1. Hydrogen 21cm Cosmology Tzu-Ching Chang (ASIAA)
Ue-Li Pen, Kiyo Masui (CITA)
Jeff Peterson, Kevin Bandura, Tabitha Voytek, Aravind Natarajan (CMU)
Pat McDonald (LBNL)
Victor Liao (ASIAA)

2. The initial condition
Cosmic Microwave Background fluctuations of 10-5 at z~1100 (surface of last-scattering)
WMAP

4. HEDEX

5. Dark Energy Dominates the current energy budget of the Universe
Causes accelerated expansion
Modifies growth of structure
Unknown origins
Most prosaic “explanation” is a cosmological constant
Well established observationally (e.g. SNe)
A sensitive probe - Baryon Acoustic Oscillation

6. Baryon Acoustic Oscillations
Sound waves in the photon- baryon plasma in the early universe propagate from density perturbations; they freeze out when universe transited from radiation to matter domination (recombination).
Thus they have a characteristic scale of 100 h- 1Mpc (~150 Mpc), corresponding to the sound horizon at recombination at z~1100.
Courtesy of D. Eisenstein

7. Baryon Acoustic Oscillation
A more subtle feature when superposed on a complex density field

8. The oscillation peaks and troughs on the CMB power spectrum are obvious

9. More subtle on the galaxy distribution at late times (z~0.35)
Sloan Digital Sky Survey

10. It causes a slight peak in the galaxy correlation function (z~0.35)
Eisenstein+ 05

11. The oscillation peaks and troughs on the large-scale matter power spectrum
Reid+ 10

12. Why is Baryon Acoustic Oscillation (BAO) interesting?
It is a direct demonstration of the gravitational instability paradigm: a feature we see in the CMB 380,000 years after the Big Bang is also seen in an evolved state in the present- day Universe, 13.6 Gyrs after.
The scale of the feature is fixed: it is determined by the scale of the sound horizon at recombination, therefore by the physics in the early Universe. It is a “standard ruler”, and thus is a direct constraint on the geometry of the Universe.
We probe dark energy via its evolution on the expansion rate of the Universe. BAO and CMB, both standard rules, provide an excellent measurement of dark energy properties.

13. BAO - great tool for precision cosmology
Komatsu+ 08

14. BAO Measurements
BAO feature is present in the distribution of large-scale structure
Can be quantified by imprints on the large- scale matter power spectrum
Galaxies are tracers of large-scale structure; traditionally, we need to measure the 3D position of millions of galaxies in a redshift survey (e.g. SDSS)
Here we propose an alternative: 21-cm

15. 21cm Line
Picture from C. Hirata
Ground-state spin-flip hyperfine transition of neutral hydrogen
Hydrogen: most abundant element, optically thin
Line transition: Probe 3D structure of the Universe
Can be seen in absorption or emission against
the CMB, depending on the spin temperature:
Ts > Tcmb: emission (z < 10)
Ts < Tcmb: absorption ( ~15? < z <~ 150)
Brightness Temperature:
300

16. The 21cm universe LSS EOR HI 21cm radiation observable up to z~150
Up to 1016 modes to z~50(Hubble/Jeans)3
Physics: Lensing, gravity waves, primordial NG, BAO, AP (Pen 04, Loeb & Zaldarriaga 04, Lewis & Challinor 07, etc.)
Astrophysics: EoR, galaxy formation & evolution
Experiment Now
EoR: GMRT, LOFAR, MWA, PAPER, 21CMA
BAO: GBT, CRT, CHIME
EOR
LSS
SDSS
Tegmark & Zaldarriaga 08

17. 21cm Large-Scale Structure10 1 Z
M. White
LSS; BAO
EOR
Large-scale HI temperature fluctuation; CMB-like, in 3D
Observed frequency: f = 1420/(1+z) MHz
0.5<z<2.5, HI traces under-lying matter distribution, can be used to measure Baryon Acoustic Oscillations (100 Mpc scale) => dark energy
6<z<10, Epoch of Reionization, ~20-50 Mpc scale, HI shows tomographic history of reionization => astrophysics

18. 21cm emission on galaxy scales
Due to small emissivity, HI in emission is difficult to detect.
Previously, HI direct detection at z~0.2 (Verheijen et al 2007), stacking at z~0.3 (Lah et al. 2007); both on galaxy scales.

