““The Dark Side of The Dark Side of the SDSS”the SDSS”
Bob NicholBob Nichol
ICG, PortsmouthICG, Portsmouth
Thanks to all my collaborators on SDSS
and other teams
OutlineOutline
• A A veryvery brief overview of Dark Energy brief overview of Dark Energy• A A veryvery brief overview of the SDSS brief overview of the SDSS• SDSS searches for the “Dark Side”SDSS searches for the “Dark Side”
SDSS SNeSDSS SNe ISW effectISW effect Cosmic magnificationCosmic magnification Baryon acoustic oscillations (BAO)Baryon acoustic oscillations (BAO)
• Future experimentsFuture experiments
WMAP: Universe at 380,000 yrs
Largest oscillations that are causally connected
(DARK) MATTER
(DA
RK
) EN
ER
GY
CMB
SN
SNe and CMB force us into a Universe ~75% DE and ~25% DM. But is this true?What is DE?
Understanding Dark EnergyUnderstanding Dark Energy(The billion dollar question)
To confirm DE we need to observe it in as many ways as possible, but there are only two broad avenues:• Geometrical tests (distances, volumes)• Growth of structure (cluster counts)
To determine what DE is, we can make progress on two simple questions:• Is DE just a cosmological constant (w(z)=-1)?
(Push observations to higher redshifts)• Is DE a new form of stress-energy with
negative effective pressure or a breakdown of General
Relativity at large distances? (Study DE using different probes)As we don’t know much, all observations are important
Fairbairn & Goobar 2005
DGP model for 5D gravity
Also, Sawicki & Carroll (2005) & Koyama (2006) show there are noticeable differences in the evolution of structure in DGP models. This maybe testable!
DGP CosmologiesDGP Cosmologies
SDSSSDSS
DR4: 849k spectra, 6670 sq degs
Done 07/2005: ~700,000 redshifts, 8000 sq degs
Extension (2005-2008): Legacy, SNe, Galaxy
• Type Ia supernovae (SNe)Type Ia supernovae (SNe)
• spectroscopically confirm spectroscopically confirm and obtain “well-measured” and obtain “well-measured” light curves of ~200 SN Ia light curves of ~200 SN Ia from z = 0.05 to ~ 0.4from z = 0.05 to ~ 0.4
• bridge low-z (z<0.05; LOSS, bridge low-z (z<0.05; LOSS, SNF) and high-z (0.3<z<1.0; SNF) and high-z (0.3<z<1.0; ESSENCE, SNLS) sourcesESSENCE, SNLS) sources
• understand and minimize understand and minimize systematics of SN Ia as systematics of SN Ia as distance indicatorsdistance indicators
• SN Ib/c, II, rare typesSN Ib/c, II, rare types
• Other transientsOther transients
SDSSII SNe SurveyExploring DE & SNe at an epoch when DE dominates
Riess et al. (2004)compilation
Astier et al. (2005)
Use the SDSS 2.5m telescopeUse the SDSS 2.5m telescope• September 1 - November 30 of 2005-2007September 1 - November 30 of 2005-2007• Scan 300 square degrees of the sky every 2 daysScan 300 square degrees of the sky every 2 days• discover supernovae and obtain multi-color light discover supernovae and obtain multi-color light
curvescurves
Survey AreaSurvey Area
N S
ARCHETMDMWHT
Subaru(NTT)
Follow-up
• Color-type SN candidates using nightly g r i data:
• make template light curves from multi-epoch spectra (Peter Nugent) and other sets of spectra of well-observed historical SNe (SUSPECT database)
• Ia, Ia-pec, II-P, II-L, IIb, Ibc, Ibc-hypernova
• fit for redshift, extinction, stretch for Ia
• Able to type with >90% efficiency after ~2 - 4 epochs
Photometric TypingPhotometric Typing
Ia Ia II
SN2005hy
II
Team of 15 “hand-scanner” visually inspected 144,000 Team of 15 “hand-scanner” visually inspected 144,000 objects selecting nearly 10k SN targets!objects selecting nearly 10k SN targets!
