1. The Accelerating Universe and the Sloan Digital Sky Survey Supernova Search
Jon Holtzman (NMSU) +
many collaborators (FNAL, U. Chicago, U. Washington, U. Penn., etc., etc.)

2. The Expanding Universe
Recession velocities of astronomical objects can be measured using the Doppler shift
Applied to galaxies, we find that all except the nearest galaxies are receding
Recession velocities are proportional to the distance to objects --> Hubble's law

3. Hubble's Law v = H d (locally)
To see that relation is linear only requires relative distances
To determine the Hubble constant (H = slope = current rate of expansion), requires absolute distance measurements
Hubble's law implies an expanding Universe

4. Cosmology and Einstein
Einstein's theory of general relativity combined with assumption of homogeneous and isotropic universe is consistent with an expanding Universe
Rate of expansion, however, changes with time depending on the contents of the Universe: how much matter/energy there is
With no matter, expansion rate is constant
With matter, the expansion rate slows down with time
Since Einstein didn't know about the expanding Universe, he also noted that an arbitrary term – the cosmological constant -- could be added to the equations to allow for a non-expanding Universe

5. that different models correspond to different ages of Universe
Expansion rate change with time for different cosmological models: note
that different models correspond to different ages of Universe
The figure above shows the scale factor vs time measured from the present for Ho = 71 km/sec/Mpc and for Ωo = 0 (green), Ωo = 1 (black), and Ωo = 2 (red) with no vacuum energy; the WMAP model with ΩM= 0.27 and ΩV = 0.73 (magenta); and the Steady State model with ΩV = 1 (blue). The ages of the Universe in these five models are 13.8, 9.2, 7.9, 13.7 and infinity Gyr. The recollapse of the Ωo = 2 model occurs when the Universe is 11 times older than it is now, and all observations indicate Ωo < 2, so we have at least 80 billion more years before any Big Crunch. (from Ned Wright's cosmology page).

6. The Accelerating Universe
Since we know there's matter in the Universe, everyone always expected that the rate of expansion has been decreasing; the big question was always how fast the deceleration was, whether it would be enough to cause an eventual recollapse of the Universe, and what the inferred age of the Universe was
But about ten years ago, observations of distant supernovae threw a very unexpected wrinkle into the picture

7. Supernovae as Cosmological Probes
Certain types of supernovae – type Ia --can be used as distance indicators
Out to intermediate redshift (z~1), SN are fainter than expected for decelerating (or even empty) Universe --> they are farther away, so Universe has been expanding faster than expected
Possible problem: are SN at earlier times intrinsically fainter? Or is there “gray” dust?
At highest redshifts (z>1), SN are brighter than expected --> probably rules out evolution.
Universe was decelerating a while ago

8. Cosmological parameters (1)
Supernovae constrain cosmological parameters via the redshift-distance relation
Supernovae by themselves indicate the need for acceleration, but don't constrain cosmological parameters uniquely
Multiple combinations of matter density and cosmological constant match SN data

9. Cosmological parameters (2)
Complementary constraints on cosmological parameters can come from observations of objects of known size, or alternatively, from statistical power at some known size, via the redshift-angular size relation
Such a size scale is imprinted on the matter/energy distribution because of acoustic oscillations in the growth of perturbations in the early Universe

10. Cosmological parameters: WMAP
Wilkinson Microwave Anisotropy Probe measures the cosmic microwave background
Angular power spectrum measures acoustic peaks at recombination
Size-redshift relation constrains cosmological parameters
Hubble constant measurements constrain things further

11. Cosmological parameters: BAO
Acoustic oscillations are also imprinted on the large scale galaxy distribution (baryon acoustic oscillations), since this evolves from the initial density perturbations
Feature in the galaxy power spectrum has been observed in SDSS galaxy sample (Eisenstein et al 2005); typical redshift z~0.35
Location of peak places strong constraint on matter density

12. Cosmological Parameters: summary
Constraints from:
Supernovae
WMAP
BAO
Hubble constant
All observations together lead to “concordance model”:

13. Dark energy What causes current acceleration?
For lack of knowledge, call it “dark energy”
Dark energy is usually parameterized by its equation of state:
Cosmological constant has w=-1 and unchanging: could result from vacuum energy but amplitude way off from simple expectations
Other models, e.g. quintessence, has w that varies with time
Major observational goal: measure w and its evolution !

