1. Expansion history of the Universe as seen by supernovae Bruno Leibundgut European Southern Observatory

2. Cosmological SN results so far Acceleration confirmed with all data sets based on more than 200 SNe Ia Consistency with flat geometry No further constraints yet on w possibility to measure time dependence of ω at the moment very limited all analyses with constant ω Systematics remain the main issue

3. Concordance ΩΛΩΛ ΩMΩM No Big Bang Empty Universum Einstein – de Sitter Lambda-dominated Universe Concordance Cosmology

4. The distant SN Hubble diagram Riess et al. 2004

5. The new challenge Ideally we would like to derive the expansion history H(z) directly (Equation of state parameter ω can substitute for the cosmological constant) with a luminosity distance (here already w=const.) assumes Ω tot =1 Miknaitis et al., in prep.

6. ESSENCE World-wide collaboration to find and characterise SNe Ia with 0.2 < z < 0.8 Search with CTIO 4m Blanco telescope Spectroscopy with VLT, Gemini, Keck, Magellan Goal: Measure distances to 200 SNe Ia with an overall accuracy of 5%  determine ω to 10% overall

7. SNLS – The SuperNova Legacy Survey World-wide collaboration to find and characterise SNe Ia with 0.2 < z < 0.8 Search with CFHT 4m telescope Spectroscopy with VLT, Gemini, Keck, Magellan Goal: Measure distances to 1000 SNe Ia with an overall accuracy of 5%  determine ω to 7% overall

8. Supernova Classification

9. Supernova models

10. Type Ia Supernovae Explosion physics relatively well understood significant progress in the past decade (especially at MPA) Radiation transport remains a big problem simplifications can provide new insight into the explosion models progress in the ab initio calculations as well –however, missing information in the atomic transitions

11. Thermonuclear Supernovae White dwarf in a binary system Growing to the Chandrasekhar mass (M Chand =1.4 M  ) by mass transfer from a nearby star The “standard model” © ESA

12. How well do we understand SNe Ia? Ejecta masses and also nickel masses are not uniform Peculiar SNe Ia with super-Chandrasekhar mass (M ejecta =2.2M  ?) Howell et al. 2006 Stritzinger et al. 2006

13. The nearby SN Ia sample and Hubble’s law Evidence for good distances

14. Are SNe Ia good distance indicators? Yes! normalisation through the light curve shape –still problems with methods! Hubble diagram of nearby SNe Ia peak luminosities of nearby supernovae

15. Essence Survey Goal: ω Monte Carlo of Special attention to systematics single photometric system calibrate zero points understand filter transformations (K-corrections) spectroscopic classification

16. A joint analysis, including results from Supernovae, CMB, BAO, and large scale structure should allow us to determine equation of state parameter ω to 10%. 

17. Controlling systematics Miknaitis et al., in prep.

18. The Experiment CTIO 4m with Mosaic imager: 150 half nights over 5 yrs (2002-2006) 3 lunations, every other night, avoiding full moon Broadband RI+V filters 32 fields, 0.36 arc-deg per field 12 sq-deg search 200s R, 400s I, + V when needed Limits at S/N=3 - 24.3 (RI) Remote observing from La Serena

19. ESSENCE goals  ~ 200 supernovae with 0.25 < z < 0.75  determine a distance modulus in each bin (of Δz = 0.1) to 2% (statistical)  ~3% photometry at peak SN brightness  spectroscopic classification  control systematics as much as possible  understand SNe Ia better

20. Sources of systematic error (ESSENCE) Photometry linearity of photometry spatial dependence of PSF on zeropoints Transfer of zeropoint across camera –chip-to-chip flux normalisation –aperture corrections 0.9m calibration error Biases in difference images photometry K-corrections Bandpass miscalibration Limitations of spectral sample “warping” of spectra to reproduce SN colours uncertainty in SED of Vega Distance estimation Biases in overall recovered cosmology –e.g. due to light curve sampling, phase coverage, S/N Cosmological sources Low-z sample zeropoint error Dust –zeropoint of Milky Way maps –Non-standard R V –Non-standard reddening of SNe Ia Lensing Sample contamination by Ib/c SNe

21. Vega is primary celestial calibrator 4% 2% 5000 A 1  m

22. An alternative calibration approach Calibrate end-to-end relative system response primary corrector optics filter detector relative to Si photodiode. Initial run at CTIO Jan 2005 was promising 10% Opotek tunable laser ~100 mW

23. Systematics Extinction by “gray” dust? Careful multicolor measurements, esp. in IR Exploit different z-dependence Look at SNe behind clusters of galaxies “Evolutionary” Effects? Use stellar populations of different ages as a proxy Selection differences in nearby vs. distant samples? Increase the sample of well-monitored SNe Calibrate detection efficiencies K-corrections, Galactic extinction, photometric zeropoints.... See Leibundgut, ARA&A, 31, 69 (2001)

24. Beyond the searches … light curves photometric zero-points extinction light curve shape corrections classifications spectroscopy K-corrections Remember: We need 3% accuracy of peak brightness!

