Transcript Dark Energy and Cosmic Sound

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
Acoustic Peaks follow
a harmonic pattern.
WMAP team (Bennett et al. 2003)
Sound Waves in the Early Universe
Before recombination:


Universe is ionized.
Photons provide enormous
pressure and restoring force.
Perturbations oscillate as
acoustic waves.
Ionized



Universe is neutral.
Photons can travel freely
past the baryons.
Peturbations collapse due
to gravity.
Recombination
z ~ 1000
~400,000 years
Time
Neutral
Today
Big Bang

After recombination:
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.
QuickTime™ and a
GIF decompressor
are needed to see this picture.
150 Mpc
(500 Mlyr)
CREDIT: WMAP & SDSS websites
CMB
GALAXIES
Looking
back
in time
in the
Universe
Looking
back
in time;
angles
imply
distance
200 kpc
SDSS
Galaxy
Redshift
Survey
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Correlations of Galaxies
Horizontal match of peak
positions allows us to
measure distance to 4%.
CDM with baryons is a good fit:
c2 = 16.1 with 17 dof.
Pure CDM rejected at Dc2 = 11.7
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 2014.
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.
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.
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:



http://cmb.as.arizona.edu/~eisenste/acousticpeak
Physics Today article by Daniel Eisenstein & Chuck
Bennett, April 2008.
http://www.sdss3.org/
Response of a point perturbation
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Remember: This is a tiny
ripple on a big background.
Based on CMBfast outputs (Seljak &
Zaldarriaga). Green’s function view
from Bashinsky & Bertschinger 2001.