14. The SDSS Supernova Survey: goals
Existing SN surveys have targetted either nearby or very distant SN
nearby SN via targetted galaxy search
distant SN via small field blind search
neither technique gets intermediate redshift objects
SDSS telescope/camera has very wide field, moderate depth --> ideally suited for intermediate redshift
Calibration uniformity is also an issue: cosmology results depend on comparing low and high redshift samples, which are taken with totally different instruments/techniques
SDSS bridges the gap
look for continuity in redshift-dist relation
uniform calibration
evolution of w

15. Supernovae as distance indicators
Several types of supernovae:
core collapse supernovae (type II, Ib, Ic)
binary star supernovae (type Ia)
None are standard candles; however, type Ia SN are “standardizable” based on light curve shape
Nagging problem: we don't exactly know what type Ia supernovae are!

16. SDSS SN search techniques
SDSS uses dedicated 2.5m telescope at Apache Point Observatory with very wide (corrected) field, very large format camera (30 science 2048x2048 CCDs)
SDSS drift scans across sky in 2.5 degree strip; two strips fill the stripe
SDSS SN survey looks at equatorial stripe during Sep-Nov , alternating strips each clear night: roughly 50 Gbytes per night

17. SDSS-SN Discovery
Candidate SN identified after subtracting template images taken earlier as part of main SDSS survey
Automatic and manual identification both play a part
Biggest contaminator is moving (solar system) objects: partly removed by time lag between filters!

18. SDSS-SN followup
Identification as type Ia supernovae requires spectroscopic followup
Candidates identified by color selection: very effective using 5 colors, 2 epochs (~90%)!

19. SDSS-SN followup spectroscopy
Multiple larger telescopes used for spectroscopic followup

20. SDSS-SN results 129 confirmed type Ia's from 2005, 193 more from 2006!
target redshift regime well sampled

21. Photometry results: lots of light curves!
Photometry extracted using “scene- modelling software developed at NMSU
Light curve fitting in progress using a variety of techniques
MLCS 2K2
Modifications for fitting in flux
Systematic effects being explored through Monte-Carlo

22. Photometry results: lots of light curves!
Light curve fitting in progress using a variety of techniques
MLCS 2K2
Modifications for fitting in flux
Systematic effects being explored through Monte-Carlo

23. SDSS-SN Cosmology No obvious departures from concordance cosmology
No discontinuity in Hubble relation

24. SDSS-SN Cosmology (2)
In conjunction with other measurements (e.g. BAO), should provide constrain on w at moderate redshift

25. Other projects: SN Ia rates
Understanding SNIa rates important for understanding of nature of Ia progenitors (which is important for using Ia's as cosmological probes!)
Rate measurement requires accurate understanding of experiment efficiency
detection efficiency obtained by inserting fake SN during initial selection
Sample efficiency from sophisticated light curve simulations
Total low redshift efficiency: / (stat) +/ (sys)
SDSS sample ideal: large numbers, blind search, well-defined (reasonably) sample definition
Will get better with 3 year sample; possible extension to higher z with “photometric Ias”

26. Other projects: photometry-only Ia's
Significantly more likely Ia light curves than those for which we have followup spectroscopy
Figuring out how to use these will help with cosmology statistics, rate evolution, etc.
Potential importance for future projects/missions

27. Other projects: host galaxies
Studying relationship of Ia properties to host galaxy properties may shed light on Ia progenitors and potential systematics
Large samples, but also spatial resolution, required
SDSS SN provides good sample
HST and SIRTF proposals for followup also submitted

28. Other projects: self-contained cosmology
Currently, Ia light curve training done from nearby sample, but this is non- homogeneous and may not have well defined photometry
Large sample of low-z SDSS SN may allow for self-consistent light curve training and application

29. Future directions
Complete 2005 SDSS analysis, prepare for full sample analysis
Variety of projects underway to understand and use type Ia SN
Note SN Factory and possible NMSU 1m contribution
Many new projects under development to contribute to understanding of dark energy
JDEM (Joint Dark Energy Mission): space mission
Mission concepts: SNAP, DESTINY, JEDI
DES (Dark Energy Survey)
SDSS AS2 (After Sloan 2) : one of the selected projects is a study that will find BAO at higher redshift