Introductory Review on BAO

Transcription

1 Introductory Review on BAO David Schlegel Lawrence Berkeley National Lab 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

2 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

3 The Acoustic Wave In the beginning (z>>1000)... The Universe at z>>1000 is a tightly coupled photon baryon plasma Consider a point overdensity - overpressured region drives out a sound wave travelling at 0.58c Baryons Photons Movie credit: Martin White

4 The Acoustic Wave Baryons Photons Movie credit: Martin White

5 The Acoustic Wave Keeps growing at 0.58c for ~ 100,000 years Baryons Photons Movie credit: Martin White

6 The Acoustic Wave At z~1100, the temperature drops sufficiently to let neutral hydrogen form (re-combination) Sound speed plummets, baryon wave stalls. Photons free stream away. 100 h -1 Mpc Baryons Photons Movie credit: Martin White

7 The Acoustic Wave on the surface of last scattering WMAP 1st-year map Fig credit: WMAP team and Tegmark, Oliveira-Costa, Hamilton (2003)

8 The Acoustic Wave At z~1100, the temperature drops sufficiently to let neutral hydrogen form. Sound speed plummets, baryon wave stalls. Photons free stream away. Baryons Photons Movie credit: Martin White

9 The Acoustic Wave Baryons Photons Movie credit: Martin White

10 The Acoustic Wave Photons are almost uniform. Baryons remain overdense in a 150 Mpc shell. The central overdensity starts to grow. Baryons Photons Movie credit: Martin White

11 The Acoustic Wave The final configuration is the central overdensity plus an echo at 150 Mpc. Initial conditions are a random field of overdensities -- observationally, statistical correlation between matter separated by 150 Mpc. Nonlinear processing of the matter broadens/shifts the field. Galaxy formation takes place in this background, but is a local process, and is essentially decoupled. First predictions: Peebles & Yu (1970), Sunyaev & Zel dovich (1970) Baryons Photons Movie credit: Martin White

12 The Acoustic Wave: What we expect to see Power spectra from galaxy surveys: Two possible standard rulers If the Universe were all baryons (huge wiggles) Closer to what we expect to see 1.) BAO peak: Sound horizon at recombinati Eisenstein & Hu ) P(k) turnover: Horizon size at matter-radiation equality

13 BAO: What tracer objects to use? ν ν ν ν ν ν ν ν z=10 11 Neutrino background (not for BAO ruler, but horizon at ν decoupling) z=1087 CMB: Planck will measure da to 0.1% z=20 H gas in 21-cm emission z=5 z=2 z=0 Ly-α emitter galaxies QSO absorption lines Galaxies, galaxy clusters, SNe, GRBs All existing BAO measurements Definitely the hard way, but it s been suggested!

14 BAO: What tracer objects to use? Important note: We need only sparsely sample these tracers Requirement: Sample linear modes at 100 h -1 Mpc Shot noise ~ (1 + 1/nP) P = power at 100 Mpc n = sampling density Shot noise small if np>3 n > 1 per (10 h -1 Mpc) 3 n If tracers are biased relative to dark matter, we need even fewer (because P>1) z=20 z=5 z=2 z=0 H gas in 21-cm emission Ly-α emitter galaxies QSO absorption lines Galaxies, galaxy clusters, SNe, GRBs

15 BAO: How does in measure dark energy? BAO is one of the Big 4 dark energy experimental techniques: Supernovae -- standard candle, 1 st results in 1998 Geometry only BAO -- standard ruler, 1 st results in 2005 Weak lensing Cluster counts Geometry + grav. growth BAO at CMB BAO from galaxies w Dark energy constraints from ESSENCE experiment (supernovae) + Sloan Digital Sky Survey (BAO) Wood-Vasey et al S N eia BAO S N eia+bao ! M

16 Acoustic Waves are Standard Candles Rulers What we like... Like supernovae, a geometrical probe of the expansion rate (and dark energy) The acoustic oscillation scale depends on the sound speed and the propagation time Anchored at recombination (z=1088) by the CMB Orientation of ruler provides two different probes Transverse rulers probes DA(z) Line of sight rulers probe H(z) These depend on the matter-to-radiation ratio (Ωmh 2 ) and the baryon-to-photon ratio (Ωbh 2 ) Only need to make 3D maps (angles + redshifts) What we don t like... Ruler is inconveniently long 150 Mpc = 450 million light years Statistical measure of a small signal Requires mapping millions of objects There is a cosmic variance limit... once we reach that, we re done!

