Type Ia Supernovae as Probes of Dark Energy

1. Type Ia Supernovae as Probes of Dark Energy Mark Sullivan University of Oxford

2. USA Andy Howell, Alex Conley, Saul Perlmutter, + … Paris Reynald Pain, Pierre Astier, Julien Guy, Nicolas Regnault, Christophe Balland, Delphine Hardin,+ … Toronto Ray Carlberg, Kathy Perrett Marseille StephaneBasa, Dominique Fouchez Victoria Chris Pritchet Oxford Mark Sullivan, Isobel Hook, + … The SNLS collaboration Full list of collaborators at: http://cfht.hawaii.edu/SNLS/

3. Nearly a century after Einstein, the “cosmological constant” is back in vogue • Deluge of astrophysical data show the expansion of the Universe is accelerating. What does this mean? • Gravity should act to slow the expansion • GR is “wrong” - modified gravity on large scales? • “Λ”, >70% of the Universe in an unknown form – “dark energy” • Characterised by an equation of state, w(z) or w(a)

4. The standard candle Measurement of flux gives distance GR: Measure apparent flux and redshift  can infer distance and cosmology Standard candle:

5. The modern day Hubble Diagram Fainter  Further  Faster expansion  More cosmological constant Different cosmological parameters make different predictions in the distance-redshift relation Fainter (Further) Brighter  Nearer  Slower expansion  Higher mass density  Less cosmological constant “Nearby” standard candles “Distance-redshift” relation For a given redshift… Universe was smaller

6. White Dwarf SNeIa: thermonuclear explosions of C-O white dwarf stars “Standard” nuclear physics Uniform triggering mass Bright: 10 billion suns, peak in optical 56Ni 56Co 56Fe powers the SN Ia light-curve Duration: a few weeks Standardizable: 6% calibration Brightness and homogeneity make them the best known measure of distance, and hence dark energy

7. SNe are not good standard candles! • Uncorrected dispersion is ~0.5mag – or 25% in distance • Empirical linear relations exist which reduce this scatter • Brighter SNe have wider light curves • Brighter SNe have a bluer optical colour

8. “Measured” maximum light magnitude Cosmology with SNe Ia SNeIa are standardised, not standard, candles: “c” –opticalcolour estimator corrects for extinction and/or intrinsic variation via β s – “stretch” corrects for light-curve shape via α “Standard” absolute magnitude  and corrections reduce scatter from25%to6% in distance

9. A Typical SNWhat we need to measure Peak brightness Colour (c) Lightcurve width (stretch)

10. SNLS: Vital Statistics >400 high-z confirmed SNeIa to measure “w” 2000 SN detections in total • 2003-2008 SN survey with “MegaCam” on CFHT • griz every 4 nights in queue mode, densely sampled SN light curves

11. SNLS3 Hubble Diagram (preliminary) ~250 distant SNLS SNeIa 128 local SNeIa 86 SDSS-SN Ia 17 from HST 476 SNetotal SNLS+flatness+w=-1: ΩM 0.271±0.017 Sullivan et al. 2009

12. SNLS3 Cosmological Constraints (Preliminary) 4.5% statistical errors WMAP-5 SNe BAO SNLS3 + BAO + WMAP5 “shifts” + Flat Sullivan et al. 2009

13. SNeIa: Systematics and Issues • “Experimental Systematics” • Photometric calibration; contamination; Malmquist biases • Non-SN systematics • Peculiar velocities; Weak lensing • SN model and K-corrections • SED uncertainties; colour relations; light curve fitters • Extinction/Colour • Effective RV; Mix of intrinsic colour and dust • Redshift evolution in the mix of SNe • “Population drift” – environment? • Evolution in SN properties • Light-curves/Colours/Luminosities Tractable, can be modelled

14. Identified systematics in SNLS3 (preliminary) Conley et al. 2009

15. SNLS3 Cosmological Constraints (Preliminary) 4.5% statistical errors WMAP-5 SNe ~5% systematic errors ~7% stat + sys errors No evidence for departures from w=-1 BAO SNLS3 + BAO + WMAP5 “shifts” + Flat Sullivan et al. 2009

16. SNLS3 Cosmological Constraints (Preliminary) 4.5% statistical errors WMAP-5 SNe ~5% systematic errors ~7% stat + sys errors No evidence for departures from w=-1 BAO SNLS3 + BAO + WMAP5 “shifts” + Flat Sullivan et al. 2009

