5Dark Energy Dominates the current energy budget of the Universe Causes accelerated expansionModifies growth of structureUnknown originsMost prosaic “explanation” is a cosmological constantWell established observationally (e.g. SNe)A sensitive probe - Baryon Acoustic Oscillation
6Baryon 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
7Baryon Acoustic Oscillation A more subtle feature when superposed on a complex density field
8The oscillation peaks and troughs on the CMB power spectrum are obvious
9More subtle on the galaxy distribution at late times (z~0.35) Sloan Digital Sky Survey
10It causes a slight peak in the galaxy correlation function (z~0.35) Eisenstein+ 05
11The oscillation peaks and troughs on the large-scale matter power spectrum Reid+ 10
12Why 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.
13BAO - great tool for precision cosmology Komatsu+ 08
14BAO MeasurementsBAO feature is present in the distribution of large-scale structureCan be quantified by imprints on the large- scale matter power spectrumGalaxies 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
1521cm LinePicture from C. HirataGround-state spin-flip hyperfine transition of neutral hydrogenHydrogen: most abundant element, optically thinLine transition: Probe 3D structure of the UniverseCan be seen in absorption or emission againstthe CMB, depending on the spin temperature:Ts > Tcmb: emission (z < 10)Ts < Tcmb: absorption ( ~15? < z <~ 150)Brightness Temperature:300
16The 21cm universe LSS EOR HI 21cm radiation observable up to z~150 Up to 1016 modes to z~50(Hubble/Jeans)3Physics: Lensing, gravity waves, primordial NG, BAO, AP (Pen 04, Loeb & Zaldarriaga 04, Lewis & Challinor 07, etc.)Astrophysics: EoR, galaxy formation & evolutionExperiment NowEoR: GMRT, LOFAR, MWA, PAPER, 21CMABAO: GBT, CRT, CHIMEEORLSSSDSSTegmark & Zaldarriaga 08
1721cm Large-Scale Structure 101ZM. WhiteLSS; BAOEORLarge-scale HI temperature fluctuation; CMB-like, in 3DObserved frequency: f = 1420/(1+z) MHz0.5<z<2.5, HI traces under-lying matter distribution, can be used to measure Baryon Acoustic Oscillations (100 Mpc scale) => dark energy6<z<10, Epoch of Reionization, ~20-50 Mpc scale, HI shows tomographic history of reionization => astrophysics
1821cm 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.
1921cm 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 regimeBrightness temperature fluctuations on the sky: just like CMB temperature field, but in 3DLow angular resolution redshift survey: economical
2021cm Observational Challenges: RFI, Galactic Synchrotron foregrounds > 103 signalHI content, distribution at high-z uncertainHaslam Map at 408 MHz
22Observing HI Large-scale Structure at GBT Green Bank Telescope: 100 meter in diameter; largest steerable single dishObserved at MHz (0.53<z<1.1) at two of the DEEP2 fields 2 x (2 x 0.5) deg2 for ~25 hoursDEEP2 survey: optical redshift survey by the Keck Telescope, ~50,000 redshifts 0.7<z<1.3Cross-correlation of HI & optical, probing 0.53 < z < 1.1Spatial resolution: Beam FWHM ~ 15’ => 9 h-1Mpc at z~0.8Spectral resolution ~ 24 kHz, rebinned to ~500 kHz => 2 h- 1MpcResolution element ~ (9 h-1Mpc)3
23HI content at z=0.8 Cross-correlating GBT HI & DEEP2 optical galaxies at z ~ 0.7-1.1 GBT radio continuum sources + HIGBT HI(after SVD foreground subtraction)DEEP2 density
24Chang, 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 scalesHI brightness temperature on these scales at z=0.8:T = 157 ± 42 μKΩHI r b = (5.5 ± 1.5) x 10- 4Highest-redshift detection of HI in emission at 4-sigma statistical significance.Chang, Pen, Bandura, Peterson, in Nature 2010
25Work 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 + HIGBT HI(after SVD foreground subtraction)zCOSMOS density
26Next Step: HI Power spectrum at z~1 Masui et al.
27Chang, Pen, Peterson, McDonald 2008 HI BAO Experiment ProspectsHI Intensity Mapping Experiment: 40,000m2 collecting area, 100 hrs of observation - competitive to DETF stage III experimentChang, Pen, Peterson, McDonald 2008
2821cm 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 structureHI auto-correlation at z~0.8: GBT on zCOSMOS fieldHI 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)
29ConclusionBAO is a powerful tool for precision measurement of dark energy properties21-cm line is a promising large-scale structure tracer at low redshifts, yielding BAO measurements21-cm would be a good probe of the observable universe.