• 126 spectroscopically confirmed SN Ia
• 13 spectroscopically probable SN Ia
• 6 SN Ib/c (3 hypernovae)
• 10 SN II (4 type IIn)• 5 AGN• ~hundreds of other
unconfirmed SNe with good light curves (galaxy spectroscopic redshifts measured for ~25 additional Ia candidates)
• Focused primarily on Ia
Results from 2005Results from 2005
<z> = 0.21
2005
spe
ctro
scop
ical
ly c
onfi
rmed
+ p
roba
ble
SN
Ia
PreliminaPreliminaryry
No reddeningNo reddening=0.27=0.27
Important training set for next Important training set for next generation of imaging surveys e.g. generation of imaging surveys e.g. DES will detect 3000 SN by ~2010DES will detect 3000 SN by ~2010
w) = 0.1 from w) = 0.1 from SDSS+ESSENCE+WMAP+LSS (statistical SDSS+ESSENCE+WMAP+LSS (statistical
errors only, constant w, flat Universe)errors only, constant w, flat Universe) SDSS data on host galaxies will allow SDSS data on host galaxies will allow study the scattering in this relation in study the scattering in this relation in
greater detailgreater detail
Lambda = 0.74
Late-time Integrated Late-time Integrated Sachs Wolfe (ISW) EffectSachs Wolfe (ISW) Effect DE also effects the growth of structure i.e. Poisson DE also effects the growth of structure i.e. Poisson
equation with dark energy:equation with dark energy:
In a flat, matter-dominated universe (CMB tells us this), In a flat, matter-dominated universe (CMB tells us this), then density fluctuations grow as:then density fluctuations grow as:
Therefore, for a flat geometry, changes in the Therefore, for a flat geometry, changes in the gravitational potential are a direct physical gravitational potential are a direct physical measurement of Dark Energy as they should be non-measurement of Dark Energy as they should be non-evolving if DE=0evolving if DE=0
[ ])(4' 12DEma
d
dGk δρδρη
π +−=Φ −
€
δρm ∝ a
Experimental Set-upExperimental Set-up
See also: Nolta et al, Boughn and Crittenden, Myers et al, Ashfordi et See also: Nolta et al, Boughn and Crittenden, Myers et al, Ashfordi et alal
WMAP vs SDSSWMAP vs SDSS
WMAP W band temperatures across 50% of SDSS areaWMAP W band temperatures across 50% of SDSS area
Density of Luminous Red Galaxies (LRGs) selected from the SDSSDensity of Luminous Red Galaxies (LRGs) selected from the SDSS
ISW and the SDSSISW and the SDSS Searching for a detectionSearching for a detection
LRG selection to z~0.8 LRG selection to z~0.8 (Eisenstein et al. 2001)(Eisenstein et al. 2001)
5300 sq degrees 5300 sq degrees Achromatic (no contamination)Achromatic (no contamination) Errors from 5000 CMB skiesErrors from 5000 CMB skies Compared to a null resultCompared to a null result
>95% for all samples>95% for all samples Low redshift sample Low redshift sample
contaminated by starscontaminated by stars Individually >2Individually >2 per redshift per redshift
sliceslice 4 redshift shells (not 4 redshift shells (not
significant overlap)significant overlap)
Yellow: “smoothed clean”, Black: “Clean”, Red: Q, Blue: W, Green: V Overall, we have detected signal at 5
ISW PredictionsISW Predictions
Halo model. Halo model. Biasing of Biasing of b=1,2,3 & 4 for LRGsb=1,2,3 & 4 for LRGs
Plus SZ on small scalesPlus SZ on small scales Data prefers DE model Data prefers DE model
over null hypothesis at over null hypothesis at the >99% confidence the >99% confidence for all combinationsfor all combinations
The measurement is The measurement is very sensitive to n(z) very sensitive to n(z) assumed and assumed and mm
Scranton et al 2003Scranton et al 2003
Future ISW directionsFuture ISW directions dg/dz is a very powerful probedg/dz is a very powerful probe Only probe of DE clustering (Hu & Only probe of DE clustering (Hu &
Scranton 2004; Pogosian 2004) and Scranton 2004; Pogosian 2004) and highly complementary to geometrical highly complementary to geometrical measures of DE (SNe etc)measures of DE (SNe etc)
Circa 2006 (SDSS)Circa 2006 (SDSS) 8000 sq degrees 8000 sq degrees
(≥3(≥3 per redshift) per redshift) Tighter redshift intervals Tighter redshift intervals
(> 5 bins)(> 5 bins) BeyondBeyond
ASTRO-F all-sky out to z~1.5ASTRO-F all-sky out to z~1.5(>4.5(>4.5 detection if there!) detection if there!)
UKIDSS+VISTA all-sky (LRG UKIDSS+VISTA all-sky (LRG selection to z>1)selection to z>1)
QSO catalogs (z out to 3)QSO catalogs (z out to 3) Cooray, Huterer, Baumann 2003
Dark Energy Survey Dark Energy Survey (DES)(DES)
5000 sq deg multiband survey of SGP using CTIO, 40 sq deg time domain search for SNe The survey will study the expansion history of the universe and
the growth of density perturbations using four distinct techniques: 1. 4000 sq deg survey in collaboration with the SPT2. weak lensing study3. galaxy angular power spectrum distance measurement study4. SNe Ia distance measurement study
Each will independently constrain the dark energy eqn of state ~10%
ISW with DES+Planck is as good as SNAP for non-constant w (Pogosian et
al. 2005)
Cosmic MagnificationCosmic Magnification
Gravitational magnification increases flux received from galaxies and hence allows us to see fainter galaxies, resulting in an increased apparent galaxy number
density. But, it also magnifies the solid angle of the projected lensed sky which results in a decrease in the apparent galaxy number density. Therefore a
competition between the two!
more flux
more solid angle
more sources come in than diluted
less source
come in than
diluted
Effe
cts
can
cel
Hunting for quasarsHunting for quasarsQuasi-stellar sources: by definition they look like stars!