17. Identified systematics in SNLS3 (preliminary) Most uncertainties arise from combining different SN samples Conley et al. 2009

18. Calibration • The single greatest challenge in SNLS3 (and probably every current SN Iasurvey…) • All SNe must be placed on the same photometric system • Different SN samples are calibrated to different systems: • Historical low-redshift samples: Observed in U,B,V,R (Landolt) • High-z: Observed in g,r,i,z- calibrate to SDSS or Landolt? • Challenges: • Zeropoints (colour terms) • Filter (system) throughput • Goal: Replace low-z sample & remove dependence on Landolt system

19. Identified systematics in SNLS3 (preliminary) When low-redshift sample is replaced, systematics should drop below 4% Need for a “rolling” low-z survey (e.g. PTF, Skymapper)

20. SNeIa: Systematics and Issues • “Experimental Systematics” • Photometric calibration; contamination; Malmquist biases • Non-SN systematics • Peculiar velocities; Weak lensing • SN model and K-corrections • SED uncertainties; colour relations; light curve fitters • Extinction/Colour • Effective RV; Mix of intrinsic colour and dust • Redshift evolution in the mix of SNe • “Population drift” – environment? • Evolution in SN properties • Light-curves/Colours/Luminosities Tractable, can be modelled “Extinction” Increasing knowledge of SN physics “Population Evolution”

21. Astrophysics – I: Colour correction • Dust would give a linear relation in log/log space • But, slope, β, << 4.1 (MW dust) • Mixture of external extinction and intrinsic relation? • Properties of the dust near SNe? • Dust in MW is different to other galaxies? Before correction β≈2.9 After correction SN Colour

22. Hubble Bubble • Latest MLCS2k2 paper (Jha 2007) • MLCS2k2 attempts to separate intrinsic colour-luminosity and reddening • 3σ decrease in Hubble constant at ≈7400 km/sec – local value of H0 high; distant SNe too faint • Local void in mass density? • Could have significant effects on w measurement SALT MLCS2k2 No Bubble with other light-curve fitters! Conley et al. (2007)

23. Observed: β ~ 2-3 Standard Dust: β ~ 4.1 “Bubble” significance versus “β” Conley et al. (2007)

24. Astrohysics– II: SN properties and environment Young Old SN light-curve shape strongly depends on host galaxy properties Strong correlation with inferred age(or morphological type)

25. Demographic shifts and cosmology residual, no s correction SN Stretch Plot cosmological residual without(s-1) correction

26. Metallicity See Timmes, Brown & Truran (2003) for full story, including role of 56Fe • CNO catalysts pile up into 14N when H-burning is completed. • During He-burning, 14N is converted into 22Ne, neutron-rich • Higher metallicity means neutron-rich SN Ia • More neutrons during SN, means stable 58Ni and less 56Ni • Fainter SN CNO cycle Slowest step

27. Howell et al. (2008) “Metallicity” No trend between HD residual and inferred metallicity

28. The next surveys • Low-z: New surveys needed to replace existing samples • Palomar Transient Factory (PTF) • 5 years, the first local rolling search (“SNLS @ low-z”) • First compete census of SNeIa in the local universe

29. The next surveys • Higher-z: • Dark Energy Survey (DES) • Starts 201X, “super-charged”-SNLS • Intriguing synergy with VISTA/VIDEO near-IR survey • Ultimately, JDEM or similar mission

30. Near-IR: Models predict smaller dispersion Kasen et al. 2006 DES/VIDEO Current JDEM/Euclid? Excludes effect of dust!

31. Summary • “SNLS3” constraints on <w>: <w>-1 to <4.5% (stat) (inc. flat Universe, BAO+WMAP-5) • Cosmological constant is completely consistent with data • Systematics ~5%; total error ~6%; dominated by z<0.1 sample • “SNLS5” statistical uncertainty will be <4%: • 400 SNLS + 200? SDSS + larger z<0.1 samples, BAO, WL Current issues: • Photometric calibration limiting factor; will improve dramatically • Mean SN Iaproperties evolve with redshift – no bias in cosmology detected • No evidence for metallicity effects • Colourcorrections poorly understood • Need for z<0.1 samples with wide wavelength coverage • Replace existing sample & disentangle SN Iacolours and progenitors • PTF underway since March 2009