Supernovae and Dark Energy Pierre Astier LPNHE / IN2P3 / CNRS , Universités Paris 6&7. Frontiers of Fundamental Physics - July 2014. P. Astier (FFP14 15/07/14)
The expansion of the universe Lemaître (1927), Hubble (1929) “The farther, the fainter” This “Hubble diagram” uses “nebulae” as tracers V = H d (from redshift) From redshift Velocity From apparent flux Distance (from flux) P. Astier (FFP14 15/07/14)
The expansion of the universe V V V V Us D D Isotropy: If distant galaxies are moving away from us, their escape velocity can only depend on distance P. Astier (FFP14 15/07/14)
The expansion of the universe V V V V Us D D Let us change the point of view V 2V V 2V Them Us D 2D P. Astier (FFP14 15/07/14)
The expansion of the universe Cosmological principle : No special direction nor special position Velocity and distance are proportional (at least not too far from us) P. Astier (FFP14 15/07/14)
So, ● V = H d tells us that the universe expands. ● It is consequence of symmetries: no dynamics get encoded there ● Dynamics (i.e. influence of content) show up at higher orders: e.g: different hypotheses for matter density Velocity Distance P. Astier (FFP14 15/07/14)
The real theory ● General relativity relates trajectories of test particles to the content of the universe ● Einstein Equations + cosmological principle → Friedman equation(s) Curvature Expansion rate Energy densities Cosmological constant P. Astier (FFP14 15/07/14)
Evolution of distances as a function of z Evolution of distances with redshift is sensitive to content P. Astier (FFP14 15/07/14)
1996: A Supernova Hubble diagram Calan-Tololo Survey (Hamuy et al, 1996) Excellent distances ! Log(distance) but redshift range too short to go beyond The Hubble law Velocity Distances to ~ 7% P. Astier (FFP14 15/07/14)
Type Ia supernovae Thermonuclear explosions of stars which appear to be reproducible ● Very luminous ● Can be identified (spectroscopy) ● Transient (rise ~ 20 days) ● Scarce (~1 /galaxy/millennium) ● Fluctuations of the peak luminosity : 40 % ● With luminosity indicators : ~14 % P. Astier (FFP14 15/07/14)
Measuring supernovae peak flux multi-band photometry => distance z spectroscopy: - identification - redshift P. Astier (FFP14 15/07/14)
So, at the end of the 90's... ● Distances to Type Ia supernovae were the best hope of measuring the distance-redshift relation ● The idea was to constrain the matter density: ● In a matter-dominated universe q 0 = Ω M /2 Matter density, today (in some unit) P. Astier (FFP14 15/07/14)
1998: the twin papers Riess et al, 1998 [High-z team] Perlmutter et al, 1999 [SCP] P. Astier (FFP14 15/07/14)
Acceleration !? Because distant supernovae are fainter than in a matter-dominated universe ==> Postulate a two-component universe : matter & dark energy Zero curvature DE density variation : ) ρ x ~ (1+z) 3(1+wx) (P x = w x ρ x Static density(i..e Λ ) d Perlmutter et al (99) Free curvature e e c t e a l r l e e l r e a c t c e a DE density varies slowly (or not at all) with time P. Astier (FFP14 15/07/14)
Fall 2011 The Nobel Prize in Physics 2011 was divided, one half awarded to Saul Perlmutter, the other half jointly to Brian P. Schmidt and Adam G. Riess " for the discovery of the accelerating expansion of the Universe through observations of distant supernovae ". P. Astier (FFP14 15/07/14)
Cosmological constant , or what ? Matter 0 Density m Constraints from SNe a t t (Perlmutter et al 1999) e r -1/3 now ?? -2/3 dark energy -1 Λ Scale factor R w : equation of state w tells how the density evolves with expansion ● Matter : w = 0 (follows expansion) ● Cosmological constant w = -1 (ignores expansion) P. Astier (FFP14 15/07/14)
From the discovery of acceleration to the characterisation of dark energy Betoule et al (2014) P. Astier (FFP14 15/07/14)