19. 21cm Intensity Mapping
“Intensity Mapping” (Chang et al 2008, Wyithe & Loeb 2008): instead of HI associated with galaxies, interested in HI associated with large-scale structure => measure the collective HI emission from a large region, more massive and luminous, without spatially resolving down to galaxy scales.
Measurement of spatially diffused spectral line, in the confusion-limited regime
Brightness temperature fluctuations on the sky: just like CMB temperature field, but in 3D
Low angular resolution redshift survey: economical

20. 21cm Observational Challenges:
RFI, Galactic Synchrotron foregrounds > 103 signal
HI content, distribution at high-z uncertain
Haslam Map at 408 MHz

21. Green Bank Telescope

22. Observing HI Large-scale Structure at GBT
Green Bank Telescope: 100 meter in diameter; largest steerable single dish
Observed at MHz (0.53<z<1.1) at two of the DEEP2 fields 2 x (2 x 0.5) deg2 for ~25 hours
DEEP2 survey: optical redshift survey by the Keck Telescope, ~50,000 redshifts 0.7<z<1.3
Cross-correlation of HI & optical, probing 0.53 < z < 1.1
Spatial resolution: Beam FWHM ~ 15’ => 9 h-1Mpc at z~0.8
Spectral resolution ~ 24 kHz, rebinned to ~500 kHz => 2 h- 1Mpc
Resolution element ~ (9 h-1Mpc)3

23. HI content at z=0.8 Cross-correlating GBT HI & DEEP2 optical galaxies at z ~ 0.7-1.1
GBT radio continuum sources + HI
GBT HI
(after SVD foreground subtraction)
DEEP2 density

24. Chang, Pen, Bandura, Peterson, in Nature 2010
HI content at z=0.8Cross-correlating GBT HI & DEEP2 optical galaxies at z ~
Measure HI & optical cross-correlation on 9 Mpc (spatial) x 2 Mpc (redshift) comoving scales
HI brightness temperature on these scales at z=0.8:
T = 157 ± 42 μK
ΩHI r b = (5.5 ± 1.5) x 10- 4
Highest-redshift detection of HI in emission at 4-sigma statistical significance.
Chang, Pen, Bandura, Peterson, in Nature 2010

25. Work in Progress: HI auto-correlation at z=0
Work in Progress: HI auto-correlation at z=0.8 Auto- & Cross-correlating GBT HI & zCOSMOS galaxies at z ~
GBT radio continuum sources + HI
GBT HI
(after SVD foreground subtraction)
zCOSMOS density

26. Next Step: HI Power spectrum at z~1
Masui et al.

27. Chang, Pen, Peterson, McDonald 2008
HI BAO Experiment Prospects
HI Intensity Mapping Experiment: 40,000m2 collecting area, 100 hrs of observation - competitive to DETF stage III experiment
Chang, Pen, Peterson, McDonald 2008

28. 21cm at z~1: current status
HI cross-correlation (with DEEP2 optical galaxies): measured at z~0.8: abundant HI at z~1; HI traces large-scale structure
HI auto-correlation at z~0.8: GBT on zCOSMOS field
HI large-scale structure redshift-space distortion: 50 deg, 300 hrs at GBT, observation and data analysis in progress (caution: foreground, calibration issues..)
Looking to build a survey instrument for Baryon Acoustic Oscillation measurement (e.g.,Chang et al. 08, Wyithe & Loeb 08, Seo et al. 10): large collecting area, compact configuration, wide-field survey (~104 m2) covering 0.5<z<2.5 ( MHz, df~0.5 MHz), resolution ~10’ (10 comoving Mpc)

29. Conclusion
BAO is a powerful tool for precision measurement of dark energy properties
21-cm line is a promising large-scale structure tracer at low redshifts, yielding BAO measurements
21-cm would be a good probe of the observable universe.