17 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

18 BAO observations from 3-D maps: Earlier maps of the Universe were simply too small! BAO scale of 150 Mpc CfA2 redshift survey (Geller & Huchra 1989) Formally, this could measure BAO with a ~0.05σ detection

19 BAO observations from 3-D maps: Earlier maps of the Universe were simply too small! BAO scale of 150 Mpc CfA2 redshift survey (Geller & Huchra 1989) Formally, this could measure BAO with a ~0.05σ detection That s not even a detection for astronomers :)

20 BAO observations from 3-D maps: SDSS Finally technologically possible Sloan Digital Sky Survey (SDSS) telescope Optical design for large focal plane: 7 deg 2 Gigapixel imaging cameras: SDSS has 120 million pixels Fiber-fed spectrographs: 640 redshifts simultaneously SDSS camera SDSS telescope, Apache Point, New Mexico

21 BAO observations from 3-D maps: SDSS SDSS spectroscopic follow-up turns 2D 3D map SDSS main galaxy survey ~1 million galaxies Too little volume for BAO

22 BAO observations from 3-D maps: Need tracers at higher z Luminous red galaxies (LRGs) are the brightest + reddest galaxies in the Universe Easily seen to z=0.6 in SDSS (most luminous 1% of galaxies)

23 BAO observations from 3-D maps: SDSS SDSS luminous red galaxies (LRGs) Sparse sampled at 10-4 galaxies/mpc 3 47,000 galaxies over 4000 deg 2 by ,000 galaxies over 8000 deg 2 by July 2008

24 BAO observations from 3-D maps: SDSS First detection SDSS luminous red galaxies (LRGs) Sparse sampled at 10-4 galaxies/mpc 3 47,000 galaxies over 4000 deg 2 by ,000 galaxies over 8000 deg 2 by July 2008 BAO peak Eisenstein et al 2005

25 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

26 BAO observations from 2-D maps: photo-z surveys Photo-z smearing ~0.03 on same scale as BAO BAO precision badly compromised: DETF figure-of-merit reduced by 5X Degradation along line-of-sight, H(z) Degradation perpendicular, DA Best existing photo-z s spectroscopic redshifts DETF FoM 5X worse

27 BAO observations from 2-D maps: SDSS photo-z s Luminous red galaxies (LRGs) are the brightest + reddest galaxies in the Universe Easily seen to z=0.6 in SDSS (brightest 1% of galaxies) Photo-z == Use colors to estimate the redshift (no spectroscopy) (This is becoming very popular)

28 BAO observations from 2-D maps: SDSS photo-z s Luminous red galaxies (LRGs) are the brightest + reddest galaxies in the Universe Easily seen to z=0.6 in SDSS (brightest 1% of galaxies) > These are old elliptical galaxies > Uniform spectra w/ strong 4000 Å > Easy to select in images > Accurate photo-z s from colors > Rare... but enough for BAO work 4000 Angstrom break

29 BAO observations from 2-D maps: SDSS photo-z s Luminous red galaxies (LRGs) are the brightest + reddest galaxies in the Universe Easily seen to z=0.6 in SDSS (brightest 1% of galaxies) How do we do this? Use colors to select LRGs Approximate redshift with color (photo-z) Divide into redshift slices Compute 2D power spectrum on each slice Complications: Photometric calibrations + Galactic extinction known to ~1% Contamination from stars Redshift errors (from photo-z s) must be small + well-understood Must train the photo-z s with fair sample of spectra! Only angular (da) measure of BAO; radial is smoothed out

