Dark Energy and Cosmic Sound Daniel Eisenstein (University of Arizona) Michael Blanton, David Hogg, Bob Nichol, Nikhil Padmanabhan, Will Percival, David Schlegel, Roman Scoccimarro, Ryan Scranton, Hee-Jong Seo, Ed Sirko, David Spergel, Max Tegmark, Martin White, Idit Zehavi, and the SDSS.

Summary  Sound waves that propagate in the first 400,000 years after the Big Bang imprint a signature that we can measure in the clustering of galaxies today: baryon acoustic oscillations.  This signature has a size that we can calculate accurately. Measuring this as an angle allows us to infer the distance to a sample of galaxies.  Measuring accurate distances is a key way to study the acceleration of the Universe and the properties of dark energy.  In the last 4 years, astronomers have detected this acoustic signature in the clustering of galaxies. Important tool to study cosmological composition.  Several new surveys being initiated to push the measurements to 1% and below.

Acoustic Oscillations in the CMB  Although there are fluctuations on all scales, there is a characteristic angular scale. Hot and cold spots tend to be about 1 degree in size.

Acoustic Oscillations in the CMB WMAP team (Bennett et al. 2003) Acoustic Peaks follow a harmonic pattern.

Sound Waves in the Early Universe Before recombination: Before recombination: Universe is ionized. Universe is ionized. Photons provide enormous pressure and restoring force. Photons provide enormous pressure and restoring force. Perturbations oscillate as acoustic waves. Perturbations oscillate as acoustic waves. After recombination: After recombination: Universe is neutral. Photons can travel freely past the baryons. Peturbations collapse due to gravity. Big Bang Today Recombination z ~ 1000 ~400,000 years Ionized Neutral Time

Sound Waves  Each initial overdensity (in DM & gas) is an overpressure that launches a spherical sound wave.  This wave travels outwards at 57% of the speed of light.  Pressure-providing photons decouple at recombination. CMB travels to us from these spheres.  Sound speed plummets. Wave stalls at a radius of 150 Mpc.  Overdensity in shell (gas) and in the original center (DM) both seed the formation of galaxies. Preferred separation of 150 Mpc. 150 Mpc (500 Mlyr)

Looking back in time in the Universe CREDIT: WMAP & SDSS websites CMB Looking back in time; angles imply distance GALAXIES

SDSS Galaxy Redshift Survey 200 kpc

Correlations of Galaxies CDM with baryons is a good fit:  2 = 16.1 with 17 dof. Pure CDM rejected at  2 = 11.7 Horizontal match of peak positions allows us to measure distance to 4%.

Chasing Sound Across Redshift Distance Errors versus Redshift

SDSS-III  SDSS-III will be the next phase of the SDSS project, operating from summer 2008 to summer  SDSS-III has 4 surveys on 3 major themes. BOSS: Largest yet redshift survey for large-scale structure. Definitive study of the low-redshift acoustic oscillations using 1.5 million galaxy redshifts. Goal: 1% measurement of cosmological distance. BOSS: Largest yet redshift survey for large-scale structure. Definitive study of the low-redshift acoustic oscillations using 1.5 million galaxy redshifts. Goal: 1% measurement of cosmological distance. SEGUE-2: Optical spectroscopic survey of stars, aimed at structure and nucleosynthetic enrichment of the outer Milky Way. SEGUE-2: Optical spectroscopic survey of stars, aimed at structure and nucleosynthetic enrichment of the outer Milky Way. APOGEE: Infrared spectroscopic survey of stars, to study the enrichment and dynamics of the whole Milky Way. APOGEE: Infrared spectroscopic survey of stars, to study the enrichment and dynamics of the whole Milky Way. MARVELS: Multi-object radial velocity planet search. MARVELS: Multi-object radial velocity planet search.  Using SDSS telescope, facilities, software.  Strong commitment to public data releases.  Collaboration is now forming. Seeking support from Sloan Foundation, DOE, NSF, and over 20 member institutions.

Conclusions  Acoustic oscillations provide a robust way to measure cosmological distance and hence probe dark energy.  SDSS LRG sample uses the acoustic signature to measure distance to 4% at one redshift.  New galaxy surveys in the coming decade will push to 1% and below over a range of redshift.  More information: Physics Today article by Daniel Eisenstein & Chuck Bennett, April Physics Today article by Daniel Eisenstein & Chuck Bennett, April

Response of a point perturbation Based on CMBfast outputs (Seljak & Zaldarriaga). Green’s function view from Bashinsky & Bertschinger Remember: This is a tiny ripple on a big background.