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Traditional approaches have used UVX approach to finding quasars, i.e., quasars are “very blue” so can be isolated in color-color space using simple hyper-planes (see Richards et al. 2002). However, there is significant contamination (~40%), thus demanding spectroscopic follow-up which is very time-consuming.
Probabilistic approachProbabilistic approach Use Kernel Density Estimation (KDE) to map color-color space
occupied by known stars and quasars (“training sets”) Use cross-validation to “optimal” smooth the 4-D SDSS color
space and obtain PDFs Fast implementation via KD-trees (Gray & Moore) ~16,000 known quasars and ~500000 stars Using a non-parametric Bayes classifier (NBC)
additional cut
95% complete95% pure
Stars
QSOs
F stars
Blue points = dataBlack line = best fitRed line = best fit + alphaGrey shading = 1sigma
195,000 quasars13.5 million
galaxies
8 detection
Now fully consistent with LCDM
Baryon OscillationBaryon Oscillation
• Gravity squeezes the gas, pressure pushes back! Gravity squeezes the gas, pressure pushes back! They oscillateThey oscillate
•When the Universe cools below 3000K these When the Universe cools below 3000K these sound waves are frozen in sound waves are frozen in
Courtesy of Wayne Hu
Cosmic Microwave Cosmic Microwave BackgroundBackground
Effect of this sound wave already discovered in relic light of the early universe That was the Universe at 400,000 years. Can we see these sound waves today?
700,000 light years
500 Million Light Years
Credit: SDSSA slice of the SDSS
The Correlation FunctionThe Correlation Function
The correlation function is the probability of finding pairs at a given separation, above that of a random distribution.
Excess of galaxies separated by 500 million
light years
What does it mean?What does it mean? We have detected the sound wave in the Universe at two very different epochs (400,000 yrs after Big Bang and present-day). This is important because our theory of gravitational structure formation predicts that such features should have been preserved. Detecting the sound wave in the galaxies is the “SMOKING GUN” that our theory is correct. Better yet, the sound wave is an object of fixed size, a “standard ruler” or “cosmic yardstick”. This means that we can measure its apparent size anywhere in the Universe, and determine how far it is away because we know its true size.
FLAT GEOMETRYCREDIT: WMAP & SDSS websites
CM
B
Looking back in time in the Universe
FLAT GEOMETRY
SD
SS
GA
LAX
IES
Looking back in time in the Universe
FLAT GEOMETRYCREDIT: WMAP & SDSS websites
SD
SS
GA
LAX
IES
CM
B
Looking back in time in the Universe
OPEN GEOMETRY
Looking back in time in the Universe
FLAT GEOMETRYCREDIT: WMAP & SDSS websites
CM
B
Looking back in time in the Universe
CLOSED GEOMETRY
SD
SS
GA
LAX
IES
UNIVERSE IS FLAT TO 1% UNIVERSE IS FLAT TO 1% PRECISIONPRECISION
WFMOSWFMOSA quantum leap in spectroscopic efficiency.
Thousands of fibres over a 1.5 degree field-of-view on an 8-meter class telescope: over an order of over an order of magnitude increase in mapping efficiency of 2dFmagnitude increase in mapping efficiency of 2dF
z~1 survey with 2 million
galaxies with twice LRG
volume
1% accuracy
KAOS purple book (Seo, Eisenstein, Blake, Glazebrook)
Will get w to <5% and w’ to <20%
ConclusionsConclusions SDSS continues for 3 more years and has been successful in SDSS continues for 3 more years and has been successful in
finding many hundreds of SNe.finding many hundreds of SNe.
The quality and quantity of SDSS data has provide several The quality and quantity of SDSS data has provide several complementary detections of dark energy and dark matter, complementary detections of dark energy and dark matter, e.g., the ISW effect provided direct physical evidence that DE e.g., the ISW effect provided direct physical evidence that DE existsexists
Detected cosmic magnification and consistent with LCDM. Detected cosmic magnification and consistent with LCDM. Powerful new probe of the UniversePowerful new probe of the Universe
SDSS has detected the baryon oscillations in the local SDSS has detected the baryon oscillations in the local Universe, the “missing link” between CMB and LSS. Now have Universe, the “missing link” between CMB and LSS. Now have a “standard ruler”a “standard ruler”