1. Moscow 2004 21cm Cosmology Collaborators: Collaborators: Rennan Barkana, Stuart Wyithe, Matias Zaldarriaga Avi Loeb Harvard University

3. Ionization History of Hydrogen REDSHIFT TIME 1000 6 Million years Billion years

4. Emergence of the First Star Clusters molecular hydrogen Yoshida et al. 2003

5. Hydrogen spin e- p p p Ground level Spin Temperature excitation rate= (atomic collisions)+(radiative coupling to CMB) Couple to Couples to Predicted Observed Predicted by Van de Hulst in 1944; Observed by Ewen &Purcell in 1951 at Harvard

6. 21 cm Absorption by Hydrogen Prior to Structure Formation Loeb & Zaldarriaga, Phys. Rev. Lett., 2004; Scott & Rees, MNRAS, 1990 Observed wavelength=21cm (1+z)  3D tomography (slicing the universe in redshift ) Fluctuations in 21cm brightness are sourced by fluctuations in gas density

7. Largest Data Set on the Sky Loeb & Zaldarriaga, Phys. Rev. Lett., 2004; astro-ph/0312134 Number of independent patches: while Silk damping limits the primary CMB anisotropies to only Noise due to foreground sky brightness:

8. Line-of-Sight Anisotropy of 21cm Flux Fluctuations terms allow separation of powers Barkana & Loeb, astro-ph/0409572; see also Bharadwaj & Ali, astro-ph/0401206 Peculiar velocity changes  Power spectrum is not isotropic (“Kaiser effect”) observer

9. Enhancement of 21cm Fluctuations During the Period of Initial Lya Coupling of the spin T to the kinetic T Fluctuations in the distribution of galaxies induce fluctuations in Lya intensity  21cm 21cm Lya Wouthuysen-Field Effect horizon Rapid evolution in retarded time Barkana & Loeb, astro-ph/0410129 n=1 n=2 saturated

10. Enhancement of 21cm Fluctuations During the Epoch of Initial Lya Coupling Sourced by fluctuations in Lya intensity from galaxies Power spectrum of 21cm brightness fluctuations Poisson fluctuations in the number of galaxies Biased fluctuations in the density of galaxies

11. redshift z=200z=30 Temperature Thermal History atomic collisions Radiative coupling to CMB Wouthuysen-Field Effect X-ray heating Sources of 21cm fluctuations Sources of 21cm fluctuations Density inhomogeneties (Loeb & Zaldarriaga 04) and peculiar velocities (Barkana & Loeb 04) Ionized bubbles (Madau,Meiksin &Rees 1997;Furlanetto et al. 2004; Gnedin & Shaver 2003) Emision from mini-halos (Iliev, Shapiro, et al. 2002) Fluctuations in Lya flux, and gas temperature (Barkana & Loeb 2004)

12. History of 21cm Brightness Fluctuations HI appears in absorption against the CMB X-ray heating to Lya coupling of spin temperature to kinetic temperature Reionization: HI gradually disappears HI appears in emission against the CMB

13. Slicing Swiss Cheese 21cm Tomography of Ionized Bubbles During Reionization is like Slicing Swiss Cheese Observed wavelength  distance H II H II H I

14. Experiments MWA (Mileura Wide-Field Array) * MWA (Mileura Wide-Field Array) MIT/ATNF/CfA LOFAR (Low-frequency Array) * LOFAR (Low-frequency Array) Netherlands PAST (Primeval Structure Telescope) * PAST (Primeval Structure Telescope) China/CITA SKA (Square Kilometer Array) * SKA (Square Kilometer Array) Enhanced VLA * Enhanced VLA CfA/NRAO

15. Mileura Wide-Field Array A very radio quiet site Antenna design MIT, ATNF, U. Melbourne, CfA

16. Characteristics of MWA demonstrator 500 16-element antennas in a compact 1.5km array Wide 80-300 MHz frequency range Digital receiver and filter chain, 32 MHz at 16 kHz resolution Full cross-correlation of all 500 antennas, 20-30 degrees field of view

17. Primary challenge: foregrounds Terrestrial: radio broadcasting Galactic synchrotron emission Extragalactic: radio sources ( Di-Matteo et al. 2004 ) Although the sky brightness (>10K) is much larger than the 21cm signal ( 10K) is much larger than the 21cm signal (<10mK), the foregrounds have a smooth frequency dependence while the signal fluctuates rapidly across small shifts in frequency (=redshift). Preliminary estimates indicate that the 21cm signal is detectable with the forthcoming generation of low-frequency arrays ( Zaldarriaga et al. astro-ph/0311514; Morales & Hewitt astro-ph/0312437 )

18. When was the Universe Reionized?

19. Empirical Hints

20. First Year Data from WMAP Polarization/temperature correlation implies an electron-scattering optical depth after cosmological recombination at z=1088 of: Implying that the universe was reionized at Only 200 million years after the big bang Kogut et al. 2003

21. Cosmic Hydrogen was significantly Neutral at z~6.3 Wyithe & Loeb, Nature, 2004; astro-ph/0401188 Size of HII region depends on neutral fraction of IGM prior to quasar activity and quasar age Ionization(Stromgren) sphere of quasar line of sight R(t)

22. The Earliest Quasar Detected: z=6.4 Fan et al. 2002

23. Likelihood for Neutral Fraction at z~6.3 SDSS J1148+5251 Wyithe & Loeb, astro-ph/0401188; Wyithe, Loeb, & Carilli, astro-ph/0411625

24. Imaging the Neutral Fraction in 21cm 21 cm absorption of CMB or 21 cm emission 21 cm absorption of CMB if IGM is cold or 21 cm emission due to heating by X-ray background or quasar X-rays (but no Lyman-alpha heating by the quasar) 21 cm Wyithe and Loeb 2004; astro-ph/0401554 Tozzi, Madau, Meiksin, & Rees 2000 quasar CMB Brightness wavelength Warm HI X-ray heated by quasar reionization Cold HI Warm HI ~100 x relic shells per known quasar

25. Emission from an X-ray Heated IGM Wyithe & Loeb 2004; astro-ph/0401554

26. The Characteristic Size of Ionized Bubbles at the End of Reionization Wyithe & Loeb, Nature, 2004 Fig. 1 SBO: Surface of Bubble Overlap SLT: Surface of Lya Transmission

27. Causality:Causality: crossing time of bubble by a 21cm photon. Cosmic variance:Cosmic variance: cosmic scatter in the formation time of ionized bubbles due to large-scale inhomogeneities SDSS quasars at z>6.1 SLT HI photon

28. (i) Should we trust numerical simulations or analytic models?

29. Cooling Rate of Primordial Gas n=0.045 cm^-3 Atomic cooling H_2 cooling

30. Virial Temperature of Halos 1-sigma2-sigma 3-sigma Atomic cooling H_2 cooling

31. Unusually Large Fluctuations in the Statistics of Galaxy Formation at High Redshifts Barkana & Loeb, ApJ, 2003; astro-ph/0310338 L Bias in numerical simulations:

33. Summary Redshifted 21cm from 30<z<200 provides the largest data set on the initial conditions of the universe Peculiar velocities provide additional power and a test for the cosmological origin of the 21cm signal The size of the ionized bubbles at the end of reionization was ~10 Mpc Foreground removal is very challenging