25. Light Curves

26. Running searches SN Legacy Survey

27. ESSENCE spectroscopy - an overview spectroscopy is vital  one axis on Hubble Diagram we now have decent spectra… e.g. from VLT, Keck and Gemini Matheson et al. (2005)

28. Redshifts

29. Checking the redshifts Blondin et al. 2006 Miknaitis et al., in prep.

30. ESSENCE spectroscopy - an overview spectroscopy is vital  one axis on Hubble Diagram (2) we now have decent spectra… e.g. from VLT, Keck and Gemini (3) investigate systematics

31. Investigating evolution Blondin et al. 2006

32. Line velocities No significant differences in the line velocity evolution observed implies similar density structure and element distribution explosion and burning physics similar Peculiarities observed in nearby SNe Ia also observed in the some distant objects detached lines The properties of distant SNe Ia are indistinguishable from the nearby ones with current observations

33. Remember, we said 3%... Control of the systematics is the difficult part understand telescope/camera combination calibration (external and internal) K-corrections

34. More systematics Reddening R V ≈3 amplifies any photometric uncertaintes what is the exact value of R V ? –e.g. Astier et al. use this as a free parameter Evolution remains difficult –not much guidance from the models –no obvious signs (still!)

35. The effect of absorption Unknown absorption law corrections are rather unsecure assume different absorption priors

36. SNLS 1 st year results Astier et al. (2006) Based on 71 distant SNe Ia: for a flat ΛCDM cosmology: Ω M =0.264±0.042 (stat) ± 0.032 (sys) Combined with BAO (Eisenstein et al. 2005) Ω M = 0.271± 0.021 (stat) ± 0.007 (sys) w = -1.02 ± 0.09 (stat) ± 0.054 (sys)

37. ESSENCE cosmology results (preliminary!) Wood-Vasey et al. (2006) Based on 92 distant SNe Ia plus 47 nearby ones Combined with BAO (Eisenstein et al. 2005) full sample w = −1.06±0.15 (stat)‘conservative’ w = − 0.95±0.13 (stat)‘aggressive’ low extinction sample w = − 0.87±0.15 (stat)‘conservative’ w = − 0.89±0.13 (stat)‘aggressive’ systematics still under investigation ‘conservative’‘aggressive’

38. Summary Type Ia Supernovae are fantastic astrophysical laboratories explosion physics becomes more resolved investigation of global parameters –Ni mass –ejecta mass provided some unexpected surprises standard candle picture is too simple

39. The SN Ia Hubble diagram Powerful tool to establish SNe Ia as good distance indicators measure the absolute scale of the universe (H 0 ) determine the amount of dark energy measure the equation of state parameter of dark energy –current best results are consistent with w=-1

40. Interesting years ahead ESSENCE will finish observing in Jan 2007 extension for one season granted SNLS will finish observing in Fall 2007 also will extend the observing for one year Results can be expected in the next couple years

41. Nature of dark energy Riess et al. 2004

42. More general ansatz The change of the expansion rate depending on the contents of the universe with several of the parameters measured accurately, we can start to determine the integrated value of w will need many supernovae per redshift bin

43. Caveat Warning to the theorists: Claims for a measurement of a change of the equation of state parameter ω with published data sets are exaggerated. Current data accuracy is inadequate for too many free parameters in the analysis.

44. Supernovae will continue … Both the US-interagency Task Force on Dark Energy and the ESO-ESA Working Group on Fundamental Cosmology recommend further studies of supernovae to investigate Dark Energy. Several surveys are under way or planned SDSS II (2005-2007; 0.1 300 SNe Ia) Carnegie Supernova Project (2004-2009; 0.1<z<0.5) PanSTARSS (2007 - ) Dark Energy Survey (2010-2015) LSST (>2013) DUNE/JDEM/SNAP etc. (>2015)

45. Time variable ω Current data sets are not sufficient in data quality size systematic control Future surveys must concentrate on the above DUNE, SNAP make use of the stability of space observatories

46. Fight the systematics … Decrease systematics to about 2% photometry to better than 1% –today about 2-3% K-corrections to better than 1% –today about 5-10% host galaxy extinction to better than 1 % –today about 10% control evolution to 1% –today estimated at 5%