30 BAO observations from 2-D maps: SDSS photo-z s First detection SDSS luminous red galaxies (LRGs) Sparse sampled at 10-4 galaxies/mpc 3 47,000 galaxies over 3700 deg ,000 galaxies over 8500 deg 2 by 2008 SDSS photo-z 600,000 LRGs over 3,500 deg 2 BAO peak Padmanabhan et al 2007

31 BAO observations: Recent compilation of all 3-D maps (Percival et al 2007) z=0.2 ΛCDM Ωm=1,Λ=0 Ωm=0.3,Λ=0 z=0.35 Ratio of BAO at z=0.35 / z=0.2 Some tension between BAO at z=0.2 and z=0.35 (2σ) SDSS SNe survey spans same z range... those results imminently Final SDSS BAO results expected end of 2008

32 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

33 BAO future experiments: What s possible? SNe as standard candles improve as N If we wait 10,000 years, there will be a SN in every galaxy. Systematic errors in SN brightness will dominate. BAO standard rulers hit a cosmic variance limit, much as the CMB Limited number of modes to measure in any redshift slice. Once measured over the observable Universe, no improvement possible (except by waiting 10 billion years to see more Universe!) (N. Padmanabhan, priv. comm.) BAO cosmic variance limit per Δz=0.2 over 10,000 deg Higher z == more volume == better BAO!! H(z) DA(z)

34 BOSS == Baryon Oscillation Spectroscopic Survey at SDSS telescope Two simultaneous spectroscopic surveys from BAO from 1.5 million galaxies at z=0.3, 0.6 BAO from 160,000 QSOs at 2.2<z<3 SDSS main galaxy survey ~1 million galaxies Too little volume for BAO SDSS luminous red galaxies (LRGs) Sparse sampled at 10-4 galaxies/mpc 3 47,000 galaxies by ,000 galaxies by deg 2 (finish in 2008) BOSS red galaxies 10,000 deg 2 5x sample density (shot noise) 2x volume Turn this photo-z sample spectro-z

35 BOSS == Baryon Oscillation Spectroscopic Survey at SDSS telescope Two simultaneous spectroscopic surveys from BAO from 1.5 million galaxies at z=0.3, 0.6 BAO from 160,000 QSOs at 2.2<z<3 Ideal 3D power (perfectly sampled) Simulation of the IGM (R. Cen) Neutral H in 25 h -1 Mpc box Sampling noise n=surface density of lines of sight (analogous to galaxy shot noise) Resolution Detector noise Lyα forest in SDSS QSO spectrum at z=3.7

36 SDSS Hardware upgrades Largest field-of-view of any large telescope -- DONE! End-to-end simulations: Galaxy spectrum at z= small-core fibers to replace existing (more objects, less sky contamination) Software development underway at LBL, Princeton, NYU Replace red CCDs w/red-sensitive LBL/SNAP CCDs, making it possible to go to higher-z Swap gratings for VPH Replace blue CCDs w/uv-sensitive e2v CCDs, making it possible for Lyα at z=2.3 3

37 BOSS: sampling the cosmic density field w/ galaxies SDSS-I and SDSS-II BOSS M. White simluation: A slice 500 h -1 Mpc across and 10 h - Mpc thick at z=0.5

38 BOSS: sampling the cosmic density field w/ QSO skewers SDSS-I and SDSS-II BOSS M. White Simulation: 500 h -1 Mpc box

39 BOSS == Baryon Oscillation Spectroscopic Survey at SDSS telescope Two simultaneous spectroscopic surveys from BAO from 1.5 million galaxies at z=0.3, 0.6 BAO from 160,000 QSOs at 2.2<z<3 (N. Padmanabhan, priv. comm.) BAO cosmic variance limit per Δz=0.2 over 10,000 deg H(z) DA(z) BOSS will be near cosmic-variance limit for z<0.7 Could improve by 2 by repeating in Southern sky An equivalent photo-z BAO survey would require 50,000 deg 2