Getting more efficient Rolling searches on large CCD mosaics Observing steps: ● Discovery in image subtraction From the same images ! ● Spectroscopic ID ● Measure light curves Implemented on 3 major surveys ● Get an image without the SN … with “classical spectroscopy” P. Astier (FFP14 15/07/14)
Major rolling searches The SDSS SN Survey The SNLS survey @ CFHT 4 deg 2 x 5 years 300 deg 2 x 3 years 0.3<z<1 0.1<z<0.45 ~1000 SNe ~2000 SNe ~500 spectra ~500 spectra P. Astier (FFP14 15/07/14)
The current SN sample (for cosmology) Low-z supernovae (z<0.1) : dominated by 2 samples: - CfA (Hicken et al 2009, 2012) - CSP (Contreras et al 2010, Strizinger et al 2011) Rolling surveys at 0.1<z<1 - ~ 2000 Sne - ~ 1000 with spectroscopic ID ~200 SNLS events still High z events with the HST: unpublished... - About 40 events in total today - About 50 % at z>1. P. Astier (FFP14 15/07/14)
The current SN sample (for cosmology) The current SN sample (for cosmology) The current SN sample (for cosmology) >700 SNe P. Astier (FFP14 15/07/14)
Cosmological information Overall brightness Slope (and beyond) Related to SN intrinsic luminosity Ratio of distances across redshifts: and distance scale: → This is what constrains dark energy → No cosmological information P. Astier (FFP14 15/07/14)
measurements We are interested in the ratio of SN luminosities at different redshifts … for similar restframe wavelengths P. Astier (FFP14 15/07/14)
measurements Each SN is measured relative to surrounding stars P. Astier (FFP14 15/07/14)
measurements Vega: historical foundation of photometric System (too bright and … variable...) Field stars are measured Relative to “calibrators” ...derived from stellar models P. Astier (FFP14 15/07/14)
accuracies Blue vs red known to ~ 4 10 -3 ~ 4 10 -3 ~10 -3 P. Astier (FFP14 15/07/14)
accuracies Distant vs nearby SN brightnesses are typically measured to ~ 6 10 -3 (Betoule et al, 2013) P. Astier (FFP14 15/07/14)
Current cosmological results (1) ● A joint effort between the two main SN surveys – Direct cross-calibration (of field stars) – Redundant paths to standard stars (Betoule et al 2103) ● A careful assessment of lightcurve empirical modelling: impact is well below calibration (Mosher et al 2014) P. Astier (FFP14 15/07/14)
Current cosmological results (2) ● 118 nearby SNe ● 366 SDSS ● 242 SNLS ● 14 HST 740 events in total Betoule et al (2014) P. Astier (FFP14 15/07/14)
Flat Λ CDM Ω m measurement independent of CMB and compatible with Planck P. Astier (FFP14 15/07/14)
Flat wCDM Planck + BAO: w = −1.01 ± 0.08 Λ Planck + SN: w = −1.018 ± 0.057 Best EoS constraint. Improvements w.r.t previous results : - improved calibration. Betoule et al (2014) - additional SDSS data - direct cross-calibration P. Astier (FFP14 15/07/14)
What's next ? ● ~130 SNe at z<0.7 from PanSTARss (2014) ● Nearby searches still running ● ~ 200 more SNe from SNLS (out in 2015) ● ~500 SNe/y from DES (z<1, 2013-2017) ● From 2020 onwards: – LSST (could cover the whole range to z=1) – Euclid and WFIRST could target the z>1 régime P. Astier (FFP14 15/07/14)
From 1999 to 2011 Guy et al (2010), Conley et al (2011), Perlmutter et al (1999) Sullivan et al (2011) P. Astier (FFP14 15/07/14)
Outlook ● The second round of SNe surveys have significantly improved the cosmological constraints. ● Λ CDM is doing fine as far as dark energy is concerned : w= -1 +/- 0.057 ● We will shortly go below +/- 0.05. SNe could reach 0.02 by the next decade. ● Sizeable efforts are devoted to improving the probe. P. Astier (FFP14 15/07/14)
More Slides P. Astier (FFP14 15/07/14)
Joint SDSS-SNLS calibration - Short and redundant paths - Direct SDSS-SNLS calibration - Direct observation of HST standards Uncertainties validated through redundancy (Betoule et al, 2013) P. Astier (FFP14 15/07/14)
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