshifts? e.g. running of spectral index: If n s = 0.96 maybe expect running ~ (1-n s ) 2 ~ Expected change in P k ~ per log k - measure running to 5 significant figures!? So worth thinking about… can we observe the baryons somehow?
How can light interact with the baryons (mostly neutral H + He)? - 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 - Need transition at much lower energy * Essentially only candidate for hydrogen is the hyperfine spin-flip transition Credit: Sigurdson triplet singlet Define spin temperature T s
Spontaneous emission: n 1 A 10 photons per unit volume per unit proper time Rate: A 10 = 2.869x /s Stimulated emission: net photons (n 1 B 10 – n 0 B 01 ) I v 0 1 h v = E 21 Net emission or absorption if levels not in equilibrium with photon distribution - observe baryons in 21cm emission or absorption if T s <> T CMB What can we observe? In terms of spin temperature: Total net number of photons:
What determines the spin temperature? Interaction with CMB photons: drives T s towards T CMB Collisions between atoms: drives T s towards gas temperature T g (+Interaction with Lyman-alpha photons - not important during dark ages) T CMB = 2.726K/a At recombination, Compton scattering makes T g =T CMB Later, once few free electrons, gas cools: T g ~ mv 2 /k B ~ 1/a 2 Spin temperature driven below T CMB by collisions: - atoms have net absorption of 21cm CMB photons
Whats the power spectrum? Use Boltzmann equation for change in CMB due to 21cm absorption: Background: Perturbation: Fluctuation in density of H atoms, + fluctuations in spin temperature CMB dipole seen by H atoms: more absorption in direction of gas motion relative to CMB l >1 anisotropies in T CMB Doppler shift to gas rest frame + reionization re-scattering terms
Solve Boltzmann equation in Newtonian gauge: Redshift distortions Main monopole source Effect of local CMB anisotropy Tiny Reionization sources Sachs-Wolfe, Doppler and ISW change to redshift For k >> aH good approximation is (+re-scattering effects)
21cm does indeed track baryons when T s < T CMB So can indirectly observe baryon power spectrum at 30< z < via 21cm z=50 Kleban et al. hep-th/
Observable angular power spectrum: White noise from smaller scales Baryon pressure support 1/N suppression within window baryon oscillations z=50 Integrate over window in frequency Small scales:
What about large scales (Ha >~ k)? <~ 1% effect at l<50 Extra terms largely negligible Narrow redshift window
Mpc Dark ages ~2500Mpc Opaque ages ~ 300Mpc Comoving distance z=30 z~1000 New large scale information? - potentials etc correlated with CMB l ~ 10
Non-linear evolution Small scales see build up of power from many larger scale modes - important But probably accurately modelled by 3 rd order perturbation theory: On small scales non-linear effects many percent even at z ~ 50 + redshift distortions, see later.
Also lensing Lensing potential power spectrum Lensed Unlensed Lewis, Challinor: astro-ph/ Modified Bessel function Wigner functions c.f. Madel & Zaldarriaga: astro-ph/
like convolution with deflection angle power spectrum; generally small effect as 21cm spectrum quite smooth C l (z=50,z=52)C l (z=50,z=50) Lots of information in 3-D (Zahn & Zaldarriaga 2006)
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 See e.g. Carilli et al: astro-ph/ , Peterson et al: astro-ph/ But: large wavelength -> crude reflectors OK Current 21cm: LOFAR, PAST, MWA: study reionization at z <20 SKA: still being planned, z< 25
Things you could do with precision dark age 21cm High-precision on small-scale primordial power spectrum (n s, running, features [wide range of k], etc.) e.g. Loeb & Zaldarriaga: astro-ph/ , Kleban et al. hep-th/ Varying alpha: A 10 ~ α 13 (21cm frequency changed: different background and perturbations) Khatri & Wandelt: astro-ph/ Isocurvature modes (direct signature of baryons; distinguish CDM/baryon isocurvature) Barkana & Loeb: astro-ph/ CDM particle decays and annihilations (changes temperature evolution) Shchekinov & Vasiliev: astro-ph/ , Valdes et al: astro-ph/ Primordial non-Gaussianity (measure bispectrum etc of 21cm: limited by non-linear evolution) Cooray: astro-ph/ , Pillepich et al: astro-ph/ Lots of other things: e.g. cosmic strings, warm dark matter, neutrino masses, early dark energy/modified gravity….
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 T s to T g - Photoheating makes gas hot at late times so signal in emission Ionizing radiation: - ionized regions have little hydrogen – regions with no 21cm signal Both 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 (~10 9 in SKA), etc.
How do we get cosmology from this mess? 1. Do gravitational lensing: measure source shears - probes line-of-sight transverse potential gradients (independently of what the sources are) Would like to measure dark-matter power on nearly-linear scales. Want to observe potentials not baryons 2. Measure the velocities induced by falling into potentials - probe line-of-sight velocity, depends on line-of-sight potential gradients Redshift distortions
A closer look at redshift distortions x y x y(z) Real spaceRedshift space
+ Density perturbed too. In redshift-space see More power in line-of-sight direction -> distinguish velocity effect Both linear (+higher) effects – same order of magnitude. Note correlated.
Linear-theory Redshift-space distance: Actual distance: n Define 3D redshift-space co-ordinate Fourier transformed: Transform using Jacobian: Redshift-space perturbation
Linear power spectrum: Messy astrophysicsDepends only on velocities -> potentials (n.k) 4 component can be used for cosmology Barkana & Loeb astro-ph/
Is linear-theory good enough? RMS velocity Radial displacement ~ MPc Megaparsec scale Looks like big non-linear effect! Real Space Redshift space BUT: bulk displacements unobservable. Need more detailed calculation. M(x + dx) M(x) + M(x) dx
Non-linear redshift distortions Power spectrum from: Exact non-linear result (for Gaussian fields on flat sky): Assume all fields Gaussian. Shaw & Lewis, 2008 also Scoccimarro 2004
Significant effect Depending on angle z=10 Small scale power boosted by superposition of lots of large-scale modes
More important at lower redshift. Not negligible even at high z. Comparable to non-linear evolution z=10
Similar effect on angular power spectrum: (sharp z=z window, z=10) (A ~ velocity covariance), u= (x-z,y-z)
What does this mean for component separation? Angular dependence now more complicated – all μ powers, and not clean. Assuming gives wrong answer… Need more sophisticated separation methods to measure small scales. z=10
Also complicates non-Gaussianity detection Redshift-distortion bispectrum Mapping redshift space -> real space nonlinear, so non-Gaussian Just lots of Gaussian integrals (approximating sources as Gaussian).. Linear-theory source Can do exactly, or leading terms are: Not attempted numerics as yet Zero if all k orthogonal to line of sight. Also Scoccimarro et al 1998, Hivon et al 1995Scoccimarro
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