40 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

41 BAO observations: Future experiments 2-D BAO from photo-z s Possible to control photometry to ~0.5% level Easy to select LRGs (luminous red galaxies) Numerous enough to reach cosmic variance limit Photo-z s accurate to <3% to (at least z=0.6) Dark Energy Survey (DES) Pan-STARRS-4 More future: SNAP/DUNE, LSST Always 5X worse than cosmic variance limit for same volume of Universe BOSS galaxies BAO cosmic variance limit per Δz=0.2 over 10,000 deg H(z) DA(z) 3-D BAO from spectro-z s It s only money: $50 per spectrum from SDSS Next-generation: WiggleZ == at Anglo-Australian (2dF upgrade) BOSS == at SDSS-III HETDEX == at Hobby Eberly Telescope PAU == Is 40-filter photo-z == spectroscopic?? More future: WFMOS, Adept, SKA

42 Current + proposed spectroscopic BAO experiments SDSS & SDSS-II ARC 2.5-m, 3º FOV 640 fibers deg 2 <z>=0.35 BOSS LRGs ARC 2.5-m, 3º FOV 1000 fibers ,000 deg 2 <z>=0.35, 0.6 BOSS QSOs ARC 2.5-m, 3º FOV 1000 fibers ,000 deg 2 <z>=2.5 LAMOST Chinese 6-m, 2º FOV 4000 fibers?????? <z>=0.7 AAOmega LRG AAT 4-m, 2º FOV 400 fibers Rejected AAOmega WiggleZ AAT 4-m, 2º FOV 400 fibers (200 nights) 1,000 deg 2 <z>=0.8 HETDEX Hobby Eberly 11-m 200 IFUs??? 200 deg 2 z=2 3.8 FMOS Subaru 8.4-m, 0.5º FOV 200 fibers (200 nights) 200 deg 2 <z>=1.4 WFMOS (previously KAOS) Subaru 8.4-m, 1º FOV ~3000 fibers 2014? (120 nights) 1000 deg 2 <z>=1 WFMOS (previously KAOS) Subaru 8.4-m, 1º FOV ~3000 fibers 2014? (60 nights) 150 deg 2 <z>=3

43 1. What are BAO? How does it measure dark energy? 2. Current observations From 3-D maps From 2-D maps (photo-z) 3. Future experiments SDSS-III BOSS: Baryon Oscillation Spectroscopic Survey Other experiments: 3-D and 2-D Cosmic variance limit + other complications

44 BAO observations: complications!? What may prevent us from reaching the cosmic variance limit? 1. Redshift-space distortions 2. Mass not tracing light 3. At late times, non-linear growth of structure Large N-body simulations needed to address Effects most important at low z Can we predict or calibrate to < 0.5%? Angulo et al 2007

45 Baryon acoustic oscillations (BAO) are a rapidly maturing method for measuring the cosmological distance scale and dark energy. Highly robust. Trigonometric method. Complementary to supernova cosmology The future? Push BAO to z>0.7 Introductory Review on BAO Conclusions BOSS (on SDSS telescope) will be the definitive low-redshift data point, reaching near the cosmic variance limit. BOSS == Dedicated BAO experiment Nearly cosmic-variance limited measure at z<0.7 First measurement at z=2.5 using QSOs No other spectro surveys planned with Area > 200 deg 2!! What is the systematics floor on da? 1%? 0.1%? Large computation problem! Requires >5 Gpc 3 simulations Many photo-z experiments in construction: Photo-z in the optical -- limit at z<1.5 Necessarily 5X worse / volume Higher z == more volume == better BAO!! BOSS galaxies BAO cosmic variance limit per Δz=0.2 over 10,000 deg H(z) DA(z)